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= Galileo project =
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Introduction
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'Galileo' was an American robotic space program that studied the
planet Jupiter and its moons, as well as several other Solar System
bodies. Named after the Italian astronomer Galileo Galilei, the
'Galileo' spacecraft consisted of an orbiter and an atmospheric entry
probe. It was delivered into Earth orbit on October 18, 1989, by on
the STS-34 mission, and arrived at Jupiter on December 7, 1995, after
gravity assist flybys of Venus and Earth, and became the first
spacecraft to orbit Jupiter. The spacecraft then launched the first
probe to directly measure its atmosphere. Despite suffering major
antenna problems, 'Galileo' achieved the first asteroid flyby, of 951
Gaspra, and discovered the first asteroid moon, Dactyl, around 243
Ida. In 1994, 'Galileo' observed Comet Shoemaker-Levy 9's collision
with Jupiter.
Jupiter's atmospheric composition and ammonia clouds were recorded, as
were the volcanism and plasma interactions on Io with Jupiter's
atmosphere. The data 'Galileo' collected supported the theory of a
liquid ocean under the icy surface of Europa, and there were
indications of similar liquid-saltwater layers under the surfaces of
Ganymede and Callisto. Ganymede was shown to possess a magnetic field
and the spacecraft found new evidence for exospheres around Europa,
Ganymede, and Callisto. 'Galileo' also discovered that Jupiter's faint
ring system consists of dust from impact events on the four small
inner moons. The extent and structure of Jupiter's magnetosphere was
also mapped.
The primary mission concluded on December 7, 1997, but the 'Galileo'
orbiter commenced an extended mission known as the 'Galileo' Europa
Mission (GEM), which ran until December 31, 1999. By the time GEM
ended, most of the spacecraft was operating well beyond its original
design specifications, having absorbed three times the radiation
exposure that it had been built to withstand. Many of the instruments
were no longer operating at peak performance, but were still
functional, so a second extension, the 'Galileo' Millennium Mission
(GMM) was authorized. On September 20, 2003, after 14 years in space
and 8 years in the Jovian system, 'Galileo' mission was terminated by
sending the orbiter into Jupiter's atmosphere at a speed of over 48
km/s to eliminate the possibility of contaminating the moons with
bacteria.
Background
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Jupiter is the largest planet in the Solar System, with more than
twice the mass of all the other planets combined. Consideration of
sending a probe to Jupiter began as early as 1959, when the National
Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory
(JPL) developed four mission concepts:
* Deep space flights would fly through interplanetary space;
* Planetary flyby missions would fly past planets close enough to
collect scientific data and could visit multiple planets on a single
mission;
* Orbiter missions would place a spacecraft in orbit around a planet
for prolonged and detailed study;
* Atmospheric entry and lander missions would explore a planet's
atmosphere and surface.
Two missions to Jupiter, 'Pioneer 10' and 'Pioneer 11', were approved
in 1969, with NASA's Ames Research Center given responsibility for
planning the missions. 'Pioneer 10' was launched in March 1972 and
passed within 200,000 km of Jupiter in December 1973. It was followed
by 'Pioneer 11', which was launched in April 1973, and passed within
34,000 km of Jupiter in December 1974, before heading on to an
encounter with Saturn. They were followed by the more advanced
'Voyager 1' and 'Voyager 2' spacecraft, which were launched on 5
September and 20 August 1977 respectively, and reached Jupiter in
March and July 1979.
Planning
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'Galileo' Project managers
Manager || Date
John R. Casani October 1977 - February 1988
Dick Spehalski February 1988 - March 1990
Bill O'Neil March 1990 - December 1997
Bob Mitchell December 1997 - June 1998
Jim Erickson June 1998 - January 2001
Eilene Theilig January 2001 - August 2003
Claudia Alexander August 2003 - September 2003
Initiation
============
Following the approval of the 'Voyager' missions, NASA's Scientific
Advisory Group for Outer Solar System Missions considered the
requirements for Jupiter orbiters and atmospheric probes. It noted
that the technology to build a heat shield for an atmospheric probe
did not yet exist, and indeed facilities to test one under the
conditions found on Jupiter would not be available until 1980. There
was also concern about the effects of radiation on spacecraft
components, which would be better understood after 'Pioneer 10' and
'Pioneer 11' had conducted their flybys. 'Pioneer 10's' flyby in
December 1973 indicated that the effects were not as severe as had
been feared. NASA management designated JPL as the lead center for the
Jupiter Orbiter Probe (JOP) Project. John R. Casani, who had headed
the 'Mariner' and 'Voyager' projects, became the first project
manager. The JOP would be the fifth spacecraft to visit Jupiter, but
the first to orbit it, and the probe the first to enter its
atmosphere.
Ames and JPL decided to use a 'Mariner' spacecraft for the Jupiter
orbiter like the ones used for 'Voyager' rather than a 'Pioneer'
spacecraft. 'Pioneer' was stabilized by spinning the spacecraft at 60
rpm, which gave a 360-degree view of the surroundings, and did not
require an attitude control system. By contrast, 'Mariner' had an
attitude control system with three gyroscopes and two sets of six
nitrogen jet thrusters. Attitude was determined with reference to the
Sun and Canopus, which were monitored with two primary and four
secondary star tracker sensors. There was also an inertial reference
unit and an accelerometer. The attitude control system allowed the
spacecraft to take high-resolution images, but the functionality came
at the cost of increased weight: a 'Mariner' weighed 722 kg compared
to just 146 kg for a 'Pioneer'.
The increase in weight had implications. The Voyager spacecraft had
been launched by Titan IIIE rockets with a 'Centaur' upper stage, but
Titan was retired afterwards. In the late 1970s, NASA was focused on
the development of the reusable Space Shuttle, which was expected to
make expendable rockets obsolete. In late 1975, NASA decreed that all
future planetary missions would be launched by the Space Shuttle. The
JOP would be the first to do so. The Space Shuttle was supposed to
have the services of a space tug to launch payloads requiring
something more than a low Earth orbit, but this was never approved.
The United States Air Force (USAF) instead developed the solid-fueled
Interim Upper Stage (IUS), later renamed the Inertial Upper Stage
(with the same acronym), for the purpose.
The IUS was constructed in a modular fashion, with two stages, a large
one with 21400 lb of propellant, and a smaller one with 6000 lb. This
was sufficient for most satellites. It could also be configured with
two large stages to launch multiple satellites. A configuration with
three stages, two large and one small, would be enough for a planetary
mission, so NASA contracted with Boeing for the development of a
three-stage IUS. A two-stage IUS was not powerful enough to launch a
payload to Jupiter without resorting to using a series of
gravity-assist maneuvers around planets to garner additional speed.
Most engineers regarded this solution as inelegant and planetary
scientists at JPL disliked it because it meant that the mission would
take months or even years longer to reach Jupiter. Longer travel times
meant that the spacecraft's components would age and possibly fail,
and the onboard power supply and propellant would be depleted. Some of
the gravity assist options also involved flying closer to the Sun,
which would induce thermal stresses that also might cause failures.
It was estimated that the JOP would cost $634 million (equivalent to
$0.634 billion in ), and it had to compete for fiscal year 1978
funding with the Space Shuttle and the Hubble Space Telescope. A
successful lobbying campaign secured funding for both JOP and Hubble
over the objections of Senator William Proxmire, the chairman of the
Independent Agencies Appropriations Subcommittee. The United States
Congress approved funding for the Jupiter Orbiter Probe on July 19,
1977, and JOP officially commenced on October 1, 1977, the start of
the fiscal year. Project manager Casani solicited suggestions for a
more inspirational name for the project from people associated with
it. The most votes went to "Galileo", after Galileo Galilei, the first
person to view Jupiter through a telescope, and the discoverer of what
are now known as the Galilean moons in 1610. It was noted at the time
that the name was also that of a spacecraft in the 'Star Trek'
television show. In February 1978, Casani officially announced the
choice of the name "Galileo".
Preparation
=============
To enhance reliability and reduce costs, the project engineers decided
to switch from a pressurized atmospheric probe to a vented one, so the
pressure inside the probe would be the same as that outside, thus
extending its lifetime in Jupiter's atmosphere, but this added 100 kg
to its weight. Another 165 kg was added in structural changes to
improve reliability. This required additional fuel in the IUS, but the
three-stage IUS was itself overweight with respect to its design
specifications, by about 7000 lb. Lifting 'Galileo' and the
three-stage IUS required a special lightweight version of the Space
Shuttle external tank, the Space Shuttle orbiter stripped of all
non-essential equipment, and the Space Shuttle main engines (SSME)
running at full power level—109 percent of their rated power level.
Running at this power level necessitated the development of a more
elaborate engine cooling system. Concerns were raised over whether the
engines could be run at 109 percent by the launch date, so a
gravity-assist maneuver using Mars was substituted for a direct
flight.
Plans called for the to launch 'Galileo' on the STS-23 mission,
tentatively scheduled for sometime between January 2 and 12, 1982,
this being the launch window when Earth, Mars and Jupiter were aligned
to permit Mars to be used for the gravity-assist maneuver. By 1980,
delays in the Space Shuttle program pushed the launch date for
'Galileo' back to 1984. While a Mars slingshot was still possible in
1984, it would no longer be sufficient.
NASA decided to launch 'Galileo' on two separate missions, launching
the orbiter in February 1984 with the probe following a month later.
The orbiter would be in orbit around Jupiter when the probe arrived,
allowing the orbiter to perform its role as a relay. This
configuration required a second Space Shuttle mission and a second
carrier spacecraft to be built for the probe to take it to Jupiter,
and was estimated to cost an additional $50 million (equivalent to $50
million in ), but NASA hoped to be able to recoup some of this through
competitive bidding. The problem was that while the atmospheric probe
was light enough to launch with the two-stage IUS, the Jupiter orbiter
was too heavy to do so, even with a gravity assist from Mars, so the
three-stage IUS was still required.
By late 1980, the price tag for the IUS had risen to $506 million
(equivalent to $0.506 billion in ). The USAF could absorb this cost
overrun on the development of the two-stage IUS (and indeed
anticipated that it might cost far more), but NASA was faced with a
quote of $179 million (equivalent to $179 million in ) for the
development of the three-stage version, which was $100 million
(equivalent to $100 million in ) more than it had budgeted for. At a
press conference on January 15, 1981, Robert A. Frosch, the NASA
Administrator, announced that NASA was withdrawing support for the
three-stage IUS, and going with a Centaur G Prime upper stage because
"no other alternative upper stage is available on a reasonable
schedule or with comparable costs."
Centaur provided many advantages over the IUS. The main one was that
it was far more powerful. The probe and orbiter could be recombined,
and the probe could be delivered directly to Jupiter in two years'
flight time. The second was that, despite this, it was gentler than
the IUS, because it had lower thrust. This reduced the chance of
damage to the payload. Thirdly, unlike solid-fuel rockets which burned
to completion once ignited, a Centaur could be switched off and on
again. This gave it flexibility, which increased the chances of a
successful mission, and permitted options like asteroid flybys.
Centaur was proven and reliable, whereas the IUS had not yet flown.
The only concern was about safety; solid-fuel rockets were considered
safer than liquid-fuel ones, especially ones containing liquid
hydrogen. NASA engineers estimated that additional safety features
might take up to five years to develop and cost up to $100 million
(equivalent to $100 million in ).
In February 1981, JPL learned that the Office of Management and Budget
(OMB) was planning major cuts to NASA's budget, and was considering
cancelling 'Galileo'. The USAF intervened to save 'Galileo' from
cancellation. JPL had considerable experience with autonomous
spacecraft that could make their own decisions. This was a necessity
for deep space probes, since a signal from Earth takes from 35 to 52
minutes to reach Jupiter, depending on the relative position of the
planets in their orbits. The USAF was interested in providing this
capability for its satellites, so that they would be able to determine
their attitude using onboard systems rather than relying on ground
stations, which were not "hardened" against nuclear weapons, and could
take independent evasive action against anti-satellite weapons. It was
also interested in the manner in which JPL was designing 'Galileo' to
withstand the intense radiation of the magnetosphere of Jupiter, as
this could be used to harden satellites against the electromagnetic
pulse of nuclear explosions. On February 6, 1981 Strom Thurmond, the
President pro tem of the Senate, wrote directly to David Stockman, the
director of the OMB, arguing that 'Galileo' was vital to the nation's
defense.
In December 1984, Casani proposed adding a flyby of asteroid 29
Amphitrite to the 'Galileo' mission. In plotting a course to Jupiter,
the engineers wanted to avoid asteroids. Little was known about them
at the time, and it was suspected that they could be surrounded by
dust particles. Flying through a dust cloud could damage the
spacecraft's optics and possibly other parts of the spacecraft as
well. To be safe, JPL wanted to avoid asteroids by at least 10000 km.
Most of the asteroids in the vicinity of the flight path like 1219
Britta and 1972 Yi Xing were only a few kilometers in diameter and
promised little scientific value when observed from a safe distance,
but 29 Amphitrite was one of the largest, and a flyby at even 10000 km
could have great value. The flyby would delay the spacecraft's arrival
in Jupiter orbit from August 29 to December 10, 1988, and the
expenditure of propellant would reduce the number of orbits of Jupiter
from eleven to ten. This was expected to add $20 to $25 million
(equivalent to $20 to $25 million in ) to the cost of the 'Galileo'
project. The 29 Amphitrite flyby was approved by NASA Administrator
James M. Beggs on December 6, 1984.
During testing, contamination was discovered in the system of metal
slip rings and brushes used to transmit electrical signals around the
spacecraft, and they were returned to be refabricated. The problem was
traced back to a chlorofluorocarbon used to clean parts after
soldering. It had been absorbed, and was then released in a vacuum
environment. It mixed with debris generated as the brushes wore down,
and caused intermittent problems with electrical signal transmission.
Problems were also detected in the performance of memory devices in an
electromagnetic radiation environment. The components were replaced,
but then a read disturb problem arose, in which reads from one memory
location disturbed the contents of adjacent locations. This was found
to have been caused by the changes made to make the components less
sensitive to electromagnetic radiation. Each component had to be
removed, retested, and replaced. All of the spacecraft components and
spare parts received a minimum of 2,000 hours of testing. The
spacecraft was expected to last for at least five years—long enough to
reach Jupiter and perform its mission. On December 19, 1985, it
departed JPL in Pasadena, California, on the first leg of its journey,
a road trip to the Kennedy Space Center in Florida. The 'Galileo'
mission was scheduled for STS-61-G on May 20, 1986, using .
Spacecraft
============
JPL built the Galileo spacecraft and managed the Galileo program for
NASA, but West Germany's Messerschmitt-Bölkow-Blohm supplied the
propulsion module, and Ames managed the atmospheric probe, which was
built by the Hughes Aircraft Company. At launch, the orbiter and probe
together had a mass of 2562 kg and stood 6.15 m tall. There were
twelve experiments on the orbiter and seven on the atmospheric probe.
The orbiter was powered by a pair of general-purpose heat source
radioisotope thermoelectric generators (GPHS-RTGs) fueled by
plutonium-238 that generated 570 watts at launch. The atmospheric
probe had a lithium-sulfur battery rated at 730 watt-hours.
Probe instruments included sensors for measuring atmospheric
temperature and pressure. There was a mass spectrometer and a
helium-abundance detector to study atmospheric composition, and a
whistler detector for measurements of lightning activity and Jupiter's
radiation belt. There were magnetometer sensors, a plasma-wave
detector, a high-energy particle detector, a cosmic and Jovian dust
detector, and a heavy ion counter. There was a near-infrared mapping
spectrometer for multispectral images for atmospheric and moon surface
chemical analysis, and an ultraviolet spectrometer to study gases.
Reconsideration
=================
On January 28, 1986, lifted off on the STS-51-L mission. A failure of
the solid rocket booster 73 seconds into flight tore the spacecraft
apart, resulting in the deaths of all seven crew members. The Space
Shuttle 'Challenger' disaster was America's worst space disaster up to
that time. The immediate impact on the 'Galileo' project was that the
May launch date could not be met because the Space Shuttles were
grounded while the cause of the disaster was investigated. When they
did fly again, 'Galileo' would have to compete with high-priority
Department of Defense launches, the tracking and data relay satellite
system, and the Hubble Space Telescope. By April 1986, it was expected
that the Space Shuttles would not fly again before July 1987 at the
earliest, and 'Galileo' could not be launched before December 1987.
The Rogers Commission into the 'Challenger' disaster handed down its
report on June 6, 1986. It was critical of NASA's safety protocols and
risk management. In particular, it noted the hazards of a Centaur-G
stage. On June 19, 1986, NASA Administrator James C. Fletcher canceled
the Shuttle-Centaur project. This was only partly due to the NASA
management's increased aversion to risk in the wake of the
'Challenger' disaster; NASA management also considered the money and
manpower required to get the Space Shuttle flying again, and decided
that there were insufficient resources to resolve lingering issues
with Shuttle-Centaur as well. The changes to the Space Shuttle proved
more extensive than anticipated, and in April 1987, JPL was informed
that 'Galileo' could not be launched before October 1989. The
'Galileo' spacecraft was shipped back to JPL.
Without Centaur, it looked like there was no means of getting
'Galileo' to Jupiter. For a time, 'Los Angeles Times' science reporter
Usha Lee McFarling noted, "it looked like 'Galileo's' only trip would
be to the Smithsonian Institution." The cost of keeping it ready to
fly in space was reckoned at $40 to $50 million per year (equivalent
to $40 to $50 million in ), and the estimated cost of the whole
project had blown out to $1.4 billion (equivalent to $1.4 billion in
).
At JPL, the 'Galileo' Mission Design Manager and Navigation Team
Chief, Robert Mitchell, assembled a team that consisted of Dennis
Byrnes, Louis D'Amario, Roger Diehl and himself, to see if they could
find a trajectory that would get 'Galileo' to Jupiter using only a
two-stage IUS. Roger Diehl came up with the idea of using a series of
gravity assists to provide the additional velocity required to reach
Jupiter. This would require 'Galileo' to fly past Venus, and then past
Earth twice. This was referred to as the Venus-Earth-Earth Gravity
Assist (VEEGA) trajectory.
The reason no one had considered the VEEGA trajectory before was that
the second encounter with Earth would not give the spacecraft any
extra energy. Diehl realised that this was not necessary; the second
encounter would merely change its direction to put it on a course for
Jupiter. In addition to increasing the flight time, the VEEGA
trajectory had another drawback from the point of view of NASA Deep
Space Network (DSN): 'Galileo' would arrive at Jupiter when it was at
the maximum range from Earth, and maximum range meant minimum signal
strength. It would have a declination of 23 degrees south instead of
18 degrees north, so the tracking station would be the Canberra Deep
Space Communication Complex in Australia, with its two 34-meter and
one 70-meter antennae. A northerly declination could have been
supported by two sites, at Goldstone and Madrid. The Canberra antennae
were supplemented by the 64-meter antenna at the Parkes Observatory.
Initially it was thought that the VEEGA trajectory demanded a November
launch, but D'Amario and Byrnes calculated that a mid-course
correction between Venus and Earth would permit an October launch as
well. Taking such a roundabout route meant that 'Galileo' would
require sixty months to reach Jupiter instead of just thirty, but it
would get there. Consideration was given to using the USAF's Titan IV
launch system with its Centaur G Prime upper stage. This was retained
as a backup for a time, but in November 1988 the USAF informed NASA
that it could not provide a Titan IV in time for the May 1991 launch
opportunity, owing to the backlog of high priority Department of
Defense missions. However, the USAF supplied IUS-19, which had
originally been earmarked for a Department of Defense mission, for use
by the 'Galileo' mission.
Nuclear concerns
==================
As the launch date of 'Galileo' neared, anti-nuclear groups, concerned
over what they perceived as an unacceptable risk to the public's
safety from the plutonium in 'Galileo's' GPHS-RTG modules, sought a
court injunction prohibiting 'Galileo' launch. RTGs were necessary for
deep space probes because they had to fly distances from the Sun that
made the use of solar energy impractical. They had been used for years
in planetary exploration without mishap: the Department of Defense's
Lincoln Experimental Satellites 8/9 had 7 percent more plutonium on
board than 'Galileo', and the two 'Voyager' spacecraft each carried 80
percent of 'Galileo' load of plutonium. By 1989, plutonium had been
used in 22 spacecraft.
Activists remembered the crash of the Soviet Union's nuclear-powered
Kosmos 954 satellite in Canada in 1978, and the 'Challenger' disaster,
while it did not involve nuclear fuel, raised public awareness about
spacecraft failures. No RTGs had ever done a non-orbital swing past
the Earth at close range and high speed, as 'Galileo' VEEGA trajectory
required it to do. This created the possibility of a mission failure
in which Galileo struck Earth's atmosphere and dispersed plutonium.
Planetary scientist Carl Sagan, a strong supporter of the 'Galileo'
mission, wrote that "there is nothing absurd about either side of this
argument."
Before the 'Challenger' disaster, JPL had conducted shock tests on the
RTGs that indicated that they could withstand a pressure of 2,000 psi
without a failure, which would have been sufficient to withstand an
explosion on the launch pad. The possibility of adding additional
shielding was considered but rejected, mainly because it would add an
unacceptable amount of extra weight. After the 'Challenger' disaster,
NASA commissioned a study on the possible effects if such an event
occurred with 'Galileo' on board. Angus McRonald, a JPL engineer,
concluded that what would happen would depend on the altitude at which
the Space Shuttle broke up. If the 'Galileo'/IUS combination fell free
from the orbiter at 90000 ft, the RTGs would fall to Earth without
melting, and drop into the Atlantic Ocean about 150 mi from the
Florida coast. On the other hand, if the orbiter broke up at an
altitude of 323,800 feet it would be traveling at 7957 ft/s and the
RTG cases and GPHS modules would melt before falling into the Atlantic
400 mi off the Florida coast.
NASA concluded that the chance of a disaster was 1 in 2,500, although
anti-nuclear groups thought it might be as high as 1 in 430. NASA
assessed the risk to an individual at 1 in 100 million, about two
orders of magnitude less than the danger of being killed by lightning.
The prospect of an inadvertent re-entry into the atmosphere during the
VEEGA maneuvers was reckoned at less than 1 in 2 million, but an
accident might have released a maximum of 11,568 Ci. This could result
in up to 9 fatalities from cancer per 10 million exposed people.
Launch
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STS-34 was the mission designated to launch 'Galileo', scheduled for
October 12, 1989, in the Space Shuttle 'Atlantis'. The spacecraft was
delivered to the Kennedy Space Center by a high-speed truck convoy
that departed JPL in the middle of the night. There were fears that
the trucks might be hijacked by anti-nuclear activists or terrorists
after the plutonium, so the route was kept secret from the drivers
beforehand, and they drove through the night and the following day and
only stopped for food and fuel.
Last-minute efforts by three environmental groups (the Christic
Institute, the Florida Coalition for Peace and Justice and the
Foundation on Economic Trends) to halt the launch were rejected by the
District of Columbia Circuit on technical grounds rather than the
merits of the case, but in a concurring opinion, Chief Justice
Patricia Wald wrote that while the legal challenge was not frivolous,
there was no evidence of the plaintiffs' claim that NASA had acted
improperly in compiling the mission's environmental assessment. On
October 16, eight protesters were arrested for trespassing at the
Kennedy Space Center; three were jailed and the remaining five
released. Federal judge Oliver Gasch ruled on October 21 that the
launch was in the public interest, as canceling it would cost the
public $164 million and increased knowledge of the Solar system.
The launch was twice delayed; first by a faulty main engine controller
that forced a postponement to October 17, and then by inclement
weather, which necessitated a postponement to the following day, but
this was not a concern since the launch window extended until November
21. 'Atlantis' finally lifted off at 16:53:40 UTC on October 18, and
went into a 213 mi orbit. 'Galileo' was successfully deployed at 00:15
UTC on October 19. Following the IUS burn, the 'Galileo' spacecraft
adopted its configuration for solo flight, and separated from the IUS
at 01:06:53 UTC on October 19. The launch was perfect, and 'Galileo'
was soon headed towards Venus at over 9000 mph. 'Atlantis' returned to
Earth safely on October 23.
Venus encounter
======================================================================
The encounter with Venus on February 9 was in view of the DSN's
Canberra and Madrid Deep Space Communications Complexes. 'Galileo's'
closest approach to Venus came at 05:58:48 UTC on February 10, 1990,
at a range of 16106 km. Due to the Doppler effect, the spacecraft's
velocity relative to Earth could be computed by measuring the change
in carrier frequency of the spacecraft's transmission compared to the
nominal frequency. Doppler data collected by the DSN allowed JPL to
verify that the gravity-assist maneuver had been successful, and the
spacecraft had obtained the expected 2.2 km/s increase in speed.
Unfortunately, three hours into the flyby, the tracking station at
Goldstone had to be shut down due to high winds, and Doppler data was
lost.
Because Venus was much closer to the Sun than the spacecraft had been
designed to operate, great care was taken to avoid thermal damage. In
particular, the X-band high gain antenna (HGA) was not deployed, but
was kept folded up like an umbrella and pointed away from the Sun to
keep it shaded and cool. This meant that the two small S-band low-gain
antennae (LGAs) had to be used instead. They had a maximum bandwidth
of 1,200 bits per second (bit/s) compared to the 134,000 bit/s
expected from the HGA. As the spacecraft moved further from Earth,
reception necessitated the use of the DSN's 70-meter dishes, to the
detriment of other users, who had lower priority than 'Galileo'. Even
so, the downlink telemetry rate fell to 40 bit/s within a few days of
the Venus flyby, and by March it was down to just 10 bit/s.
Venus had been the focus of many automated flybys, probes, balloons
and landers, most recently the 1989 'Magellan' spacecraft, and
'Galileo' had not been designed with Venus in mind. Nonetheless, there
were useful observations that it could make, as it carried some
instruments that had never flown on spacecraft to Venus, such as the
near-infrared mapping spectrometer (NIMS). Telescopic observations of
Venus had revealed that there were certain parts of the infrared
spectrum that the greenhouse gases in the Venusian atmosphere did not
block, making them transparent on these wavelengths. This permitted
the NIMS to both view the clouds and obtain maps of the equatorial-
and mid-latitudes of the night side of Venus with three to six times
the resolution of Earth-based telescopes. The ultraviolet spectrometer
(UVS) was also deployed to observe the Venusian clouds and their
motions.
Another set of observations was conducted using Galileo's
energetic-particles detector (EPD) when 'Galileo' moved through the
bow shock caused by Venus's interaction with the solar wind. Earth's
magnetic field causes the bow shock to occur at around 65000 km from
its center, but Venus's weak magnetic field causes it to occur nearly
on the surface, so the solar wind interacts with the atmosphere. A
search for lightning on Venus was conducted using the plasma-wave
detector, which noted nine bursts likely to have been caused by
lightning, but efforts to capture an image of lightning with the
solid-state imaging system (SSI) were unsuccessful.
Flybys
========
'Galileo' made two course corrections on April 9 to 12 and May 11 to
12, 1990, to alter its velocity by 35 m/s. The spacecraft flew by
Earth twice; the first time at a range of 960 km at 20:34:34 UTC on
December 8, 1990. This was 5 mi higher than predicted, and the time of
the closest approach was within a second of the prediction. It was the
first time that a deep space probe had returned to Earth from
interplanetary space. A second flyby of Earth was at 304 km at
15:09:25 UTC on December 8, 1992. This time the spacecraft passed
within a kilometer of its aiming point over the South Atlantic. This
was so accurate that a scheduled course correction was cancelled,
thereby saving 5 kg of propellant.
Earth's bow shock and the solar wind
======================================
The Earth encounters provided an opportunity for a series of
experiments. A study of Earth's bow shock was conducted as 'Galileo'
passed by Earth's day side. The solar wind travels at 200 to and is
deflected by Earth's magnetic field, creating a magnetic tail on
Earth's dark side over a thousand times the radius of the planet.
Observations were made by 'Galileo' when it passed through the
magnetic tail on Earth's dark side at a distance of 56000 km from the
planet. The magnetosphere was quite active at the time, and 'Galileo'
detected magnetic storms and whistlers caused by lightning strikes.
The NIMS was employed to look for mesospheric clouds, which were
thought to be caused by methane released by industrial processes. The
water vapor in the clouds breaks down the ozone in the upper
atmosphere. Normally the clouds are only seen in September or October,
but 'Galileo' was able to detect them in December, an indication of
possible damage to Earth's ozone layer.
Remote detection of life on Earth
===================================
Carl Sagan, pondering the question of whether life on Earth could be
easily detected from space, devised a set of experiments in the late
1980s using 'Galileo' remote sensing instruments during the mission's
first Earth flyby in December 1990. After data acquisition and
processing, Sagan published a paper in 'Nature' in 1993 detailing the
results of the experiment. 'Galileo' had indeed found what are now
referred to as the "Sagan criteria for life". These included strong
absorption of light at the red end of the visible spectrum (especially
over continents) by chlorophyll in photosynthesizing plants;
absorption bands of molecular oxygen as a result of plant activity;
infrared bands caused by the approximately 1 micromole per mole of
methane (a gas which must be replenished by volcanic or biological
activity) in the atmosphere; and modulated narrowband radio wave
transmissions uncharacteristic of any known natural source. 'Galileo'
experiments were thus the first ever scientific controls in the
newborn science of astrobiological remote sensing.
Lunar observations
====================
File:Moon-galileo-color.jpg|Mare Orientale on Earth's Moon|alt=The
maria are large areas with less cratering
File:The Moon from Galileo - GPN-2000-000473.jpg|'Galileo' shot of the
lunar north pole|alt=The far side is cratered; maria on the near side
File:Moon Crescent - False Color Mosaic.jpg|False-color mosaic by
'Galileo' showing compositional variations of the Moon's surface
|alt=refer to caption
En route to 'Galileo' second gravity-assist flyby of Earth, the
spacecraft flew over the lunar north pole on December 8, 1992, at an
altitude of 110,000 km. The north pole had been photographed before,
by 'Mariner 10' in 1973, but 'Galileo' cameras, with their 1.1 km per
pixel imagery, provided new information about a region that still held
some scientific mysteries. The infrared spectromer surveyed the
surface minerals and revealed that the region was more
minerallogically diverse than expected. There was evidence that the
Moon had been volcanically active earlier than originally thought, and
the spectrometer clearly distinguished different lava flows on the
Mare Serenitatis. Areas where titanium-rich material had been blasted
from vents, like the one sampled by Apollo 17, showed up clearly.
''Galileo'' Optical Experiment
================================
During the second Earth flyby, another experiment was performed.
Optical communications in space were assessed by detecting light
pulses from powerful lasers with 'Galileo' CCD. The experiment, dubbed
'Galileo' Optical Experiment or GOPEX, used two separate sites to beam
laser pulses to the spacecraft, one at Table Mountain Observatory in
California and the other at the Starfire Optical Range in New Mexico.
The Table Mountain site used a Nd:YAG laser operating at a
frequency-doubled wavelength of 532 nm, with a repetition rate of 15
to 30 Hertz and a pulse power full width at half maximum (FWHM) in the
tens of megawatts range, which was coupled to a 0.6 m Cassegrain
reflector telescope for transmission to 'Galileo'. The Starfire range
site used a similar setup with a larger 1.5 m transmitting telescope.
Long-exposure (~0.1 to 0.8 s) images using 'Galileo' 560 nm centered
green filter produced images of Earth clearly showing the laser pulses
even at distances of up to 6 e6km.
Adverse weather conditions, restrictions placed on laser transmissions
by the U.S. Space Defense Operations Center (SPADOC) and a pointing
error caused by the scan platform on the spacecraft not being able to
change direction and speed as quickly as expected (which prevented
laser detection on all frames with less than 400 ms exposure times)
contributed to a reduction in the number of successful detections of
the laser transmission to 48 of the total 159 frames taken.
Nonetheless, the experiment was considered a resounding success and
the data acquired were used to design laser downlinks to send large
volumes of data very quickly from spacecraft to Earth. The scheme was
studied in 2004 for a data link to a future Mars-orbiting spacecraft.
On December 5, 2023, NASA's Deep Space Optical Communications
experiment on the 'Psyche' spacecraft used infrared lasers for two-way
communication between Earth and the spacecraft.
High-gain antenna problem
======================================================================
Once 'Galileo' headed beyond Earth, it was no longer risky to employ
the , so on April 11, 1991, 'Galileo' was ordered to unfurl it. This
was done using two small dual drive actuator (DDA) motors to drive a
worm gear, and was expected to take 165 seconds, or 330 seconds if one
actuator failed. The antenna had 18 graphite-epoxy ribs; when the
driver motor started and put pressure on the ribs, they were supposed
to pop out of the cup their tips were held in, and the antenna would
unfold like an umbrella. When it reached the fully deployed
configuration, redundant microswitches would shut down the motors.
Otherwise they would run for eight minutes before being automatically
shut down to prevent them from overheating.
Through telemetry from 'Galileo', investigators determined that the
electric motors had stalled at 56 seconds. The spacecraft's spin rate
had decreased due to an increase in its moment of inertia and its
wobble increased, indicative of an asymmetric unfolding. Only 15 ribs
had popped out, leaving the antenna looking like a lop-sided,
half-open umbrella. It was not possible to re-fold the antenna and try
the opening sequence again; although the motors were capable of
running in reverse, the antenna was not designed for this, and human
assistance was required when it was done on Earth to ensure that the
wire mesh did not snag.
The first thing the 'Galileo' team tried was to rotate the spacecraft
away from the Sun and back again on the assumption that the problem
was with friction holding the pins in their sockets. If so, then
heating and cooling the ribs might cause them to pop out of their
sockets. This was done seven times, but with no result. They then
tried swinging LGA-2 (which faced in the opposite direction to the HGA
and LGA-1) 145 degrees to a hard stop, thereby shaking the spacecraft.
This was done six times with no effect. Finally, they tried shaking
the antenna by pulsing the DDA motors at 1.25 and 1.875 Hertz. This
increased the torque by up to 40 percent. The motors were pulsed
13,000 times over a three-week period in December 1992 and January
1993, but only managed to move the ballscrew by one and a half
revolutions beyond the stall point.
Investigators concluded that during the 4.5 years that 'Galileo' spent
in storage after the 'Challenger' disaster, the lubricants between the
tips of the ribs and the cup were eroded. They were then worn down by
vibration during the three cross-country journeys by truck between
California and Florida for the spacecraft. The failed ribs were those
closest to the flat-bed trailers carrying 'Galileo' on these trips.
The use of land transport was partly to save costs—air transport would
have cost an additional $65,000 (65000) or so per trip—but also to
reduce the amount of handling required in loading and unloading the
aircraft, which was considered a major risk of damage. The spacecraft
was also subjected to severe vibration in a vacuum environment by the
IUS. Experiments on Earth with the test HGA showed that having a set
of stuck ribs all on one side reduced the DDA torque produced by up to
40 percent.
The antenna lubricants were applied only once, nearly a decade before
launch. Furthermore, the HGA was not subjected to the usual rigorous
testing, because there was no backup unit that could be installed in
'Galileo' in case of damage. The flight-ready HGA was never given a
thermal evaluation test, and was unfurled only a half dozen or so
times before the mission. Testing might not have revealed the problem
in any case; the Lewis Research Center was never able to replicate the
problem on Earth, and it was assumed to be the combination of loss of
lubricant during transportation, vibration during launch by the IUS,
and a prolonged period of time in the vacuum of space where bare metal
touching could undergo cold welding. Whatever the cause, the HGA was
rendered useless.
The two LGAs were capable of transmitting information back to Earth,
but since it transmitted its signal over a cone with a 120-degree
half-angle, allowing it to communicate even when not pointed at Earth,
its bandwidth was significantly less than that of the HGA would have
been, as the HGA transmitted over a half-angle of one-sixth of a
degree. The HGA was to have transmitted at 134 kilobits per second,
whereas LGA-1 was only intended to transmit at about 8 to 16 bits per
second. LGA-1 transmitted with a power of about 15 to 20 watts, which
by the time it reached Earth and had been collected by one of the
large aperture 70-meter DSN antennas, had a total power of about 1020
watts. The change to mission plan required a series of software
changes to be uploaded.
Image data collected was buffered and collected in 'Galileo's' Command
and Data Subsystem (CDS) memory. This represented 192 kilobytes of the
384 kilobyte CDS storage, and had been added late, out of concern that
the 6504 Complementary metal-oxide-semiconductor (CMOS) memory devices
might not be reliable during a mission. As it happened, they gave no
trouble, but the CDS memory could store up to 31 minutes of data from
the Radio Relay Hardware (RRH) channels. To conserve bandwidth,
data-compression software was implemented. Image compression used an
integer approximation of the discrete cosine transform, while other
data were compressed with variant of the Lempel-Ziv-Welch algorithm.
Using compression, the arraying of several Deep Space Network
antennas, and sensitivity upgrades to the receivers used to listen to
'Galileo' signal, data throughput was increased to a maximum of 160
bits per second. By further using data compression, the effective
bandwidth could be raised to 1,000 bits per second.
The data collected on Jupiter and its moons were stored in the
spacecraft's onboard tape recorder, and transmitted back to Earth
during the long apoapsis portion of the probe's orbit using the
low-gain antenna. At the same time, measurements were made of
Jupiter's magnetosphere and transmitted back to Earth. The reduction
in available bandwidth reduced the total amount of data transmitted
throughout the mission, but William J. O'Neil, 'Galileo' project
manager from 1992 to 1997, expressed confidence that 70 percent of
'Galileo' science goals could still be met. The decision to use
magnetic tape for storage was a conservative one, taken in the late
1970s when the use of tape was common. Conservatism was not restricted
to engineers; a 1980 suggestion that the results of 'Galileo' could be
distributed electronically instead of on paper was regarded as
ridiculous by geologists, on the grounds that storage would be
prohibitively expensive; some of them thought that taking measurements
on a computer involved putting a wooden ruler up to the screen.
951 Gaspra
============
Two months after entering the asteroid belt, 'Galileo' performed the
first asteroid encounter by a spacecraft. 'Galileo' passed 951 Gaspra,
an S-type asteroid, at a distance of 1604 km at 22:37 UTC on October
29, 1991, at a relative speed of about 8 km/s. Fifty-seven images of
Gaspra were taken with the SSI, covering about 80 percent of the
asteroid. Without the HGA, the bit rate was only about 40 bit/s, so an
image took up to 60 hours to transmit back to Earth. The 'Galileo'
project was able to secure 80 hours of Canberra's 70-meter dish time
between 7 and 14 November 1991, but most of images taken, including
low-resolution images of more of the surface, were not transmitted to
Earth until November 1992.
The imagery revealed a cratered and irregular body, measuring about 19
by. Its shape was not remarkable for an asteroid of its size.
Measurements were taken using the NIMS to indicate the asteroid's
composition and physical properties. While Gaspra has plenty of small
craters—over 600 of them ranging in size from 100 to—it lacks large
ones, hinting at a relatively recent origin, although it is possible
that some of the depressions were eroded craters. Several relatively
flat planar areas were found, suggesting that Gaspra was formed from
another body by a collision. Measurements of the solar wind in the
vicinity of the asteroid showed it changing direction a few hundred
kilometers from Gaspra, which hinted that Gaspra might have a magnetic
field, but this was not certain.
243 Ida and Dactyl
====================
Following the second Earth encounter, 'Galileo' performed close
observations of another asteroid, 243 Ida. A slight trajectory
correction was made to enable this on August 26, 1993. With four hours
to go before the encounter with Ida, 'Galileo' spontaneously abandoned
the observation configuration and resumed its cruise configuration.
Engineers were able to correct the problem and have the instruments
ready by 16:52:04 UTC on August 28, 1993, when 'Galileo' flew past Ida
at a range of 2410 km. High-resolution images were taken to create a
color mosaic of one side of the asteroid, with the highest resolution
image taken at a range of 10,500 mi. Measurements were taken using SSI
and NIMS.
Transmission was still limited to the 40 bit/s data rate available
during the Gaspra flyby. At that rate, it took thirty hours to send
each of the five frames. In September, the line of sight between
'Galileo' and Earth was close to the Sun, so there was only time to
send one mosaic before it was blocked by the Sun on September 29,
1993; the rest of the mosaics were transmitted in February and March,
after Earth had come around the Sun. 'Galileo' tape recorder was used
to store the images, but tape space was also required for the primary
Jupiter mission. A technique was developed whereby only image
fragments of two or three lines out of every 330 were initially sent.
A determination could then be made as to whether the image was of 243
Ida or of empty space. Ultimately, only about 16 percent of the SSI
data recorded could be sent back to Earth.
When astronomer Ann Harch examined the images on February 17, 1994,
she found that Ida had a small moon measuring around 1.6 km in
diameter, which appeared in 47 images. A competition was held among
'Galileo' project members to select a name for the moon, which was
ultimately dubbed Dactyl after the legendary Dactyls, mythical beings
which lived on Mount Ida, the geographical feature on Crete the
asteroid was named for. Craters on Dactyl were named after individual
dactyloi. Regions on 243 Ida were named after cities where Johann
Palisa, the discover of 243 Ida, made his observations, while ridges
on 243 Ida were named in honor of deceased 'Galileo' team members.
Dactyl was the first asteroid moon to be discovered. Moons of
asteroids had been assumed to be rare, but the discovery of Dactyl
hinted that they might in fact be quite common. From subsequent
analysis of this data, Dactyl appeared to be an S-type asteroid, and
spectrally different from 243 Ida, although Ida is also an S-type
asteroid. It was hypothesized that both may have been produced by the
breakup of a Koronis parent body.
Comet Shoemaker–Levy 9
========================
'Galileo' prime mission was a two-year study of the Jovian system, but
on March 26, 1993, while it was en route, astronomers Carolyn S.
Shoemaker, Eugene M. Shoemaker and David H. Levy discovered fragments
of a comet orbiting Jupiter, the remains of a comet that had passed
within Jupiter's Roche limit and had been torn apart by tidal forces.
It was named Comet Shoemaker-Levy 9. Calculations indicated that it
would crash into the planet sometime between July 16 and 24, 1994.
Although 'Galileo' was still 238 e6km away, Jupiter was 66 pixels wide
in its camera, and it was perfectly positioned to observe this event.
Terrestrial telescopes had to wait to see the impact event sites as
they rotated into view because it would occur on Jupiter's night side.
Instead of burning up in Jupiter's atmosphere as expected, the first
of the 21 comet fragments struck the planet at around 320000 km/h and
exploded with a fireball 3000 km high, easily discernible to
Earth-based telescopes even though it was on the night side of the
planet. The impact left a series of dark scars on the planet, some two
or three times as large as the Earth, that persisted for weeks. When
'Galileo' observed an impact in ultraviolet light, the fireballs
lasted for about ten seconds, but in the infrared they persisted for
90 seconds or more. When a fragment hit the planet, it increased
Jupiter's overall brightness by about 20 percent. The NIMS observed
one fragment create a fireball 7 km in diameter that burned with a
temperature of 8000 K, which was hotter than the surface of the Sun.
Probe deployment
==================
The 'Galileo' probe separated from the orbiter at 03:07 UTC on July
13, 1995, five months before its rendezvous with the planet on
December 7. At this point, the spacecraft was 83 e6km from Jupiter,
but 664 e6km from Earth, and telemetry from the spacecraft,
transmitted at the speed of light, took 37 minutes to reach JPL. A
tiny frequency change in the radio signal indicated that the
separation had been accomplished. The 'Galileo' orbiter was still on a
collision course with Jupiter. Previously, course corrections had been
made using the twelve 10 N thrusters, but with the probe on its way,
the 'Galileo' orbiter could now fire its 400 N
Messerschmitt-Bölkow-Blohm main engine which had been covered by the
probe until then. At 07:38 UTC on July 27, it was fired for the first
time to place the 'Galileo' orbiter on course to enter orbit around
Jupiter, whence it would act as a communications relay for the
'Galileo' probe. The 'Galileo' probe's project manager, Marcie Smith
at the Ames Research Center, was confident that the LGAs could be used
as relays. The burn lasted for five minutes and eight seconds, and
changed the velocity of the 'Galileo' orbiter by 61.9 m/s.
Dust storms
=============
In August 1995, the 'Galileo' orbiter encountered a severe dust storm
63 e6km from Jupiter that took several months to traverse. Normally
the spacecraft's dust detector picked up a dust particle every three
days; now it detected up to 20,000 particles a day. Interplanetary
dust storms had previously been encountered by the 'Ulysses' probe,
which had passed by Jupiter three years before on its mission to study
the Sun's polar regions, but those encountered by 'Galileo' were more
intense. The dust particles were 5 to 10 nm in size, about the same as
those in cigarette smoke, and had speeds ranging from 90,000 to
depending on their size. The existence of the dust storms had come as
a complete surprise to scientists when 'Ulysses' encountered them.
While data from both 'Ulysses' and 'Galileo' hinted that they
originated somewhere in the Jovian system, it was a mystery how they
had been created and how they had escaped from Jupiter's strong
gravitational and electromagnetic fields.
Tape recorder anomaly
=======================
The failure of 'Galileo' high-gain antenna meant that data storage to
the tape recorder for later compression and playback was crucial in
order to obtain any substantial information from the flybys of Jupiter
and its moons. The four-track, 114-megabyte digital tape recorder was
manufactured by Odetics Corporation. On October 11, it was stuck in
rewind mode for 15 hours before engineers learned what had happened
and were able to send commands to shut it off. Although the recorder
itself was still in working order, the malfunction had possibly
damaged a length of tape at the end of the reel. This section of tape
was declared "off limits" to any future data recording, and was
covered with 25 more turns of tape to secure the section and reduce
any further stresses, which could tear it. Because it happened only
weeks before 'Galileo' entered orbit around Jupiter, the anomaly
prompted engineers to sacrifice data acquisition of almost all of the
Io and Europa observations during the orbit insertion phase in order
to focus on recording data sent from the atmospheric probe during its
descent.
Arrival
=========
The 'Galileo' orbiter's magnetometers reported that the spacecraft had
encountered the bow shock of Jupiter's magnetosphere on November 16,
1995, when it was 15 e6km from Jupiter. The bow shock moved to and fro
in response to solar wind gusts, and was therefore crossed multiple
times between 16 and 26 November, by which time 'Galileo' was 9 e6km
from Jupiter.
On December 7, 1995, the orbiter arrived in the Jovian system. That
day it made a 32500 km flyby of Europa at 11:09 UTC, and then an 890
km flyby of Io at 15:46 UTC, using Io's gravity to reduce its speed,
and thereby conserve propellant for use later in the mission. At 19:54
it made its closest approach to Jupiter. The orbiter's electronics had
been heavily shielded against radiation, but the radiation surpassed
expectations, and nearly exceeded the spacecraft's design limits. One
of the navigational systems failed, but the backup took over. Most
robotic spacecraft respond to failures by entering safe mode and
awaiting further instructions from Earth, but this was not possible
for 'Galileo' during the arrival sequence due to the great distance
and consequent long turnaround time.
Atmospheric probe
===================
The descent probe awoke in response to an alarm at 16:00 UTC and began
powering up its instruments. It passed through the rings of Jupiter
and encountered a previously undiscovered radiation belt ten times as
strong as Earth's Van Allen radiation belt 31,000 mi above Jupiter's
cloud tops. It had been predicted that the probe would pass through
three layers of clouds; an upper one consisting of ammonia-ice
particles at a pressure of 0.5 to; a middle one of ammonium
hydrosulfide ice particles at a pressure of 1.5 to; and one of water
vapor at 4 to. The atmosphere through which the probe descended was
much denser and hotter than expected. Jupiter was also found to have
only half the amount of helium expected and the data did not support
the three-layered cloud structure theory: only one significant cloud
layer was measured by the probe, at a pressure of around 1.55 bar but
with many indications of smaller areas of increased particle densities
along the whole length of its trajectory.
The descent probe entered Jupiter's atmosphere, defined for the
purpose as being 450 km above the 1 bar pressure level, without any
braking at 22:04 UTC on December 7, 1995. At this point it was moving
at 170700 km/h relative to Jupiter. This was by far the most difficult
atmospheric entry yet attempted by any spacecraft; the probe had to
withstand a peak deceleration of 228 g0. The rapid flight through the
atmosphere produced a plasma with a temperature of about 14,000 C, and
the probe's carbon phenolic heat shield lost more than half of its
mass, 80 kg, during the descent. As the probe passed through Jupiter's
cloud tops, it started transmitting data to the orbiter, 215000 km
above. The data was not immediately relayed to Earth, but a single bit
was transmitted from the orbiter as a notification that the signal
from the probe was being received and recorded, which would then take
days to be transmitted using the LGA.
The atmospheric probe deployed its 2.5 m parachute fifty-three seconds
later than anticipated, resulting in a small loss of upper-atmospheric
readings. This was attributed to wiring problems with an accelerometer
that determined when to begin the parachute deployment sequence. The
probe then dropped its heat shield, which fell into Jupiter's
interior. The parachute reduced the probe's speed to 430 km/h. The
signal from the probe was no longer detected by the orbiter after 61.4
minutes, at an elevation of 112 miles below the cloud tops and a
pressure of 22.7 atm. It was believed that the probe continued to fall
at terminal velocity, as the temperature increased to 1700 C and the
pressure to 5000 atm, destroying it.
File:Galileo Probe - AC81-0174.jpg|Artist's impression of the probe's
entry into Jupiter's atmosphere |alt=refer to caption
Image:Galileo atmospheric probe.jpg|Timeline of the probe's
atmospheric entry |alt=Probe enters, deploys parachute, transmission
ends 61.4 minutes after entry where the pressure is ~
File:Jupiter's clouds.jpg|Jupiter's clouds - expected and actual
results of 'Galileo's' atmospheric probe mission |alt=The clouds of
ammonia and ammonium sulfide were much thinner than expected, and
clouds of water vapor were not detected.
The probe detected less lightning, less water, but stronger winds than
expected. Scientists had expected to find wind speeds of up to 220
mph, but winds of up to 330 mph were detected. The implication was
that the winds are not produced by heat generated by sunlight (as
Jupiter gets less sunlight than Earth) or the condensation of water
vapor (the main causes on Earth), but are due to an internal heat
source. It was already well known that the atmosphere of Jupiter was
mainly composed of hydrogen, but the clouds of ammonia and ammonium
sulfide were much thinner than expected, and clouds of water vapor
were not detected. This was the first observation of ammonia clouds in
another planet's atmosphere. The atmosphere creates ammonia-ice
particles from material coming up from lower depths.
The atmosphere was more turbulent than expected. Wind speeds in the
outermost layers were 290 to, in agreement with previous measurements
from afar, but those wind speeds increased dramatically at pressure
levels of 1 to, then remaining consistently high at around 170 m/s.
The abundance of nitrogen, carbon and sulfur was three times that of
the Sun, raising the possibility that they had been acquired from
other bodies in the Solar system, but the low abundance of water cast
doubt on theories that Earth's water had been acquired from comets.
There was far less lightning activity than expected, only about a
tenth of the level of activity on Earth, but this was consistent with
the lack of water vapor. More surprising was the high abundance of
noble gases (argon, krypton and xenon), with abundances up to three
times that found in the Sun. For Jupiter to trap these gases, it would
have had to be much colder than today, around -240 C, which suggested
that either Jupiter had once been much further from the Sun, or that
the interstellar debris that the Solar system had formed from was much
colder than had been thought.
Orbiter
=========
With the probe data collected, the 'Galileo' orbiter's next task was
to slow down in order to avoid heading off into the outer solar
system. A burn sequence commencing at 00:27 UTC on December 8 and
lasting 49 minutes reduced the spacecraft's speed by 600 m/s and it
entered a parking orbit with an orbital period of 198 days. The
'Galileo' orbiter thus became the first artificial satellite of
Jupiter. Most of its initial orbit was occupied transmitting the data
from the probe back to Earth. When the orbiter reached its apojove on
March 26, 1996, the main engine was fired again to increase the orbit
from four times the radius of Jupiter to ten times. By this time the
orbiter had received half the radiation allowed for in the mission
plan, and the higher orbit was to conserve the instruments for as long
as possible by limiting the radiation exposure.
The spacecraft traveled around Jupiter in elongated ellipses, each
orbit lasting about two months. The differing distances from Jupiter
afforded by these orbits allowed 'Galileo' to sample different parts
of the planet's extensive magnetosphere. The orbits were designed for
close-up flybys of Jupiter's largest moons. A naming scheme was
devised for the orbits: a code with the first letter of the moon being
encountered on that orbit (or "J" if none was encountered) plus the
orbit number.
Mission extension
===================
After the primary mission concluded on December 7, 1997, most of the
mission staff departed, including O'Neil, but about a fifth of them
remained. The 'Galileo' orbiter commenced an extended mission known as
the 'Galileo' Europa Mission (GEM), which ran until December 31, 1999.
This was a low-cost mission, with a budget of $30 million (equivalent
to $30 million in ). The reason for calling it as the "Europa" mission
rather than the "Extended" mission was political; although it was
wasteful to scrap a spacecraft that was still functional and capable
of performing a continuing mission, Congress took a dim view of
requests for more money for projects that had already been fully
funded. This was avoided through rebranding.
The smaller GEM team did not have the resources to deal with problems,
but when they arose it was able to temporarily recall former team
members for intensive efforts to solve them. The spacecraft performed
several flybys of Europa, Callisto and Io. On each one the spacecraft
collected only two days' worth of data instead of the seven it had
collected during the prime mission. The radiation environment near Io,
which 'Galileo' approached to within 201 km on November 26, 1999, on
orbit I25, was very unhealthy for 'Galileo' systems, and so these
flybys were saved for the extended mission when loss of the spacecraft
would be more acceptable.
By the time GEM ended, most of the spacecraft was operating well
beyond its original design specifications, having absorbed more than
600 kilorads in between 1995 and 2002, three times the radiation
exposure that it had been built to withstand. Many of the instruments
were no longer operating at peak performance, but were still
functional, so a second extension, the 'Galileo' Millennium Mission
(GMM) was authorized. This was intended to run until March 2001, but
it was subsequently extended until January 2003. GMM included return
visits to Europa, Io, Ganymede and Callisto, and for the first time to
Amalthea. The total cost of the original 'Galileo' mission was about
(equivalent to $1.39 billion in ). Of this, (equivalent to $892
million in ) was spent on spacecraft development. Another $110 million
(equivalent to $110 million in ) was contributed by international
agencies.
Io
====
The innermost of the four Galilean moons, Io is roughly the same size
as Earth's moon, with a mean radius of 1821.3 km. It is in orbital
resonance with Ganymede and Europa, and tidally locked with Jupiter,
so just as the Earth's Moon always has the same side facing Earth, Io
always has the same side facing Jupiter. It has a faster orbit though,
with a rotation period of 1.769 days. As a result, the rotational and
tidal forces on Io are 220 times as great as those on Earth's moon.
These frictional forces are sufficient to melt rock, creating
volcanoes and lava flows. Although only a third of the size of Earth,
Io generates twice as much heat. While geological events occur on
Earth over periods of thousands or even millions of years, cataclysmic
events are common on Io. Visible changes occurred between orbits of
'Galileo'. The colorful surface is a mixture of red, white and yellow
sulfur compounds.
'Galileo' flew past Io, but in the interest of protecting the tape
recorder, O'Neil decided to forego collecting images. To use the SSI
camera meant operating the tape recorder at high speed, with sudden
stops and starts, whereas the fields and particles instruments only
required the tape recorder to run continuously at slow speeds, and it
was believed that it could handle this. This was a crushing blow to
scientists, some of whom had waited years for the opportunity. No
other Io encounters were scheduled during the prime mission because it
was feared that the high radiation levels close to Jupiter would
damage the spacecraft. However, valuable information was still
obtained; Doppler data used to measure Io's gravitational field
revealed that Io had a core of molten iron and iron sulfide.
Another opportunity to observe Io arose during the 'Galileo' Europa
Mission (GEM), when 'Galileo' flew past Io on orbits I24 and I25, and
it would revisit Io during the 'Galileo' Millennium Mission (GMM) on
orbits I27, I31, I32 and I33. As 'Galileo' approached Io on I24 at
11:09 UTC on October 11, 1999, it entered safe mode. Apparently,
high-energy electrons had altered a bit on a memory chip. When it
entered safe mode, the spacecraft turned off all non-essential
functions. Normally it took seven to ten days to diagnose and recover
from a safe mode incident; this time the 'Galileo' Project team at JPL
had nineteen hours before the encounter with Io. After a frantic
effort, they managed to diagnose a problem that had never been seen
before, and restore the spacecraft systems with just two hours to
spare. Not all of the planned activities could be carried out, but
'Galileo' obtained a series of high-resolution color images of the
Pillan Patera, and Zamama, Prometheus, and Pele volcanic eruption
centers.
When 'Galileo' next approached Io on I25 at 03:40 UTC on November 26,
1999, JPL were eating their Thanksgiving dinner at the 'Galileo'
Mission Control Center when, with the encounter with Io just four
hours away, the spacecraft again entered safe mode. This time the
problem was traced to a software patch implemented to bring 'Galileo'
out of safe mode during I24. Fortunately, the spacecraft had not shut
down as much as it had on I24, and the team at JPL were able to bring
it back online. During I24 they had done so with two hours to spare;
this time, they had just three minutes. Nonetheless, the flyby was
successful, with 'Galileo' NIMS and SSI camera capturing an erupting
volcano that generated a 20 mi long plume of lava that was
sufficiently large and hot to have also been detected by the NASA
Infrared Telescope Facility atop Mauna Kea in Hawaii. While such
events were more common and spectacular on Io than on Earth, it was
extremely fortuitous to have captured it; planetary scientist Alfred
McEwen estimated the odds at 1 in 500.
The safe-mode incidents on I24 and I25 left some gaps in the data,
which I27 targeted. This time 'Galileo' passed 198 km over the surface
of Io. At this time, the spacecraft was nearly at the maximum distance
from Earth, and there was a solar conjunction, a period when the Sun
blocked the line of sight between Earth and Jupiter. As a consequence,
three quarters of the observations had to be taken over a period of
three hours. NIMS images revealed fourteen active volcanoes in a
region thought to contain just four. Images of Loki Patera showed that
in the four and half months between I24 and I27, some 10000 km2 had
been covered in fresh lava. A series of observations of extreme
ultraviolet (EUV) had to be cancelled due to yet another safe-mode
event. Radiation exposure caused a transient bus reset, a computer
hardware error resulting in a safe mode event. A software patch
implemented after the Europa encounter on orbit E19 guarded against
this when the spacecraft was within 15 Jupiter radii of the planet,
but this time it occurred at 29 Jupiter radii. The safe mode event
also caused a loss of tape playback time, but the project managers
decided to carry over some Io data into orbit G28, and play it back
then. This limited the amount of tape space available for that
Ganymede encounter, but the Io data was considered to be more
valuable.
The discovery of Io's iron core raised the possibility that it had a
magnetic field. The I24, I25 and I27 encounters had involved passes
over Io's equator, which made it difficult to determine whether Io had
its own magnetic field or one induced by Jupiter. Accordingly, on
orbit I31, 'Galileo' passed within 200 km of the surface of the north
pole of Io, and on orbit I32 it flew 181 km over the south pole. After
examining the magnetometer results, planetary scientist Margaret G.
Kivelson, announced that Io had no intrinsic magnetic field, which
meant that its molten iron core did not have the same convective
properties as that of Earth.
On I31 'Galileo' sped through an area that had been in the plume of
the Tvashtar Paterae volcano, and it was hoped that the plume could be
sampled. This time, Tvashtar was quiet, but the spacecraft flew
through the plume of another, previously unknown, volcano 600 km away.
What had been assumed to be hot ash from the volcanic eruption turned
out to be sulfur dioxide snowflakes, each consisting of 15 to 20
molecules clustered together. 'Galileo' final return to Io on orbit
I33 was marred by another safe mode incident, and much of the
hoped-for data was lost.
Europa
========
Although the smallest of the four Galilean moons, with a radius of
1565 km, Europa is the sixth-largest moon in the solar system.
Observations from Earth indicated that it was covered in ice. Like Io,
Europa is tidally locked with Jupiter. It is in orbital resonance with
Io and Ganymede, with its 85-hour orbit being twice that of Io, but
half that of Ganymede. Conjunctions with Io always occur on the
opposite side of Jupiter to those with Ganymede. Europa is therefore
subject to tidal effects. There is no evidence of volcanism like on
Io, but 'Galileo' revealed that the surface ice was covered in cracks.
Some observations of Europa were made during orbits G1 and G2. On C3,
'Galileo' conducted a 34,800 km "nontargeted" encounter of
Europa—meaning a secondary flyby at a distance of up to 100,000 km—on
November 6, 1996. During E4 from December 15 to 22, 1996, 'Galileo'
flew within 692 km of Europa, but data transmission was hindered by a
Solar occultation that blocked transmission for ten days.
'Galileo' returned to Europa on E6 in January 1997, this time at a
height of 586 km, to analyze oval-shaped features in the infrared and
ultraviolet spectra. Occultations by Europa, Io and Jupiter provided
data on the atmospheric profiles of them, and measurements were made
of Europa's gravitational field. On E11 from November 2 to 9, 1997,
data was collected on the magnetosphere. Due to the problems with the
HGA, only about two percent of the anticipated number of images of
Europa were obtained by the primary mission. On the GEM, the first
eight orbits (E12 through E19) were all dedicated to Europa, and
'Galileo' paid it a final visit on E26 during the GMM.
Images of Europa also showed few impact craters. It seemed unlikely
that it had escaped the meteor and comet impacts that scarred Ganymede
and Callisto, so this indicated Europa has an active geology that
renews the surface and obliterates craters. Astronomer Clark Chapman
argued that, assuming a 20 km crater occurs in Europa once every
million years, and given only about twenty have been spotted on
Europa, the implication is that the surface must only be about 10
million years old. With more data on hand, in 2003 a team led by Kevin
Zahle at NASA's Ames Research Center arrived at a figure of 30 to 70
million years. Tidal flexing of up to 100 m per day was the most
likely culprit. But not all scientists were convinced; Michael Carr, a
planetologist from the US Geological Survey, argued that, on the
contrary, Europa's surface age was closer to a billion years. He
compared the craters on Ganymede with those on Earth's moon, and
concluded that the satellites of Jupiter were not subject to the same
amount of cratering.
Evidence of surface renewal hinted at the possibility of a viscous
layer below the surface of warm ice or liquid water. NIMS observations
by 'Galileo' indicated that the surface of Europa appeared to contain
magnesium- and sodium-based salts. A likely source was brine below the
ice crust. Further evidence was provided by the magnetometer, which
reported that the magnetic field was induced by Jupiter. This could be
explained by the existence of a spherical shell of conductive material
like salt water. Since the surface temperature on Europa was -162 C,
any water breaching the surface ice would instantly freeze over. Heat
required to keep water in a liquid state could not come from the Sun,
which at that distance had only 4 percent of the intensity it had on
Earth, but ice is a good insulator, and the heat could be provided by
the tidal flexing. 'Galileo' also yielded evidence that the crust of
Europa had slipped over time, moving south on the hemisphere facing
Jupiter, and north on the far side.
There was acrimonious debate among scientists over the thickness of
the ice crust, and those who presented results indicating that it
might be thinner than the 20 to proposed by the accredited scientists
on the 'Galileo' Imaging Team faced intimidation, scorn, and reduced
career opportunities. The 'Galileo' Imaging Team was led by Michael J.
Belton from the Kitt Peak National Observatory. Scientists who planned
imaging sequences had the exclusive right to the initial
interpretation of the 'Galileo' data, most which was performed by
their research students. The scientific community did not want a
repetition of the 1979 Morabito incident, when Linda A. Morabito, an
engineer at JPL working on 'Voyager 1', discovered the first active
extraterrestrial volcano on Io. The Imaging Team controlled the manner
in which discoveries were presented to the scientific community and
the public through press conferences, conference papers and
publications.
Observations by the Hubble Space Telescope in 1995 reported that
Europa had a thin oxygen atmosphere. This was confirmed by 'Galileo'
in six experiments on orbits E4 and E6 during occultations when Europa
was between 'Galileo' and the Earth. This allowed Canberra and
Goldstone to investigate the ionosphere of Europa by measuring the
degree to which the radio beam was diffracted by charged particles.
This indicated the presence of water ions, which were most likely
water molecules that had been dislodged from the surface ice and then
ionized by the Sun or the Jovian magnetosphere. The presence of an
ionosphere was sufficient to deduce the existence of a thin atmosphere
on Europa.
On December 11, 2013, NASA reported, based on results from the
'Galileo' mission, the detection of "clay-like minerals"
(specifically, phyllosilicates), often associated with organic
materials, on the icy crust of Europa. The presence of the minerals
may have been the result of a collision with an asteroid or comet.
Ganymede
==========
The largest of the Galilean moons with a radius of 2620 km, Ganymede
is larger than Earth's moon, the dwarf planet Pluto or the planet
Mercury. It is the largest of the moons in the Solar system that are
characterized by large amounts of water ice, which also includes
Saturn's moon Titan, and Neptune's moon Triton. Ganymede has three
times as much water for its mass as Earth has.
When 'Galileo' entered Jovian orbit, it did so at an orbital
inclination to the Jovian equator, and therefore in the orbital plane
of the four Galilean moons. To transfer orbit while conserving
propellant, two slingshot maneuvers were performed. On G1, the gravity
of Ganymede was used to slow the spacecraft's orbital period from 210
to 72 days to allow for more encounters and to take 'Galileo' out of
the more intense regions of radiation. On G2, the gravity assist was
employed to put it into a coplanar orbit to permit subsequent
encounters with Io, Europa and Callisto.
Although the primary purpose of G1 and G2 was navigational, the
opportunity to make some observations was not missed. The plasma-wave
experiment and the magnetometer detected a magnetic field with a
strength of about 750 nT, more than strong enough to create a separate
magnetosphere within that of Jupiter. This was the first time that a
magnetic field had ever been detected on a moon contained within the
magnetosphere of its host planet. This discovery led naturally to
questions about its origin. The evidence pointed to an iron or iron
sulfide core and mantle 400 to below the surface, encased in ice.
Margaret Kivelson, the scientist in charge of the magnetometer
experiment, contended that the induced magnetic field required an iron
core, and speculated that an electrically conductive layer was
required, possibly a brine ocean 200 km below the surface.
'Galileo' returned to Ganymede on orbits G7 and G9 in April and May
1997, and on G28 and G29 in May and December 2000 on the GMM. Images
of the surface revealed two types of terrain: highly cratered dark
regions and grooved terrain sulcus. Images of the Arbela Sulcus taken
on G28 made Ganymede look more like Europa, but tidal flexing could
not provide sufficient heat to keep water in liquid form on Ganymede,
although it may have made a contribution. One possibility was
radioactivity, which might provide sufficient heat for liquid water to
exist 50 to below the surface. Another possibility was volcanism.
Slushy water or ice reaching the surface would quickly freeze over,
creating areas of a relatively smooth surface.
Callisto
==========
Callisto is the outermost of the Galilean moons, and the most
pockmarked, indeed the most of any body in the Solar system. So many
craters must have taken billions of years to accumulate, which gave
scientists the idea that its surface was as much as four billion years
old, and provided a record of meteor activity in the Solar system.
'Galileo' visited Callisto on orbits C3, C9 and C100 during the prime
mission, and then on C20, C21, C22 and C23 during the GEM. When the
cameras observed Callisto close up, there was a puzzling absence of
small craters. The surface features appeared to have been eroded,
indicating that they had been subject to active geological processes.
'Galileo' flyby of Callisto on C3 marked the first time that the Deep
Space Network operated a link between its antennae in Canberra and
Goldstone that allowed them to operate as a gigantic array, thereby
enabling a higher bit rate. With the assistance of the antenna at
Parkes, this raised the effective bandwidth to as much as 1,000 bits
per second.
Data accumulated on C3 indicated that Callisto had a homogeneous
composition, with heavy and light elements intermixed. This was
estimated to be composed of 60 percent silicate, iron and iron sulfide
rock and 40 percent water ice. This was overturned by further radio
Doppler observations on C9 and C10, which indicated that rock had
settled towards the core, and therefore that Callisto indeed has a
differentiated internal structure, although not as much so as the
other Galilean moons.
Observations made with 'Galileo' magnetometer indicated that Callisto
had no magnetic field of its own, and therefore lacked an iron core
like Ganymede's, but that it did have an induced field from Jupiter's
magnetosphere. Because ice is too poor a conductor to generate this
effect, it pointed to the possibility that Callisto, like Europa and
Ganymede, might have a subsurface ocean of brine. 'Galileo' made its
closest encounter with Callisto on C30, when it made a 138 km pass
over the surface, during which it photographed the Asgard, Valhalla
and Bran craters. This was used for slingshot maneuvers to set up the
final encounters with Io on I31 and I32.
Amalthea
==========
Although 'Galileo' main mission was to explore the Galilean moons, it
also captured images of four of the inner moons, Thebe, Adrastea,
Amalthea, and Metis. Such images were only possible from a spacecraft;
to Earth-based telescopes they were merely specks of light. Two years
of Jupiter's intense radiation took its toll on the spacecraft's
systems, and its fuel supply was running low in the early 2000s.
'Galileo' cameras were deactivated on January 17, 2002, after they had
sustained irreparable radiation damage.
NASA engineers were able to recover the damaged tape-recorder
electronics, and 'Galileo' continued to return scientific data until
it was deorbited in 2003, performing one last scientific experiment: a
measurement of Amalthea's mass as the spacecraft swung by it. This was
tricky to arrange; to be useful, 'Galileo' had to fly within 300 km of
Amalthea, but not so close as to crash into it. This was complicated
by its irregular 146 by potato-like shape. It was tidally locked,
pointing its long axis towards Jupiter. A successful flyby meant
knowing which direction the asteroid was pointed in relation to
'Galileo' at all times.
'Galileo' flew by Amalthea on November 5, 2002, during its 34th orbit,
allowing a measurement of the moon's mass as it passed within 160 km
of its surface. The results startled the scientific team; they
revealed that Amalthea had a mass of 2.08e18 kg, and with a volume of
2.43e6 km3, it therefore had a density of 857 ± 99 kilograms per cubic
meter, less than that of water.
A final discovery occurred during the last two orbits of the mission.
When the spacecraft passed the orbit of Amalthea, the star scanner
detected unexpected flashes of light that were reflections from seven
to nine moonlets. None of the individual moonlets was reliably sighted
twice, so no orbits were determined. It is believed that they were
most likely debris ejected from Amalthea that formed a tenuous, and
perhaps temporary, ring around Jupiter.
Star scanner
==============
'Galileo' star scanner was a small optical telescope that provided an
absolute attitude reference, but it made several scientific
discoveries serendipitously. In the prime mission, it was found that
the star scanner was able to detect high-energy particles as a noise
signal. This data was eventually calibrated to show the particles were
predominantly >2 MeV electrons that were trapped in the Jovian
magnetic belts, and released to the Planetary Data System.
A second discovery occurred in 2000. The star scanner was observing a
set of stars that included the second magnitude star Delta Velorum. At
one point, this star dimmed for 8 hours below the star scanner's
detection threshold. Subsequent analysis of 'Galileo' data and work by
amateur and professional astronomers showed that Delta Velorum is the
brightest known eclipsing binary, brighter at maximum than Algol. It
has a primary period of 45 days and the dimming is just visible with
the naked eye.
Radiation-related anomalies
=============================
Jupiter's uniquely harsh radiation environment caused over 20
anomalies over the course of 'Galileo' mission, in addition to the
incidents expanded upon below. Despite having exceeded its radiation
design limit by at least a factor of three, the spacecraft survived
all these anomalies. Work-arounds were found eventually for all of
these problems, and 'Galileo' was never rendered entirely
non-functional by Jupiter's radiation. The radiation limits for
'Galileo' computers were based on data returned from 'Pioneer 10' and
'Pioneer 11', since much of the design work was underway before the
two 'Voyagers' arrived at Jupiter in 1979.
A typical effect of the radiation was that several of the science
instruments suffered increased noise while within about 700000 km of
Jupiter. The SSI camera began producing totally white images when the
spacecraft was hit by the exceptional Bastille Day coronal mass
ejection in 2000, and did so again on subsequent close approaches to
Jupiter. The quartz crystal used as the frequency reference for the
radio suffered permanent frequency shifts with each Jupiter approach.
A spin detector failed, and the spacecraft gyro output was biased by
the radiation environment.
The most severe effects of the radiation were current leakages
somewhere in the spacecraft's power bus, most likely across brushes at
a spin bearing connecting rotor and stator sections of the orbiter.
These current leakages triggered a reset of the onboard computer and
caused it to go into safe mode. The resets occurred when the
spacecraft was either close to Jupiter or in the region of space
magnetically downstream of Jupiter. A change to the software was made
in April 1999 that allowed the onboard computer to detect these resets
and autonomously recover, so as to avoid safe mode.
Tape recorder problems
========================
Routine maintenance of the tape recorder involved winding the tape
halfway down its length and back again to prevent it sticking. In
November 2002, after the completion of the mission's only encounter
with Jupiter's moon Amalthea, problems with playback of the tape
recorder again plagued 'Galileo'. About 10 minutes after the closest
approach of the Amalthea flyby, 'Galileo' stopped collecting data,
shut down all of its instruments, and went into safe mode, apparently
as a result of exposure to Jupiter's intense radiation environment.
Though most of the Amalthea data was already written to tape, it was
found that the recorder refused to respond to commands telling it to
play back data.
After weeks of troubleshooting of an identical flight spare of the
recorder on the ground, it was determined that the cause of the
malfunction was a reduction of light output in three infrared Optek
OP133 light-emitting diodes (LEDs) located in the drive electronics of
the recorder's motor encoder wheel. The gallium arsenide LEDs had been
particularly sensitive to proton-irradiation-induced atomic lattice
displacement defects, which greatly decreased their effective light
output and caused the drive motor's electronics to falsely believe the
motor encoder wheel was incorrectly positioned.
'Galileo' flight team then began a series of "annealing" sessions,
where current was passed through the LEDs for hours at a time to heat
them to a point where some of the crystalline lattice defects would be
shifted back into place, thus increasing the LED's light output. After
about 100 hours of annealing and playback cycles, the recorder was
able to operate for up to an hour at a time. After many subsequent
playback and cooling cycles, the complete transmission back to Earth
of all recorded Amalthea flyby data was successful.
End of mission and deorbit
============================
When the exploration of Mars was being considered in the early 1960s,
Carl Sagan and Sidney Coleman produced a paper concerning
contamination of the red planet. In order that scientists could
determine whether native life forms existed before the planet became
contaminated by micro-organisms from Earth, they proposed that space
missions should aim at a 99.9 percent chance that contamination should
not occur. This figure was adopted by the Committee on Space Research
(COSPAR) of the International Council of Scientific Unions in 1964,
and was subsequently applied to all planetary probes.
The danger was highlighted in 1969 when the Apollo 12 astronauts
returned components of the Surveyor 3 spacecraft that had landed on
the Moon three years before, and it was found that microbes were still
viable even after three years in that harsh climate. An alternative
was the Prime Directive, a philosophy of non-interference with alien
life forms enunciated by the original 'Star Trek' television series
that prioritized the interests of the life forms over those of
scientists. Given the (admittedly slim) prospect of life on Europa,
scientists Richard Greenberg and Randall Tufts proposed that a new
standard be set of no greater chance of contamination than that which
might occur naturally by meteorites.
'Galileo' had not been sterilized prior to launch and could
conceivably have carried bacteria from Earth. Therefore, a plan was
formulated to send the probe directly into Jupiter, in an intentional
crash to eliminate the possibility of an impact with Jupiter's moons,
particularly Europa, and prevent a forward contamination. On April 14,
2003, the 'Galileo' orbiter reached its greatest orbital distance from
Jupiter for the entire mission since orbital insertion, 26 e6km,
before plunging back towards the gas giant for its final impact. At
the completion of J35, its final orbit around the Jovian system,
'Galileo' struck Jupiter in darkness just south of the equator on
September 21, 2003, at 18:57 UTC. Its impact speed was approximately
30 mi/s.
Major findings
================
# The composition of Jupiter differs from that of the Sun, indicating
that Jupiter has evolved since the formation of the Solar System.
# 'Galileo' made the first observation of ammonia clouds in another
planet's atmosphere. The atmosphere creates ammonia ice particles from
material coming up from lower depths.
# Io was confirmed to have extensive volcanic activity that is 100
times greater than that found on Earth. The heat and frequency of
eruptions are reminiscent of early Earth.
# Complex plasma interactions in Io's atmosphere create immense
electrical currents which couple to Jupiter's atmosphere.
# Several lines of evidence from 'Galileo' support the theory that
liquid oceans exist under Europa's icy surface.
# Ganymede possesses its own, substantial magnetic field - the first
satellite known to have one.
# 'Galileo' magnetic data provided evidence that Europa, Ganymede and
Callisto have a liquid salt water layer under the visible surface.
# Evidence exists that Europa, Ganymede, and Callisto all have a thin
atmospheric layer known as a "surface-bound exosphere".
# Jupiter's ring system is formed by dust kicked up as interplanetary
meteoroids smash into the planet's four small inner moons. The
outermost ring is actually two rings, one embedded with the other.
There is probably a separate ring along Amalthea's orbit as well.
# The 'Galileo' spacecraft identified the global structure and
dynamics of a giant planet's magnetosphere.
Follow-on missions
======================================================================
There was a spare 'Galileo' spacecraft that was considered by the
NASA-ESA Outer Planets Study Team in 1983 for a mission to Saturn, but
it was passed over in favor of a newer design, which became
'Cassini-Huygens'. While 'Galileo' was operating, 'Ulysses' passed by
Jupiter in 1992 on its mission to study the Sun's polar regions, and
'Cassini-Huygens' coasted by the planet in 2000 and 2001 en route to
Saturn. 'New Horizons' passed close by Jupiter in 2007 for a gravity
assist en route to Pluto, and it too collected data on the planet.
''Juno''
==========
The next mission to orbit Jupiter was NASA's 'Juno' spacecraft, which
was launched on August 5, 2011, and entered Jovian orbit on July 4,
2016. Although intended for a two-year mission, it is still active in
2024 and expected to continue until September 2025. 'Juno' provided
the first views of Jupiter's north pole and new insights into
Jupiter's aurorae, magnetic field, and atmosphere. Information
gathered about Jovian lightning prompted revision of earlier theories,
and analysis of the frequency of interplanetary dust impacts
(primarily on the backs of the solar panels), as 'Juno' passed between
Earth and the asteroid belt, indicated that this dust comes from Mars,
rather than from comets or asteroids, as was previously thought.
Jupiter Icy Moons Explorer
============================
The European Space Agency is planning to return to the Jovian system
with the Jupiter Icy Moons Explorer (JUICE). This was launched from
Europe's Spaceport in French Guiana on April 14, 2023, and is expected
to reach Jupiter in July 2031.
''Europa Clipper''
====================
Even before 'Galileo' concluded, NASA considered the Europa Orbiter,
but it was canceled in 2002. A lower-cost version was then studied,
which led to 'Europa Clipper' being approved in 2015. This mission
launched from Kennedy Space Center on October 14, 2024 and is expected
to reach Jupiter in April 2030.
''Europa Lander''
===================
A lander, simply called 'Europa Lander' was assessed by the Jet
Propulsion Laboratory. , this mission remains a concept, although some
funds were released for instrument development and maturation.
External links
======================================================================
*
[https://web.archive.org/web/20151114055422/http://solarsystem.nasa.gov/missions/Galileo
'Galileo' mission site] by NASA's Solar System Exploration
* [http://solarsystem.nasa.gov/galileo/ 'Galileo' legacy site] by
NASA's Solar System Exploration
* [http://rpif.asu.edu/Galileo/ 'Galileo' Satellite Image Mosaics] by
Arizona State University
* [https://www.flickr.com/photos/kevinmgill/albums/72157651267182078
Galileo image album] by Kevin M. Gill
License
=========
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Original Article: http://en.wikipedia.org/wiki/Galileo_project