The Galileo spacecraft
Posted: September 20, 2003

The Galileo orbiter weighed 2,223 kilograms at launch (2-1/2 tons) and measured 5.3 meters (17 feet) from the top of the low-gain antenna to the bottom of the probe. The orbiter features an innovative "dual-spin" design. Most spacecraft are stabilized in flight either by spinning around a major axis, or by maintaining a fixed orientation in space, referenced to the Sun and another star. As the first dual-spin planetary spacecraft, Galileo combines these techniques. A spinning section rotates at about 3 rpm, and a "despun" section is counter-rotated to provide a fixed orientation for cameras and other remote sensors. A star scanner on the spinning side determines orientation and spin rate; gyroscopes on the despun side provide the basis for measuring turns and pointing instruments.

The power supply, propulsion module and most of the computers and control electronics are mounted on the spinning section. The spinning section also carries instruments to study magnetic fields and charged particles. These instruments include magnetometer sensors mounted on an 11-meter (36-foot) boom to minimize interference from the spacecraft's electronics; a plasma instrument to detect low-energy charged particles; and a plasma-wave detector to study electromagnetic waves generated by the particles. There is also a high-energy particle detector and a detector of cosmic and Jovian dust, an extreme ultraviolet detector associated with the ultraviolet spectrometer, and a heavy ion counter to assess potentially hazardous charged-particle environments the spacecraft flies through.

Galileo's de-spun section carries instruments that need to be held steady. These instruments include the camera system; the near-infrared mapping spectrometer to make multispectral images for atmosphere and surface chemical analysis; the ultraviolet spectrometer to study gases; and the photopolarimeter-radiometer to measure radiant and reflected energy. The camera system obtains images of Jupiter's satellites at resolutions from 20 to 1,000 times better than the best possible from NASA's Voyager spacecraft; its charge-coupled-device (CCD) sensor is much more sensitive than previous spacecraft cameras and is able to detect a broader color band. Galileo's de-spun section also carries a dish antenna that picked up the descent probe's signals during its fall into Jupiter's atmosphere.

The spacecraft's propulsion module consists of twelve 10-newton (2.25-pound-force) thrusters and a single 400-newton (90-pound-force) engine which use monomethylhydrazine fuel and nitrogen-tetroxide oxidizer. The propulsion system was developed and built by Messerschmitt-Bolkow-Blohm (MBB) and provided by the Federal Republic of Germany as NASA's major international partner on Galileo.

Because radio signals take more than one hour to travel from Earth to Jupiter and back, the Galileo spacecraft was designed to operate from computer instructions sent to it in advance and stored in spacecraft memory. A single master sequence of commands can cover a period ranging from weeks to months of quiet operations between flybys of Jupiter's moons. During busy encounter operations, one sequence of commands covers only about a week.

These sequences operate through flight software installed in the spacecraft computers, with built-in automatic fault protection software designed to put Galileo in a safe state in case of computer glitches or other unforeseen circumstance. Electrical power is provided by two radioisotope thermoelectric generators. Heat produced by natural radioactive decay of plutonium is converted to electricity (570 watts at launch, 485 at the end of the mission) to operate the orbiter spacecraft's equipment. This is the same type of power source used on other NASA missions including Viking to Mars, Voyager and Pioneer to the outer planets, Ulysses to study the Sun, and Cassini to Saturn.

Descent probe
Galileo's descent probe had a mass of 339 kilograms (750 pounds), and included a deceleration module to slow and protect the descent module. The deceleration module consisted of an aeroshell and an aft cover designed to block heat generated by friction during atmospheric entry. Inside the aeroshells were the descent module and its 2.5- meter (8-foot) parachute. The descent module carried a radio transmitter and seven scientific instruments. These were devices to measure temperature, pressure and deceleration, atmospheric composition, clouds, particles, and light and radio emissions from lightning and energetic particles in Jupiter's radiation belts.

Orbiter instruments
The Galileo orbiter spacecraft carries 11 scientific instruments. Another seven were on the descent probe. One engineering instrument on the orbiter, originally for measurements to aid design of future spacecraft, also collects scientific information.

Goodbye Galileo
Timeline - Times and descriptions of Galileo's descent into Jupiter.

Entry preview - Story on Galileo's demise.

Mission overview - A look back at Galileo's voyage.

Top 10 - Chart of the leading science achievements by Galileo mission.

Story on stage
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