The Galileo spacecraft was the first to enter orbit around Jupiter. It carried a suite of instruments to investigate the giant planet’s atmosphere, ring structure, moons, and radiation and plasma environments.
One of the first interplanetary missions scheduled to be launched from an American space shuttle, Galileo was approved to be an orbiter and atmospheric probe to study the Jovian system (the system of the planet Jupiter) up close and in depth. Galileo suffered a tortured developmental phase. Originally, it was meant to be propelled directly to Jupiter using a Centaur liquid-fueled upper-stage space vehicle. However, program delays and the Challenger space shuttle accident
Liquid-fueled boosters such as the high-performance Centaur were banned from the shuttle after the Challenger accident for safety reasons. This meant that Galileo would have to use lower-performance solid-fueled upper stages to escape Earth orbit, and these did not have sufficient performance to take such a large spacecraft as Galileo directly to the Jovian system. As a result, Galileo would have to first take advantage of multiple gravity assists while flying about the inner solar system. Thermal protection for certain components had to be added to the spacecraft as a result of having to spend many months close to the Sun. A sunshade was added atop the orbiter’s umbrella-like high-gain antenna, a structure that was folded up at launch and would be deployed after Galileo’s first gravitational boost from Earth. This particular aspect of the spacecraft’s redesign would lead to a serious problem with data transmission rates from Galileo.
The Galileo spacecraft consisted of two major parts, one maintained in stable attitude using thrusters and another that was spin-stabilized at a rotation rate of three revolutions per minute. An atmospheric probe was mounted at the base of the towering spacecraft beneath the 400-Newton main engine. Operating too far from the Sun to conveniently use solar power, Galileo was outfitted with two radioisotope thermoelectric generators filled with plutonium. Galileo’s sixteen instruments, weighing 118 kilograms (260 pounds), were distributed around the spacecraft structure according to function, or in the atmospheric probe. Those mounted in the despun section included a solid-state imager, a near-infrared mapping spectrometer, an ultraviolet/extreme ultraviolet spectrometer, and a photopolarimeter/radiometer. Those mounted on the spun section included a magnetometer boom, a heavy ion counter, a plasma subsystem, an energetic particle detector, a dust detection subsystem, and a plasma wave subsystem. The 339-kilogram (747-pound) probe incorporated an atmospheric structure instrument group, a neutral mass spectrometer, a helium-abundance interferometer, a nephelometer, a net flux radiometer, and a lightning/radio-emission instrument.
Galileo was deployed from the space shuttle Atlantis
Months after Galileo flew past Earth for the first time, in April, 1991, it was time to unfurl the spacecraft’s 4.8-meter-diameter high-gain antenna. The antenna was designed to handle transmission rates as high as 134 kilobits per second, much faster than previous spacecraft imaging the outer solar system. Unfortunately, at least three of the ribs on the antenna remained stuck to a restraint attached to the thermal shield structure; despite numerous approaches to deploy the antenna, it remained only partially opened and as such unable to transmit usefully. Data would have to be transmitted at the snail’s pace of 8 to 16 bits per second available through use of the low-gain antenna. With data compression techniques, the transmission rate was eventually ramped up to 160 bits per second. Obviously, that would greatly increase the time it would take to send the information contained in each image. Data had to be recorded and then played back slowly when observations were not being made.
This composite of the Jovian system includes the edge of Jupiter, with its Great Red Spot, and the planet’s four largest moons, known as the Galilean satellites. From top to bottom, they are Io, Europa, Ganymede, and Callisto.
Galileo next passed through the asteroid belt, coming close enough to a pair of asteroids to train its cameras and instruments on them. On October 29, 1991, the spacecraft passed within 1,600 kilometers (994 miles) of Gaspra, and on August 28, 1993, Galileo passed within 2,400 kilometers (1,491 miles) of Ida, discovering that it had a small moon, which was given the name Dactyl. In 1994, the spacecraft was in an advantageous position to observe the unique celestial collision of pieces of Comet Shoemaker-Levy 9
After years of coasting along the Venus-Earth-Earth-Gravity-Assist (VEEGA) trajectory with only intermittent scientific observations, Galileo experienced a very active year in 1995. As Galileo neared the Jovian system, spacecraft controllers dealt with slipping tape within the onboard data recorder, the high-gain antenna issue, and a leaky valve in the main propulsion system. None of these problems precluded independent deployment of the atmospheric probe on July 13 or the orbital insertion maneuver of the orbiter in December coupled with the coordination of the orbiter with the probe for the probe’s atmospheric plunge. The probe was spun up to ten revolutions per minute prior to its release in order to provide a stable attitude for its independent motion. Then the probe flew on its own trajectory for five months, powering up at the precise moment to begin collecting data as it descended through the upper atmosphere of Jupiter on December 7. Hitting the outer fringes at 170,000 kilometers per hour (106,000 miles per hour), the probe experienced an initial deceleration period sufficient to allow deployment of a large Dacron parachute; the probe continued to descend in the thick atmosphere for an hour before being crushed by the tremendous pressure.
Meanwhile, Galileo made its closest planned approach to Io,
In early 1610, the Italian physicist, mathematician, and astronomer Galileo, using a crude, homemade telescope, became the first to observe the orbital movement of moons around Jupiter. In his honor, these four large moons are now referred to as the Galilean moons. In the twentieth century, the United States dispatched a spacecraft to Jupiter bearing the name of this great thinker.
Some of the spacecraft Galileo’s major observations included determining that Io has a hundred times as much volcanic activity as Earth, providing circumstantial evidence of liquid water beneath Europa’s icy surface, finding that Ganymede has a magnetic field, determining that Jupiter’s ring is created from dust ejected by meteorite impacts on four small inner moons, measuring the dynamics of Jupiter’s magnetosphere, and finding a liquid-saltwater layer on Europa, Ganymede, and Callisto.
Galileo’s primary mission in the Jovian system concluded after two years, but the spacecraft remained in excellent condition. Several mission extensions were approved, each focusing on Europa and Io, and taking greater risks as time proceeded. Going deeper and deeper into the Jovian radiation environment, Galileo’s camera eventually suffered irreparable radiation damage and was deactivated on January 17, 2002. Then, on September 21, 2003, during its thirty-fifth orbit, Galileo executed a destructive entry into Jupiter’s atmosphere, thereby precluding a potential impact on and plutonium or bacteriological contamination of either Europa or Ganymede.
Galileo data and observations rewrote planetary science textbooks, answering many of the questions raised by Voyager data, and raised new questions that could only be answered by more direct observations from perhaps an orbiter around or a lander on Europa. Galileo observations led to a host of proposals for next-generation missions to investigate icy Europa and perhaps even plumb the depths of the ocean believed to be under that thick ice.
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Bagenai, Fran, et al. Jupiter: The Planet, Satellite, and Magnetosphere. New York: Cambridge University Press, 2004. A series of papers cover all aspects of our understanding of Jupiter, including telescope observations and spacecraft data. Includes a CD with color images. -
Harland, David M. Jupiter Odyssey: The Story of NASA’s Galileo Mission. New York: Springer, 2000. Relates the entire history of the Galileo project and explains its scientific results. -
Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Brooks/Cole, 2005. Accessible textbook on planetary science for high school and college undergraduates; includes Galileo data and imagery.
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