Galileo (1989-2003)

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Overview

The Galileo craft was sent to study Jupiter in great detail. It consisted of an orbiter and an atmospheric probe. The probe was launched into Jupiter's atmosphere upon arrival (see below), and the orbiter studied Jupiter for about 14 years. The orbiter chiefly studies Jupiter's atmosphere, four largest moons, and its magnetosphere.

Mission Highlights

The Galileo craft was supposed to be launched in 1986, and take a more direct two-year journey to Jupiter. Unfortunately, the space shuttle Challenger's explosion delayed the craft until 1989.

Once the craft reached Jupiter, it released the atmospheric probe. The atmospheric probe provided the first direct evidence as to the make-up and dynamics of Jupiter's atmosphere. On December 9, 1995, at about 5:04 P.M. (EDT), the 346 kg (760 lbs) probe opened its 2.5 m (8 ft) parachute and fell through Jupiter's atmosphere at speeds exceeding 160,000 kph (100,000 mph). Two minutes later it dropped its heat shield so that it could collect data on atmospheric structure, temperature, cloud and chemical composition, while also detecting lightning within 12,000 km (8,000 miles) of the probe's entry point (the probe found that lightning occurs about one tenth as often as on Earth).

Some of the probe's findings indicated that there was less water than previously expected, but further study showed that the probe had merely encountered a dry spot. The probe also encountered winds of up to 530 kph (330 mph) with intense turbulence, which suggested that Jupiter's winds are driven by heat escaping from the planet's interior. The probe found less helium, neon, carbon, oxygen, and sulfur than expected. As expected, the probe encountered no solid objects or surfaces during its entire 600 km (373 mile) voyage. After 57 minutes, the extreme temperature and pressure of Jupiter's atmosphere destroyed the probe.

The Galileo orbiter consisted of two main sections. One section spun several times per minute, and this helped to stabilize the craft. Included on this spinning section were instruments which detected low-energy charged particles, high-energy and potentially dangerous charged particles, and cosmic and Jovian dust. Other instruments included on this section studied waves generated in planetary magnetospheres and by lightning discharges. Galileo's magnetometer sensors, designed to measure planetary magnetic fields, were mounted on a boom 11 m (36 ft) long; the boom was long as to escape interference from the spacecraft.

The second part of the orbiter was stationary, and contained the instruments which required stability. They consisted of a high-resolution camera system, a near-infrared mapping spectrometer, an ultraviolet spectrometer to help analyze the chemistry of Jupiter's atmosphere, a photo-polarimeter radiometer to measure radiant and reflected energy, and a dish antenna which was used to track the above mentioned probe as it entered Jupiter's atmosphere while the orbiter relayed the data to Earth. The orbiter was powered by converting the natural radioactive decay of plutonium 238 dioxide into electricity.

There was a single tape recorder on board the spacecraft; it was a four-track digital model manufactured by Odetics Corporation that could store up to 914,489,344 bits of data (that's about 109 Mb, or about 300,000 pages of text).

The Galileo orbiter could only complete approximately 70% of its original science objectives. This is because in 1991 its 4.8 m (16 ft) diameter High Gain Antenna became stuck, and was unable to open completely. This forced NASA to only utilize the much slower Low Gain Antenna. The Low Gain Antenna could only transmit information at about 10 bits per second - about 100 times slower than the High Gain Antenna (if you are viewing this site with a 56k modem, then it is about 5,500 times faster than the Low Gain Antenna). The graphics-sensitive weather monitoring suffered the most.

Galileo has imaged the four Galilean Satellites, named after Galileo Galilei. Also, in 1995, Galileo's tape recorder got stuck in the "rewind" position for 15 hours, which permanently damaged a section of the tape.

The orbiter was expected to make over twenty total passes of Jupiter before its extended mission ends in December 1999 (its planned mission ended in December of 1997, two years after arrival). The mission has since been extended again, and is still operating as of December 2002.

One of the most important discoveries made is that the moon Europa might contain a liquid ocean of water underneath its thin ice crust. Galileo has also shown that the three large moons Ganymede, Europa, and Io have fairly strong magnetic fields, which means that the moons probably have cores of liquid metal. Molten metal cores provide heat that could make the moons hospitable to some forms of life.

The Galileo orbiter was terminated in a controlled entry of Jupiter's atmosphere on September 21, 2003. This entry destroyed the craft and any possibility for the craft to contaminate the moons, especially Europa, which scientists believe is one of the best candidates to find extraterrestrial life in the solar system.