Galileo to Jupiter

The Voyager probes had provided a tantalising taste of Jupiter and its varied moons. The next challenge was to put a probe into orbit around the giant planet and conduct a long-term study of the Jovian system.

GREAT INFRARED SPOT

Galileo's infrared cameras allowed it to map the depth of Jupiter's clouds. In this image of the famous Great Red Spot, deep clouds are colour coded blue and black, dense high clouds are white, and high thin hozes ore pink.

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LAUNCHED BY SHUTTLE

The Shuttle Atlantis blasts off from Cape Canaveral on 18 October 1989, carrying Galileo at the start of its long journey to Jupiter.

13 July 1995

Galileo releases its atmospheric probe towards Jupiter.

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LAUNCHED BY SHUTTLE

The Shuttle Atlantis blasts off from Cape Canaveral on 18 October 1989, carrying Galileo at the start of its long journey to Jupiter.

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The atmospheric probe enters Jupiter's atmosphere, sending back data for 59 minutes.

7 December 1997

After two years, Galileo completes its primary mission and moves into an extended phase, making closer flybys of lo and Europa.

7 December 1999

The Galileo mission enters a new phase known as the Galileo Millennium Mission.

30 December 2000

The Saturn-bound Cassini probe makes its closest approach to Jupiter, and mission controllers use both probes to study Jupiter simultaneously.

15 October 2001

Galileo makes its closest flyby of lo, passing just 180km (112 miles) above the surface.

21 September 2003

Galileo plunges into the Jovian atmosphere and is destroyed.

VOLCANO WORLD

Galileo's pictures of lo revealed that it is peppered with countless volcanoes and coloured by sulphur compounds from their eruptions. All this volcanic activity is driven by tidal forces from nearby Jupiter.

Although it did not reach Jupiter until 1995, the Galileo probe had a long gestation. Development of a Jupiter orbiter spacecraft with an atmospheric probe began even before the launch of the Voyagers in 1977. Germany joined the project in the same year, providing Galileo's propulsion systems in return for participation in the scientific programme.

Even after it was ready for launch, Galileo suffered a series of delays that ultimately pushed the mission back by eight whole years. At first, it was to be deployed in 1981, but the Space Shuttle's teething troubles put paid to that plan. The launch was rescheduled for 1984, but debates about how to boost the probe from Earth orbit saw it miss that chance. Finally scheduled for a May 1986 launch, Galileo then fell victim to the delays following the Challenger disaster. It set off at last in October 1989.

Planetary pinball

Safety restrictions introduced after Challenger limited the size of Galileo's booster rocket, so in order to get up speed for the trip to Jupiter, it was sent through a tortuous series of gravity assists involving a flyby of Venus and two further flybys of Earth. This Venus-Earth-Earth Gravity Assist (VEEGA) put Galileo on course to reach its target six years after its launch.

VOLCANO WORLD

Galileo's pictures of lo revealed that it is peppered with countless volcanoes and coloured by sulphur compounds from their eruptions. All this volcanic activity is driven by tidal forces from nearby Jupiter.

Even while the probe was on its long flight to Jupiter, its controllers and technicians were kept busy. Following the first Earth flyby, Galileo's high-gain antenna was supposed to unfold in order to allow long-distance communication, but the mechanism jammed and it was rendered useless. For a while, it looked as if Galileo might be an expensive failure, but fortunately the engineers found a way to send data through the emergency low-gain antenna at a slower rate. Galileo's onboard computers and the Earth-based receivers had to be reprogrammed to handle the new procedures. With this problem solved, Galileo was directed towards close encounters with two asteroids as it passed through the asteroid belt between Mars and Jupiter (see p.272). Then, while the probe was

LAUNCH FROM ATLANTIS

Deployment from the Shuttle meant that Galileo could only use on Inertiol Upper Stage booster rocket, instead of a powerful Centaur upper stage that would hove allowed a more direct route to Jupiter.

13 July 1995

Galileo releases its atmospheric probe towards Jupiter.

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7 Oecember 1995

The atmospheric probe enters Jupiter's atmosphere, sending back data for 59 minutes.

7 December 1999

The Galileo mission enters a new phase known as the Galileo Millennium Mission.

15 October 2001

Galileo makes its closest flyby of lo, passing just 180km (112 miles) above the surface.

21 September 2003

Galileo plunges into the Jovian atmosphere and is destroyed.

LAUNCH FROM ATLANTIS

Deployment from the Shuttle meant that Galileo could only use on Inertiol Upper Stage booster rocket, instead of a powerful Centaur upper stage that would hove allowed a more direct route to Jupiter.

GREAT INFRARED SPOT

Galileo's infrared cameras allowed it to map the depth of Jupiter's clouds. In this image of the famous Great Red Spot, deep clouds are colour coded blue and black, dense high clouds are white, and high thin hozes ore pink.

still a year from Jupiter, it had a spectacular view when Comet Shoemaker-Levy crashed into the giant planet in July 1994. Arriving at last, Galileo fired its main engine and dropped into orbit around Jupiter. During final approach, the atmospheric probe was released from the main spacecraft and plunged into the atmosphere (see panel, right).

Eight years of discovery

Now Galileo began its long scientific mission. It orbited ptfA Jupiter roughly once every two months, on long ■ ellipses that brought it

ICY EUROPA

Europa's brown stains ore thought to show where its icy crust has cracked, allowing chemicol-loden water to seep in and heal the scars.

ICY EUROPA

Europa's brown stains ore thought to show where its icy crust has cracked, allowing chemicol-loden water to seep in and heal the scars.

close to all of the major moons. The original plan for the mission assumed the probe might function for about two years - Jupiter is surrounded by radiation belts far fiercer than those around the Earth, which damage even the most robust electronics over time. As it was, Galileo beat even the most optimistic forecasts, surviving for more than eight years.

Over time, the probe was gradually brought closer to Jupiter itself, spending more time in the radiation zone. A final series of close encounters with lo saw its cameras damaged beyond repair in January 2002. During its time in orbit, Galileo had revolutionized ideas about the Jupiter system. In particular, it had revealed the huge extent of volcanic eruptions on lo (now known to be the most volcanic world in the Solar System) and bolstered the evidence that Europa's crust of scarred ice hides a global saltwater ocean kept warm by volcanic activity. Such an ocean might even harbour life, and in order to prevent contamination Galileo was steered to a fiery end in Jupiter's atmosphere on 21 September 2003.

TECHNOLOGY

S STORY

Galileo's atmospheric probe entered the Jovian atmosphere at a speed of 47km (29 miles) per second. As it fell, it was slowed to below the speed of sound (0.35km/0.2 miles per second) in just two minutes. Friction with the atmosphere turned the probe into a flaming bullet and burned away more than half the mass of its 152kg (334lb) heat shield. Only then did the probe deploy its parachute and begin transmitting data. Fifty-nine minutes of temperature and pressure information were relayed

via the main Galileo spacecraft to Earth, along with information about the chemical composition of the atmosphere, which turned out to be drier than expected, with less water vapour. The probe finally overheated and stopped transmitting at a depth of 146km (91 miles).

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