Editing Galileo project (section)

==Jupiter==
{{see also|Timeline of Galileo (spacecraft)|l1=Timeline of ''Galileo'' (spacecraft)}}
{{multiple image |align=center |direction=horizontal |total_width=900
 |image1=E4 True and False Color Jupiter Hot Spot Mosaic (PIA00602).jpg
 |width1=1300 |height1=1000 |caption1=True and false color images of Jupiter's cloud layers |alt1=refer to caption
 |image2=Great Red Spot at Four Different Wavelengths (PIA00721).jpg
 |width2=2800 |height2=1600 |caption2=Jupiter's [[Great Red Spot]] at 757&nbsp;nm, 415&nbsp;nm, 732&nbsp;nm, and 886&nbsp;nm |alt2=refer to caption
 |image3=Jupiter lightnings.jpg
 |width3=1700 |height3=900 |caption3=Jovian lightning amidst clouds lit by Io's moonlight |alt3=lightning is just small dots
}}

===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 {{convert|15|e6km|e6mi|abbr=off|sp=us}} 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 {{convert|9|e6km|e6mi|abbr=off|sp=us}} from Jupiter.{{sfn|Meltzer|2007|pp=201–202}}

On December 7, 1995, the orbiter arrived in the Jovian system. That day it made a {{convert|32500|km|sp=us|adj=on}} flyby of Europa at 11:09 UTC, and then an {{convert|890|km|sp=us|adj=on}} 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 (spacecraft)|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.{{sfn|Meltzer|2007|pp=201–202}}

===Atmospheric probe===
[[File:Descent Module.jpeg|thumb|right|Inner descent module of the ''Galileo'' entry probe|alt=Spherical spaccreaft with some portuding instruments]]
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]] {{convert|31,000|mi|order=flip|sp=us}} above Jupiter's cloud tops.{{sfn|Ragent|Colburn|Avrin|Rages|1996|pp=854–856 }}{{sfn|Meltzer|2007|pp=202–204}} 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 {{convert|0.5|to|0.6|bar|psi}}; a middle one of [[ammonium hydrosulfide]] ice particles at a pressure of {{convert|1.5|to|2|bar|psi}}; and one of water vapor at {{convert|4|to|5|bar|psi}}.{{sfn|Meltzer|2007|p=212}} 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 {{convert|1.55|bar|psi}} but with many indications of smaller areas of increased particle densities along the whole length of its trajectory.{{sfn|Ragent|Colburn|Avrin|Rages|1996|pp=854–856 }}

The descent probe entered [[atmosphere of Jupiter|Jupiter's atmosphere]], defined for the purpose as being {{convert|450|km|sp=us}} above the {{convert|1|bar|psi|adj=on}} pressure level,{{sfn|Young|1998|p=22,776}} without any braking at 22:04 UTC on December 7, 1995. At this point it was moving at {{convert|170700|km/h|sp=us}} relative to Jupiter.<ref name="Probe Events">{{cite web |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_events.html |archive-url=https://web.archive.org/web/20070102143553/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_events.html |archive-date=January 2, 2007 |title=Galileo Probe Mission Events |publisher=NASA |url-status=dead |date=June 14, 1996 }}</ref> This was by far the most difficult [[atmospheric entry]] yet attempted by any spacecraft; the probe had to withstand a peak [[deceleration]] of {{convert|228|g0|lk=in|abbr=on}}.{{sfn|Heppenheimer|2009|p=257}}<ref>{{cite news |url=http://www.space.com/searchforlife/070719_seti_probing.html |title=Probing Planets: Can You Get There From Here? |first=Lisa |last=Chu-Thielbar |date=July 19, 2007 |publisher=[[Space.com]] |access-date=2007-07-27 |archive-date=February 12, 2009 |archive-url=https://web.archive.org/web/20090212005251/http://www.space.com/searchforlife/070719_seti_probing.html |url-status=live }}</ref> The rapid flight through the atmosphere produced a plasma with a temperature of about {{convert|14,000|C}}, and the probe's [[carbon phenolic]] heat shield lost more than half of its mass, {{convert|80|kg|sp=us}}, during the descent.{{sfn|Meltzer|2007|p=118}}<ref>{{cite web|url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Heat_Shield.html |title=''Galileo'' Probe Heat Shield Ablation |first=Julio |last=Magalhães |publisher=NASA Ames Research Center |date=1997-09-17 |access-date=2006-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20060929185050/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Heat_Shield.html |archive-date=2006-09-29 }}</ref><ref>{{cite web |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_spacecraft.html |title=The ''Galileo'' Probe Spacecraft |first=Julio |last=Magalhães |publisher=NASA Ames Research Center |date=1996-12-06 |access-date=2006-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20070101114453/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/probe_spacecraft.html |archive-date=2007-01-01 }}</ref> As the probe passed through Jupiter's cloud tops, it started transmitting data to the orbiter, {{convert|215000|km|sp=us}} above.{{sfn|Harland|2000|p=105}} 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.{{sfn|Meltzer|2007|pp=202–204}}

The atmospheric probe deployed its {{convert|2.5|m|ft|abbr=off|adj=on|sp=us}} [[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.{{sfn|Harland|2000|p=105}}<ref name="gpr">{{cite web |url=http://www2.jpl.nasa.gov/sl9/gll38.html |title=Galileo Probe Science Results |date=January 22, 1996 |first1=Douglas |last1=Isbell |first2=David |last2=Morse |publisher=NASA/Jet Propulsion Laboratory |access-date=March 4, 2016 |archive-url=https://web.archive.org/web/20220424134149/http://www2.jpl.nasa.gov/sl9/gll38.html |archive-date=24 April 2022 }}</ref><ref>{{cite web |title=The Galileo Probe Mission Events Timeline |publisher=NASA |url=http://ccf.arc.nasa.gov/galileo_probe/htmls/probe_mission_events.html |archive-url=https://web.archive.org/web/19990424091538/http://ccf.arc.nasa.gov/galileo_probe/htmls/probe_mission_events.html |archive-date=1999-04-24 |url-status=dead |df=mdy-all}}</ref><ref>{{cite web |title=In Depth |publisher=Galileo – NASA Solar System Exploration |url=https://solarsystem.nasa.gov/missions/galileo/in-depth/ |access-date=6 March 2021 |archive-date=February 19, 2018 |archive-url=https://web.archive.org/web/20180219070520/https://solarsystem.nasa.gov/missions/galileo/in-depth/ |url-status=live }}</ref> The parachute reduced the probe's speed to {{convert|430|km/h|sp=us}}. The signal from the probe was no longer detected by the orbiter after 61.4 minutes, at an elevation of {{convert|112|miles|order=flip|sp=us}} below the cloud tops and a pressure of {{convert|22.7|atm|bar psi atm|order=out}}.<ref>{{cite web |title=In Depth Galileo Probe |url=https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/ |website=NASA Solar System Exploration |access-date=3 July 2023 |archive-date=January 27, 2023 |archive-url=https://web.archive.org/web/20230127141502/https://solarsystem.nasa.gov/missions/galileo-probe/in-depth/ |url-status=live }}</ref> It was believed that the probe continued to fall at [[terminal velocity]], as the temperature increased to {{convert|1700|C}} and the pressure to {{convert|5000|atm|bar psi atm|order=out}}, destroying it.{{sfn|Meltzer|2007|pp=204–205}}

<gallery class="center" mode="packed" heights="240px">
File:Galileo Probe - AC81-0174.jpg|Artist's impression of the probe's entry into [[Atmosphere of Jupiter|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 ~<!--(The Probe transmitted data to the Orbiter continuously for 57.6 minutes reaching a depth of 23 bars but the relay link to the Orbiter began at four minutes after entry, so transmission ended 61.4 minutes after entry.) -->
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.
</gallery>

The probe detected less lightning, less water, but stronger winds than expected. Scientists had expected to find wind speeds of up to {{convert|220|mph|order=flip|sp=us}}, but winds of up to {{convert|330|mph|order=flip|sp=us}} 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.<ref name="endkit">{{cite web |url=https://solarsystem.nasa.gov/system/downloadable_items/1028_galileo-end_presskit.pdf |title=Galileo End of Mission Press Kit |access-date=October 29, 2011 |archive-date=October 23, 2020 |archive-url=https://web.archive.org/web/20201023104131/https://solarsystem.nasa.gov/system/downloadable_items/1028_galileo-end_presskit.pdf |url-status=live }}</ref>

The atmosphere was more turbulent than expected. Wind speeds in the outermost layers were {{convert|290|to|360|km/h|sp=us}}, in agreement with previous measurements from afar, but those wind speeds increased dramatically at pressure levels of {{convert|1|to|4|bar|psi}}, then remaining consistently high at around {{convert|170|m/s|km/h|disp=flip|sp=us}}.{{sfn|Atkinson|Ingersoll|Seiff|1997|pages=649–650}} 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,<ref>{{cite web |title=Galileo Probe Mission Science Summary |publisher=NASA |url=http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Science_summary.html |archive-url=https://web.archive.org/web/20060221225640/http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Science_summary.html |archive-date=2006-02-21 |url-status=dead |df=mdy-all}}</ref><ref name=gpr/> but the low abundance of water cast doubt on theories that Earth's water had been acquired from comets.<ref>{{cite news |title=Jupiter Retains Atmosphere of Mystery; Surprise Galileo Data Could Force New Theories of Planetary Formation|last=Sawyer |first=Kathy |newspaper=[[The Washington Post]] |date=January 23, 1996 |page=A.03 |url=https://www.washingtonpost.com/archive/politics/1996/01/23/jupiter-retains-atmosphere-of-mystery/07c4f386-749d-43d5-8adf-c9fa3fb1cc28/ |access-date=23 May 2024}}</ref>

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 gas]]es ([[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 {{convert|-240|C|F|0}}, 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.<ref>{{cite press release |publisher=NASA/Jet Propulsion Laboratory |title=Galileo Probe Results Suggest Jupiter Had an Ancient, Chilly Past |url=https://www.jpl.nasa.gov/news/galileo-probe-results-suggest-jupiter-had-an-ancient-chilly-past/ |date=November 17, 1999 |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130054857/https://www.jpl.nasa.gov/news/galileo-probe-results-suggest-jupiter-had-an-ancient-chilly-past |url-status=live }}</ref>

===Orbiter===
[[File:Animation of Galileo trajectory around Jupiter.gif|thumb|right|Animation of ''Galileo''{{'s}} trajectory around Jupiter from August 1, 1995, to September 30, 2003<br/>{{legend2|magenta|''Galileo''}}{{·}}{{legend2|Lime|[[Jupiter]]}}{{·}}{{legend2|OrangeRed|Io}}{{·}}{{legend2|RoyalBlue|Europa}}{{·}}{{legend2|Gold|[[Ganymede (moon)|Ganymede]]}}{{·}}{{legend2|Cyan|[[Callisto (moon)|Callisto]]}} |alt=refer to caption]]
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 {{convert|600|m/s|sp=us}} and it entered a [[parking orbit]] with an [[orbital period]] of 198 days. The ''Galileo'' orbiter thus became the first artificial satellite of Jupiter.{{sfn|Meltzer|2007|pp=208–209}}<ref>{{cite web |url=http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo&Display=ReadMore |title=Solar System Exploration – Galileo |archive-url=https://web.archive.org/web/20121006010150/http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo&Display=ReadMore |archive-date=October 6, 2012 |publisher=NASA |url-status=dead |access-date=April 24, 2012}}</ref> 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.{{sfn|Meltzer|2007|pp=208–209}}

The spacecraft traveled around Jupiter in elongated [[ellipse]]s, 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.{{sfn|Meltzer|2007|pp=232–233}}

===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 ${{Inflation|US-GDP|30|1997}} million in {{Inflation/year|US-GDP}}).{{sfn|Meltzer|2007|pp=234–237}} 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.{{sfn|Greenberg|2005|pp=337–338}}

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 (moon)|Europa]], [[Callisto (moon)|Callisto]] and [[Io (moon)|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 {{convert|201|km|sp=us}} on November 26, 1999, on orbit I25, was very unhealthy for ''Galileo''{{'s}} systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.{{sfn|Meltzer|2007|pp=234–237}}

By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed more than 600 [[kilorad]]s in between 1995 and 2002,{{sfn|Fieseler|Ardalan|Frederickson|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 (moon)|Amalthea]].{{sfn|Meltzer|2007|pp=237–238}} The total cost of the original ''Galileo'' mission was about {{US$|1.39 billion}} (equivalent to ${{Inflation|US-GDP|1.39|2003}} billion in {{Inflation/year|US-GDP}}). Of this, {{US$|892 million}} (equivalent to ${{Inflation|US-GDP|892|2003}} million in {{Inflation/year|US-GDP}}) was spent on spacecraft development.<ref name="galileo-arrival" /> Another $110 million (equivalent to ${{Inflation|US-GDP|110|2003}} million in {{Inflation/year|US-GDP}}) was contributed by international agencies.<ref>{{cite web |url=https://solarsystem.nasa.gov/missions/galileo/overview/#otp_quick_facts |title=Galileo: Quick Facts |publisher=NASA |access-date=January 19, 2021 |archive-date=July 19, 2009 |archive-url=https://web.archive.org/web/20090719111109/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/2Probe.html#otp_quick_facts |url-status=live }}</ref>

===Io===
The innermost of the four Galilean moons, Io is roughly the same size as Earth's moon, with a [[mean radius]] of {{convert|1821.3|km|sp=us}}. It is in [[orbital resonance]] with Ganymede and Europa, and [[tidal locking|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.{{sfn|Anderson|Sjogren|Schubert|1996|p=709}} 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.{{sfn|Meltzer|2007|pp=242–244}}

[[File:Io - Tvashtar Catena.jpg|thumb|right|upright=2.0|[[Tvashtar Paterae|Tvashtar]] [[Crater chain|Catena]] on Io, showing changes in hot spots between 1999 and 2000. Infrared imaging shows a hot lava flow more than {{convert|60|km|mi}} long. |alt=Different lava flows]]
''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.{{sfn|Meltzer|2007|pp=245–246}} 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.{{sfn|Meltzer|2007|p=231}} 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]].{{sfn|Anderson|Sjogren|Schubert|1996|p=709}}<ref>{{cite press release |date=May 3, 1996 |publisher=NASA/Jet Propulsion Laboratory |title=NASA's Galileo Finds Giant Iron Core in Jupiter's Moon Io |url=https://www.jpl.nasa.gov/news/nasas-galileo-finds-giant-iron-core-in-jupiters-moon-io/ |access-date=January 19, 2021 |archive-date=February 1, 2021 |archive-url=https://web.archive.org/web/20210201080131/https://www.jpl.nasa.gov/news/nasas-galileo-finds-giant-iron-core-in-jupiters-moon-io/ |url-status=live }}</ref>

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.{{sfn|Meltzer|2007|pp=240–241}} 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 (volcano)|Zamama]], [[Prometheus (volcano)|Prometheus]], and [[Pele (volcano)|Pele]] volcanic eruption centers.{{sfn|Meltzer|2007|pp=246–248}}

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''{{'s}} NIMS and SSI camera capturing an erupting volcano that generated a {{convert|20|mi|order=flip|sp=us|adj=on}} 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.<ref>{{cite press release |date=December 17, 1999 |publisher=NASA/Jet Propulsion Laboratory |title=Galileo Sees Dazzling Lava Fountain on Io |url=https://www.jpl.nasa.gov/news/galileo-sees-dazzling-lava-fountain-on-io/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130071105/https://www.jpl.nasa.gov/news/galileo-sees-dazzling-lava-fountain-on-io |url-status=live }}</ref>

[[File:Io rotating 2.ogv|thumb|left|Io in sped-up motion; a rotation actually takes 1.769 days |alt=refer to caption]]
The safe-mode incidents on I24 and I25 left some gaps in the data, which I27 targeted. This time ''Galileo'' passed {{convert|198|km|sp=us}} 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 {{convert|10000|km2|sp=us}} 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 (computing)|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.{{sfn|Meltzer|2007|pp=249–250}}

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 {{convert|200|km|sp=us}} of the surface of the north pole of Io, and on orbit I32 it flew {{convert|181|km|sp=us}} over the south pole.{{sfn|Meltzer|2007|pp=251–253}}<ref>{{cite press release |first=Guy |last=Webster |id=2001-161 |publisher=NASA/Jet Propulsion Laboratory |title=Spacecraft to Fly Over Source of Recent Polar Eruption on Io |date=August 1, 2001 |url=https://www.jpl.nasa.gov/news/spacecraft-to-fly-over-source-of-recent-polar-eruption-on-io |access-date=April 16, 2024 |archive-date=April 28, 2024 |archive-url=https://web.archive.org/web/20240428014501/https://www.jpl.nasa.gov/news/spacecraft-to-fly-over-source-of-recent-polar-eruption-on-io |url-status=live }}</ref> 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 [[convection (heat transfer)|convective]] properties as that of Earth.<ref>{{cite press release |title=Jupiter's Io Generates Power and Noise, But No Magnetic Field |date=December 10, 2001 |first=Guy |last=Webster |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/jupiters-io-generates-power-and-noise-but-no-magnetic-field/ |access-date=November 29, 2020 |archive-date=February 1, 2021 |archive-url=https://web.archive.org/web/20210201071553/https://www.jpl.nasa.gov/news/jupiters-io-generates-power-and-noise-but-no-magnetic-field/ |url-status=live }}</ref>

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 [[effusive eruption|quiet]], but the spacecraft flew through the plume of another, previously unknown, volcano {{convert|600|km|sp=us}} 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.{{sfn|Meltzer|2007|pp=251–253}}<ref>{{cite web |title=Dashing through the Snows of Io |publisher=NASA |url=https://science.nasa.gov/science-news/science-at-nasa/2001/ast16oct_1 |access-date=November 29, 2020 |archive-url=https://web.archive.org/web/20221207005035/https://science.nasa.gov/science-news/science-at-nasa/2001/ast16oct_1 |archive-date=7 December 2022}}</ref><ref>{{cite web |title=Eruption at Tvashtar Catena, Io |publisher=The Planetary Society |url=https://www.planetary.org/space-images/pia02550 |access-date=May 30, 2024}}</ref> ''Galileo''{{'s}} final return to Io on orbit I33 was marred by another safe mode incident, and much of the hoped-for data was lost.<ref>{{cite press release |title=Farewell, Io; Galileo Paying Last Visit to a Restless Moon |date=January 15, 2002 |first=Guy |last=Webster |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/farewell-io-galileo-paying-last-visit-to-a-restless-moon/ |access-date=January 19, 2021}}</ref>

===Europa===
[[File:Europa-moon-with-margins.jpg|thumb|right|Europa photographed by ''Galileo''|alt=Europe is criss-crossed by lines]]
Although the smallest of the four Galilean moons, with a radius of {{convert|1565|km|sp=us}}, Europa is the sixth-largest moon in the solar system.<ref name="Europa - Jupiter's Icy Moon">{{cite web |title=Europa – Jupiter's Icy Moon |publisher= Teachlink @ Utah State University |url=http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |access-date=December 3, 2020 |archive-date=February 4, 2021 |archive-url=https://web.archive.org/web/20210204170152/http://teacherlink.ed.usu.edu/tlnasa/OtherPRINT/poster/Europa_Poster.pdf |url-status=dead }}</ref> Observations from Earth indicated that it was covered in ice.{{sfn|Greenberg|2005|p=9}} 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.{{sfn|Greenberg|2005|pp=51–52}} Europa is therefore subject to tidal effects.{{sfn|Greenberg|2005|pp=49–51}} There is no evidence of volcanism like on Io, but ''Galileo'' revealed that the surface ice was covered in cracks.{{sfn|Greenberg|2005|pp=12–14}}

Some observations of Europa were made during orbits G1 and G2. On C3, ''Galileo'' conducted a {{convert|34,800|km|sp=us|adj=on}} "nontargeted" encounter of Europa—meaning a secondary flyby at a distance of up to {{convert|100,000|km|sp=us}}—on November 6, 1996. During E4 from December 15 to 22, 1996, ''Galileo'' flew within {{convert|692|km|sp=us}} of Europa, but data transmission was hindered by a Solar [[occultation]] that blocked transmission for ten days.{{sfn|Meltzer|2007|pp=254–256}}

''Galileo'' returned to Europa on E6 in January 1997, this time at a height of {{convert|586|km|sp=us}}, 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.{{sfn|Meltzer|2007|pp=254–256}} 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.{{sfn|Greenberg|2005|p=160}} 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.{{sfn|Meltzer|2007|pp=256–259}}

[[File:PIA03002 Blocks in the Europan Crust Provide More Evidence of Subterranean Ocean.jpg|thumb|left|This false color image on the left shows a region of Europa's crust made up of blocks which are thought to have broken apart and "rafted" into new positions.|alt=refer to caption]]
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.{{sfn|Greenberg|2005|pp=12–14}}<ref name="Europa - Jupiter's Icy Moon" /> Astronomer [[Clark Chapman]] argued that, assuming a {{convert|20|km|adj=on|sp=us}} 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.{{sfn|Meltzer|2007|pp=260–216}} 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.{{sfn|Zahnle|Schenk|Levison|Dones|2003|p=277}} [[Tidal flexing]] of up to {{convert|100|m|sp=us}} per day was the most likely culprit.<ref name="Off kilter">{{cite press release |title=Long-stressed Europa Likely Off-kilter at One Time |date=September 18, 2013 |publisher=NASA/Jet Propulsion Laboratory |first1=Jia-Rui |last1=Cook |first2=Elizabeth |last2=Zubritsky |first3=Nancy |last3=Neal-Jones |url=https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time/ |access-date=December 4, 2020 |archive-date=February 19, 2021 |archive-url=https://web.archive.org/web/20210219074046/https://www.jpl.nasa.gov/news/long-stressed-europa-likely-off-kilter-at-one-time |url-status=live }}</ref> 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.{{sfn|Meltzer|2007|pp=260–261}}<ref name="New Images Hint">{{cite press release |id=97-66 |date=April 9, 1997 |first1=Donald |last1=Savage |first2=Jane |last2=Platt |title=New Images Hint at Wet and Wild History For Europa |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |access-date=April 2, 2024 |archive-date=April 2, 2024 |archive-url=https://web.archive.org/web/20240402205836/https://www.jpl.nasa.gov/news/new-images-hint-at-wet-and-wild-history-for-europa |url-status=live }}</ref>

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 {{convert|-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.<ref name="New Images Hint" />{{sfn|Meltzer|2007|pp=261–263}} ''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.<ref name="Off kilter" />{{sfn|Greenberg|2005|pp=173–178}}{{sfn|Sarid et al.|2002|p=24}}

[[File:Plate Tectonics on Europa.jpg|thumb|right|Plate tectonics on Europa|alt=Illustration of model of Europa with a liquid ocaen surrounded by warmer ice and then an outer layer of a cold ice shell, with outbreaks of cryolarva.]]
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 {{convert|20|to|30|km|sp=us}} proposed by the accredited scientists on the ''Galileo'' Imaging Team faced intimidation, scorn, and reduced career opportunities.{{sfn|Greenberg|2005|pp=313–321}} 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.{{sfn|Greenberg|2005|pp=31–32}} 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.{{sfn|Chaisson|1994|p=102}} 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.{{sfn|Greenberg|2005|pp=31–32}}

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.<ref>{{cite press release |title=Galileo finds Europa has an Atmosphere |publisher=NASA/Jet Propulsion Laboratory |first=Jane |last=Platt |date=July 18, 1997 |url=https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130052248/https://www.jpl.nasa.gov/news/galileo-spacecraft-finds-europa-has-atmosphere |url-status=live }}</ref>

On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of "[[Clay mineral|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], on the icy crust of [[Europa (moon)|Europa]]. The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]].<ref name="NASA-20131211">{{cite web |last=Cook |first=Jia-Rui c. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=https://www.jpl.nasa.gov/news/clay-like-minerals-found-on-icy-crust-of-europa/ |date=December 11, 2013 |publisher=NASA/Jet Propulsion Laboratory |access-date=January 19, 2021 |archive-date=May 28, 2019 |archive-url=https://web.archive.org/web/20190528024022/https://www.jpl.nasa.gov/news/news.php?release=2013-362 |url-status=live }}</ref>

{{Clear}}

===Ganymede===
[[File:Ganymede - June 26 1996 (26781123830).jpg|thumb|right|Ganymede, photographed on June 26, 1996|alt=Ganymede looks like the Moon, with craters and darker and lighter grey regions]]
The largest of the Galilean moons with a radius of {{convert|2620|km|sp=us}}, Ganymede is larger than Earth's moon, the [[dwarf planet]] [[Pluto]] or the planet [[Mercury (planet)|Mercury]].{{sfn|Meltzer|2007|pp=267–268}} 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 (moon)|Titan]], and Neptune's moon [[Triton (moon)|Triton]]. Ganymede has three times as much water for its mass as Earth has.{{Sfn|Stevenson|1996|pp=511–512}}

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.{{sfn|Meltzer|2007|pp=267–268}}

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 {{convert|750|nT|nT|lk=on|disp=out|abbr=off}}, more than strong enough to create a separate magnetosphere within that of Jupiter.{{efn|Earth's magnetic field varies from 22,000 to 67,000 nanoteslas.<ref>{{cite web |title=An Overview of the Earth's Magnetic Field |publisher=British Geological Survey |url=http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |access-date=16 April 2024 |archive-date=September 7, 2023 |archive-url=https://web.archive.org/web/20230907044147/http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |url-status=live }}</ref> }} This was the first time that a magnetic field had ever been detected on a moon contained within the magnetosphere of its host planet.{{sfn|Kivelson|Khurana|Russell|Walker|1996|pp=537–541}}<ref>{{cite press release |title=Galileo Makes Discoveries at Ganymede |publisher=NASA/Jet Propulsion Laboratory |date=October 7, 1996 |url=https://www2.jpl.nasa.gov/releases/96/gany1res.html |access-date=December 5, 2020 |archive-url=https://web.archive.org/web/20221202092949/https://www2.jpl.nasa.gov/releases/96/gany1res.html |archive-date=2 December 2022 }}</ref><ref>{{cite press release|title= New Discoveries From Galileo – Big Icy Moon of Jupiter Found to Have a 'Voice' After All; Europa Flyby Next for Galileo |date=December 12, 1996 |first1=Douglas |last1=Isbell |first2=Mary Beth |last2=Murrill |id=96-255|publisher=NASA/Jet Propulsion Laboratory |url=http://www2.jpl.nasa.gov/galileo/status961212.html |archive-url=https://web.archive.org/web/20100602211623/http://www2.jpl.nasa.gov/galileo/status961212.html |archive-date=2010-06-02 |url-status=dead |df=mdy-all}}</ref> This discovery led naturally to questions about its origin. The evidence pointed to an iron or iron sulfide core and [[mantle (geology)|mantle]] {{convert|400|to|1,300|km|sp=us}} 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 {{convert|200|km|sp=us}} below the surface.{{sfn|Meltzer|2007|pp=270–272}}<ref name="Hidden ocean">{{cite press release |title=Solar System's Moon Likely Has a Hidden Ocean |publisher=NASA |first=Guy |last=Webster |date=December 16, 2000 |url=https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |access-date=December 5, 2020 |archive-date=October 18, 2020 |archive-url=https://web.archive.org/web/20201018122955/https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |url-status=live }}</ref>

[[File:Ganymede diagram.svg|thumb|left|The internal structure of Ganymede]]
''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.{{sfn|Meltzer|2007|pp=268–270}} Images of the surface revealed two types of terrain: highly cratered dark regions and grooved terrain [[Sulcus (geology)|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 {{convert|50|to|200|km|sp=us}} below the surface.<ref name="Hidden ocean" />{{sfn|Meltzer|2007|pp=271–273}} Another possibility was volcanism. Slushy water or ice reaching the surface would quickly freeze over, creating areas of a relatively smooth surface.<ref>{{cite magazine |last=Cowen |first=Ron |title=Images Suggest Icy Eruptions on Ganymede |magazine=[[Science News]] |issn=0036-8423 |date=March 3, 2001 |volume=159 |issue=9 |page=133 |doi=10.2307/3981750 |jstor=3981750 }}</ref>
{{Clear}}

===Callisto===
[[File:Callisto, moon of Jupiter, NASA.jpg|thumb|right|Callisto, photographed by ''Galileo'']]
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.{{sfn|Meltzer|2007|pp=273–277}}<ref name="The Galileo Mission to Jupiter and Its Moons">{{cite magazine |last=Johnson |first=Torrence V. |title=The Galileo Mission to Jupiter and Its Moons |magazine=Scientific American |issn=0036-8733 |volume=282 |issue=2 |date=February 2000 |pages=40–49 |doi=10.1038/scientificamerican0200-40 |jstor=26058599 |pmid=10710785 |bibcode=2000SciAm.282b..40J }}</ref>

''Galileo''{{'s}} 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 [[aperture synthesis|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.<ref>{{cite press release |title=Galileo makes close pass by Callisto |date=November 4, 1996 |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/galileo-makes-close-pass-by-callisto/ |access-date=January 19, 2021 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130052441/https://www.jpl.nasa.gov/news/galileo-makes-close-pass-by-callisto |url-status=live }}</ref>

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.{{sfn|Harland|2000|p=172}}<ref>{{cite press release |title=Galileo Returns New Insights into Callisto and Europa |date=May 23, 1997 |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/news/galileo-returns-new-insights-into-callisto-and-europa/ |access-date=December 6, 2020 |archive-date=January 25, 2021 |archive-url=https://web.archive.org/web/20210125212928/https://www.jpl.nasa.gov/news/galileo-returns-new-insights-into-callisto-and-europa/ |url-status=live }}</ref> 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.{{sfn|Meltzer|2007|pp=273–277}}<ref>{{cite press release |first=Jane |last=Pratt |title=Galileo Mission Finds Strange Interior of Jovian Moon |publisher=NASA |url=https://solarsystem.nasa.gov/news/141/galileo-mission-finds-strange-interior-of-jovian-moon/ |access-date=December 6, 2020 |archive-date=October 17, 2020 |archive-url=https://web.archive.org/web/20201017184949/https://solarsystem.nasa.gov/news/141/galileo-mission-finds-strange-interior-of-jovian-moon/ |url-status=live }}</ref>

[[File:Callisto diagram.svg|thumb|left|The internal structure of Callisto|alt=Cut away diagram of Ganymede, with a solid iron core successively surrounded by liquid iron and iron sulfide, rocky mantle, tetragonal ice, salt water and hexagonal ice.]]
Observations made with ''Galileo''{{'s}} 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.{{sfn|Meltzer|2007|pp=273–277}}<ref>{{cite press release |first=Jane |last=Pratt |title=Jupiter's Moon Callisto May Hide Salty Ocean |publisher=NASA |url=https://solarsystem.nasa.gov/news/145/jupiters-moon-callisto-may-hide-salty-ocean/ |access-date=December 6, 2020 |archive-date=December 6, 2020 |archive-url=https://web.archive.org/web/20201206005510/https://solarsystem.nasa.gov/news/145/jupiters-moon-callisto-may-hide-salty-ocean/ |url-status=live }}</ref> ''Galileo'' made its closest encounter with Callisto on C30, when it made a {{convert|138|km|adj=on|sp=us}} pass over the surface, during which it photographed the [[Asgard (crater)|Asgard]], [[Valhalla (crater)|Valhalla]] and Bran craters.{{sfn|Meltzer|2007|pp=273–277}} This was used for slingshot maneuvers to set up the final encounters with Io on I31 and I32.<ref>{{cite press release |first=Guy |last=Webster |title=Galileo Succeeds in its Closest Flyby of a Jovian Moon |publisher=NASA |url=https://solarsystem.nasa.gov/news/196/galileo-succeeds-in-its-closest-flyby-of-a-jovian-moon/ |access-date=December 6, 2020 |archive-date=December 26, 2020 |archive-url=https://web.archive.org/web/20201226145659/https://solarsystem.nasa.gov/news/196/galileo-succeeds-in-its-closest-flyby-of-a-jovian-moon/ |url-status=live }}</ref>{{Clear}}

===Amalthea===
[[File:Galileo Amalthea artwork.jpg|thumb|right|Artist's concept of Galileo passing near Jupiter's small inner moon Amalthea|alt=Amalthea looks like a large rock.]]
Although ''Galileo''{{'s}} main mission was to explore the Galilean moons, it also captured images of four of the inner moons, [[Thebe (moon)|Thebe]], [[Adrastea (moon)|Adrastea]], [[Amalthea (moon)|Amalthea]], and [[Metis (moon)|Metis]]. Such images were only possible from a spacecraft; to Earth-based telescopes they were [[Diffraction-limited system|merely specks of light]].<ref name="The Galileo Mission to Jupiter and Its Moons" /> 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''{{'s}} cameras were deactivated on January 17, 2002, after they had sustained irreparable radiation damage.<ref>{{cite web |title= 30 Years Ago: Galileo off to Orbit Jupiter |date= October 17, 2019 |publisher= NASA |url= https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter |access-date= December 6, 2020 |archive-date= August 31, 2020 |archive-url= https://web.archive.org/web/20200831053435/https://www.nasa.gov/feature/30-years-ago-galileo-off-to-orbit-jupiter/ |url-status= live }}</ref>

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 {{convert|300|km|sp=us}} of Amalthea, but not so close as to crash into it. This was complicated by its irregular {{convert|146|by|262|km|sp=us|adj=on}} 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.<ref name="The Long Goodbye">{{cite magazine |last=Carroll |first=M. |year=2003 |title=The Long Goodbye |magazine=[[Astronomy (magazine)|Astronomy]] |issn=0091-6358 |volume=31 |issue=10 |pages=36–41 |via=ProQuest |url-access=subscription |url=https://www.proquest.com/docview/215919054 |access-date=23 May 2024|id={{ProQuest|215919054}} }}</ref>

''Galileo'' flew by Amalthea on November 5, 2002, during its 34th orbit<!-- 21 September 2002 11:58:55.818 UTC to 28 January 2003 00:58:55.815 UTC -->, allowing a measurement of the moon's mass as it passed within {{convert|160|km|mi|abbr=on}} of its surface<!-- S030916A.LBL uses "Altitude"; presumably the uncertainty comes from Amalthea's irregular shape -->.{{sfn|Meltzer|2007|p=280}} The results startled the scientific team; they revealed that Amalthea had a mass of {{convert|2.08e18|kg}}, and with a volume of {{convert|2.43e6|km3|sp=us}}, it therefore had a density of 857 ± 99 kilograms per cubic meter, less than that of water.<ref name="The Long Goodbye" />{{sfn|Anderson et al.|2005|pp=1291–1293}}

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.{{sfn|Fieseler et al.|2004|pp=399–400}}

{{multiple image |align=center |direction=horizontal |total_width=900
 |image1=Jupiter's Main Ring and Halo (PIA01622).jpg
 |width1=1019 |height1=577 |caption1=Jupiter's rings. Enhanced top image shows the halo of ring particles suspended by Jupiter's powerful electromagnetic field. |alt1=refer to caption
 |image2=Jupiter's moon Amalthea photographed by Galileo.jpg
 |width2=798 |height2=573 |caption2=Inner moon Amalthea |alt2=refer to caption
 |image3=Thebe.jpg
 |width3=229 |height3=229 |caption3=Inner moon Thebe |alt3=refer to caption
}}

===Star scanner===
''Galileo''{{'s}} 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 >{{convert|2|MeV|abbr=on|lk=on}} electrons that were trapped in the Jovian magnetic belts, and released to the Planetary Data System.<ref>{{cite web |url=http://www.mindspring.com/~feez/ |title=Science with The Galileo Star Scanner |archive-url=https://web.archive.org/web/20080719195042/http://www.mindspring.com/~feez/ |archive-date=2008-07-19 |date=July 19, 2008 |website=Mindspring.com |access-date=December 8, 2012}}</ref>

A second discovery occurred in 2000. The star scanner was observing a set of stars that included the second [[Magnitude (astronomy)|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.<ref>{{cite web |title=Galileo Mystery Solved: The Star, Not The Instrument, Was On The Blink |publisher=ScienceDaily |url=https://www.sciencedaily.com/releases/2001/03/010326072946.htm |access-date=April 7, 2024 |archive-date=April 7, 2024 |archive-url=https://web.archive.org/web/20240407031918/https://www.sciencedaily.com/releases/2001/03/010326072946.htm |url-status=live }}</ref>

===Radiation-related anomalies===
[[File:Jupiter's Magnetosphere animation.png|thumb|left|Jupiter's inner magnetosphere and radiation belts|alt=lines of magnetism come from the poles and loop around.]]
Jupiter's uniquely harsh radiation environment caused over 20 anomalies over the course of ''Galileo''{{'s}} 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''{{'s}} 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.{{sfn|Tomayko|1988|p=200}}

A typical effect of the radiation was that several of the science instruments suffered increased [[signal-to-noise ratio|noise]] while within about {{convert|700000|km|mi|abbr=on}} of Jupiter. The SSI camera began producing totally white images when the spacecraft was hit by the exceptional [[Bastille Day event|Bastille Day coronal mass ejection]] in 2000, and did so again on subsequent close approaches to Jupiter.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2748–2751}} The quartz crystal used as the frequency reference for the radio suffered permanent frequency shifts with each Jupiter approach.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2743–2744}} A spin detector failed, and the spacecraft gyro output was biased by the radiation environment.{{sfn|Fieseler|Ardalan|Frederickson|2002|pp=2744–2746}}

The most severe effects of the radiation were current leakages somewhere in the spacecraft's power bus, most likely across [[Brush (electric)|brushes]] at a [[Bearing (mechanical)|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.<ref>{{cite web |url=http://starbase.jpl.nasa.gov/go-a-nims-3-tube-v1.0/go_1117/catalog/insthost.cat~ |title=Instrument Host Overview |publisher=NASA/Jet Propulsion Laboratory |date=1999 |access-date=November 29, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20160315192151/http://starbase.jpl.nasa.gov/go-a-nims-3-tube-v1.0/go_1117/catalog/insthost.cat~ |archive-date=March 15, 2016}}</ref>

===Tape recorder problems===
Routine maintenance of the tape recorder involved winding the tape halfway down its length and back again to prevent it sticking.{{sfn|Meltzer|2007|p=226}} 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.<ref>{{cite press release |first=Guy |last=Webster |id=2002-213 |url=https://www.jpl.nasa.gov/news/galileo-millennium-mission-status-3 |title=Galileo Millennium Mission Status |publisher=NASA/Jet Propulsion Laboratory |date=November 25, 2002 |access-date=April 27, 2024 |archive-date=April 28, 2024 |archive-url=https://web.archive.org/web/20240428014837/https://www.jpl.nasa.gov/news/galileo-millennium-mission-status-3 |url-status=live }}</ref>

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 diode]]s (LEDs) located in the drive electronics of the recorder's motor [[rotary encoder|encoder]] wheel. The [[gallium arsenide]] LEDs had been particularly sensitive to [[proton]]-irradiation-induced [[crystal|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.{{sfn|Swift|Levanas|Ratliff|Johnston|2003|pp=1991–1993}}

''Galileo''{{'s}} flight team then began a series of "[[Annealing (metallurgy)|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.{{sfn|Swift|Levanas|Ratliff|Johnston|2003|pp=1993–1997}}

===End of mission and deorbit===
[[File:Galileo End.jpg|thumb|right|Illustration of ''Galileo'' entering Jupiter's atmosphere|alt=refer to caption]]
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.<ref name="Macroscope">{{cite magazine |last1=Greenberg |first1=Richard |last2=Tufts |first2=B. Randall |title=Macroscope: Infecting Other World |magazine=[[American Scientist]] |issn=0003-0996 |date=July–August 2001 |volume=89 |issue=4 |pages=296–299 |doi=10.1511/2001.28.3356 |jstor=27857494 }}</ref>

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 [[Star Trek: The Original Series|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.<ref name="Macroscope" />

''Galileo'' had not been [[sterilization (microbiology)|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, {{convert|26|e6km|e6mi|abbr=unit}}, before plunging back towards the gas giant for its final impact.<ref>{{cite web |url=http://solarsystem.nasa.gov/galileo/ |title=Galileo Legacy Site |publisher=NASA |year=2010 |access-date=April 24, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120819145316/http://solarsystem.nasa.gov/galileo/ |archive-date=August 19, 2012}}</ref> At the completion of J35, its final orbit<!-- 13 September 2003 13:58:55.818 UTC to 30 September 2003 11:58:55.818 UTC -- note that the last 9 days did not occur --> around the Jovian system, ''Galileo'' struck Jupiter in darkness just south of the equator on September 21, 2003, at 18:57&nbsp;UTC. Its impact speed was approximately {{convert|30|mi/s|km/s|abbr=on|order=flip}}.<ref name="spref20030919"/><ref>{{cite news |first=Peter |last=Bond |title=Galileo spacecraft crashes into Jupiter |url=http://spaceflightnow.com/galileo/030921galileogone.html |publisher=Spaceflight Now |date=September 21, 2003 |access-date=December 5, 2020 |archive-date=December 5, 2020 |archive-url=https://web.archive.org/web/20201205122235/https://spaceflightnow.com/galileo/030921galileogone.html |url-status=live }}</ref>

===Major findings===
# The composition of Jupiter differs from that of the Sun, indicating that Jupiter has evolved since the formation of the Solar System.<ref name="endkit" /><ref name="Key Science Discoveries">{{cite web |publisher=NASA |title=Galileo – 10 Key Science Discoveries |url=https://solarsystem.nasa.gov/missions/galileo/overview/ |access-date=November 29, 2020 |archive-date=July 19, 2009 |archive-url=https://web.archive.org/web/20090719111109/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/2Probe.html |url-status=live }}</ref>
# ''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.<ref name="endkit" />
# 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.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Complex plasma interactions in Io's atmosphere create immense electrical currents which couple to Jupiter's atmosphere.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Several lines of evidence from ''Galileo'' support the theory that liquid oceans exist under Europa's icy surface.<ref name="endkit" /><ref name="Key Science Discoveries" />
# Ganymede possesses its own, substantial magnetic field – the first satellite known to have one.<ref name="endkit" /><ref name="Key Science Discoveries" />
# ''Galileo'' magnetic data provided evidence that Europa, Ganymede and Callisto have a liquid salt water layer under the visible surface.<ref name="endkit" />
# Evidence exists that Europa, Ganymede, and Callisto all have a thin atmospheric layer known as a "surface-bound [[exosphere]]".<ref name="endkit" /><ref name="Key Science Discoveries" />
# Jupiter's [[ring system]] is formed by dust kicked up as interplanetary [[meteoroid]]s smash into the planet's [[Inner satellites of Jupiter|four small inner moons]]. The outermost ring is actually two rings, one embedded with the other. There is probably a separate ring along [[Amalthea (moon)|Amalthea]]'s orbit as well.<ref name="endkit" /><ref name="Key Science Discoveries" />
# The ''Galileo'' spacecraft identified the global structure and dynamics of a giant planet's [[magnetosphere]].<ref name="endkit" />