Number 21: 05/08/2003
A scientific publication by SGF and NEODyS

NEOs' Spacemissions : Galileo

Galileo Mission fast facts: an introduction

It all started on October 18, 1989, at Cape Canveral: Galielo spacecraft was launched in a Space Shuttle Atlantis, ready to begin a journey that would last 6 years, headed toward Jupiter, the outer planets and the external frontier of our solar system.

The main objective

Its main objective was to study the planet Jupiter and its moons in more detail than it was ever done before. The task was very challenging: when the Pioneer and Voyager spacecrafts traveled to Jupiter in the nineteen-seventies, they merely flew by the planet, and there was a limit to what they could tell us.
Today, Jupiter is far from being well known: up to now, the planet holds clues to help us understand how the Sun and planets formed over four-and-a-half billion years ago. Its moons are also very misterious: one has active volcanoes, and others have strange icy terrain (see the 3d models of Jupiter and its moons).
Galileo was therfore projected to flyby the planet for 2 years and drop a probe through Jupiter's cloudtops to collect data as it descended.

figure 1. This is an artist impression of spacecraft Galileo flying toward Jupiter. On the left of the spacecraft, Io is visible, with a volcanic eruption.

figure 2. In this 'family portrait,' the four Galilean Satellites are shown to scale. These four largest moons of Jupiter shown in increasing distance from Jupiter are (left to right) Io, Europa, Ganymede, and Callisto. These global views show the side of volcanically active Io which always faces away from Jupiter, icy Europa, the Jupiter-facing side of Ganymede, and heavily cratered Callisto. The appearances of these neighboring satellites are amazingly different even though they are relatively close to Jupiter (350,000 kilometers for Io; 1, 800,000 kilometers for Callisto). These images were acquired on several orbits at very low "phase" angles.

figure 3. This overview of NASA's Galileo mission to Jupiter shows the heliocentric flight path of the spacecraft from Earth launch in October 1989 to Jupiter arrival in December 1995 and continuing through its many orbits as the first artificial satellite of the largest planet in the solar system. The small images on the left half of the picture mark special events in the spacecraft's mission.

Copyright of images: NASA -

The cruise

The cruise Galileo followed to reach Jupiter is known with the nickname "VEEGA", being made of three Gravity Assists (Venus-Earth-Earth) to have enough power to reach distant Jupiter.
After the first Earth flyby, Galileo had some technical problems, since its' umbrella-shaped high-gain antenna did not open as planned. But there was still a way to get data, though more slowly, using Galileo's smaller antenna and reprogramming the onboard computer and the Deep Space Network's. In this way, Galileo's team saved the mission, managing to gather all the information originally planned.

About minor bodies

During its long cruise, Galileo played an important role in the science of minor bodies. On the first trip through the asteroid belt, Galileo captured close-up images and made a flyby of asteroid Gaspra, while on its second cruise, the spacecraft discovered a miniature "moon", soon named Dactyl, orbiting asteroid Ida. In 1994, Galileo was perfectly positioned to watch the fragments of comet Shoemaker-Levy 9 crash into Jupiter while Earth-based telescopes had to wait to see the impact sites as they rotated into view (to know more, read "The Galileo Spacecraft's Studies of Asteroids and Comets" by Clark R. Chapman).

The probe

Finally, Galileo arrived at Jupiter in December 1995 and began its prime mission: a two-year study of the Jovian system.
While being inorbit around the planet, the probe sliced into Jupiter's atmosphere collecting fifty-eight minutes of data on the local weather. The results were very interesting: it appeared that Jupiter's atmosphere is drier than we thought. Measurements from the probe showed few clouds, and lightning only in the distance. Only later, Galileo scientists discovered that the probe had entered an area called a "hot spot."
Towards the end of the 58 minute descent, the probe measured winds of four-hundred-and-fifty miles per hour - stronger than anything on Earth. The probe was finally melted and vaporized by the intense heat of the atmosphere.

The extended phases

Galileo's prime mission ended on December 7, 1997. With more to learn, and the spacecraft in good health, NASA approved a first extension, a two-year study called "GEM"- the Galileo Europa Mission-. During this period Galileo studied the icy moon Europa, supporting the theory that an ocean of water currently exists below the surface.
Next target, was Io, the innermost moon. This encounter was saved until last because arriving at Io, meant surviving Jupiter's intense radiation that -as scientists had foreseen- upset the spacecraft's computer. Despite the technical problems, Galileo came through again, and even managed to discover in this period a lava fountain erupting on Io.
These successful flybys led to another exciting mission, a second extended phase named "the Galileo Millennium Mission" that lasted untill 2001. More data on Io and Europa, was collected and studies of the effects of strong radiation on a spacecraft were made. This phase also gave birth to a particular phenomenon: on its way to Saturn, the Cassini spacecraft went by Jupiter in late 2000 and for a few weeks, both spacecrafts observed the giant of our Solar System.
The Galileo mission will end on September 21, 2003 after 8 years from its beginning, when the spacecraft will be made to impact Jupiter.

Images: copyright NASA