Editing Galileo project (section)

===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>