The Galileo mission to Jupiter


* La misión Galileo a Júpiter

Jupiter is the largest planet in the Solar System. Eleven times larger than Earth, this gas giant was studied closely for the first time by the spacecraft Pioneer 10 and Pioneer 11, and Voyager 1 and Voyager 2 in the 70s. In 1977 a proposal to investigate Jupiter's atmosphere with a probe that would fall by parachute appeared inside NASA. This atmospheric probe would be carried by a mother ship, which after transmitting to Earth the data collected during the descent, would stay orbiting the planet, studying satellites, rings, the magnetic field and radiations for two years. The mission was called the Galileo Mission, in honor of the great scientist.

With a total mass at launch of 2584 kg and a total height of 5,3 m, the mother ship had at its main body instruments to investigate Jupiter's magnetosphere. The spacecraft would turn three revolutions per minute to facilitate data collection and provide further stability, like a big spinning top. To aim the remote sensing cameras, an electric motor prevented the bottom of the ship from spinning. Nuclear batteries (of plutonium-243) would be used because solar panels could not provide enough electricity, because Jupiter is five times farther from the Sun than Earth. The 335-kg small atmospheric capsule was at the base of the spacecraft.

It was decided to launch the Galileo by the Space Shuttle, with a final push provided by a small but powerful Centaur rocket, carried on board. It was planned to be launched within five years, but delays with the shuttle program moved the date to 1984 and then to 1986, until the Challenger tragedy in January of that year postponed the launch again. Technicians were forced to think of a rocket with a fuel safer than Centaur's hydrogen, though less powerful: the IUS solid-fuel rocket. As the IUS could not get to provide the Galileo the push needed to reach Jupiter, the engineers thought of a solution: to use the force of gravity and the movement of Venus and the Earth around the Sun to "drag" the Galileo and increase its speed. The road was longer, and the trip would take more time, but in return the Galileo would study three planets instead of one, and up to two asteroids.


Finally the Galileo spacecraft was launched on 18 October 1989 by the STS-34 mission of the space shuttle Atlantis. Six hours later and traveling at 28 000 km/h, the IUS rocket (carrying the Galileo at its tip) was released from the cargo bay of Atlantis. Shortly afterwards, the IUS accelerated the Galileo to 42 200 km/h, in the direction of Venus.

Five months later the Galileo passed by the neighboring planet. Venus was investigated with more sophisticated tools than previous spacecraft. The Galileo saw the different layers of clouds, and even came to perceive the surface in infrared. It also confirmed the presence of lightnings.

The mission had one of its worst moments in 1991, after the first of two Earth flybys. The main antenna, stored like an umbrella during launch, got stuck when trying to open it. The technicians devised a system to process the data in the spacecraft itself before sending them through the much, much slower secondary antenna, and this, together with improvements in the receiving antennas on Earth, managed to save at least 70% of the scientific research.

The Galileo's trajectory led to a historic encounter in 1991: it became the first spacecraft to study an asteroid, in this case (951) Gaspra.

During its first Earth fly-by ​a somewhat speculative but nontheless key test was made: whether a modern interplanetary space probe of ours could detect life on the planets it studies. The result was positive: the Galileo detected liquid water on Earth, abundant oxygen (produced by plants), more methane than normal (due to digestion by ruminants), chlorophyll, and radio signals not of natural origin. Besides this, it studied the Moon, making more complete maps of the distribution of lunar minerals than previous ones.

In 1993 came the second encounter with an asteroid, (243) Ida. In the photos by the Galileo, an unprecedented discovery: the great rock of 53 km in size had another of 1 km in size revolving around it, a natural satellite!

By 1994 the spacecraft's trajectory placed it in the best position to observe the collision of Comet Shoemaker-Levy 9 with Jupiter, something never-before documented. Sequences of images taken with an interval of 2,5 seconds directly showed explosions when comet fragments collided with the dark side of the planet.


Five months before reaching Jupiter, the atmospheric probe separated from the orbiter. That period of time was needed for the orbiter to be able to alter its trajectory and avoid falling into Jupiter along with the small probe. This atmospheric probe continued traveling in free fall, with all their instruments off to save power of its lithium chemical batteries. A countdown clock would wake it up 6 hours before arrival.

Meanwhile, the Hubble Space Telescope photographed the giant gaseous planet regularly. It was important to have a last detailed image before the entry of the atmospheric probe, because cloud changes occur a few hours apart.

The arrival day, 7 December 1995, was one of the most hectic of the mission. The orbiter flew by the major satellites Europa and then by Io, entering a region where the radiation can kill a person in 15 minutes. Simultaneously, the probe also arrived. Technicians feared that this radiation would damage its computers, but they endured. Triumphantly, the small probe entered the atmosphere and forwarded during 1 hour its findings to the orbiter, for a later relay to Earth. After another hour the orbiter fired its main engine to approximate its speed to that of the planet. Thus it was captured by this and began to orbit it.

The entry of its little daughter probe to Jupiter's atmosphere was the most violent in the history of Astronautics. Attracted by the enormous gravitational pull of Jupiter, the probe collided with the atmosphere at 170 000 km/h, or 50 kilometers per second. The ceramic shield reached a temperature of 15 000 kelvins, more than twice as hot as the surface of the Sun.  In less than 3 minutes and after suffering a very violent deceleration of 230 g, the speed of the atmospheric probe went down to 1600 km/h. The parachute opened and what was left of the heat shield was ejected. Immediately the atmospheric probe began collecting and transmitting data to the orbiter, as it descended through the different layers of clouds. It sank more and more and the pressure and temperature increased. An hour later the transmitter could not withstand those conditions anymore and stopped working. Finally the entire atmospheric probe started turning into smoke, but not before sending valuable data about Jupiter.

On the other hand, the critical orbit insertion maneuver around Jupiter consumed most of the load of 925 kg of fuel, representing nearly 40% of the total mass of the main spacecraft. The Galileo then began to describe a series of orbits around Jupiter, highly elliptical and with duration of 1-2 months each. Normally at the end of each orbit the spacecraft encountered at least one natural satellite, and at each encounter the satellite gravity would alter the spacecraft's orbit towards the next encounter. During the first two years the technicians tried to keep away the Galileo from satellite Io though, because this was in the region of excessive radiation. At the end of the main mission, in 1997, the spacecraft's good health allowed a series of extensions, usually focusing on studies of satellite Europa.


The atmospheric probe, in the hours before the impact, studied the planet's inner magnetosphere, including a "doughnut" that surrounds Io's orbit where Jupiter's magnetic field keeps particles released by volcanoes prisoner. After impact, the atmospheric probe transmitted data down to a depth of about 150 km, where the pressure was approximately 22 atmospheres and the temperature 450 kelvins (177 degrees C). Three dense layers of clouds were expected: one made of ammonia crystals, one of ammonia hydrosulfide and one of H2O, but the water did not appear and the others were very thin. Today it is thought that this analyzed place, in particular, was atypical. The measured winds reached at least 750 km/h, but not many thunderbolts were detected. The orbiter, meanwhile, confirmed that Jupiter does contain a lot of H2O. These white clouds are visible in the pictures, and some scientists even believe that underneath it could be raining. The atmospheric probe showed that Jupiter's interior is convective, agitated like a boiling broth in a pot, and it transmits to the surface twice the amount of heat than comes from the Sun. Few organic molecules exist in this atmosphere.

Turning to the huge four major moons, it was found that Io, Europa and Ganymede have metal cores and rock mantles. Europa, Ganymede and Callisto also have a layer of ice. The crust of Ganymede and Callisto is rock and ice, as deep inside Callisto, which has no core. Perhaps there are oceans of liquid water in the subsurface of Europa, and even in Callisto. All four have very thin atmospheres, almost imperceptible, but the only one that has a magnetic field by itself is Ganymede.

Several minor satellites of Jupiter were also photographed. The Galileo confirmed that in this case the origin of the rings is dust torn out from the surface of these satellites by meteorite impacts.

Finally, in 2003, at more than 25 years since the start of the project, it was time that the spacecraft end its days. Europa was so interesting because of the possibility of life in its likely oceans, that it was decided to "bury" the Galileo into Jupiter to avoid any accidental contamination of the natural satellite by an unsterilized spacecraft. On Sunday 21 September 2003 the orbiter continued sending scientific reports without stopping, as it disappeared into the gas giant. 

The Galileo mission was undoubtedly one of the most complex interplanetary missions, but also one of the most successful.

Aldo Loup.

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Based on a lecture given at USP, on 30 September 2000. Originally published in ABC Color on 18 February 2007. Illustration: The "Inertial Upper Stage" (IUS), in turn a dual-stage rocket, separates from the space shuttle Atlantis, on the night of 18 to 19 October 1989, UTC, to help the Galileo spacecraft on its tip follow its journey with Jupiter as final destination. Credit: Ken Hodges / NASA.