The Galileo Spacecraft Mission to Jupiter
Galileo is a NASA spacecraft mission to Jupiter, launched on 18 October, 1989, and designed to study the planet's atmosphere, satellites and surrounding magnetosphere. It was named after the Italian Renaissance scientist Galileo Galilei, who discovered Jupiter's major moons in 1610 with the first astronomical telescope.
This mission was the first to make direct measurements from an instrumented probe within Jupiter's atmosphere, and the first to conduct long-term observations of the planet and its magnetosphere and satellites from orbit around Jupiter. It is the first probe to go into orbit around and investigate the atmosphere of an outer planet.
In order to reach Jupiter, Galileo had to pick up the necessary speed by flying past Venus and twice by the Earth, using the gravity-assist technique employed by Voyager to reach the planets Saturn, Uranus and Neptune. (By flying close to a massive body, such as a planet, a spacecraft will have its direction of motion changed. This change can be used to redirect the craft towards a new destination and is often called a `gravity-assist'.)
These gravity-assist encounters provided the opportunity for Galileo to conduct brief scientific observations of Venus in 1990 and the Earth-Moon system in 1990 and 1992. In addition, Galileo's flight through the asteroid belt provided opportunities for the first close-up observation of asteroids. In 1991 it encountered the asteroid Gaspra and in 1993 a second encounter took place with the asteroid Ida.
In December 1995 the Galileo atmospheric probe conducted a direct examination of Jupiter's atmosphere, while the larger part of the craft, the orbiter, began a 22-month, 10-orbit tour of the major satellites and the magnetosphere, including long- term observations of Jupiter throughout this phase.
Launch Operations
The Space Shuttle Atlantis carried the Galileo spacecraft into Earth orbit on October 18 1989. A two-stage solid fuel rocket accelerated the spacecraft out of Earth orbit toward the planet Venus.
The Galileo mission was previously designed for a direct flight of about 2 ½ years to Jupiter. Changes in the launch system after the Challenger accident precluded this direct flight. Trajectory engineers designed a new interplanetary flight path using gravity assists, once with Venus and twice with Earth, to build up the speed to reach Jupiter, taking a total of just over 6 years.
A sun-shield was added to the space-craft to protect it from the heat of the Sun during its trip to Venus. The deployment of the high-gain antenna, which would be used to transmist images, video and scientific data back to Earth, was postponed by 18 months until after the first Earth pass. Unfortunately the antenna jammed and did not unfurl properly. About 4 of the 18 graphite ribs of the 16-foot antenna remain connected to the central mast by their support pins. The spacecraft now communicates with the Earth using its low-gain antenna which transmits data at a much slower rate. Despite this setback, the mission has been an overwhelming success.
Galileo communicates with Earth via NASA's Deep Space Network (DSN), which has a complex of large antennas with receivers and transmitters located in the California desert, in Australia and in Spain, linked to a network control center at JPL in Pasadena, California. The spacecraft receives commands, sends science and engineering data and is tracked through this network.
First Asteroid: Gaspra
Nine months after the Earth passage, Galileo entered the asteroid belt, and 2 months after that it performed the world's first asteroid encounter. Gaspra is believed to be a fairly representative main-belt asteroid, about 15 kilometres across, probably similar in composition to stony meteorites.
The spacecraft passed about 1,600 kilometres from Gaspra at a relative speed of about 29,000 kilometres per hour; several pictures of Gaspra were taken as well as measurements to indicate composition and physical properties.
Earth (Second Pass)
Thirteen months after the Gaspra encounter, the spacecraft completed its 2-year elliptical orbit around the Sun and arrived back at Earth. It needs a much larger elliptical orbit (with a 6-year period) to reach as far as Jupiter, and the second flyby of Earth increased the orbit up to that size.
Second Asteroid Ida
Ida is 56 kilometres long. Like Gaspra, it is believed to represent the majority of main-belt asteroids in composition, though there are believed to be differences between the two. One of the surprising discoveries was that Ida has a small moon, christened Dactyl. This small moon is 1.5 km across and from measurements of its orbit around Ida the mass of the asteroid could be determined. The result was surprisingly low and indicated that Ida must contain many holes in its interior.
At Jupiter
Early in December 1995 the Galileo orbiter and probe approached Jupiter, having separated some months earlier. A few hours later, the probe entered the upper atmosphere, about 6 degrees north of Jupiter's equator, at more than 160,000 kilometres per hour, and slowed down by aerodynamic braking for about 2 minutes before deploying its parachute and dropping its heat shields. It then floated down about 200 kilometres through the clouds, passing from a pressure of 1/10 that on Earth's surface to about 25 Earth atmospheres in 75 minutes. The probe batteries were not expected to last beyond this point, and the radio-communications link was terminated.
Spacecraft
The Galileo mission and systems were designed to investigate three broad aspects of the Jupiter system: the planet's atmosphere, the satellites and the magnetosphere.
The spacecraft was constructed in three main parts. The atmospheric probe is a non-spinning section of the orbiter carrying cameras and other remote sensors. The spinning main section of the orbiter spacecraft which includes the fields and particles instruments, was designed to sense and measure the environment directly as the spacecraft flies through it. The spinning section also carries the main communications antenna, the propulsion module, flight computers and most support systems.
Atmospheric Probe
The probe weighed about 340 kilograms. It included a deceleration module to slow and protect the descent module, which carries out the scientific mission.
Instruments on the probe measured the temperature and pressure in Jupiter's atmosphere and the rate at which the probe was slowed down. Measurements were then made of the composition of the atmosphere and the clouds. A study was made of the radiation associated with both lightning storms on Jupiter and with the enormous radiation belts around Jupiter.
Results from the atmospheric probe
Results from the probe indicate that the proportion of the elements is very similar to that found in the Sun. This suggests that Jupiter's overall composition has not changed since it was formed four and a half billion years ago. In particular the abundance of helium is now put at 24%, a value close to the Sun's 25% which is in agreement with the primordial abundance predicted from theories of the Big Bang.
The revised abundances for elements such as carbon, nitrogen and sulphur are significantly higher than those found in the Sun. This is interpreted as being due to enhancement of these elements by collisions of meteors and other small bodies (including objects like comet Shoemaker Levy).
Only very small amounts of carbon compounds, organic molecules, were found. This reduces the possibility that any kind of exotic life forms may be found in the Jovian atmosphere. In addition, almost no water was detected. This has been explained as being due to the probe entering a very dry part of Jupiter's atmosphere where there were very few clouds.
Galileo Orbiter
The orbiter, in addition to supporting the probe activities, supported all the scientific investigations of Jupiter's satellites and magnetosphere, and remote observation of the giant planet itself, including those carried out on the way to Jupiter.
Results from the Galileo orbiter
The orbiter obtained numerous images of Jupiter and the moons Amalthea, Io, Europa, Ganymede and Callisto. Shortly after arrival in 1995 it observed the impact of the comet Shoemaker-Levy 9 with the Jovian atmosphere.
Images of Io showed extensive volcanic resurfacing since the Voyager flybys in 1979 and over a period of 4 months from November 1999 to February 2000. Galileo discovered a magnetic field around Io and Ganymede and a thin oxygen atmosphere around Europa. One of the most exciting results is the evidence of a possible liquid water ocean below the surface of Europa. This is deduced from the cracks in the visible ice sheet which resemble the movement of pack ice on Earth. There is also some evidence for a salty ocean beneath the surface of Callisto. Liquid water oceans could be an ideal environment in which to search for life and the proposed Europa Lander will investigate this in the next decade.
Galileo Update (June 2000)
After the successful end to its initial mission in December of 1997, the Galileo mission was extended for a further two years. During 1998 and 1999, it observed and studied the moons which orbit Jupiter , with special reports on the moons Europa and Io. The new mission was named the Galileo Europa Mission (GEM).
Europa, which has a diameter of 3138 km, is slightly smaller than our Moon. It has a very smooth surface made up largely of water ice and we see large plains criss-crossed with cracks. The absence of craters suggests that it is quite young, possibly less than 30 million years old. It has a tenuous atmosphere of oxygen and recent images have provided evidence of a liquid ocean beneath the crust.
The main objective of GEM was to conduct a detailed 14-month study of Europa and then to plunge repeatedly through the plasma torus connecting Io and Jupiter to reach Io itself. The study of Europa included finding out about the crust, atmosphere and about the reality of the sub-surface ocean. Eight consecutive close encounters with Europa were scheduled.
After the intensive study of Europa, Galileo came into close contact with Callisto, another moon of Jupiter, en-route to its next destination, Io. As Galileo swung through Io's atmosphere the make-up of Io's torus was explored. A close fly-by took place during October 1999, when it studied the volcanic activity, atmosphere and magnetospheric environment. A second fly-by took place in December 1999 and a third in February 2000. A further extension, the Galileo Millennium mission, is now in progress. The Cassini spacecraft will pass Jupiter in December 2000, when the two probes will observe the planet simultaneously.
When the mission ends, it is possible that the probe will be directed to burn up in the Jovian atmosphere to avoid contamination of the surfaces of any of the moons.
