Editing Galileo (spacecraft)

{{About|the Jupiter probe|the navigation satellites|Galileo (satellite navigation)}}
{{DISPLAYTITLE: ''Galileo'' (spacecraft)}}
{{Infobox spaceflight
| name                  = ''Galileo''
| image                 = File:Artwork Galileo-Io-Jupiter.JPG
| image_size            = 260px
| image_caption         = Artist's concept of ''Galileo'' at Io; the high-gain antenna is fully deployed. 
| insignia              = Galileo mission patch fx.png
| insignia_size         = 140px

| mission_type          = [[Jupiter]] orbiter
| operator              = [[NASA]]<ref>[http://space.skyrocket.de/doc_sdat/galileo.htm Galileo] Gunter's Spcae Page. Retrivied 11 May 2016.</ref> 
| website               = {{url|http://www2.jpl.nasa.gov/galileo/}}
| COSPAR_ID             = 1989-084B
| SATCAT                = 20298
| mission_duration      = 14 years in space <br /> 8&nbsp;years in Jovian orbit

| manufacturer          = [[Jet Propulsion Laboratory]] <br /> [[Messerschmitt-Bolkow-Blohm]]<ref name="Galileo Project"/><br />General Electric<ref name="Galileo Project">[http://astro.if.ufrgs.br/solar/galfs.htm Galileo Project]</ref><br />[[Hughes Aircraft Company]]<ref name="Galileo Project"/>
| dry_mass              = {{convert|2380|kg}}<br/>Probe: {{convert|339|kg}}
| launch_mass           = {{convert|5712|kg|lb}}
| power                 = Orbiter: 570&nbsp;watts<br/>Probe: 580&nbsp;watts

| launch_date           = {{start-date|October 18, 1989, 16:53:40|timezone=yes}}&nbsp;UTC
| launch_rocket         = {{nowrap|{{OV|104}}}}<br/>[[STS-34]] / [[Inertial Upper Stage|IUS]]
| launch_site           = [[Kennedy Space Center|Kennedy]] [[Kennedy Space Center Launch Complex 39|LC-39B]]
| launch_contractor     = 
| entered_service       = December 8, 1995{{citation needed|date=August 2013}}

| disposal_type         = Deorbited
| decay_date            = {{end-date|September 21, 2003, 18:57:00|timezone=yes}}&nbsp;UTC

| orbit_epoch           = 
| orbit_reference       = Zenocentric orbit
| orbit_periapsis       = 
| orbit_apoapsis        = 
| orbit_inclination     =  
| orbit_period          = 
| apsis                 = zene

|interplanetary         ={{Infobox spaceflight/IP
   |type                = flyby
   |note                = gravity assist
   |object              = [[Venus]]
   |distance            = {{convert|16106|km|mi}}
   |arrival_date        = February 10, 1990
}}
{{Infobox spaceflight/IP
   |type                = flyby
   |note                = gravity assist
   |object              = [[Earth]]
   |distance            = {{convert|960|km|mi}}
   |arrival_date        = December 8, 1990
}}
{{Infobox spaceflight/IP
   |type                = flyby
   |note                = incidental
   |object              = {{ats|951|Gaspra}}
   |distance            = {{convert|1600|km|mi}}
   |arrival_date        = October 29, 1991
}}
{{Infobox spaceflight/IP
   |type                = flyby
   |note                = gravity assist
   |object              = [[Earth]]
   |distance            = {{convert|300|km|mi}}
   |arrival_date        = December 8, 1992
}}
{{Infobox spaceflight/IP
   |type                = flyby
   |note                = incidental
   |object              = {{ats|243|Ida}}
   |distance            = {{convert|2400|km|mi}}
   |arrival_date        = August 28, 1993
}}
{{Infobox spaceflight/IP
   |type                = atmospheric
   |component           = [[Galileo Probe|Probe]]
   |object              = [[Jupiter]]
   |location            = {{Coord|06|05|00|N|04|04|00|W|globe:jupiter|name=Galileo Probe (Galileo)}}<br/><small>at entry interface</small>
   |arrival_date        = December 7, 1995, 22:04&nbsp;UTC<br/><small>Operated for 57 minutes</small>
}}
{{Infobox spaceflight/IP
   |type                = orbiter
   |component           = Orbiter
   |object              = [[Jupiter]]
   |orbits              = 
   |arrival_date        = December 8, 1995, 01:20:00&nbsp;UTC
}}
}}
<!-- [[File:Galileo Deployment (high res).jpg|right|thumb|250px|''Galileo'' and its Inertial Upper Stage (IUS) booster being deployed by the [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] on the [[STS-34]] mission in October 1989.]]
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'''''Galileo''''' was an American [[unmanned spacecraft]] that studied the planet [[Jupiter]] and [[Moons of Jupiter|its moons]], as well as several other Solar System bodies. Named after the astronomer [[Galileo Galilei]], it consisted of an orbiter and entry probe. It was launched on October 18, 1989, carried by [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']], on the [[STS-34]] mission. ''Galileo'' arrived at Jupiter on December 7, 1995, after [[Gravity assist|gravitational assist]] flybys of [[Venus]] and [[Earth]], and became the first spacecraft to orbit Jupiter. It launched the first probe into Jupiter, directly measuring its [[celestial body atmosphere|atmosphere]].<ref name="endkit">{{cite web|url=http://solarsystem.nasa.gov/missions/docs/galileo-end.pdf |title=Galileo End of Mission Press Kit |format=PDF |accessdate=2011-05-15}}</ref> Despite suffering ma 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.<ref name="endkit"/>

Jupiter's atmospheric composition and [[ammonia]] clouds were recorded, the clouds possibly created by outflows from the lower depths of the atmosphere. [[Io (moon)|Io]]'s [[volcanism]] and [[Plasma (physics)|plasma]] interactions with Jupiter's atmosphere were also recorded. The data ''Galileo'' collected supported [[Europa (moon)#Subsurface ocean|the theory of a liquid ocean]] under the icy surface of [[Europa (moon)|Europa]], and there were indications of similar liquid-[[saline water|saltwater]] layers under the surfaces of [[Ganymede (moon)|Ganymede]] and [[Callisto (moon)|Callisto]]. Ganymede was shown to possess a [[magnetic field]] and the spacecraft found new evidence for [[exosphere]]s around Europa, Ganymede, and Callisto.<ref name="endkit"/> ''Galileo'' also discovered that Jupiter's faint [[planetary rings|ring system]] consists of dust from impacts on the four small inner moons. The extent and structure of Jupiter's [[magnetosphere]] was also mapped.<ref name="endkit"/>

On September 21, 2003, after 14 years in space and 8 years in the Jovian system, ''Galileo''{{'s}} mission was terminated by sending it into Jupiter's atmosphere at a speed of over {{convert|48|km|mi|0|sp=us}} per second, eliminating the possibility of [[Interplanetary contamination|contaminating local moons]] with terrestrial bacteria.

==Background==
[[File:Great Red Spot From Voyager 1.jpg|thumb|Jupiter seen by Voyager 1 in 1979]] 
gtg gg gdg rg ffsfefrefdfere Jupiter was rated as the number one priority in the [[Planetary Science Decadal Survey]] published in the summer of 1968.<ref name=ssb1968>{{cite book|title=Report on Space Science|year=1968|publisher=Space Studies Board|url=https://books.google.com/books?id=0jwrAAAAYAAJ&printsec=frontcover&dq=Planetary+Exploration:1968-1975&source=bl&ots=eLpuUiVSV4&sig=aGVPiswA1JKecK7aMavCkP36C38&hl=en&ei=n16BTcuyKsW40QHNxMn6CA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBoQ6AEwAA#v=onepage&q&f=false|oclc=254442711}}</ref> 
In the early 1970s the first flybys of Jupiter were achieved by [[Pioneer 10]] and [[Pioneer 11]], and before the decade was out it was also visited by the more advanced [[Voyager 1]] and [[Voyager 2]] spacecraft.

==Mission overview==

Work on the spacecraft began at Jet Propulsion Laboratory in 1977, while the ''[[Voyager 1]]'' and ''[[Voyager 2|2]]'' missions were still being prepared for launch. Early plans called for a launch on [[Space Shuttle]] [[Space Shuttle Columbia|''Columbia'']] on what was then codenamed [[Cancelled Space Shuttle missions#STS-23 (Columbia)|STS-23]] in January 1982, but delays in the development of the Space Shuttle allowed more time for development of the probe. As the shuttle program got underway, ''Galileo'' was scheduled for launch in 1984, but this later slipped to 1985 and then to 1986.<ref name=tom>{{cite web|last=Tomayko|first=James E.|title=Computers in Spaceflight: The NASA Experience|url=http://history.nasa.gov/computers/Ch6-3.html|publisher=NASA History Office|accessdate=November 7, 2012|date=March 1988}}</ref> The mission was initially called the ''Jupiter Orbiter Probe''; it was christened ''Galileo'' in 1978.<ref name=explore>[http://www.nasa.gov/exploration/whyweexplore/Why_We_26.html Why We Explore]. NASA.gov. 29 May 2007. Retrieved 12 November 2012.</ref>

Once the spacecraft was complete, its launch was scheduled for [[Cancelled Space Shuttle missions#STS-61-G (Atlantis)|STS-61-G]] on-board [[Space Shuttle Atlantis|''Atlantis'']] in 1986. The [[Inertial Upper Stage]] booster was going to be used at first, but this changed to the Centaur booster, then back to IUS after ''Challenger''.<ref name=tom/>

The [[Centaur (rocket stage)|Centaur-G]] liquid hydrogen-fueled booster stage allowed a direct trajectory to Jupiter. However, the mission was further delayed by the hiatus in launches that occurred after the [[Space Shuttle Challenger disaster]]. New safety protocols introduced as a result of the disaster prohibited the use of the Centaur-G stage on the Shuttle, forcing ''Galileo'' to use a lower-powered Inertial Upper Stage solid-fuel booster. The mission was re-profiled in 1987 to use several [[gravitational slingshot]]s, referred to as the "VEEGA" or Venus Earth Earth Gravity Assist maneuvers, to provide the additional velocity required to reach its destination.
[[File:STS-34 Launch 1.jpg|thumb|right|220px|''Galileo'' launching.]]
[[File:Galileo Deployment (high res).jpg|thumb|right|220px|The ''Galileo'' (black) aboard [[Space Shuttle Atlantis|Atlantis]] ready for launch toward [[Jupiter]]. Attached to it, the small rocket [[Inertial Upper Stage]] (white).]]
It was finally launched on October 18, 1989, by the [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] on the [[STS-34]] mission.

[[Venus]] was flown by at 05:58:48 UT on February 10, 1990 at a range of 16,106&nbsp;km. Having gained 8,030&nbsp;km per hour in speed, the spacecraft flew by [[Earth]] twice, the first time at a range of 960&nbsp;km at 20:34:34 UT on 8 December 1990 before approaching the [[S-type asteroid|S-type]] asteroid [[951 Gaspra]] to a distance of 1,604&nbsp;km at 22:37 UT on 29 October 1991. ''Galileo'' then performed a second flyby of Earth at 303.1&nbsp;km at 15:09:25 UT on 8 December 1992, adding 3.7&nbsp;km per second to its cumulative speed. ''Galileo'' performed close observation of a second asteroid, [[243 Ida]], at 16:51:59 UT on 28 August 1993 at a range of 2,410&nbsp;km. The spacecraft discovered Ida has a moon [[Dactyl (asteroid)|Dactyl]], the first discovery of a natural satellite orbiting an asteroid. In 1994, ''Galileo'' was perfectly positioned to watch the fragments of the [[comet Shoemaker-Levy 9]] crash into Jupiter, whereas terrestrial telescopes had to wait to see the impact sites as they rotated into view. After releasing its atmospheric probe on 13 July 1995, the ''Galileo'' orbiter became the first man-made satellite of Jupiter at 00:27 UT on 8 December 1995 when it fired its main engine to enter a 198-day parking orbit.<ref>[http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo&Display=ReadMore Solar System Exploration – Galileo]. NASA. Retrieved 2012-04-24.</ref>
[[File:Repeated Flybys Yield a Pole-to-Pole View of Europa.jpg|thumb|upright|Multiple spacecraft flybys collected the data for this [[Europa (moon)|Europa]] mosaic]]

''Galileo{{'s}}'' prime mission was a two-year study of the Jovian system. 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. Once the prime mission concluded, an extended mission started on December 7, 1997; the spacecraft made a number of flybys of [[Europa (moon)|Europa]] and [[Io (moon)|Io]]. The closest approach was {{convert|180|km|mi|abbr=on}} on October 15, 2001. The radiation environment near Io 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.

''Galileo{{'s}}'' 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&nbsp;—a measurement of the moon [[Amalthea (moon)|Amalthea]]'s mass as the spacecraft swung by it.

On December 11, 2013, NASA reported, based on results from the ''Galileo'' mission, the detection of "[[Clay minerals|clay-like minerals]]" (specifically, [[phyllosilicates]]), often associated with [[organic materials]], on the icy crust of [[Europa (moon)|Europa]], moon of [[Jupiter]].<ref name="NASA-20131211">{{cite web |last=Cook |first=Jia-Rui c. |title=Clay-Like Minerals Found on Icy Crust of Europa |url=http://www.jpl.nasa.gov/news/news.php?release=2013-362 |date=December 11, 2013 |work=[[NASA]] |accessdate=December 11, 2013}}</ref> The presence of the minerals may have been the result of a collision with an [[asteroid]] or [[comet]] according to the scientists.<ref name="NASA-20131211" />

==Spacecraft==
The [[Jet Propulsion Laboratory]] built the ''Galileo'' spacecraft and managed the ''Galileo'' mission for NASA. Germany supplied the propulsion module. NASA's [[Ames Research Center]] managed the probe, which was built by [[Hughes Aircraft Company]].
[[File:Galileo in 1983.jpg|thumb|right|upright|300px|Galileo with its main antenna open]]

At launch, the orbiter and probe together had a mass of {{convert|2,564|kg|lb|abbr=off}} and stood seven metres tall. One section of the spacecraft rotated at 3&nbsp;rpm, keeping ''Galileo'' stable and holding six instruments that gathered data from many different directions, including the fields and particles instruments. The other section of the spacecraft was an antenna, and data were periodically transmitted to it. Back on the ground, the mission operations team used software containing 650,000 lines of programming code in the orbit sequence design process; 1,615,000 lines in the telemetry interpretation; and 550,000 lines of code in navigation.

===Command and Data Handling (CDH)===
The CDH subsystem was actively redundant, with two parallel [[System bus|data system buses]] running at all times.<ref>{{cite book|last=Siewiorek|first=Daniel|title=Reliable Computer Systems|date=1998|publisher=A K Peters|location=Natick, Massachusetts, USA|isbn=1-56881-092-X|pages=683}}</ref>   Each data system bus (a.k.a. string) was composed of the same functional elements, consisting of multiplexers (MUX), high-level modules (HLM), low-level modules (LLM), power converters (PC), bulk memory (BUM), data management subsystem bulk memory (DBUM), timing chains (TC), [[Phase-locked loop|phase locked loops]] (PLL), [[Binary Golay code|Golay]] coders (GC), hardware command decoders (HCD) and critical controllers (CRC).

The CDH subsystem was responsible for maintaining the following functions:
#decoding of uplink commands
#execution of commands and sequences
#execution of system-level fault-protection responses
#collection, processing, and formatting of telemetry data for downlink transmission
#movement of data between subsystems via a data system bus

The spacecraft was controlled by six [[RCA 1802]] COSMAC microprocessor [[central processing unit|CPUs]]: four on the spun side and two on the despun side. Each CPU was clocked at about 1.6&nbsp;MHz, and fabricated on [[sapphire]] ([[silicon on sapphire]]), which is a [[radiation hardening|radiation-and static-hardened]] material ideal for spacecraft operation. This microprocessor was the first low-power [[CMOS]] processor chip, quite on a par with the 8-bit 6502 that was being built into the [[Apple II]] [[desktop computer]] at that time.

The Galileo Attitude and Articulation Control System (AACSE) was controlled by two [[Itek]] Advanced Technology Airborne Computers (ATAC), built using radiation-hardened [[AMD Am2900|2901s]].
The AACSE could be reprogrammed in flight by sending the new program through the Command and Data Subsystem.
''Galileo'''s attitude control system software was written in the [[HAL/S]] programming language,<ref name=tom/> also used in the [[Space Shuttle program]].

Memory capacity provided by each BUM was 16K of [[RAM]], while the DBUMs each provided 8K of RAM. There were two BUMs and two DBUMs in the CDH subsystem and they all resided on the spun side of the spacecraft. The BUMs and DBUMs provided storage for sequences and contain various buffers for telemetry data and interbus communication.

Every HLM and LLM was built up around a single 1802 microprocessor and 32K of RAM (for HLMs) or 16K of RAM (for LLMs). Two HLMs and two LLMs resided on the spun side while two LLMs were on the despun side.

Thus, total memory capacity available to the CDH subsystem was 176K of RAM: 144K allocated to the spun side and 32K to the despun side.

Each HLM was responsible for the following functions:
#uplink command processing
#maintenance of the spacecraft clock
#movement of data over the data system bus
#execution of stored sequences (time-event tables)
#telemetry control
#error recovery including system fault-protection monitoring and response

Each LLM was responsible for the following functions:
#collect and format engineering data from the subsystems
#provide the capability to issue coded and discrete commands to spacecraft users
#recognize out-of-tolerance conditions on status inputs
#perform some system fault-protection functions

The HCD receives command data from the modulation/demodulation subsystem, decodes these data and transfers them to the HLMs and CRCs.

The CRC controls the configuration of CDH subsystem elements. It also controls access to the two data system buses by other spacecraft subsystems. In addition, the CRC supplies signals to enable certain critical events (e.g. probe separation).

The GCs provide [[Binary Golay code|Golay]] encoding of data via hardware.

The TCs and PLLs establish timing within the CDH subsystem.

===Propulsion===
The Propulsion Subsystem consisted of a 400&nbsp;N main engine and twelve 10&nbsp;N thrusters, together with propellant, storage and pressurizing tanks and associated plumbing. The 10&nbsp;N thrusters were mounted in groups of six on two 2-meter booms. The fuel for the system was 925&nbsp;kg of [[monomethylhydrazine]] and nitrogen tetroxide. Two separate tanks held another 7&nbsp;kg of helium pressurant. The Propulsion Subsystem was developed and built by [[DaimlerChrysler Aerospace|Daimler Benz Aero Space AG (DASA)]] (formerly [[Messerschmitt|Messerschmitt–Bölkow–Blohm (MBB)]]) and provided by Germany, the major international partner in Project ''Galileo''.<ref>[http://www.resa.net/nasa/engineer.htm Engineering] {{wayback|url=http://www.resa.net/nasa/engineer.htm |date=20080613233904}}</ref>

===Electrical power===
At the time, [[Photovoltaic module|Solar panels]] were not practical at Jupiter's distance from the Sun (it would have needed a minimum of {{convert|65|m2|sqft|sp=us}} of solar panels). Chemical batteries would likewise be prohibitively massive due to the technological limitations. The solution was two [[radioisotope thermoelectric generator]]s (RTGs) which powered the spacecraft through the radioactive decay of plutonium-238. The heat emitted by this decay was converted into electricity through the solid-state [[Seebeck effect]]. This provided a reliable and long-lasting source of electricity unaffected by the cold environment and high-radiation fields in the Jovian system.

Each [[GPHS-RTG]], mounted on a 5-meter long boom, carried {{convert|7.8|kg|lb|abbr=off}} of <sup>238</sup>[[Plutonium|Pu]].<ref name="RTG">{{cite web|url=http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTG.html|title=What's in an RTG? |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref> Each RTG contained 18 separate heat source modules, and each module encased four pellets of [[plutonium dioxide]], a [[ceramic]] material resistant to fracturing. The modules were designed to survive a range of hypothetical accidents: launch vehicle explosion or fire, re-entry into the atmosphere followed by land or water impact, and post-impact situations. An outer covering of graphite provided protection against the structural, thermal, and eroding environments of a potential re-entry. Additional graphite components provided impact protection, while [[iridium]] cladding of the fuel cells provided post-impact containment. The RTGs produced about 570 watts at launch. The power output initially decreased at the rate of 0.6 watts per month and was 493 watts when ''Galileo'' arrived at Jupiter.
[[File:Galileo Diagram.jpg|thumb|left|A diagram of ''Galileo'''s main components.]]
As the launch of ''Galileo'' neared, anti-nuclear groups, concerned over what they perceived as an unacceptable risk to the public's safety from ''Galileo's'' RTGs, sought a court injunction prohibiting ''Galileo's'' launch. RTGs had been used for years in planetary exploration without mishap: the [[Lincoln Experimental Satellite]]s 8/9, launched by the U.S. Department of Defense, had 7% more plutonium on board than ''Galileo'', and the two [[Voyager spacecraft]] each carried 80% as much plutonium as ''Galileo'' did. However, activists remembered the messy crash of the Soviet Union's nuclear-powered [[Cosmos 954]] satellite in Canada in 1978, and though was not nuclear-powered, the 1986 [[Challenger accident]] raised public awareness about spacecraft failures. In addition, no RTGs had ever done a non-orbital swing past the Earth at close range and high speed, as ''Galileo's'' Venus-Earth-Earth Gravity Assist trajectory required it to do. This created a novel mission failure modality that might plausibly have entailed total dispersal of ''Galileo's'' plutonium in the Earth's atmosphere. Scientist [[Carl Sagan]], for example, a strong supporter of the ''Galileo'' mission, said in 1989 that "there is nothing absurd about either side of this argument." <ref name="Sagan">Sagan, Carl. "Benefit outweighs risk: Launch ''Galileo'' craft," USA Today, Inquiry Page, Tuesday, October 10, 1989 [http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html]</ref>

After ''Challenger'', a study considered additional shielding but rejected it, in part because such a design significantly increased the overall risk of mission failure and only shifted the other risks around (for example, if a failure on orbit had occurred, additional shielding would have significantly increased the consequences of a ground impact).<ref name="RTG"/>

===Instrumentation overview===
[[File:Jupiter equatorial hot spot.jpg|thumb|upright|Jovian "Hotspot" in visible (top) and near infrared (bottom)]]
Scientific instruments to measure fields and particles were mounted on the spinning section of the spacecraft, together with the main [[antenna (electronics)|antenna]], power supply, the propulsion module and most of ''Galileo'''s computers and control electronics. The sixteen instruments, weighing 118&nbsp;kg altogether, included [[magnetometer]] sensors mounted on an 11 m boom to minimize interference from the spacecraft; a [[Plasma physics|plasma]] instrument for detecting low-energy charged particles and a plasma-wave detector to study waves generated by the particles; a high-energy particle detector; and a detector of cosmic and Jovian dust. It also carried the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged particle environments the spacecraft flew through, and an added Extreme [[Ultraviolet]] detector associated with the UV spectrometer on the scan platform.

The despun section's instruments included the camera system; the near infrared mapping spectrometer to make multi-spectral images for atmospheric and moon surface chemical analysis; the ultraviolet spectrometer to study gases; and the photo-polarimeter radiometer to measure radiant and reflected energy. The camera system was designed to obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than [[Voyager program|''Voyager'']]'s best, because ''Galileo'' flew closer to the planet and its inner moons, and because the more modern [[Charge-coupled device|CCD]] sensor in ''Galileo'''s camera was more sensitive and had a broader color detection band than the [[vidicon]]s of ''Voyager''.

===Instrumentation details===
The following information was taken directly from NASA's ''Galileo'' legacy site.<ref>{{cite web|url=http://galileo.jpl.nasa.gov/resources.cfm |title=Solar System Exploration: '&#39;Galileo'&#39; Legacy Site |publisher=Galileo.jpl.nasa.gov |accessdate=2011-05-15}}</ref>

====Despun section====
[[File:Detailed Drawing of the Galileo Spacecraft.png|thumb|right|Detailed diagram of ''Galileo'&nbsp;''s instruments and subsystems]]

=====Solid State Imager (SSI)=====
[[File:Galileo Solid- State Imaging.jpg|thumb|left|The spacecraft's Solid-State Imager]]
The SSI was an 800-by-800-pixel solid state camera consisting of an array of silicon sensors called a "[[charge coupled device]]" (CCD). Galileo was one of the first spacecraft to be equipped with a CCD camera.{{Citation needed|date=February 2011}}  The optical portion of the camera was built as a [[Cassegrain telescope]]. Light was collected by the primary mirror and directed to a smaller secondary mirror that channeled it through a hole in the center of the primary mirror and onto the CCD. The CCD sensor was shielded from radiation, a particular problem within the harsh Jovian magnetosphere. The shielding was accomplished by means of a 10&nbsp;mm thick layer of [[tantalum]] surrounding the CCD except where the light enters the system. An eight-position filter wheel was used to obtain images at specific wavelengths. The images were then combined electronically on Earth to produce color images. The spectral response of the SSI ranged from about 0.4 to 1.1 micrometres. The SSI weighed 29.7 kilograms and consumed, on average, 15 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/ssi.html |title=SSI – Solid State Imaging |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www2.jpl.nasa.gov/galileo/sepo/ SSI Imaging Team site].</ref>

=====Near-Infrared Mapping Spectrometer (NIMS)=====
The NIMS instrument was sensitive to 0.7-to-5.2-micrometre wavelength IR light, overlapping the wavelength range of the SSI. The telescope associated with NIMS was all reflective (using only mirrors and no lenses) with an aperture of 229&nbsp;mm. The [[spectrometer]] of NIMS used a grating to disperse the light collected by the telescope. The dispersed spectrum of light was focused on detectors of [[indium]] [[antimonide]] and [[silicon]]. The NIMS weighed 18 kilograms and used 12 watts of power on average.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/nims.html |title=NIMS – Near-Infrared Mapping Spectrometer |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://jumpy.igpp.ucla.edu/~nims/ NIMS Team site].</ref>

=====Ultraviolet Spectrometer / Extreme Ultraviolet Spectrometer (UVS/EUV)=====
[[File:Galileo - UVS photo - uvs1.jpg|thumb|Ultraviolet Spectrometer]]
The [[Cassegrain telescope]] of the UVS had a 250&nbsp;mm aperture and collected light from the observation target. Both the UVS and EUV instruments used a ruled [[grating]] to disperse this light for spectral analysis. This light then passed through an exit slit into [[photomultiplier]] tubes that produced pulses or "sprays" of electrons. These electron pulses were counted, and these count numbers constituted the data that were sent to Earth. The UVS was mounted on ''Galileo'''s scan platform and could be pointed to an object in inertial space. The EUV was mounted on the spun section. As ''Galileo'' rotated, EUV observed a narrow ribbon of space perpendicular to the spin axis. The two instruments combined weighed about 9.7 kilograms and used 5.9 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/euv.html |title=EUVS – Extreme Ultraviolet Spectrometer |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://lasp.colorado.edu/galileo/ EUV Team site].</ref>

=====Photopolarimeter-Radiometer (PPR)=====
The PPR had seven radiometry bands. One of these used no filters and observed all incoming radiation, both solar and thermal. Another band allowed only solar radiation through. The difference between the solar-plus-thermal and the solar-only channels gave the total thermal radiation emitted. The PPR also measured in five broadband channels that spanned the spectral range from 17 to 110 micrometres. The radiometer provided data on the temperatures of Jupiter's atmosphere and satellites. The design of the instrument was based on that of an instrument flown on the [[Pioneer Venus]] spacecraft. A 100&nbsp;mm aperture reflecting telescope collected light and directed it to a series of filters, and, from there, measurements were performed by the detectors of the PPR. The PPR weighed 5.0 kilograms and consumed about 5 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/ppr.html |title=PPR – Photopolarimeter-Radiometer |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www.lowell.edu/users/ppr/ PPR Team site].</ref>

====Spun section====

=====Dust Detector Subsystem (DDS)=====
The Dust Detector Subsystem (DDS) was used to measure the mass, electric charge, and velocity of incoming particles. The masses of dust particles that the DDS could detect go from 10<sup>−16</sup> to 10<sup>−7</sup> grams. The speed of these small particles could be measured over the range of 1 to 70 kilometers per second. The instrument could measure impact rates from 1 particle per 115 days (10 megaseconds) to 100 particles per second. Such data was used to help determine dust origin and dynamics within the [[magnetosphere]]. The DDS weighed 4.2 kilograms and used an average of 5.4 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/dds.html |title=DDS – Dust Detector Subsystem |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www.mpi-hd.mpg.de/dustgroup/galileo/galileo.html "Cosmic Dust: Messengers from Distant Worlds"]. DSI via Stuttgart University. Retrieved 10 December 2012.</ref>

=====Energetic Particles Detector (EPD)=====
The Energetic Particles Detector (EPD) was designed to measure the numbers and energies of ions and electrons whose energies exceeded about 20 [[keV]] (3.2 fJ). The EPD could also measure the direction of travel of such particles and, in the case of ions, could determine their composition (whether the ion is oxygen or sulfur, for example). The EPD used silicon solid state detectors and a [[time-of-flight]] detector system to measure changes in the energetic particle population at Jupiter as a function of position and time. These measurements helped determine how the particles got their energy and how they were transported through Jupiter's magnetosphere. The EPD weighed 10.5 kilograms and used 10.1 watts of power on average.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/epd.html |title=EPD – Energetic Particles Detector |publisher=JPL |accessdate=2011-05-15}}</ref><ref>[http://sd-www.jhuapl.edu/Galileo_EPD/ Galileo EPD]. JHUAPL.edu.</ref>

=====Heavy Ion Counter (HIC)=====
[[File:Galileo Heavy Ion Counter.jpg|thumb|right|''Galileo'&nbsp;''s Heavy Ion Counter.]]
The HIC was in effect a repackaged and updated version of some parts of the flight spare of the [[Voyager program|Voyager]] Cosmic Ray System. The HIC detected heavy [[ion]]s using stacks of single crystal silicon wafers. The HIC could measure heavy ions with energies as low as 6 MeV (1 pJ) and as high as 200 MeV (32 pJ) per nucleon. This range included all atomic substances between [[carbon]] and nickel. The HIC and the EUV shared a communications link and, therefore, had to share observing time. The HIC weighed 8 kilograms and used an average of 2.8 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/hic.html |title=HIC – Heavy Ion Counter |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www.srl.caltech.edu/galileo/galHIC.html HIC Team site].</ref>

=====Magnetometer (MAG)=====
The [[spacecraft magnetometer|magnetometer]] (MAG) used two sets of three sensors. The three sensors allowed the three orthogonal components of the [[magnetic field]] section to be measured. One set was located at the end of the magnetometer boom and, in that position, was about 11 m from the spin axis of the spacecraft. The second set, designed to detect stronger fields, was 6.7 m from the spin axis. The boom was used to remove the MAG from the immediate vicinity of ''Galileo'' to minimize magnetic effects from the spacecraft. However, not all these effects could be eliminated by distancing the instrument. The rotation of the spacecraft was used to separate natural magnetic fields from engineering-induced fields. Another source of potential error in measurement came from the bending and twisting of the long magnetometer boom. To account for these motions, a calibration coil was mounted rigidly on the spacecraft to generate a reference magnetic field during calibrations. The magnetic field at the surface of the Earth has a strength of about 50,000 [[Tesla (unit)|nT]]. At Jupiter, the outboard (11 m) set of sensors could measure magnetic field strengths in the range from ±32 to ±512 nT, while the inboard (6.7 m) set was active in the range from ±512 to ±16,384 nT. The MAG experiment weighed 7 kilograms and used 3.9 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/mag.html |title=MAG – Magnetometer |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www.igpp.ucla.edu/galileo/ MAG Team site].</ref>
[[File:Galileo - PWS photo - pws1.jpg|thumb|The Plasma Wave Subsystem.]]

=====Plasma Subsystem (PLS)=====
The PLS used seven fields of view to collect [[Particle radiation|charged particle]]s for energy and mass analysis. These fields of view covered most angles from 0 to 180 degrees, fanning out from the spin axis. The rotation of the spacecraft carried each field of view through a full circle. The PLS measured particles in the energy range from 0.9 eV to 52 keV (0.1 aJ to 8.3 fJ). The PLS weighed 13.2 kilograms and used an average of 10.7 watts of power.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/pls.html |title=PLS – Plasma Subsystem |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www-pi.physics.uiowa.edu/www/pls/ PLS Team site].</ref>

=====Plasma Wave Subsystem (PWS)=====
An electric [[dipole antenna]] was used to study the electric fields of [[Plasma physics|plasma]]s, while two search coil magnetic antennas studied the magnetic fields. The electric dipole antenna was mounted at the tip of the magnetometer boom. The search coil magnetic antennas were mounted on the high-gain antenna feed. Nearly simultaneous measurements of the electric and magnetic field spectrum allowed electrostatic waves to be distinguished from electromagnetic waves. The PWS weighed 7.1 kilograms and used an average of 9.8 watts.<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/instruments/pws.html |title=PWS – Plasma Wave Subsystem |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref><ref>[http://www-pw.physics.uiowa.edu/plasma-wave/galileo/home.html "Galileo PWS"]. UIowa.edu. Retrieved 4 December 2012.</ref>
{{clear}}

==Galileo Probe==
{{main|Galileo Probe}}
The '''Galileo Probe''' was an atmospheric-entry probe carried by the main Galileo spacecraft on its way to [[Jupiter]]. It separated from the main spacecraft in July 1995, five months before its rendezvous with the planet on 7 December. After a rough deceleration, the Descent Module started to return data to the main spacecraft hovering high above Jupiter.<ref name=gpr>[http://www2.jpl.nasa.gov/sl9/gll38.html Galileo Probe Science Results]</ref> The {{convert|339|kg|lb|adj=on}} probe was built by Hughes Aircraft Company<ref>{{cite web|url=http://www.flightglobal.com/pdfarchive/view/1989/1989%20-%200347.html|accessdate=2011-05-15|publisher=flightglobal.com|title=Hughes Science/Scope Press Release and Advertisement, retrieved from Flight Global Archives May 23, 2010}}</ref> at its El Segundo, California plant, measured about {{convert|1.3|m|ft|sp=us}} across. Inside the probe's [[heat shield]], the Descent Module with its scientific instruments were protected from extreme heat and pressure during its high-speed journey into the Jovian atmosphere, entering at {{convert|47.8|km|mi|sp=us}} per second.

During the 57 minutes of data collecting, the Galileo probe returned some surprising data on Jupiter's atmospheric conditions and composition and also some new discoveries.

==Jupiter science==
{{Main|Timeline of Galileo Jupiter orbiter tour}}
[[File:Europa Pwyll.jpg|thumb|[[Pwyll (crater)|Pwyll]] crater on Europa.]]
[[File:Ganymede terrain.jpg|thumb|Terrain on Ganymede. This region is about {{convert|213|by|97|km|mi|abbr=on}}<ref>[http://photojournal.jpl.nasa.gov/catalog/PIA02577 PIA02577]</ref>]]

After arriving on December 7, 1995 and completing 35 orbits around Jupiter throughout a nearly eight-year mission, the ''Galileo'' Orbiter was destroyed during a controlled impact with Jupiter on September 21, 2003. During that intervening time, ''Galileo'' forever changed the way scientists saw Jupiter and provided a wealth of information on the moons orbiting the planet which will be studied for years to come. Culled from NASA's press kit, the top orbiter science results were:

* ''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.
* The moon [[Io (moon)|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 provide evidence that Europa, Ganymede and Callisto have a liquid-saltwater layer under the visible surface.
* Evidence exists that [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|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 [[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.
* The ''Galileo'' spacecraft identified the global structure and dynamics of a giant planet's [[magnetosphere]].

==Other science conducted by ''Galileo''==

===Remote detection of life on Earth===
[[File:Galileo Earth - PIA00114.jpg|thumb|''Galileo'' image of Earth, taken in December 1990]]
The astronomer [[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'&nbsp;''s remote sensing instruments during the mission's first Earth flyby in December 1990. After data acquisition and processing, Sagan et al. published a paper in ''[[Nature (journal)|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]]) which was caused by absorption by chlorophyll in photosynthesizing plants, absorption bands of molecular oxygen which is also a result of plant activity, infrared absorption bands caused by the ~1 micromole per [[mole (unit)|mole]] (µmol/mol) of methane in Earth's atmosphere (a gas which must be replenished by either volcanic or biological activity), and modulated narrowband radio wave transmissions uncharacteristic of any known natural source. ''Galileo'&nbsp;''s experiments were thus the first ever controls in the newborn science of [[astrobiological]] remote sensing.
<ref>{{cite journal
|author1=C. Sagan |author2=W. R. Thompson |author3=R. Carlson |author4=D. Gurnett |author5=C. Hord | title = A search for life on Earth from the ''Galileo'' spacecraft
| journal = Nature
| date = 1993
| volume = 365
| pages = 715–721
| doi =  10.1038/365715a0
| pmid = 11536539
| issue = 6448|bibcode = 1993Natur.365..715S}}</ref>

===The ''Galileo'' optical experiment===
[[File:GOPEX(galileo optical experiment).jpg|left|thumb|200px|An image of Earth taken by ''Galileo'' during the GOPEX test, showing laser pulses coming from a transmitting telescope on the night side. ''Galileo'''s imager was panned downward during the exposure to separate the pulses, thus blurring Earth's image on the right.]]
In December 1992, during ''Galileo'&nbsp;''s second gravity-assist [[planetary flyby]] of Earth, another groundbreaking experiment was performed. Optical communications in space was assessed by detecting light pulses from powerful lasers with ''Galileo'&nbsp;''s [[Charge-coupled device|CCD]]. The experiment, dubbed Galileo OPtical EXperiment or GOPEX,<ref>{{cite web|url=http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf |title=GOPEX SPIE 1993 (Edited) |format=PDF |accessdate=2011-05-15}}</ref> 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 [[nonlinear optics|frequency doubled]] [[Neodymium]]-[[Yttrium]]-Aluminium [[Garnet]] ([[Nd-YAG laser|Nd:YAG]]) laser operating at 532&nbsp;nm with a repetition rate of ~15 to 30&nbsp;Hz and a pulse power ([[Full width at half maximum|FWHM]]) in the tens of megawatts range, which was coupled to a 0.6 meter Cassegrain telescope for transmission to ''Galileo''; the Starfire range site used a similar setup with a larger transmitting telescope (1.5 m). Long exposure (~0.1 to 0.8 s) images using ''Galileo'''s 560&nbsp;nm centered green filter produced images of Earth clearly showing the laser pulses even at distances of up to 6,000,000&nbsp;km. Adverse weather conditions, restrictions placed on laser transmissions by the U.S. Space Defense Operations Center ([[Cheyenne Mountain nuclear bunker|SPADOC]]) and a pointing error caused by the scan platform acceleration on the spacecraft being slower than expected (which prevented laser detection on all frames with less than 400 ms exposure times) all contributed to the reduction of 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 will likely be used in the future to design laser "downlinks" which will send large volumes of data very quickly, from spacecraft to Earth. The scheme is already being studied (as of 2004) for a data link to a future Mars orbiting spacecraft.<ref>{{cite web|url=http://www.space.com/spacenews/businessmonday_041115.html |title=NASA To Test Laser Communications With Mars Spacecraft |publisher=Space.com |date=2004-11-15 |accessdate=2011-05-15}}</ref>

===Lunar observations===
{|
|[[File:Earth's Moon.jpg|thumb|200px|Galileo shot of Earth's Moon]]
|[[File:Moon Crescent - False Color Mosaic.jpg|left|thumb|200px|upright|An [[false-color|artificially]] coloured mosaic constructed from a series of 53 images taken through three [[Filter (optics)|spectral filters]] by ''Galileo'&nbsp;''s imaging system as the spacecraft flew over the northern regions of the Moon on December 7, 1992.]]
|[[File:Moon-galileo-color.jpg|thumb|The Moon by ''Galileo'' spacecraft]]
|}
{{clear}}

===Star scanner===
''Galileo'&nbsp;''s star scanner was a small optical telescope that provided an absolute attitude reference. It also made several scientific discoveries serendipitously.<ref>[http://www.mindspring.com/~feez/ "Science with The Galileo Star Scanner"]. Mindspring.com. Retrieved 8 December 2012.</ref> 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.

A second discovery occurred in 2000. The star scanner was observing a set of stars which 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 even [[Algol]].<ref>{{cite web|url=http://www.konkoly.hu/cgi-bin/IBVS?4999 |title=IBVS 4999 (7 December 2000) |publisher=Konkoly.hu |accessdate=2011-05-15}}</ref> It has a primary period of 45 days and the dimming is just visible with the naked eye.

A final discovery occurred during the last two orbits of the mission. When the spacecraft passed the orbit of Jupiter's moon [[Amalthea (moon)|Amalthea]], the star scanner detected unexpected flashes of light that were reflections from moonlets. None of the individual moonlets were reliably sighted twice, hence no orbits were determined and the moonlets did not meet the International Astronomical Union requirements to receive designations.<ref name=IAU_2003/> It is believed that these moonlets most likely are debris ejected from Amalthea and form a tenuous, and perhaps temporary, ring.<ref>{{Cite journal | doi = 10.1016/j.icarus.2004.01.012 | bibcode = 2004Icar..169..390F| title = The Galileo star scanner observations at Amalthea| journal = Icarus| volume = 169| issue = 2| pages = 390| year = 2004| last1 = Fieseler | first1 = P. D. | last2 = Adams | first2 = O. W. | last3 = Vandermey | first3 = N. | last4 = Theilig | first4 = E. E. | last5 = Schimmels | first5 = K. A. | last6 = Lewis | first6 = G. D. | last7 = Ardalan | first7 = S. M. | last8 = Alexander | first8 = C. J.}}</ref>

===Asteroid encounters===
[[File:951 Gaspra.jpg|thumb|[[951 Gaspra]] (enhanced colorization).]]
[[File:243 ida.jpg|thumb|left|[[243 Ida]]. The bright dot to the right is its moon, Dactyl.]]

====First asteroid encounter: 951 Gaspra====
On October 29, 1991, two months after entering the asteroid belt, ''Galileo'' performed the first asteroid encounter by a human spacecraft, passing approximately {{convert|1,600|km|mi|sp=us}} from [[951 Gaspra]] at a relative speed of about 8 kilometers per second (18,000&nbsp;mph). Several pictures of Gaspra were taken, along with measurements using the NIMS instrument to indicate composition and physical properties. The last two images were relayed back to Earth in November 1991 and June 1992. The imagery revealed a cratered and very irregular body, measuring about {{convert|19|by|12|by|11|km|mi|abbr=off|sp=us}}. The remainder of data taken, including low-resolution images of more of the surface, were transmitted in late November 1992.<ref>{{cite journal
| title = Galileo's Encounter with 951 Gaspra: Overview
|author1=Veverka, J. |author2=Belton, M. |author3=Klaasen, K. |author4=Chapman, C. | journal = Icarus
| date = 1994
| volume = 107
| issue = 1
| pages = 2–17
| doi = 10.1006/icar.1994.1002
| bibcode=1994Icar..107....2V
}}</ref>

====Second asteroid encounter: 243 Ida and Dactyl====
On August 28, 1993, ''Galileo'' flew within {{convert|2,400|km|mi|sp=us}} of the asteroid [[243 Ida]]. The probe discovered that Ida had a small moon, dubbed Dactyl, measuring around {{convert|1.4|km|mi|sp=us}} in diameter; this was the first [[asteroid moon]] discovered. Measurements using ''Galileo'''s solid state imager, magnetometer and NIMS instrument were taken. From subsequent analysis of this data, Dactyl appears to be an SII subtype S type asteroid, and is spectrally different from 243 Ida. It is hypothesized that Dactyl may have been produced by partial melting within a [[Koronis family|Koronis]] parent body, while the 243 Ida region escaped such igneous processing.

==Mission challenges==
[[File:Galileo encounter with Io.png|thumb|right|Artist's impression of ''Galileo'' flying past [[Io (moon)|Io]]]]
Some of the mission challenges that had to be overcome included intense radiation at Jupiter and hardware wear-and-tear, as well as dealing with the unexpected conditions of discovery.

===Radiation-related anomalies===
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 exceeding 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 off data returned from [[Pioneer 10|Pioneers 10]] and [[Pioneer 11|11]], since much of the design work was underway before the two Voyagers arrived at Jupiter in 1979.<ref>{{cite book|last=Tomayko|first=James|title=Computers in Spaceflight: The NASA Experience|date=1988|publisher=NASA History Office|pages=200|url=http://www.hq.nasa.gov/pao/History/computers/Ch6-3.html}}</ref>

A typical effect of the radiation was that several of the science instruments suffered increased [[signal-to-noise|noise]] while within about {{convert|700,000|km|mi}} 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. 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 effect of the radiation were current leakages somewhere in 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 the Earth. 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>[http://starbase.jpl.nasa.gov/go-a-nims-3-tube-v1.0/go_1117/catalog/insthost.cat~ "Instrument Host Overview"]. NASA. 1999. Retrieved 29 November 2012.</ref>

===Main antenna problem===
[[File:Galileo arrival at Jupiter.gif|thumb|Artist's impression of ''Galileo'' arriving at Jupiter.]]
''Galileo'&nbsp;''s [[high-gain antenna]] failed to fully deploy after its first flyby of Earth.
The antenna had 18 ribs, like an umbrella and 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. Only 15 popped out, leaving the antenna looking like a lop-sided, half-open umbrella. Investigators concluded that during the 4.5 years that ''Galileo'' spent in storage after the [[Challenger disaster|1986 ''Challenger'' disaster]], the [[lubricant]]s between the tips of the ribs and the cup evaporated and no one thought to renew them.

Engineers tried thermal-cycling the antenna, rotating the spacecraft up to its maximum spin rate of 10.5 rpm, and "hammering" the antenna deployment motor — turning it on and off repeatedly — over 13,000 times, but all attempts failed to open the high-gain antenna.

The associated problem mission managers faced was if one rib popped free, there would be increased pressure on the remaining two, and if one of them popped out the last would be under so much pressure it would never release.
The second part of the problem was due to Galileo's revised flight plan. The probe had never been intended to approach the Sun any closer than the orbit of Earth, but sending it to Venus would expose it to temperatures at least 50 degrees higher than at Earth distance. So the probe had to be protected from that extra heat, part of which involved adapting some of the computer functions. Forty-one drivers had been programmed into the computer, but with no room for any more, the mission planners had to decide which driver they could use in association with the heat protection. They chose the antenna motor reverse driver.

Even with the dry grease at the antenna rib tips, had the antenna motor been able to run backwards, as well as forwards, the ribs would have eventually popped out.

Fortunately, ''Galileo'' possessed an additional [[low-gain antenna]] that was capable of transmitting information back to Earth, although since it transmitted a signal [[isotropic]]ally, the low-gain antenna's [[bandwidth (computing)|bandwidth]] was significantly less than the high-gain antenna's would have been; the high-gain antenna was to have transmitted at 134&nbsp;[[kilobit]]s per second, whereas the low-gain antenna was only intended to transmit at about 8 to 16&nbsp;bits per second. ''Galileo'&nbsp;''s low-gain antenna transmitted with a power of about 15 to 20&nbsp;watts, which, by the time it reached Earth, and had been collected by one of the large aperture (70 m) DSN antennas, had a total power of about −170 dBm or 10 zeptowatts (10 × 10<sup>−21</sup>&nbsp;watts).<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/faqhga.html |title='&#39;Galileo'&#39; FAQ – '&#39;Galileo'&#39;'s Antennas |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref> Through the implementation of sophisticated technologies, the arraying of several [[Deep Space Network]] antennas and sensitivity upgrades to the receivers used to listen to ''Galileo'&nbsp;''s signal, data throughput was increased to a maximum of 160 bits per second.<ref name=DeepSpaceNasaDataRate>[http://deepspace.jpl.nasa.gov/technology/95_20/gll_case_study.html Advanced Systems Program and the Galileo Mission to Jupiter ]</ref> By further using data compression, the effective data rate could be raised to 1,000&nbsp;bits per second.<ref name=DeepSpaceNasaDataRate /><ref>[http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1989-084B Galileo Orbiter Telecommunications Description]</ref> The data collected on Jupiter and its moons was stored in the spacecraft's onboard [[tape recorder]], and transmitted back to Earth during the long [[apsis|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, although 70% of ''Galileo'&nbsp;''s science goals could still be met.<ref>[http://webcache.googleusercontent.com/search?q=cache:http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22009/1/97-0449.pdf Galileo’s Telecom Using The Low-Gain Spacecraft Antenna (PDF)]. NASA/JPL, 1996 (cached). Retrieved 2012-01-29.</ref>

===Tape recorder anomalies and remote repair===
[[File:Galileo Descent Full.jpg|thumb|An artist's impression of the ''Galileo'' probe descending into Jupiter's atmosphere.]]

The failure of ''Galileo'&nbsp;''s high-gain antenna meant that data storage to the tape recorder for later compression and playback was absolutely crucial in order to obtain any substantial information from the flybys of Jupiter and its moons. In October 1995, ''Galileo'&nbsp;''s four-track, 114-[[megabyte]]<ref>{{cite web|url=http://www2.jpl.nasa.gov/galileo/faqtape.html#capacity |title='&#39;Galileo'&#39; FAQ – Tape Recorder |publisher=.jpl.nasa.gov |accessdate=2011-05-15}}</ref> digital tape recorder, which was manufactured by [[Odetics Corporation]], remained stuck in rewind mode for 15 hours before engineers learned what had happened and sent commands to shut it off. Though the recorder itself was still in working order, the malfunction possibly damaged a length of tape at the end of the reel. This section of tape was subsequently 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 (moon)|Io]] and [[Europa (moon)|Europa]] observations during the orbit insertion phase, in order to focus solely on recording data sent from the Jupiter probe descent.

In November 2002, after the completion of the mission's only encounter with Jupiter's moon [[Amalthea (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 (spacecraft)|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 [[LED|light emitting diodes]] located in the drive electronics of the recorder's motor [[rotary encoder|encoder]] wheel. The [[GaAs]] 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. ''Galileo'&nbsp;''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.

===Probe parachute deployment===
[[File:Parachutes - Galileo Spacecraft - GPN-2000-001911.jpg|thumb|upright|The parachute for the entry probe is tested on Earth]]
The atmospheric probe deployed its 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.<ref>{{cite book|author=David M. Harland|title=Jupiter Odyssey: The Story of NASA's Galileo Mission|url=https://books.google.com/books?id=QtxVTDhrRAEC&pg=PA105|date=1 October 2000|publisher=Springer Science & Business Media|isbn=978-1-85233-301-0|pages=105–}}</ref>

==End of mission and deorbit==
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 had not been sterilized prior to launch and could potentially 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 any impact with Jupiter's moons and prevent a [[forward contamination]].

In order to crash into Jupiter, ''Galileo'' flew by [[Amalthea (moon)|Amalthea]] on November 5, 2002,<ref>Michael Meltzer, [http://history.nasa.gov/sp4231.pdf ''Mission to Jupiter: a History of the ''Galileo'' Project''], NASA SP 2007–4231, p. 238</ref> during its 34th orbit<!-- 21-SEP-2002 11:58:55.818 UTC to 28-JAN-2003 00:58:55.815 UTC -->, allowing a measurement of the moon's mass as it passed within {{convert|163.0|km|mi|sp=us}} ± {{convert|11.7|km|mi|sp=us}} of its surface<!-- S030916A.LBL uses "Altitude"; presumably the uncertainty comes from Amalthea's irregular shape -->. On April 14, 2003, ''Galileo'' reached its greatest distance from Jupiter for the entire mission after orbital insertion, {{convert|26000000|km|mi|sp=us}}, before plunging back towards the gas giant for its final impact.<ref>[http://solarsystem.nasa.gov/galileo/ Galileo Legacy Site]. NASA, 2010. Retrieved 2012-04-24.</ref> At the completion of its 35th and final circuit<!-- 13-SEP-2003 13:58:55.818 UTC to 30-SEP-2003 11:58:55.818 UTC -- note that the last 9 days did not occur --> around the Jovian system, ''Galileo'' impacted the gas giant in darkness just south of the equator on September 21, 2003, at 18:57 GMT. Its impact speed was approximately {{convert|173736|kph|mph}}.<ref>Peter Bond, [http://spaceflightnow.com/galileo/030921galileogone.html Spaceflight Now], 21 September 2003.</ref> The total mission cost was about US$1.4 billion.<ref>[http://history.nasa.gov/sp4231.pdf Kathy Sawyer (17 December 1991). "Galileo Antenna Apparently Still Stuck". ''Washington Post'': A14; Kathy Sawyer (18 December 1991). "$1.4 Billion Galileo Mission Appears Crippled." ''Washington Post'': A3] in ''Mission to Jupiter''. p.180.</ref><ref>[http://solarsystem.nasa.gov/galileo/facts.cfm Galileo: Facts & Figures]. NASA.gov. Retrieved 12 November 2012.</ref>

==Images from Jupiter==
[[File:The Galilean satellites (the four largest moons of Jupiter).tif|thumb|500px|center|<center>The [[Galilean moons|four largest moons]] of [[Jupiter]], as imaged by ''Galileo''.</center>]]
{{clear}}
[[File:PIA19048 realistic color Europa mosaic.jpg|thumb|500px|center|<Center>A color mosaic of [[Europa (moon)|Europa]] released by NASA in 2014</center>]]
{{Clear}}
<center>
{|
|[[File:SL9ImpactGalileo.jpg|thumb|left|''Galileo'' was serendipitously positioned to view [[Comet Shoemaker–Levy 9]]'s impact with Jupiter. These images, taken several seconds apart, show fragment W colliding (1994)]]
|[[File:Io - Tvashtar Catena.jpg|thumb|left|''Galileo'' captures a dynamic eruption at [[Tvashtar Catena]], a chain of volcanic bowls on Jupiter's moon [[Io (moon)|Io]].]]
|[[File:Amalthea (moon).png|thumb|90px|<center>[[Amalthea (moon)|Amalthea]]</center>]]
|}</center>

==Follow-on missions==
[[File:Europa Clipper orbit away from Jupiter radiation.jpg|thumb|The proposed [[Europa Clipper]] would attempt to circle beyond the radiation belts, shown here from another angle]]

===Juno===
{{main|Juno (spacecraft)}}
The next mission for an orbiter for Jupiter is the NASA [[Juno (spacecraft)|Juno]] spacecraft, launched in 2011 and expected to arrive in mid 2016.<ref>{{cite web | url=http://www.nasa.gov/missions/highlights/schedule.html | title=NASA's Shuttle and Rocket Launch Schedule |publisher=NASA| accessdate=February 17, 2011}}</ref>

===Europa Orbiter (canceled)===
There was a spare Galileo spacecraft that the Europeans considered using for a mission to Saturn, however this was passed over in favor of a newer design which became the [[Cassini (spacecraft)]].<ref>[U.S.-European Collaboration in Space Science - Page 61]</ref> Even before Galileo concluded, NASA considered the [[Europa Orbiter]],<ref>[http://hdl.handle.net/2014/20516 The Europa Orbiter Mission Design]</ref> which was a mission to [[Jupiter]]'s Moon [[Europa (moon)|Europa]] but it was canceled in 2002.<ref>[http://www.space.com/news/nasa_budget_020204.html NASA Kills Europa Orbiter]</ref>

===JUICE===
The ESA also is planning to return to the Jovian system with the [[Jupiter Icy Moons Explorer]] (JUICE), which is designed to orbit Ganymede in the 2020s.<ref>[http://sci.esa.int/juice/55055-juice-mission-gets-green-light-for-next-stage-of-development/ JUICE mission gets green light for next stage of development]</ref> There have been several other attempts at missions that were either dedicated to, or included the Jupiter system as part of their mission plan but did not make it out of the planning stages.

===Europa Multiple Flyby Mission===
Following the cancellation of Europa Orbiter, lower-cost versions were studied, initially named Europa Clipper. These studies led to the [[Europa Multiple Flyby Mission]] which was approved in 2015 and is currently planned for launch in the mid-2020s.

==Galileo team==
[[File:1989 s34 Galileo Deploy 5.jpg|thumb|200px|Galileo floating free into space after release from Space Shuttle ''Atlantis'', 1989]]
Investigators and researchers came from the following institutions, and included famous scientists such as [[Carl Sagan]] and [[James Van Allen]]<ref>[http://astro.if.ufrgs.br/solar/galfs.htm#investigator Galileo Probe (mainly bottom of page)]</ref>

* [[Ames Research Center]]
* [[Bell Labs|Bell Particles Laboratory]]
* [[Bonn University]] (Germany)
* Cornell University
* California Institute of Technology (aka Caltech)
* [[Goddard Institute for Space Studies]]
* [[Goddard Spaceflight Center]]
* Harvard University
* [[Jet Propulsion Laboratory]]
* [[Max Planck Institute for Chemistry]]
* [[NOAO]]
* [[State University of New York]]
* University of Arizona
* University of Colorado
* University of California at Los Angeles (UCLA)
* University of Hawaii
* University of Iowa 
* University of Wisconsin
* Johns Hopkins [[Applied Physics Laboratory]] (APL)

Makers:<ref>[http://astro.if.ufrgs.br/solar/galfs.htm#investigator Galileo Probe]</ref>
*Ames Research Center
*General Electric
*Jet Propulsion Laboratory
*Messerschmitt-Bolkow-Blohm 
*United States Department of Energy

[[File:Sts-34-patch.png|thumb|Patch for the STS-34 Mission]]
Crew of STS-34:
*[[Donald E. Williams]]
*[[Michael J. McCulley]]
*[[Shannon W. Lucid]]
*[[Franklin R. Chang-Diaz]]
*[[Ellen S. Baker]]

==See also==
* [[Cassini–Huygens]], Saturn orbiter and moon lander
* [[Exploration of Jupiter]]
* [[List of missions to the outer planets]]
* [[List of missions to Venus]]

==References==
{{Reflist|30em|refs=
<ref name=IAU_2003>{{cite web
 |url         = http://www.cbat.eps.harvard.edu/iauc/08100/08107.html#Item2
 |title       = Objects near Jupiter V (Amalthea)
 |author1=Fieseler P. D. |author2=Ardalan S. M. |work        = International Astronomical Union Circular 8107
 |publisher   = Central Bureau for Astronomical Telegrams
 |accessdate  = 2014-10-12
 |date        = 2003-04-04
 |archiveurl  = https://web.archive.org/web/20140302034543/http://www.cbat.eps.harvard.edu/iauc/08100/08107.html
 |archivedate = 2014-03-02
}} ({{bibcode|2003IAUC.8107....2F}})</ref>
}}

==External links==
{{Commons|Category:Galileo mission}}
*[http://www.jpl.nasa.gov/galileo/ ''Galileo'' home page]
*[http://solarsystem.nasa.gov/missions/profile.cfm?MCode=Galileo ''Galileo'' Mission Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
*[http://simkin.asu.edu/galileo/ ''Galileo'' Satellite Image Mosaics]
*[http://www.cs.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html Site explaining the LGA bandwidth upgrades from the Parkes Observatory]
*[http://lasers.jpl.nasa.gov/PAPERS/GOPEX/gopex_s2.pdf GOPEX site from JPL]
*[http://science.nasa.gov/newhome/headlines/ast21may99_1.htm NASA site on ''Galileo'' life detection experiments]
*[http://history.nasa.gov/sp4231.pdf ''Mission to Jupiter: a History of the ''Galileo'' Project'', by Michael Meltzer, NASA SP 2007–4231] (on-line book)
*[http://www.lpi.usra.edu/expmoon/galileo/galileo.html Exploring the Moon: ''Galileo'' Mission]
*[http://descanso.jpl.nasa.gov/DPSummary/Descanso5--Galileo_new.pdf JPL guide to Galileo Telecommunications]
*[http://www.boston.com/bigpicture/2008/07/views_of_jupiter.html Gallery of Jupiter system photos, including some by Galileo]
*[http://www.nasa.gov/topics/solarsystem/features/pia16827.html View of Europa from Galileo flybys]

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{{DEFAULTSORT:Galileo (Spacecraft)}}
[[Category:Galileo (spacecraft)|Galileo]]
[[Category:Destroyed space probes|Galileo (destroyed September 21, 2003)]]
[[Category:Missions to Jupiter]]
[[Category:NASA space probes]]
[[Category:Flagship Program]]
[[Category:Laser communication in space]]
[[Category:Spacecraft launched by the Space Shuttle]]
[[Category:Missions to asteroids]]
[[Category:Spacecraft launched in 1989]]
[[Category:Lunar flybys]]
[[Category:NASA programs]]
[[Category:Space programme of Germany]]