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Galileo probe

Galileo probe being deployed
during the STS 34 flight (NASA)

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The Galileo probe was an unmanned probe sent by NASA to study the planet Jupiter and its moons. Named after the astronomer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis and arrived at Jupiter on December 7, 1995. Galileo's launch had been significantly delayed by the hiatus in Space Shuttle launches that occurred after the Space Shuttle Challenger disaster, and new safety protocols that were implemented as a result forced Galileo to use a lower-powered upper stage booster rocket to send it from Earth orbit to Jupiter; several additional gravitational slingshots (once by Venus and twice by Earth) were required in order to give it enough velocity to reach its target. Along the way, Galileo performed close observation of the asteroids 951 Gaspra and 243 Ida, and discovered Ida's moon Dactyl. In 1994, Galileo was perfectly positioned to watch the fragments of comet Shoemaker-Levy 9 crash into Jupiter. Earth-based telescopes had to wait to see the impact sites as they rotated into view.

Galileo's prime mission was a two-year study of the Jovian system. Galileo traveled around Jupiter in elongated ellipses; each orbit lasted about two months. By traveling at different distances from Jupiter, Galileo could sample different parts of the planet's extensive magnetosphere. The orbits were designed for close-up flybys of Jupiter's largest moons. Once Galileo's primary mission was concluded, an extended mission followed starting on December 7 1997; the spacecraft made a number of daring close flybys of Jupiter's moons Europa and Io. The radiation environment near Io in particular was very unhealthy for Galileo's systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.

At launch, the spacecraft and probe together had a mass of almost six thousand pounds, about as much as two sport utility vehicles. Galileo is over twenty feet tall. The spacecraft is a "dual-spin" design; a controlled spin keeps Galileo stable. One section of the spacecraft rotates at 3rpm. On this section, six instruments rapidly gather data from many different directions. The other section of the spacecraft holds steady for the four instruments that must point accurately while Galileo is flying through space.

The Galileo mission and systems were designed to investigate three broad aspects of the Jovian system: the planet's atmosphere, the satellites and the magnetosphere. The spacecraft was constructed in three segments, which help focus on these areas: 1) the atmospheric probe; 2) a non-spinning section of the orbiter carrying cameras and other remote sensors; 3) the spinning main section of the orbiter spacecraft which includes the fields and particles instruments, designed to sense and measure the environment directly as the spacecraft flies through it.

This innovative "dual spin" design allows part of the orbiter to rotate constantly at three rpm, and part of the spacecraft to remain fixed. This means that the orbiter can easily accommodate magnetospheric experiments (which need to take measurements while rapidly sweeping about) while also providing stability and a fixed orientation for cameras and other sensors.

Scientific instruments to measure fields and particles, together with the main antenna, the power supply, the propulsion module, most of the computers and control electronics, are mounted on the spinning section. The instruments include magnetometer sensors, mounted on an 11-meter (36-foot) boom to minimize interference from the spacecraft; a plasma instrument detecting low-energy charged particles and a plasma-wave detector to study waves generated by the particles; a high-energy particle detector; and a detector of cosmic and Jovian dust. It also carries the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged-particle environments the spacecraft flies through, and an added Extreme Ultraviolet detector associated with the UV spectrometer on the scan platform.

The despun section carries instruments and other equipment whose operation depends on a steady pointing capability. The instruments include the camera system; the near-infrared mapping spectrometer to make multispectral images for atmospheric and moon surface chemical analysis; the ultraviolet spectrometer to study gases; and the photopolarimeter-radiometer to measure radiant and reflected energy. The camera system will obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best, largely because it will be closer. The CCD sensor in Galileo's camera is more sensitive and has a broader color detection band than the vidicons[?] of Voyager.

The 700-pound atmospheric probe measured about four feet across. Inside the heat shield, the scientific instruments were protected from ferocious heat during entry. The probe had to withstand extreme heat and pressure on its high-speed journey at 106,000 miles per hour. The probe was released from the main spacecraft in July 1995, five months before reaching Jupiter, and entered Jupiter's atmosphere with no braking beforehand. It slowed, released its parachute, and dropped its heat shield. As the probe descended through ninety-five miles of the top layers of the atmosphere, it collected fifty-eight minutes of data on the local weather. The data were sent to the spacecraft overhead, then transmitted back to Earth. The probe was finally melted and vaporized by the intense heat of the atmosphere.

The Jet Propulsion Laboratory built the Galileo Spacecraft and manages the Galileo mission for NASA. Germany supplied the propulsion module. NASA's Ames Research Center managed the probe, which was built by Hughes Aircraft Company[?].

For reasons which are not currently known, and in all likelihood will never be known with certainty, Galileo's high-gain antenna failed to fully deploy after its first flyby of Earth. Investigators speculate that during the time that Galileo spent in storage after the Challenger disaster lubricants evaporated, or the system was otherwise damaged. Fortunately Galileo had an additional low-gain antenna that was capable of transmitting information back to Earth, but the low-gain antenna's bandwidth was significantly less than the high-gain antenna's would have been; the high-gain antenna was to have transmitted at 134 kilobits per second whereas the low-gain antenna's bandwidth was only 160 bits per second. The data collected on Jupiter and the moons are stored on the on-board tape recorder, and transmitted back to Earth during the long apogee portion of the probe's orbit using the low-gain antenna. At the same time, measurements are made of Jupiter's magnetosphere and transmitted back to Earth. The reduction in available bandwidth reduced the number of pictures that were transmitted significantly; in all, only 14,000 images were returned.

Galileo's cameras were deactivated on January 17, 2002 after funding for camera operation was used up. Galileo continues to return other scientific data, and is scheduled to be deorbited in September 2003.


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