Editing Galileo project (section)

==Planning==
{| class="wikitable floatright" style="font-size: 91%" 
|+ ''Galileo'' Project managers{{sfn|Meltzer|2007|p=265}}
! Manager || Date
|-
| [[John R. Casani]]|| October 1977 – February 1988
|-
| Dick Spehalski    || February 1988 – March 1990
|-
| Bill O'Neil       || March 1990 – December 1997
|-
| Bob Mitchell      || December 1997 – June 1998
|-
| Jim Erickson      || June 1998 – January 2001
|-
| Eilene Theilig    || January 2001 – August 2003
|-
| Claudia Alexander || August 2003 – September 2003
|}

===Initiation===
Following the approval of the ''[[Voyager program|Voyager]]'' missions, NASA's Scientific Advisory Group for Outer Solar System Missions considered the requirements for Jupiter orbiters and atmospheric probes. It noted that the technology to build a [[heat shield]] for an atmospheric probe did not yet exist, and indeed facilities to test one under the conditions found on Jupiter would not be available until 1980. There was also concern about the effects of radiation on spacecraft components, which would be better understood after ''Pioneer 10'' and ''Pioneer 11'' had conducted their flybys. ''Pioneer 10''{{'}}s flyby in December 1973 indicated that the effects were not as severe as had been feared.{{sfn|Meltzer|2007|pp=29–30}} NASA management designated JPL as the lead center for the Jupiter Orbiter Probe (JOP) Project.{{sfn|Meltzer|2007|pp=32–33}} [[John R. Casani]], who had headed the ''[[Mariner program|Mariner]]'' and ''Voyager'' projects, became the first project manager.<ref>{{cite web |publisher=NASA |title=NASA's 50 Year Men and Women |url=https://www.nasa.gov/50th/50th_magazine/50yearsEmployees.html |access-date=October 28, 2020 |archive-date=March 19, 2010 |archive-url=https://web.archive.org/web/20100319002842/https://www.nasa.gov/50th/50th_magazine/50yearsEmployees.html |url-status=dead }}</ref> The JOP would be the fifth spacecraft to visit Jupiter, but the first to orbit it, and the probe the first to enter its atmosphere.{{sfn|Dawson|Bowles|2004|pp=190–191}}

Ames and JPL decided to use a ''Mariner'' spacecraft for the Jupiter orbiter like the ones used for ''Voyager'' rather than a ''Pioneer'' spacecraft. ''Pioneer'' was stabilized by spinning the spacecraft at 60 [[rpm]], which gave a 360-degree view of the surroundings, and did not require an [[attitude control system]]. By contrast, ''Mariner'' had an attitude control system with three [[gyroscopes]] and two sets of six [[nitrogen]] jet thrusters. Attitude was determined with reference to the Sun and [[Canopus]], which were monitored with two primary and four secondary [[star tracker]] sensors. There was also an [[inertial reference unit]] and an [[accelerometer]]. The attitude control system allowed the spacecraft to take high-resolution images, but the functionality came at the cost of increased weight: a ''Mariner'' weighed {{convert|722|kg}} compared to just {{convert|146|kg}} for a ''Pioneer''.{{sfn|Meltzer|2007|pp=30–32}}

The increase in weight had implications. The Voyager spacecraft had been launched by [[Titan IIIE]] rockets with a ''[[Centaur (rocket stage)|Centaur]]'' upper stage, but Titan was retired afterwards. In the late 1970s, NASA was focused on the development of the reusable [[Space Shuttle]], which was expected to make expendable rockets obsolete.<ref>{{cite news |newspaper=[[The New York Times]] |title=Test Rocket for Planetary Exploration Rolled Out |first=John Noble |last=Wilford |author-link=John Noble Wilford |date=3 October 1973 |url=https://www.nytimes.com/1973/10/03/archives/test-rocket-for-planetary-exploration-rolled-out-forthcoming.html |access-date=8 October 2020 |archive-date=August 10, 2020 |archive-url=https://web.archive.org/web/20200810122304/https://www.nytimes.com/1973/10/03/archives/test-rocket-for-planetary-exploration-rolled-out-forthcoming.html |url-status=live }}</ref> In late 1975, NASA decreed that all future planetary missions would be launched by the Space Shuttle. The JOP would be the first to do so.{{sfn|Mudgway|2001|p=294}} The Space Shuttle was supposed to have the services of a [[space tug]] to launch payloads requiring something more than a [[low Earth orbit]], but this was never approved. The [[United States Air Force]] (USAF) instead developed the [[solid-fueled]] Interim Upper Stage (IUS), later renamed the [[Inertial Upper Stage]] (with the same acronym), for the purpose.{{sfn|Meltzer|2007|pp=32–33}}

The IUS was constructed in a modular fashion, with two stages, a large one with {{convert|21400|lb|order=flip}} of propellant, and a smaller one with {{convert|6000|lb|order=flip}}. This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites.{{sfn|Heppenheimer|2002|pp=330–335}} A configuration with three stages, two large and one small, would be enough for a planetary mission, so NASA contracted with [[Boeing]] for the development of a three-stage IUS.{{sfn|Heppenheimer|2002|pp=368–370}} A two-stage IUS was not powerful enough to launch a payload to Jupiter without resorting to using a series of [[gravity-assist]] maneuvers around planets to garner additional speed. Most engineers regarded this solution as inelegant and planetary scientists at JPL disliked it because it meant that the mission would take months or even years longer to reach Jupiter.{{sfn|Bowles|2002|p=420}}{{sfn|Heppenheimer|2002|pp=368–370}} Longer travel times meant that the spacecraft's components would age and possibly fail, and the onboard power supply and propellant would be depleted. Some of the gravity assist options also involved flying closer to the Sun, which would induce thermal stresses that also might cause failures.{{sfn|Meltzer|2007|p=82}}

It was estimated that the JOP would cost $634 million (equivalent to ${{Inflation|US-GDP|0.634|1979|r=3}} billion in {{Inflation/year|US-GDP}}), and it had to compete for [[fiscal year]] 1978 funding with the Space Shuttle and the [[Hubble Space Telescope]]. A successful lobbying campaign secured funding for both JOP and Hubble over the objections of [[United States Senate|Senator]] [[William Proxmire]], the chairman of the Independent Agencies Appropriations Subcommittee.{{sfn|Meltzer|2007|p=38}} The [[United States Congress]] approved funding for the Jupiter Orbiter Probe on July 19, 1977,<ref>{{cite web |title=Congressional Record |publisher=United States Congress |url=https://www.congress.gov/95/crecb/1977/07/19/GPO-CRECB-1977-pt19-3-1.pdf |access-date=11 July 2024}}</ref> and JOP officially commenced on October 1, 1977, the start of the fiscal year.{{sfn|Meltzer|2007|pp=33–36}} Project manager Casani solicited suggestions for a more inspirational name for the project from people associated with it. The most votes went to "Galileo", after [[Galileo Galilei]], the first person to view Jupiter through a telescope, and the discoverer of what are now known as the [[Galilean moons]] in 1610. It was noted at the time that the name was also that of a [[Galileo (Star Trek)|spacecraft]] in the ''[[Star Trek: The Original Series|Star Trek]]'' television show. In February 1978, Casani officially announced the choice of the name "Galileo".{{sfn|Meltzer|2007|p=38}}

===Preparation===
To enhance reliability and reduce costs, the project engineers decided to switch from a pressurized [[Galileo Probe|atmospheric probe]] to a vented one, so the pressure inside the probe would be the same as that outside, thus extending its lifetime in Jupiter's atmosphere, but this added {{convert|100|kg}} to its weight. Another {{convert|165|kg}} was added in structural changes to improve reliability. This required additional fuel in the IUS, but the three-stage IUS was itself overweight with respect to its design specifications, by about {{convert|7000|lb|order=flip}}.<ref name="Hurdles">{{cite news |title=More Hurdles Rise In Galileo Project To probe Jupiter |first=Thomas |last=O'Toole |author-link=Thomas O'Toole |newspaper=[[The Washington Post]] |date=11 August 1979 |url=https://www.washingtonpost.com/archive/politics/1979/08/15/more-hurdles-rise-in-galileo-project-to-probe-jupiter/a30ddbe5-d805-418f-8a93-5d8deaa69f93/ |access-date=11 October 2020 |archive-date=May 23, 2021 |archive-url=https://web.archive.org/web/20210523182517/https://www.washingtonpost.com/archive/politics/1979/08/15/more-hurdles-rise-in-galileo-project-to-probe-jupiter/a30ddbe5-d805-418f-8a93-5d8deaa69f93/ |url-status=live }}</ref>{{sfn|Finley|1988|p=23}}{{sfn|Meltzer|2007|pp=41–43}} Lifting ''Galileo'' and the three-stage IUS required a special lightweight version of the [[Space Shuttle external tank]], the [[Space Shuttle orbiter]] stripped of all non-essential equipment, and the [[Space Shuttle main engine]]s (SSME) running at full power level—109 percent of their rated power level.{{sfn|Heppenheimer|2002|pp=368–370}}{{efn|The rated power level (RPL) is the power at which an engine can be normally operated. In the case of the Space Shuttle, the specification called for 27,000 seconds operation at 100 percent of the RPL, or 14,000 seconds at 109 percent of the RPL, which was designated full power level (FPL).{{sfn|Jenkins|2016|p=II-158}} }} Running at this power level necessitated the development of a more elaborate engine cooling system. Concerns were raised over whether the engines could be run at 109 percent by the launch date, so a gravity-assist maneuver using Mars was substituted for a direct flight.{{sfn|Meltzer|2007|pp=41–43}}

[[File:Galileo Preparations - GPN-2000-000672.jpg|thumb|left|In the Vertical Processing Facility (VPF), ''Galileo'' is prepared for mating with the [[Inertial Upper Stage]] booster.|alt=High gain antenna is folded. ]]
Plans called for the {{OV|Columbia}} to launch ''Galileo'' on the [[Canceled Space Shuttle missions#STS-23 (Columbia)|STS-23 mission]], tentatively scheduled for sometime between January 2 and 12, 1982,<ref>{{cite magazine |last=Portree |first=David S. F. |title=What Shuttle Should Have Been: The October 1977 Flight Manifest |date=March 24, 2012 |magazine=[[Wired (magazine)|Wired]] |issn=1059-1028 |url=https://www.wired.com/2012/03/what-shuttle-should-have-been-the-october-1977-flight-manifest/ |access-date=October 30, 2020 |archive-date=March 17, 2014 |archive-url=https://web.archive.org/web/20140317100957/http://www.wired.com/wiredscience/2012/03/what-shuttle-should-have-been-the-october-1977-flight-manifest |url-status=live }}</ref> this being the launch window when Earth, Mars and Jupiter were aligned to permit Mars to be used for the gravity-assist maneuver.{{sfn|Finley|1988|p=23}} By 1980, delays in the Space Shuttle program pushed the launch date for ''Galileo'' back to 1984.<ref>{{cite web |title=STS Flight Assignment Baseline |publisher=John H. Evans Library Digital Collections |url=https://digcollections.lib.fit.edu/items/show/45381 |access-date=October 31, 2020 |archive-date=November 30, 2021 |archive-url=https://web.archive.org/web/20211130095340/https://digcollections.lib.fit.edu/items/show/45381 |url-status=live }}</ref> While a Mars slingshot was still possible in 1984, it would no longer be sufficient.{{sfn|Meltzer|2007|pp=46–47}}

NASA decided to launch ''Galileo'' on two separate missions, launching the orbiter in February 1984 with the probe following a month later. The orbiter would be in orbit around Jupiter when the probe arrived, allowing the orbiter to perform its role as a relay. This configuration required a second Space Shuttle mission and a second carrier spacecraft to be built for the probe to take it to Jupiter, and was estimated to cost an additional $50 million (equivalent to ${{Inflation|US-GDP|50|1979}} million in {{Inflation/year|US-GDP}}), but NASA hoped to be able to recoup some of this through competitive bidding. The problem was that while the atmospheric probe was light enough to launch with the two-stage IUS, the Jupiter orbiter was too heavy to do so, even with a gravity assist from Mars, so the three-stage IUS was still required.<ref name="Deferring">{{cite web |title=NASA Weighs Deferring 1982 Mission to Jupiter |first=Thomas |last=O'Toole |author-link=Thomas O'Toole |newspaper=The Washington Post |date=September 19, 1979 |url=https://www.washingtonpost.com/archive/politics/1979/09/04/nasa-weighs-deferring-1982-mission-to-jupiter/bfe8bb4a-20fe-41f5-af14-d6b1c003b470/ |access-date=11 October 2020 |archive-date=August 27, 2017 |archive-url=https://web.archive.org/web/20170827161312/https://www.washingtonpost.com/archive/politics/1979/09/04/nasa-weighs-deferring-1982-mission-to-jupiter/bfe8bb4a-20fe-41f5-af14-d6b1c003b470/ |url-status=live }}</ref>{{sfn|Meltzer|2007|pp=46–47}}

By late 1980, the price tag for the IUS had risen to $506 million (equivalent to ${{Inflation|US-GDP|0.506|1979|r=3}} billion in {{Inflation/year|US-GDP}}).{{sfn|Heppenheimer|2002|pp=330–335}} The USAF could absorb this cost overrun on the development of the two-stage IUS (and indeed anticipated that it might cost far more), but NASA was faced with a quote of $179 million (equivalent to ${{Inflation|US-GDP|179|1979}} million in {{Inflation/year|US-GDP}}) for the development of the three-stage version,{{sfn|Heppenheimer|2002|pp=368–370}} which was $100 million (equivalent to ${{Inflation|US-GDP|100|1979}} million in {{Inflation/year|US-GDP}}) more than it had budgeted for.{{sfn|Meltzer|2007|p=43}} At a press conference on January 15, 1981, [[Robert A. Frosch]], the [[NASA Administrator]], announced that NASA was withdrawing support for the three-stage IUS, and going with a [[Centaur G Prime]] upper stage because "no other alternative upper stage is available on a reasonable schedule or with comparable costs."{{sfn|Janson|Ritchie|1990|p=250}}

[[File:Model of Centaur G with Galileo probe (upright).jpg|thumb|right|Model of ''Galileo'' atop a [[Centaur G Prime]] upper stage in the [[San Diego Air and Space Museum]] |alt=refer to caption]]
Centaur provided many advantages over the IUS. The main one was that it was far more powerful. The probe and orbiter could be recombined, and the probe could be delivered directly to Jupiter in two years' flight time.{{sfn|Heppenheimer|2002|pp=368–370}}{{sfn|Bowles|2002|p=420}} The second was that, despite this, it was gentler than the IUS, because it had lower thrust. This reduced the chance of damage to the payload. Thirdly, unlike solid-fuel rockets which burned to completion once ignited, a Centaur could be switched off and on again. This gave it flexibility, which increased the chances of a successful mission, and permitted options like asteroid flybys. Centaur was proven and reliable, whereas the IUS had not yet flown. The only concern was about safety; solid-fuel rockets were considered safer than liquid-fuel ones, especially ones containing [[liquid hydrogen]].{{sfn|Heppenheimer|2002|pp=368–370}}{{sfn|Bowles|2002|p=420}} NASA engineers estimated that additional safety features might take up to five years to develop and cost up to $100 million (equivalent to ${{Inflation|US-GDP|100|1979}} million in {{Inflation/year|US-GDP}}).{{sfn|Meltzer|2007|p=43}}<ref name="Deferring" />

In February 1981, JPL learned that the [[Office of Management and Budget]] (OMB) was planning major cuts to NASA's budget, and was considering cancelling ''Galileo''. The USAF intervened to save ''Galileo'' from cancellation. JPL had considerable experience with autonomous spacecraft that could make their own decisions.{{sfn|Meltzer|2007|pp=49–50}} This was a necessity for deep space probes, since a signal from Earth takes from 35 to 52 minutes to reach Jupiter, depending on the relative position of the planets in their orbits.<ref>{{cite web |title=Seeing in the Dark. Astronomy Topics. Light as a Cosmic Time Machine |publisher=PBS |url=https://www.pbs.org/seeinginthedark/astronomy-topics/light-as-a-cosmic-time-machine.html |access-date=12 October 2020 |archive-date=October 26, 2020 |archive-url=https://web.archive.org/web/20201026023044/https://www.pbs.org/seeinginthedark/astronomy-topics/light-as-a-cosmic-time-machine.html |url-status=live }}</ref> The USAF was interested in providing this capability for its satellites, so that they would be able to determine their [[orientation (geometry)|attitude]] using onboard systems rather than relying on [[ground station]]s, which were not "hardened" against [[nuclear weapons]], and could take independent evasive action against [[anti-satellite weapon]]s. It was also interested in the manner in which JPL was designing ''Galileo'' to withstand the intense radiation of the [[magnetosphere of Jupiter]], as this could be used to harden satellites against the [[electromagnetic pulse]] of nuclear explosions. On February 6, 1981 [[Strom Thurmond]], the [[President pro tempore of the United States Senate|President pro tem of the Senate]], wrote directly to [[David Stockman]], the director of the OMB, arguing that ''Galileo'' was vital to the nation's defense.{{sfn|Waldrop|1982|p=1013}}{{sfn|Meltzer|2007|pp=50–51}}{{efn|[[Sandia National Laboratories]] produced 12,000 microprocessor and integrated circuit components for ''Galileo'' that were hardened against radiation.{{sfn|Olmstead|Johnson|2024|p=246}} }}

[[File:Astronauts John Fabian and Dave Walker pose in front of a model of the Shuttle-Centaur.jpg|thumb|left|Astronauts [[John M. Fabian]] and [[David M. Walker (astronaut)|David M. Walker]] pose in front of a model of the [[Shuttle-Centaur]] with ''Galileo'' in mid-1985 |alt=refer to caption]]
In December 1984, Casani proposed adding a flyby of asteroid [[29 Amphitrite]] to the ''Galileo'' mission. In plotting a course to Jupiter, the engineers wanted to avoid asteroids. Little was known about them at the time, and it was suspected that they could be surrounded by dust particles. Flying through a dust cloud could damage the spacecraft's optics and possibly other parts of the spacecraft as well. To be safe, JPL wanted to avoid asteroids by at least {{convert|10000|km|sp=us}}. Most of the asteroids in the vicinity of the flight path like [[1219 Britta]] and [[1972 Yi Xing]] were only a few kilometers in diameter and promised little scientific value when observed from a safe distance, but 29 Amphitrite was one of the largest, and a flyby at even {{convert|10000|km|sp=us}} could have great value. The flyby would delay the spacecraft's arrival in Jupiter orbit from August 29 to December 10, 1988, and the expenditure of propellant would reduce the number of orbits of Jupiter from eleven to ten. This was expected to add $20 to $25 million (equivalent to ${{Inflation|US-GDP|20|1984}} to ${{Inflation|US-GDP|25|1984}} million in {{Inflation/year|US-GDP}}) to the cost of the ''Galileo'' project. The 29 Amphitrite flyby was approved by NASA Administrator [[James M. Beggs]] on December 6, 1984.<ref>{{cite press release |id=1062 |date=January 17, 1985 |title=Asteroid 29 Flyby Approved |publisher=NASA/Jet Propulsion Laboratory |url=https://www.jpl.nasa.gov/releases/80s/release_1985_1062.html |archive-url=https://web.archive.org/web/20081006064236/https://www.jpl.nasa.gov/releases/80s/release_1985_1062.html |archive-date=2008-10-06 |url-status=dead |df=mdy-all}}</ref>{{sfn|Meltzer|2007|pp=66–67}}

During testing, contamination was discovered in the system of metal [[slip ring]]s and brushes used to transmit electrical signals around the spacecraft, and they were returned to be refabricated. The problem was traced back to a [[chlorofluorocarbon]] used to clean parts after soldering. It had been absorbed, and was then released in a vacuum environment. It mixed with debris generated as the brushes wore down, and caused intermittent problems with electrical signal transmission. Problems were also detected in the performance of memory devices in an electromagnetic radiation environment. The components were replaced, but then a [[read disturb]] problem arose, in which reads from one memory location disturbed the contents of adjacent locations. This was found to have been caused by the changes made to make the components less sensitive to electromagnetic radiation. Each component had to be removed, retested, and replaced. All of the spacecraft components and spare parts received a minimum of 2,000 hours of testing. The spacecraft was expected to last for at least five years—long enough to reach Jupiter and perform its mission. On December 19, 1985, it departed JPL in [[Pasadena, California]], on the first leg of its journey, a road trip to the [[Kennedy Space Center]] in [[Florida]].{{sfn|Meltzer|2007|pp=68–69}} The ''Galileo'' mission was scheduled for [[STS-61-G]] on May 20, 1986, using {{OV|Atlantis|full=nolink}}.{{sfn|Hitt|Smith|2014|pp=282–285}}<ref>{{cite press release |title=NASA Names Flight Crews for ''Ulysses'', ''Galileo'' Missions |id=85-022 |date=31 May 1985 |first=Steve |last=Nesbitt |publisher=NASA |url=https://www.nasa.gov/centers/johnson/pdf/83137main_1985.pdf |access-date=17 October 2020 |archive-url=https://web.archive.org/web/20221101030918/https://www.nasa.gov/centers/johnson/pdf/83137main_1985.pdf |archive-date=1 November 2022 }}</ref>

===Spacecraft===
{{main|Galileo (spacecraft)|l1=''Galileo'' (spacecraft)}}
JPL built the Galileo spacecraft and managed the Galileo program for NASA, but West Germany's [[Messerschmitt-Bölkow-Blohm]] supplied the propulsion module, and Ames managed the atmospheric probe, which was built by the [[Hughes Aircraft Company]]. At launch, the orbiter and probe together had a mass of {{convert|2562|kg|lb|abbr=on}} and stood {{convert|6.15|m|ft|abbr=on}} tall. There were twelve experiments on the orbiter and seven on the atmospheric probe. The orbiter was powered by a pair of [[GPHS-RTG|general-purpose heat source radioisotope thermoelectric generator]]s (GPHS-RTGs) fueled by [[plutonium-238]] that generated 570&nbsp;watts at launch. The atmospheric probe had a [[lithium–sulfur battery]] rated at 730&nbsp;watt-hours.{{efn|A 12-Volt car battery has about 600 Watt-Hours.<ref>{{cite web |title=How Many Watt Hours in a Car Battery |publisher=Large Power |url=https://www.large.net/news/90u43pm.html |access-date=13 April 2024 |archive-date=April 9, 2024 |archive-url=https://web.archive.org/web/20240409192148/https://www.large.net/news/90u43pm.html |url-status=live }}</ref>}}<ref name="Press Kit" />

Probe instruments included sensors for measuring atmospheric temperature and pressure. There was a [[mass spectrometer]] and a [[helium]]-abundance detector to study atmospheric composition, and a [[Whistler (radio)|whistler]] detector for measurements of lightning activity and Jupiter's radiation belt. There were magnetometer sensors, a plasma-wave detector, a [[high-energy particle]] detector, a [[cosmic dust|cosmic]] and Jovian dust detector, and a [[heavy ion]] counter. There was a [[near-infrared spectroscopy|near-infrared mapping spectrometer]] for [[multispectral images]] for atmospheric and moon surface chemical analysis, and an [[ultraviolet–visible spectroscopy|ultraviolet spectrometer]] to study gases.<ref name="Press Kit">{{cite web |url=http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |title=Galileo Jupiter Arrival |type=Press Kit |publisher=NASA{{\}}Jet Propulsion Laboratory |date=December 1995 |access-date=December 22, 2016 |archive-date=November 16, 2001 |archive-url=https://web.archive.org/web/20011116223923/http://www.jpl.nasa.gov/news/press_kits/gllarpk.pdf |url-status=live }}</ref>

===Reconsideration===
On January 28, 1986, {{OV|Challenger|full=nolink}} lifted off on the [[STS-51-L]] mission. A failure of the solid rocket booster 73 seconds into flight tore the spacecraft apart, resulting in the deaths of all seven crew members.{{sfn|Meltzer|2007|pp=72–77}} The [[Space Shuttle Challenger disaster|Space Shuttle ''Challenger'' disaster]] was America's worst space disaster up to that time.{{sfn|Dawson|Bowles|2004|pp=206–207}} The immediate impact on the ''Galileo'' project was that the May launch date could not be met because the Space Shuttles were grounded while the cause of the disaster was investigated. When they did fly again, ''Galileo'' would have to compete with high-priority [[United States Department of Defense|Department of Defense]] launches, the [[tracking and data relay satellite]] system, and the Hubble Space Telescope. By April 1986, it was expected that the Space Shuttles would not fly again before July 1987 at the earliest, and ''Galileo'' could not be launched before December 1987.{{sfn|Meltzer|2007|p=78}}

[[File:Animation of Galileo trajectory.gif|thumb|right|Animation of ''Galileo''{{'s}} trajectory from October 19, 1989, to September 30, 2003 <br/>{{legend2|magenta|''Galileo''}}{{·}}{{legend2|lime|
[[Jupiter]]}}{{·}}{{legend2|royalblue|[[Earth]]}}{{·}}{{legend2|PaleGreen|[[Venus]]}}{{·}}{{legend2|Gold|[[951 Gaspra]]}}{{·}}{{legend2|Cyan|[[243 Ida]]}} |alt=Depicts slingshot maneuvers around Venus and Earth.]]
The [[Rogers Commission]] into the ''Challenger'' disaster handed down its report on June 6, 1986.{{sfn|Meltzer|2007|p=78}} It was critical of NASA's safety protocols and risk management.{{sfn|Rogers|1986|pp=160–162}} In particular, it noted the hazards of a Centaur-G stage.{{sfn|Meltzer|2007|pp=176–177}} On June 19, 1986, NASA Administrator [[James C. Fletcher]] canceled the Shuttle-Centaur project.{{sfn|Meltzer|2007|p=79}} This was only partly due to the NASA management's increased aversion to risk in the wake of the ''Challenger'' disaster; NASA management also considered the money and manpower required to get the Space Shuttle flying again, and decided that there were insufficient resources to resolve lingering issues with Shuttle-Centaur as well.{{sfn|Dawson|Bowles|2004|pp=216–218}} The changes to the Space Shuttle proved more extensive than anticipated, and in April 1987, JPL was informed that ''Galileo'' could not be launched before October 1989.{{sfn|Meltzer|2007|p=93}} The ''Galileo'' spacecraft was shipped back to JPL.{{sfn|Meltzer|2007|p=177}}

Without Centaur, it looked like there was no means of getting ''Galileo'' to Jupiter. For a time, ''[[Los Angeles Times]]'' science reporter [[Usha Lee McFarling]] noted, "it looked like ''Galileo''{{'}}s only trip would be to the [[Smithsonian Institution]]."<ref name="McFarling">{{cite news |first=Usha Lee |last=McFarling |author-link=Usha Lee McFarling |date=September 22, 2003 |title=Stalwart Galileo Is Vaporized Near Jupiter |newspaper=[[Los Angeles Times]] |url=https://www.latimes.com/archives/la-xpm-2003-sep-22-me-galileo22-story.html |access-date=May 19, 2024 |archive-date=May 19, 2024 |archive-url=https://web.archive.org/web/20240519195233/https://www.latimes.com/archives/la-xpm-2003-sep-22-me-galileo22-story.html |url-status=live }}</ref> The cost of keeping it ready to fly in space was reckoned at $40 to $50 million per year (equivalent to ${{Inflation|US-GDP|40|1986}} to ${{Inflation|US-GDP|50|1986}} million in {{Inflation/year|US-GDP}}), and the estimated cost of the whole project had blown out to $1.4 billion (equivalent to ${{Inflation|US-GDP|1.4|1986}} billion in {{Inflation/year|US-GDP}}).<ref>{{cite web |title=NASA's Galileo mission clears hurdles for Jupiter voyage. In flying past Venus, probe could learn much about 'greenhouse effect' |newspaper=Christian Science Monitor |date=December 3, 1987 |first=Peter N. |last=Spotts |url=https://www.csmonitor.com/1987/1203/agal.html |access-date=November 7, 2020 |archive-date=December 7, 2021 |archive-url=https://web.archive.org/web/20211207041458/https://www.csmonitor.com/1987/1203/agal.html |url-status=live }}</ref>

At JPL, the ''Galileo'' Mission Design Manager and Navigation Team Chief, Robert Mitchell, assembled a team that consisted of Dennis Byrnes, Louis D'Amario, Roger Diehl and himself, to see if they could find a trajectory that would get ''Galileo'' to Jupiter using only a two-stage IUS. Roger Diehl came up with the idea of using a series of gravity assists to provide the additional velocity required to reach Jupiter. This would require ''Galileo'' to fly past Venus, and then past Earth twice. This was referred to as the Venus-Earth-Earth Gravity Assist (VEEGA) trajectory.{{sfn|Meltzer|2007|pp=293–294}}

[[File:Galileo probe deployed (large).jpg|thumb|left|''Galileo'' is prepared for release from {{OV|Atlantis}}. The [[Inertial Upper Stage]] (white) is attached.|alt=refer to caption]]
The reason no one had considered the VEEGA trajectory before was that the second encounter with Earth would not give the spacecraft any extra energy. Diehl realised that this was not necessary; the second encounter would merely change its direction to put it on a course for Jupiter.{{sfn|Meltzer|2007|pp=293–294}} In addition to increasing the flight time, the VEEGA trajectory had another drawback from the point of view of [[NASA Deep Space Network]] (DSN): ''Galileo'' would arrive at Jupiter when it was at the maximum range from Earth, and maximum range meant minimum signal strength. It would have a [[declination]] of 23 degrees south instead of 18 degrees north, so the tracking station would be the [[Canberra Deep Space Communication Complex]] in Australia, with its two 34-meter and one 70-meter antennae. A northerly declination could have been supported by two sites, at [[Goldstone Deep Space Communications Complex|Goldstone]] and [[Madrid Deep Space Communications Complex|Madrid]]. The Canberra antennae were supplemented by the 64-meter antenna at the [[Parkes Observatory]].{{sfn|Mudgway|2001|p=301}}{{sfn|Taylor|Cheung|Seo|2002|p=23}}

Initially it was thought that the VEEGA trajectory demanded a November launch, but D'Amario and Byrnes calculated that a mid-course correction between Venus and Earth would permit an October launch as well.{{sfn|Meltzer|2007|p=157}} Taking such a roundabout route meant that ''Galileo'' would require sixty months to reach Jupiter instead of just thirty, but it would get there.<ref name="McFarling" /> Consideration was given to using the USAF's [[Titan IV]] launch system with its Centaur G Prime upper stage.{{sfn|Dawson|Bowles|2004|p=215}} This was retained as a backup for a time, but in November 1988 the USAF informed NASA that it could not provide a Titan IV in time for the May 1991 launch opportunity, owing to the backlog of high priority Department of Defense missions.{{sfn|Office of Space Science and Applications|1989|p=2{{hyphen}}19}} However, the USAF supplied IUS-19, which had originally been earmarked for a Department of Defense mission, for use by the ''Galileo'' mission.{{sfn|Bangsund|Knutson|1988|p=10{{hyphen}}12}}

===Nuclear concerns===
As the launch date of ''Galileo'' neared, [[Anti-nuclear movement in the United States|anti-nuclear groups]], concerned over what they perceived as an unacceptable risk to the public's safety from the [[plutonium]] in ''Galileo''{{'}}s GPHS-RTG modules, sought a court injunction prohibiting ''Galileo''{{'s}} launch.<ref name="Groups Protest Use of Plutonium">{{cite news |newspaper=[[The New York Times]] |title=Groups Protest Use of Plutonium on Galileo |first=William J. |last=Broad |author-link=William Broad |date=October 10, 1989 |url=https://www.nytimes.com/1989/10/10/science/groups-protest-use-of-plutonium-on-galileo.html |access-date=November 4, 2020 |archive-date=February 12, 2021 |archive-url=https://web.archive.org/web/20210212025607/http://www.nytimes.com/1989/10/10/science/groups-protest-use-of-plutonium-on-galileo.html |url-status=live }}</ref> RTGs were necessary for deep space probes because they had to fly distances from the Sun that made the use of solar energy impractical.<ref name="Sagan" /> They had been used for years in planetary exploration without mishap: the Department of Defense's [[Lincoln Experimental Satellite]]s 8/9 had 7 percent more plutonium on board than ''Galileo'', and the two [[Voyager program|''Voyager'' spacecraft]] each carried 80 percent of ''Galileo''{{'s}} load of plutonium.<ref name="RTG">{{cite web |title=What's in an RTG? |publisher=NASA/Jet Propulsion Laboratory |url=http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTG.html |access-date=May 15, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20100411024748/http://www2.jpl.nasa.gov/galileo/messenger/oldmess/RTG.html |archive-date=April 11, 2010 }}</ref> By 1989, plutonium had been used in 22 spacecraft.<ref>{{cite magazine |title=Plutonium gets a lift from Galileo |magazine=[[New Scientist]] |issn=0262-4079 |url=https://www.newscientist.com/article/mg12216683-100-plutonium-gets-a-lift-from-galileo/ |date=June 10, 1989 |access-date=November 4, 2020 |archive-date=October 28, 2020 |archive-url=https://web.archive.org/web/20201028151529/https://www.newscientist.com/article/mg12216683-100-plutonium-gets-a-lift-from-galileo/ |url-status=live }}</ref>

Activists remembered the crash of the [[Soviet Union]]'s nuclear-powered [[Kosmos 954]] satellite in Canada in 1978, and the ''Challenger'' disaster, while it did not involve nuclear fuel, raised public awareness about spacecraft failures. No RTGs had ever done a non-orbital swing past the Earth at close range and high speed, as ''Galileo''{{'s}} VEEGA trajectory required it to do. This created the possibility of a mission failure in which Galileo struck Earth's atmosphere and dispersed plutonium. [[Planetary scientist]] [[Carl Sagan]], a strong supporter of the ''Galileo'' mission, wrote that "there is nothing absurd about either side of this argument."<ref name="Sagan">{{cite web |last=Sagan |first=Carl |author-link=Carl Sagan |title=Galileo: To Launch or not to Launch? |date=October 9, 1989 |url=http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html |access-date=November 4, 2020 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126152333/http://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html |url-status=dead }}</ref>

Before the ''Challenger'' disaster, JPL had conducted shock tests on the RTGs that indicated that they could withstand a pressure of {{convert|2,000|psi|order=flip}} without a failure, which would have been sufficient to withstand an explosion on the launch pad. The possibility of adding additional shielding was considered but rejected, mainly because it would add an unacceptable amount of extra weight.{{sfn|Meltzer|2007|p=77}} After the ''Challenger'' disaster, NASA commissioned a study on the possible effects if such an event occurred with ''Galileo'' on board. Angus McRonald, a JPL engineer, concluded that what would happen would depend on the altitude at which the Space Shuttle broke up. If the ''Galileo''/IUS combination fell free from the orbiter at {{convert|90000|ft|order=flip|sp=us}}, the RTGs would fall to Earth without melting, and drop into the Atlantic Ocean about {{convert|150|mi|order=flip|sp=us}} from the Florida coast. On the other hand, if the orbiter broke up at an altitude of {{convert|323,800|feet|order=flip|sp=us}} it would be traveling at {{convert|7957|ft/s|order=flip|sp=us}} and the RTG cases and GPHS modules would melt before falling into the Atlantic {{convert|400|mi|order=flip|sp=us}} off the Florida coast.<ref>{{cite magazine |last=Portree |first=David S. F. |title=If Galileo Had Fallen to Earth (1988) |date=December 18, 2012 |magazine=[[Wired (magazine)|Wired]] |issn=1059-1028 |url=https://www.wired.com/2012/12/galileo-and-an-uncontrolled-shuttle-orbiter-reentry-1988/ |access-date=November 4, 2020 |archive-date=October 27, 2020 |archive-url=https://web.archive.org/web/20201027072205/https://www.wired.com/2012/12/galileo-and-an-uncontrolled-shuttle-orbiter-reentry-1988/ |url-status=live }}</ref><ref>{{cite report |title=Galileo: Uncontrolled STS Orbiter Reentry |id=JPL D-4896 |first=Angus D. |last=McRonald |publisher=NASA |date=April 15, 1988 |url=https://www.nasa.gov/pdf/24889main_JPL_Report.pdf |access-date=November 4, 2020 |archive-url=https://web.archive.org/web/20221111183533/https://www.nasa.gov/pdf/24889main_JPL_Report.pdf |archive-date=11 November 2022 }}</ref>

NASA concluded that the chance of a disaster was 1 in 2,500, although anti-nuclear groups thought it might be as high as 1 in 430.<ref name="Groups Protest Use of Plutonium" />{{sfn|Office of Space Science and Applications|1989|p=2-23}} NASA assessed the risk to an individual at 1 in 100 million, about two orders of magnitude less than the danger of being killed by lightning.{{sfn|Office of Space Science and Applications|1989|p=2{{hyphen}}24}} The prospect of an inadvertent re-entry into the atmosphere during the VEEGA maneuvers was reckoned at less than 1 in 2 million,<ref name="RTG"/> but an accident might have released a maximum of {{convert|11,568|Ci|lk=on}}. This could result in up to 9 fatalities from cancer per 10 million exposed people.{{sfn|Office of Space Science and Applications|1989|pp=2{{hyphen}}21, 4{{hyphen}}18}}