Editing Galileo project (section)

==High-gain antenna problem==
[[File:Galileo orbiter arrival at Jupiter (cropped).jpg|thumb|right|Illustration of ''Galileo'' with antenna not fully deployed|alt=refer to caption]]
After the Venus flyby and ''Galileo'' passing beyond Earth’s orbit, it was no longer risky to employ the {{abbr|HGA|high-gain antenna}}, so on April 11, 1991, ''Galileo'' was ordered to unfurl it. This was done using two small dual drive actuator (DDA) motors to drive a [[worm gear]], and was expected to take 165 seconds, or 330 seconds if one actuator failed. The antenna had 18 [[carbon-fiber reinforced polymer|graphite-epoxy]] ribs; when the driver motor started and put pressure on the ribs, they were supposed to pop out of the cup their tips were held in, and the antenna would unfold like an umbrella. When it reached the fully deployed configuration, redundant [[microswitch]]es would shut down the motors. Otherwise they would run for eight minutes before being automatically shut down to prevent them from overheating.{{sfn|Johnson|1994|pp=362–366}}{{sfn|Meltzer|2007|pp=171–172}}

Through telemetry from ''Galileo'', investigators determined that the electric motors had stalled at 56 seconds. The spacecraft's spin rate had decreased due to an increase in its [[moment of inertia]] and its wobble increased, indicative of an asymmetric unfolding. Only 15 ribs had popped out, leaving the antenna looking like a lop-sided, half-open umbrella. It was not possible to re-fold the antenna and try the opening sequence again; although the motors were capable of running in reverse, the antenna was not designed for this, and human assistance was required when it was done on Earth to ensure that the wire mesh did not snag.{{sfn|Johnson|1994|p=372}}{{sfn|Meltzer|2007|pp=173–174}}

The first thing the ''Galileo'' team tried was to rotate the spacecraft away from the Sun and back again on the assumption that the problem was with friction holding the pins in their sockets. If so, then heating and cooling the ribs might cause them to pop out of their sockets. This was done seven times, but with no result. They then tried swinging LGA-2 (which faced in the opposite direction to the HGA and LGA-1) 145 degrees to a hard stop, thereby shaking the spacecraft. This was done six times with no effect. Finally, they tried shaking the antenna by pulsing the DDA motors at 1.25 and 1.875 Hertz. This increased the torque by up to 40 percent. The motors were pulsed 13,000 times over a three-week period in December 1992 and January 1993, but only managed to move the ballscrew by one and a half revolutions beyond the stall point.{{sfn|Johnson|1994|p=372}}{{sfn|Meltzer|2007|p=181}}

[[File:Galileo in 1983.jpg|thumb|left|''Galileo'' with its high gain antenna open in the VPF on Earth |alt=refer to caption]]
Investigators concluded that during the 4.5 years that ''Galileo'' spent in storage after the ''Challenger'' disaster, the [[lubricant]]s between the tips of the ribs and the cup were eroded. They were then worn down by [[vibration]] during the three cross-country journeys by truck between California and Florida for the spacecraft. The failed ribs were those closest to the flat-bed trailers carrying ''Galileo'' on these trips.{{sfn|Meltzer|2007|pp=177–178}} The use of land transport was partly to save costs—air transport would have cost an additional $65,000 ({{Inflation|US-GDP|65000|r=-3|1989|fmt=eq}}) or so per trip—but also to reduce the amount of handling required in loading and unloading the aircraft, which was considered a major risk of damage.{{sfn|Meltzer|2007|p=183}} The spacecraft was also subjected to severe vibration in a vacuum environment by the IUS. Experiments on Earth with the test HGA showed that having a set of stuck ribs all on one side reduced the DDA torque produced by up to 40 percent.{{sfn|Meltzer|2007|pp=177–178}}

The antenna lubricants were applied only once, nearly a decade before launch. Furthermore, the HGA was not subjected to the usual rigorous testing, because there was no backup unit that could be installed in ''Galileo'' in case of damage. The flight-ready HGA was never given a thermal evaluation test, and was unfurled only a half dozen or so times before the mission. Testing might not have revealed the problem in any case; the [[Lewis Research Center]] was never able to replicate the problem on Earth, and it was assumed to be the combination of loss of lubricant during transportation, vibration during launch by the IUS, and a prolonged period of time in the vacuum of space where bare metal touching could undergo [[cold welding]]. Whatever the cause, the HGA was rendered useless.{{sfn|Meltzer|2007|pp=182–183}}

The two LGAs were capable of transmitting information back to Earth, but since it transmitted its signal over a cone with a 120-degree [[half-angle]], allowing it to communicate even when not pointed at Earth, its [[bandwidth (computing)|bandwidth]] was significantly less than that of the HGA would have been, as the HGA transmitted over a half-angle of one-sixth of a degree. The HGA was to have transmitted at 134&nbsp;[[kilobit]]s per second, whereas LGA-1 was only intended to transmit at about 8 to 16&nbsp;bits per second. LGA-1 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-meter DSN antennas, had a total power of about 10<sup>{{nbh}}20</sup>&nbsp;watts.<ref>{{cite web |url=http://www2.jpl.nasa.gov/galileo/faqhga.html |title=''Galileo'' FAQ – ''Galileo''{{'s}} Antennas |publisher=NASA/Jet Propulsion Laboratory |access-date=May 15, 2011 |archive-url=https://web.archive.org/web/20100528014039/http://www2.jpl.nasa.gov/galileo/faqhga.html |archive-date=May 28, 2010}}</ref> The change to mission plan required a series of software changes to be uploaded.{{sfn|Marr|1994|pp=150–157}}

Image data collected was buffered and collected in ''Galileo''{{'}}s Command and Data Subsystem (CDS) memory. This represented 192 kilobytes of the 384 kilobyte CDS storage, and had been added late, out of concern that the 6504 Complementary metal–oxide–semiconductor ([[CMOS]]) memory devices might not be reliable during a {{abbr|VEEGA|Venus-Earth-Earth Gravity Assist}} mission. As it happened, they gave no trouble, but the CDS memory could store up to 31 minutes of data from the Radio Relay Hardware (RRH) channels.{{sfn|Marr|1994|pp=150–157}} To conserve bandwidth, [[data compression|data-compression]] software was implemented. Image compression used an integer approximation of the [[discrete cosine transform]], while other data were compressed with variant of the [[Lempel–Ziv–Welch]] algorithm.{{sfn|Cheung|Tong|1993|p=99}} Using compression, the arraying of several Deep Space Network antennas, and sensitivity upgrades to the receivers used to listen to ''Galileo''{{'s}} signal, data throughput was increased to a maximum of 160&nbsp;bits per second.<ref name="parkes.tracks">{{cite web |url=https://www.scss.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html |title=The Parkes Galileo Tracks |publisher=Trinity College Dublin |first=John M. |last=Sarkissian |date=November 1997 |access-date=January 19, 2021 |archive-date=June 18, 2021 |archive-url=https://web.archive.org/web/20210618152442/https://www.scss.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html |url-status=live }}</ref><ref name="DeepSpaceNasaDataRate">{{cite web |url=http://deepspace.jpl.nasa.gov/technology/95_20/gll_case_study.html |title=Advanced Systems Program and the Galileo Mission to Jupiter |publisher=NASA/Jet Propulsion Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20110614133445/http://deepspace.jpl.nasa.gov/technology/95_20/gll_case_study.html |archive-date=June 14, 2011}}</ref> By further using data compression, the effective bandwidth could be raised to 1,000&nbsp;bits per second.<ref name=DeepSpaceNasaDataRate /><ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1989-084B |title=NASA – NSSDCA – Spacecraft – PDMP Details |publisher=NASA/Goddard Space Flight Center |url-status=dead |archive-url=https://web.archive.org/web/20090404070908/http://nssdc.gsfc.nasa.gov/nmc/spacecraftTelemetry.do?id=1989-084B |archive-date=April 4, 2009}}</ref>

The data collected on Jupiter and its moons were stored in the spacecraft's onboard tape recorder, and transmitted back to Earth during the long [[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,<ref name="parkes.tracks" /> but William J. O'Neil, ''Galileo''{{'s}} project manager from 1992 to 1997,{{sfn|Meltzer|2007|p=201}} expressed confidence that 70 percent of ''Galileo''{{'s}} science goals could still be met.{{sfn|Mudgway|2001|p=312}}<ref>{{cite web |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22009/1/97-0449.pdf |title=Galileo's Telecom Using The Low-Gain Spacecraft Antenna |archive-url=https://web.archive.org/web/20111124054217/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/22009/1/97-0449.pdf |archive-date=2011-11-24 |date=November 24, 2011 |publisher=NASA/Jet Propulsion Laboratory |orig-year=1996 |url-status=dead |access-date=January 29, 2012}}</ref> The decision to use magnetic tape for storage was a conservative one, taken in the late 1970s when the use of tape was common. Conservatism was not restricted to engineers; a 1980 suggestion that the results of ''Galileo'' could be distributed electronically instead of on paper was regarded as ridiculous by geologists, on the grounds that storage would be prohibitively expensive; some of them thought that taking measurements on a computer involved putting a wooden ruler up to the screen.{{sfn|Greenberg|2005|pp=40–41}}