The Galileo project began in October 1977 as the Jupiter Orbiter probe mission, and was launched from Space Shuttle Atlantis (mission STS 34 ) on October 18,1989. Named after the man who first observed Jupiter's satellites, the spacecraft's mission was to further explore Jupiter and the Galilean satellites. This mission will come to an end on September 21, 2003 when the spacecraft dives deep into Jupiter's crushing atmosphere.

Nuts and bolts

The Galileo spacecraft is composed of an orbiter, developed by the Jet Propulsion Laboratory and an atmospheric probe developed by the Ames Research Center. Two science experiments as well as the propulsion subsystem were provided by the German government.

The mission goals for the probe were to:

  1. Determine the chemical composition of the Jovian atmosphere.
  2. Characterise the structure of the atmosphere to a depth of at least 10 bars.
  3. Investigate the nature of cloud particles and the location and structure of cloud layers.
  4. Examine the Jovian radiative heat balance.
  5. Study the nature of Jovian lightning activity.
  6. Measure the flux of energetic charged particles down to the top of the atmosphere.
The probe weighs roughly 340 kilograms, including parachutes and heat shields. The electrical subsystems are powered by batteries. The probe separated from the orbiter on July 13, 1995. This early separation served a dual purpose: to save fuel during later manoeuvres and to allow the main engine to fire (the probe covered it up). The probe entered Jupiter's atmosphere on December 7, 1995 at 22:04 UTC and ceased transmitting roughly 58 minutes later after having transmitted 3.5 megabits of data from the probe's 6 instruments. This was the first time such an experiment was performed, and valuable data about the Jupiter's atmosphere was obtained.

The orbiter's mission goals were to:

  1. Investigate the circulation and dynamics of the Jovian atmosphere.
  2. Investigate the upper Jovian atmosphere and ionosphere.
  3. Characterize the morphology, geology, and physical state of the Galilean satellites.
  4. Investigate the composition and distribution of surface minerals on the Galilean satellites.
  5. Determine the gravitational and magnetic fields and dynamic properties of the Galilean satellites.
  6. Study the atmospheres, ionospheres, and extended gas clouds of the Galilean satellites.
  7. Study the interaction of the Jovian magnetosphere with the Galilean satellites.
  8. Characterize the vector magnetic field and the energy spectra, composition, and angular distribution of energetic particles and plasma to a distance of 150 Rj.
The orbiter is significantly larger than the probe, weighing 2223 kg (not counting the upper-stage-rocket adapter but including about 925 kilograms of usable rocket propellant). The propulsion module consists of 12 thrusters and a main engine. Like many spacecraft operating at long distances from the sun, the orbiter draws its power from 2 radioisotope thermoelectric generators providing around 500W of power. The orbiter's main communcation device is its 4.8 meter high-gain antenna, capable of transmitting data at speeds of up to 134 kilobits per second (unfortunately the high-gain antenna was not successfuly deployed). This would have allowed a single 800x800 @8bits/pixel image from the SSI camera to be transmitted in a little under a minute. Two low-gain antennas are also included. All this is controlled by the onboard computer and its software which comprises around 70,000 lines of code. Galileo uses 18 microprocessors, comparable to the 6502 processor used in the Apple II. A 109 megabyte tape recorder is used for storing data. The orbiter also features an innovative dual spin system: Most of it spins constantly at 3 rpm which helps stabilise the spacecraft and is necessary for certain measurements (the rate of rotation can be increased to 10.5 rpm when needed). A special section can be "despun" for instruments that need to remain fixed (cameras for example).

The Journey

Like many spacecraft Galileo used the gravitational fields of other planets in order to save fuel. After passing Venus (in February 1990) and 2 flybys of Earth (the last one in December 1992 at an altitude of only 300km), Galileo set off for Jupiter which it arrived at in December 1995, where it became the first spacecraft to go into orbit around the planet. It had already made a number of firsts, included the world's first close-up images of an asteroid when it flew by Gaspra in October 1991 and the first observation of an asteroid moon, Dactyl when it flew past the asteroid Ida in August 1993. Last but not least it was the only observer of the far side of Jupiter during the collision of the Shoemaker-Levy comet with the planet in July 1994.

Bad Luck

The Galileo project was not blessed by the best of fortunes. Like many projects it was delayed by the 1986 Challenger disaster and the subsequent grounding of the Space Shuttle fleet. Originally the spacecraft had been designed to use a liquid fuel. After Challenger volatile liquid fuels were no longer desirable in a Shuttle cargo bay. A redesign was needed in order to allow the probe use a more stable but less powerful solid fuel. The original direct flight to Jupiter was replaced with the Venus-Earth-Earth gravity assist, more than doubling the flight time.

A major hit to the spacecraft was when its high-gain antenna failed to deploy properly. The antenna was supposed to unfurl umbrella style, but when this was attempted on April 11, 1991 friction from some of the ribs of antenna stopped deployment. For almost 2 years the flight team tried many possible solutions but were eventually forced to give up. The low gain antenna had a significantly lower throughput (initally 10 bits/s, eventually boosted to 160 bits/s) which meant that the probe had to store data on its onboard tape recorder before transmitting it. The tape recorder itself malfunctionned in October 1995. The tape jammed during a rewind operation and continued trying to rewind for several hours. It was feared that this portion of the tape (fortunately at the end of the reel) would have been weakened by the hours of tension and was subsequently marked off limits. Unfortunately this resulted in the loss of some data but ensured that the device continued functioning throughout the rest of the mission.


Galileo's prime mission was a 2 year survey of Jupiter and the Galilean moons. When this mission ended in December 1997 the spacecraft was still in relatively good condition, and eager to learn more NASA extended the mission by 2 years. This mission focused on ice, wind and fire: the icy moon Europa, now thought to have an ocean below the surface, the violent thunderstorms of Jupiter and the active volcanism of Io. In 1999, the probe's lifetime was extended by 2 years again as part of a mission called the Galileo Millennium Mission. Many more flybys of Europa and Io were conducted as well as studies of the effects of Jupiter's radiation on the spacecraft. By 2002 the probe was showing signs of its age, and NASA began planning its plunge into Jupiter.


Over the years, Galileo gathered reams of data about Jupiter and the Galilean satellites, listing all of the important results it obtained would be very long and beyond the scope of this node. Scientific highlights of the mission include:

  • Io's volcanic activity: In January 2002 it made its closest flyby of Io, passing within 62 km of the moon's service. No photos were taken on that occasion as the intense radiation from Jupiter forced the probe into safe mode. However in October of that year pictures were taken from an altitude of only 181 km. Voyager closest approach was around 20,000 km. Voyager brought the first visual evidence of Io's volcanism, but Galileo discovered many more volcanoes and revealed just how active the moon was: the landscape had extensively changed since the photos taken by Voyager over 15 years before.
  • Composition of Jupiter's atmosphere: Far less water was found than was estimated from observations made by the Voyager probes. The percentage of helium was found to be very close to that in the sun, and was less than half what was expected. No definitive answer to the question of the origin of the colours of Jupiter's atmosphere was found from probe data.
  • Discovery of storms in Jupiter's atmosphere: Galileo discovered that the planet has thunderstorms many times larger that those found on earth with violent winds exceeding 600 km/h. Overall lightning activity was found to be roughly a tenth of that found on earth (per unit of area), although individual events are more violent than on earth.
  • Discovery of the origin of Jupiter's rings: the dust particles that make up the rings are fragments of the 4 innermost satellites, blasted off by the impacts of meteorites.
  • Strong evidence in favour of an ocean below Europa's icy surface. Data from Galileo's magnetometer is consistent with large volumes of water, close to the surface. Callisto and Ganymede could also have subsurface oceans.
  • Discovery of Ganymede's magnetic field: It is believed to be caused by internal tidal friction.

The End

Galileo is showing its age, battered by intense radiation fields and general old age its systems are gradually failing. One of its cameras is offline and its tape recorder has been giving signs of failing. Fuel levels are also low. In February 2003 the flight team programmed the crash of Galileo for September 21, 2003 and disbanded.

One might wonder why NASA is intentionally crashing Galileo into Jupiter rather than letting it drift aimlessly though space as has been done with so many probes before it. The answer is Europa. Many hope that the oceans below its surface may harbour primitive life and it could be disastrous if Galileo were to contaminate the moon by crashing into it. When Galileo enters Jupiter's atmosphere, any organism that hitched a ride from earth will be destroyed by the intense heat of re-entry.

Galileo's mission is over, but already the next mission to the gas giant is being planned. The Jupiter Icy Moons Orbiter would orbit the 3 planet-sized moons of Jupiter thought to possibly harbour oceans below the surface: Callisto, Ganymede and Europa. The mission would launch in 2012 at the earliest.

Sources: (countdown to impact)

Update, September 21, 2003. As planned, Galileo crashed into Jupiter at 18:57 UTC travelling at around 50 km/s. Farewell.