II.

Spacecraft

The 2,222-kg (4,899-lb) Galileo orbiter has two sections. One section spins at a rate of several times per minute to help stabilize the spacecraft. Instruments to detect low-energy charged particles, high-energy and potentially dangerous charged particles, and cosmic and Jovian dust are mounted on this rotating segment. Other instruments 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 11 m (36 ft) in length to escape interference from the spacecraft.

A second section of the orbiter is stationary and carries the instruments that require stability: 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 to track the probe as it enters Jupiter’s atmosphere while relaying data to Earth. The orbiter is powered by converting the natural radioactive decay of plutonium 238 dioxide into electricity.

The Galileo orbiter delivered a 346-kg (760-lb) atmospheric probe to Jupiter and relayed data collected by the probe to Earth. The 86-cm (34-in) diameter probe was powered by a lithium-sulfur battery. It contained instruments to study Jupiter’s atmospheric structure, temperatures, clouds, and chemical composition, and to detect the presence of lightning within 12,000 km (8,000 mi) of the entry point.