Galileo (spacecraft)

• 1. Introduction

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  On the 21st of September 2003, the Galileo spacecraft was deliberately aimed into Jupiter's atmosphere and destroyed. The probe had spent nearly eight years orbiting the solar system's largest planet. Its destruction was not a malfunction. NASA chose to end Galileo's life to prevent contamination of Europa, one of Jupiter's moons, where conditions might support life. How did an American spacecraft make it so far from the Sun, where solar power cannot function? What kind of engineering kept it stable through almost a decade of radiation-saturated space? And what choices, made decades before the launch, determined what Galileo could and could not do?

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• 2. The Mission Takes Shape

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  Consideration of sending a probe to Jupiter began as early as 1959. NASA's Scientific Advisory Group for Outer Solar System Missions confronted an immediate obstacle. The technology to build a heat shield for an atmospheric probe did not yet exist. Facilities to test one under Jovian conditions would not be available until 1980. Jupiter was the most ambitious possible target. With more than twice the mass of all the other planets combined, it was the dominant object in the outer solar system.

  John R. Casani had previously managed both the Mariner and Voyager programs. He became the first project manager for what was then called the Jupiter Orbiter Probe. He solicited ideas for a more inspiring name, and the most votes went to Galileo, after the Italian astronomer Galileo Galilei. Galilei was the first person to view Jupiter through a telescope. His 1610 discovery of four moons orbiting the planet served as important evidence for the Copernican model of the solar system. The name also happened to match a spacecraft in the Star Trek television show, a coincidence noted at the time. NASA adopted it officially in February 1978.

  At 146 kilograms, Pioneer probes were the lighter design option. They were stabilized by spinning at 60 revolutions per minute and required no attitude control system. Mariner spacecraft weighed 722 kilograms. They carried three gyroscopes, nitrogen jet thrusters, and sensors calibrated to the Sun and the star Canopus. The heavier design enabled high-resolution imaging, and that capability drove the decision. Galileo would follow the Mariner model. At launch, the orbiter and probe together weighed 2,562 kilograms and stood 6.15 meters tall.

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• 3. The Dual-Spin Design

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  Three revolutions per minute was the steady rotation rate of Galileo's spinning section. This kept the probe stable while six instruments swept continuously across a full arc of space. The other section was despun, locked in a fixed orientation for instruments that required steady pointing. Most spacecraft used one stabilization method or the other. Galileo used both simultaneously.

  The rotating section held the fields and particles instruments and the magnetometer sensors. The sensors were mounted on an 11-meter boom to minimize interference from the spacecraft itself. The despun section held the camera system, the near-infrared mapping spectrometer, the ultraviolet spectrometer, and the photopolarimeter-radiometer. The camera, known as the solid-state imager, was an 800-by-800-pixel CCD device. It was designed to capture images of Jupiter's moons at resolutions 20 to 1,000 times better than the best images Voyager had produced. Galileo flew closer to Jupiter's inner moons than Voyager had. Its CCD sensor was also more sensitive and covered a broader range of wavelengths than Voyager's older vidicons.

  Six RCA 1802 COSMAC microprocessors ran the spacecraft. Each was clocked at about 1.6 megahertz and fabricated on silicon on sapphire, a radiation- and static-hardened material suited to deep space operation. This 8-bit chip was the first low-power CMOS processor of its kind. Its design was comparable to the processor then being used in the Apple II computer. On the ground, mission software contained 650,000 lines of code for orbit sequence design and 550,000 lines for navigation. Telemetry interpretation alone required 1,615,000 lines. Every spacecraft component and spare part was tested for a minimum of 2,000 hours. The attitude control system software was written in HAL/S, the same programming language used in the Space Shuttle program.

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• 4. Powered by Plutonium

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  At least 65 square meters of solar panels would have been needed to power a spacecraft at Jupiter's distance from the Sun. Chemical batteries were equally impractical given the technology of the time. The solution was two radioisotope thermoelectric generators, each mounted on a 5-meter boom. They converted heat from the radioactive decay of plutonium-238 into electricity through the solid-state Seebeck effect. This provided a reliable source of power unaffected by cold or radiation.

  Each RTG contained 18 separate heat source modules. Each module held four pellets of plutonium oxide, a ceramic chosen for its resistance to fracturing. The plutonium was enriched to about 83.5 percent plutonium-238. The modules were engineered to survive several categories of potential accident: launch explosion, fire, re-entry into the atmosphere, and land or water impact. An outer layer of graphite protected against the thermal and eroding conditions of a potential re-entry. Iridium cladding provided containment after any impact. At launch, the RTGs produced about 570 watts. The output declined at 0.6 watts per month, reaching 493 watts when Galileo arrived at Jupiter.

  Anti-nuclear groups, concerned about the plutonium's risk to the public, sought a court injunction to prohibit the launch. Deep-space probes had no practical alternative to RTG power at distances where sunlight was too faint. The launch proceeded. Galileo carried a separate probe designed for release into Jupiter's atmosphere. That probe ran on 13 lithium sulfur dioxide batteries, each the size of a standard D cell. Honeywell's Power Sources Center in Horsham, Pennsylvania manufactured them.

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• 5. The Atmospheric Plunge

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  Hughes Aircraft Company's Space and Communications Group built the entry probe at its plant in El Segundo, California. The probe weighed 339 kilograms and stood 86 centimeters tall. Its task was to fall into Jupiter's atmosphere at 48 kilometers per second, taking measurements as it descended. Data was transmitted from the probe up to the orbiter above. Temperatures during entry reached around 16,000 degrees Celsius.

  Carbon phenolic, an ablative material, formed the probe's heat shield. NASA constructed a special facility, the Giant Planet Facility, to simulate the heat load the shield would face during entry. That heating environment resembled what an intercontinental ballistic missile warhead encounters during re-entry into the atmosphere. Despite those conditions, the shield was designed to protect seven scientific instruments housed inside.

  Alvin Seiff of Ames Research Center led the atmospheric structure instrument, which measured temperature, pressure, and deceleration. Hasso Niemann of Goddard Space Flight Center ran the neutral mass spectrometer, built to analyze atmospheric gas composition. Ulf von Zahn of the University of Bonn contributed the helium abundance detector, an interferometer for atmospheric composition studies. Boris Ragent, also of Ames Research Center, led the nephelometer for cloud location and cloud-particle observations. Louis Lanzerotti of Bell Laboratories directed the lightning and radio emission detector, which measured light, radio emissions, and energetic particle fluxes. David Atkinson of the University of Idaho managed the radio equipment, tracking wind speeds and atmospheric absorption. L. Sromovsky of the University of Wisconsin ran the net-flux radiometer, measuring the difference between upward and downward radiant energy at each altitude.

  Beyond those seven instruments, the heat shield itself carried sensors to measure the ablation of its own surface during descent. The shield was designed to record its own destruction as it burned away.

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• 6. Launch and Arrival

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  The Space Shuttle Challenger disaster made Galileo's planned May launch impossible. The mission was rescheduled to the 12th of October. A faulty main engine controller forced a further postponement to the 17th. Bad weather pushed it once more, to the 18th. The launch window extended to the 21st of November, so none of the delays was critical. Atlantis lifted off at 16:53:40 UTC on the 18th of October. The spacecraft and its upper stage reached a 213-mile orbit around Earth. Galileo was deployed at 00:15 UTC on the 19th of October. The spacecraft separated from its upper stage at 01:06:53 UTC and was soon heading toward Venus at more than 9,000 miles per hour.

  Gravitational assist flybys of Venus and Earth gave Galileo the velocity it needed to reach the outer solar system. During the same journey, the spacecraft also made close passes of two asteroids, Gaspra and Ida. Galileo arrived at Jupiter on the 7th of December 1995 and became the first spacecraft to orbit an outer planet. Its large high-gain antenna had failed to fully deploy, so data was transmitted through the low-gain antenna at reduced bit rates. The next spacecraft to orbit Jupiter was Juno, which arrived on the 5th of July 2016.

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