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.

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 760 pound (346 kg) probe opened its eight foot (2.5 m) parachute and fell through Jupiter's atmosphere at speeds more than 100,000 mph (160,000 kph). 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 8,000 miles (12,000 km) 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 mearly encountered a dry spot. The probe also encountered winds of up to 330 mph (530 kph) 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 373 mile (600 km) voyage. After fifty-seven minutes, the extreme temperature and pressure of Jupiter's atmosphere destroyed the probe.

The Galileo orbiter weighs in at about 2.5 tons (2222 kg), and consists of two main sections. One section spins several times per minute, and this helps to stabilize the craft. Included on this spinning section are instruments which detect low-energy charged particles, high-energy and potentially dangerous charged particles, and cosmic and Jovian dust. Other instruments included on this section study waves generated in planetary magnetospheres and by lightning discharges. Galileo's magnetometer sensors, designed to measure planetary magnetic fields, are mounted on a boom 36 feet (11 m) long; The boom is long as to escape interference from the spacecraft.

The second part of the orbiter is stationary. This section boasts the instruments which require stability. They consist of a high-resolution camera system, a near-infrared mapping spectrometer and 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 is powered by converting the natural radioactive decay of plutonium 238 dioxide into electricity.

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

The Galileo orbiter can only complete approximately seventy per cent of its original science objectives. This is because in 1991 its sixteen-foot-wide (4.8-meter-wide) High Gain Antenna became stuck, and was unable to open completely. This has forced NASA to only utilize the much slower Low Gain Antenna. The Low Gain Antenna can only transmit information at about 10 bits per second, which is about 100 times slower than the High Gain Antenna (if you are viewing this site with a 14.4k modem, then that is about 1,400 times faster than the Low Gain Antenna, and 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 is suffering 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 fifteen hours, which permanently damaged a section of the tape.

The orbiter is 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).