The timeline of the Galileo spacecraft spans its launch in 1989 to the conclusion of its mission when it dove into and destroyed itself in the atmosphere of Jupiter in 2003.
Primary mission (1995–1997)
The trip from Earth to Jupiter, the probe's exploration of the Jovian atmosphere, and an orbiter tour consisting of 11 orbits of Jupiter constituted Galileos primary mission.
On Jupiter Arrival Day (7 December 1995), the Galileo spacecraft was given a gravity-assist from Io and then subjected to the Jupiter orbit insertion (JOI) maneuver, which slowed the spacecraft down so that the planet could "catch" it. These two actions placed the orbiter on its proper trajectory to tour the Jovian moons. The Jupiter orbit insertion maneuver involved an orbit around the planet, which is referred to as the spacecraft's "zeroth" orbit. The spacecraft's "first," and by far longest, orbit around Jupiter followed the JOI and lasted nearly seven months. On 27 June 1996, this initial orbit culminated in a close encounter with Ganymede, the largest of the four Galilean satellites.
After the first Jupiter orbit of seven months, subsequent orbits were much shorter, ranging from one to two and a half months.
The orbiter tour included four close encounters with Ganymede, three with Europa, and three with Callisto. No Io encounters were planned for the primary mission (besides the flyby on arrival day) because mission scientists feared that the high radiation levels so close to Jupiter could damage the spacecraft and possibly end the project.
The primary mission ended in December 1997, two years after Jupiter arrival.Michael Meltzer, Mission to Jupiter: a History of the Galileo Project, NASA SP 2007–4231, p. 223-282
The Galileo mission used a two-character code to specify each orbit. The first character was the first letter of the name of the moon that would receive a flyby on the orbit, while the second character indicated the number of the orbit.
Orbit: C: Callisto; E: Europa; G: Ganymede; I: Io; J: Jovian Orbit Closest approach in kilometers (miles) Date in Pacific time Observations Jupiter Arrival DayI0 1,000 (600) 7 December 1995 No imaging of Io and Europa due to tape-recorder problems. Because of the high radiation levels around Io and their effect on sensitive instruments, the scientists knew that Arrival Day might be the only time they would get a detailed look at Io—the only time they could obtain high-resolution, closeup images. But concerns regarding Galileos malfunctioning tape recorder made it necessary to cancel all remote sensing operations during the flyby. Only fields-and-particles instruments were allowed to take and record data. Remote sensing instruments such as the SSI camera required the recorder to run discontinuously, with abrupt starts and stops, and at high speeds. That kind of operation might well have permanently crippled the recorder and, with it, the entire mission. G1 835 (519) 27 June 1996 A gravity-assist during G1 reduced Galileos orbital period from 210 to 72 days, which allowed more orbits and close encounters each year. The perijove of orbit (point of closest approach to Jupiter) was increased to keep the spacecraft out of the most intense radiation regions. A radio-science experiment analyzed Ganymede's gravitational field and internal structure. The instruments detected evidence of a self-generated magnetosphere around the moon. G2 260 (161) 6 September 1996 A Ganymede gravity-assist put Galileo into coplanar orbit with other Galilean satellites, permitting subsequent encounters with them. A radio-science experiment analyzed Ganymede's gravitational field and internal structure. G1 and G2 radio-science and other data revealed that Ganymede had an interior that was probably differentiated into a core and a mantle. The plasma wave experiment and magnetometer data gave evidence of an internally generated magnetic field. C3 1,136 (705) 4 November 1996 Observations supported the theory that Callisto has a homogeneous internal structure, 60 percent rock and 40 percent ice. E4 692 (429) 19 December 1996 Galileos primary science objectives during E4 were to conduct remote sensing observations of Europa's surface, collect data on the moon's interactions with Jupiter's magnetosphere, and analyze Jovian atmospheric features. It included occultations of the Sun and Earth by both Jupiter and Europa, which provided an opportunity to search for indications of an ionosphere and atmosphere on the moon. The return of data from E4 was limited by a solar conjunction on 19 January 1997, occurring approximately midway between the E4 and E6 encounters. J5 no close flyby No close encounter to a Jovian moon was designed because Earth and Jupiter were in solar conjunction about the time that the closest approach would have occurred, and there would have been minimal communication capability between the spacecraft and Earth. E6 586 (363) 20 February 1997 The main scientific objective was to conduct high-resolution coverage of Europa. This was a similar objective to E4, but with some new Europa surface terrain observed. Monitoring of Io was also conducted. Jupiter atmospheric observations during E4 involved a coordinated effort by all of the Orbiter's remote sensing instruments to analyze white oval features in the infrared and ultraviolet regions of the spectrum. Four occultations of Earth occurred during E6—two by Europa, one by Io, and one by Jupiter. The radio science occultation measurements made during these events provided data on atmospheric profiles of the moons and Jupiter, and also on Europa's gravitational field. G7 3,102 (1,926) 5 April 1997 The spacecraft flew over the high latitudes of Ganymede and took high-resolution observations of high-energy impact regions, as well as Jupiter magnetosphere and aurora observations. G8 1,603 (995) 7 May 1997 The spacecraft passed over the midlatitudes of Ganymede, with closest approach longitudes 180° apart from those of the G7 encounter, allowing new terrain to be imaged. C9 418 (260) 25 June 1997 The spacecraft passed through and studied the magnetotail region of the Jovian magnetosphere during the period between the C9 and C10 flybys. Analysis of the C3, C9, and C10 data suggests that Callisto may have a subsurface, salty ocean that is responsible for a variable magnetic field induced by Jupiter's field. C10 539 (335) 17 September 1997 C10 data suggest that the internal structure of the moon is not homogeneous, but partially differentiated, with a higher percentage of rock than ice having settled toward the center of the satellite. Callisto is probably less differentiated than the other Galilean moons. E11 2,042 (1,266) 6 November 1997 The encounter included the longest recording to date, lasting almost 3 hours, of Jovian magnetospheric data close to Europa. The data were helpful not only in the study of Europa, but also for analyzing Io's plasma torus, whose charged particles are strongly influenced by the magnetic fields they encounter. Primary science objectives of E11 included more remote sensing of the moon's surface and more Jovian atmospheric observations. Another objective was to obtain the highest resolution images yet taken of four small, inner Jovian satellites: Thebe, Metis, Amalthea, and Adrastea.
Galileo Europa Mission (1997–1999)
The Galileo project would have been considered a success even if the spacecraft had stayed operational only through the end of the primary mission on 7 December 1997, two years after Jupiter arrival. The orbiter was an extremely robust machine, however, with many backup systems. It showed no sign of quitting at the end of the primary mission, so it was given a highly focused set of new exploration objectives, defined in part by the findings of the primary mission. As some of these new objectives centered on investigating Europa in great detail, the new mission was appropriately called the "Galileo Europa Mission" (GEM). Mission objectives were not limited to Europa, however; they included analyses of other satellites, as well as of Jovian fields and particles and atmospheric characteristics. During GEM, some of the most important and spectacular observations of the volcanic moon Io were taken.
GEM ran for slightly over two years, from 8 December 1997 to 31 December 1999. It was a low-cost mission with a budget of only $30 million. At the end of the primary mission, most of the 200 Galileo staff members left for other assignments. The remaining bare-bones crew, about one-fifth the size of the primary mission, was left to run GEM and achieve the objectives of four separate studies: Europa campaign. Io campaign. Io plasma torus study. Jupiter water study.
On each flyby, the spacecraft took only two days of data versus the seven days it had taken during the primary mission. Minimal Jovian magnetic field data were collected. The GEM team did not include the expertise to deal with unexpected problems, as the primary mission had. When issues arose, specialists who had gone on to other missions were temporarily brought back and placed on "tiger teams" to work through the problems quickly.