Galileo was an American robotic space program that studied the planet Jupiter and its moons, as well as several other Solar System bodies. Named after the Italian astronomer Galileo Galilei, the Galileo spacecraft consisted of an orbiter and an atmospheric entry probe. It was delivered into Earth orbit on October 18, 1989, by on the STS-34 mission, and arrived at Jupiter on December 7, 1995, after gravity assist flybys of Venus and Earth, and became the first spacecraft to orbit Jupiter. The spacecraft then launched the first probe to directly measure its atmosphere. Despite suffering major 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.
Jupiter's atmospheric composition and ammonia clouds were recorded, as were the volcanism and plasma interactions on Io with Jupiter's atmosphere. The data Galileo collected supported the theory of a liquid ocean under the icy surface of Europa, and there were indications of similar liquid-saltwater layers under the surfaces of Ganymede and Callisto. Ganymede was shown to possess a magnetic field and the spacecraft found new evidence for exospheres around Europa, Ganymede, and Callisto. Galileo also discovered that Jupiter's faint ring system consists of dust from impact events on the four small inner moons. The extent and structure of Jupiter's magnetosphere was also mapped.
The primary mission concluded on December 7, 1997, but the Galileo orbiter commenced an extended mission known as the Galileo Europa Mission (GEM), which ran until December 31, 1999. By the time GEM ended, most of the spacecraft was operating well beyond its original design specifications, having absorbed three times the radiation exposure that it had been built to withstand. Many of the instruments were no longer operating at peak performance, but were still functional, so a second extension, the Galileo Millennium Mission (GMM) was authorized. On September 20, 2003, after 14 years in space and 8 years in the Jovian system, Galileo mission was terminated by sending the orbiter into Jupiter's atmosphere at a speed of over to eliminate the possibility of contaminating the moons with bacteria.
Background Jupiter is the largest planet in the Solar System, with more than twice the mass of all the other planets combined. Consideration of sending a probe to Jupiter began as early as 1959, when the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) developed four mission concepts: Deep space flights would fly through interplanetary space; Planetary flyby missions would fly past planets close enough to collect scientific data and could visit multiple planets on a single mission; Orbiter missions would place a spacecraft in orbit around a planet for prolonged and detailed study; Atmospheric entry and lander missions would explore a planet's atmosphere and surface.
Two missions to Jupiter, Pioneer 10 and Pioneer 11, were approved in 1969, with NASA's Ames Research Center given responsibility for planning the missions. Pioneer 10 was launched in March 1972 and passed within of Jupiter in December 1973. It was followed by Pioneer 11, which was launched in April 1973, and passed within of Jupiter in December 1974, before heading on to an encounter with Saturn. They were followed by the more advanced Voyager 1 and Voyager 2 spacecraft, which were launched on 5 September and 20 August 1977 respectively, and reached Jupiter in March and July 1979.
Planning + Galileo Project managers 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 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 10s flyby in December 1973 indicated that the effects were not as severe as had been feared. NASA management designated JPL as the lead center for the Jupiter Orbiter Probe (JOP) Project. John R. Casani, who had headed the Mariner and Voyager projects, became the first project manager. 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.
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 compared to just for a Pioneer.
The increase in weight had implications. The Voyager spacecraft had been launched by Titan IIIE rockets with a 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. 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. 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.
The IUS was constructed in a modular fashion, with two stages, a large one with of propellant, and a smaller one with . This was sufficient for most satellites. It could also be configured with two large stages to launch multiple satellites. 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. 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. 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.
It was estimated that the JOP would cost $634 million (equivalent to $ billion in ), 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 Senator William Proxmire, the chairman of the Independent Agencies Appropriations Subcommittee. The United States Congress approved funding for the Jupiter Orbiter Probe on July 19, 1977, and JOP officially commenced on October 1, 1977, the start of the fiscal year. 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 spacecraft in the Star Trek television show. In February 1978, Casani officially announced the choice of the name "Galileo".
Preparation To enhance reliability and reduce costs, the project engineers decided to switch from a pressurized 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 to its weight. Another 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 . 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 engines (SSME) running at full power level—109 percent of their rated power level. 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.
Plans called for the to launch Galileo on the STS-23 mission, tentatively scheduled for sometime between January 2 and 12, 1982, this being the launch window when Earth, Mars and Jupiter were aligned to permit Mars to be used for the gravity-assist maneuver. By 1980, delays in the Space Shuttle program pushed the launch date for Galileo back to 1984. While a Mars slingshot was still possible in 1984, it would no longer be sufficient.
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 $ million in ), 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.
By late 1980, the price tag for the IUS had risen to $506 million (equivalent to $ billion in ). 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 $ million in ) for the development of the three-stage version, which was $100 million (equivalent to $ million in ) more than it had budgeted for. 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."