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Galileo probe

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

Galileo was an unmanned probe sent by NASA to study the planet Jupiter and its moonss. Named after the astronomer and Renaissance man Galileo Galilei, it was launched on October 18 1989 by the Space Shuttle Atlantis and arrived at Jupiter on December 7 1995.

On September 21, 2003, after 14 years of flight time and 8 years of service in the Jovian system, Galileo's mission was terminated by sending the probe into Jupiter's crushing atmosphere at a speed of nearly 50 kilometres per second to avoid any chance of it contaminating local moons with bacteria from Earth. Of particular concern was the ice-crusted moon Europa, which, thanks to Galileo, scientists now suspect harbors a salt water ocean—and possibly microbial life—beneath its surface.

Mission overview

Galileo's launch had been significantly delayed by the hiatus in Space Shuttle launches that occurred after the Space Shuttle Challenger disaster. New safety protocols that were implemented as a result of the explosion 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 (October 29, 1991) 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 (closest approach was 112 miles on October 15, 2001). 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.

Galileo's cameras were deactivated on January 17 2002 after they had sustained irrecoverable radiation damage. NASA engineers were able to recover the damaged tape recorder electronics, and once more Galileo continued to return other scientific data until it was deorbited on September 21 2003 by impacting Jupiter in order to avoid an uncontrolled collision of the unsterilized probe and a Jovian moon. This was done because it is thought that some Jovian moons might harbor microbial life and a crash of Galileo on one of these moons would contaminate any future investigation and analysis.

The Galileo spacecraft

The Jet Propulsion Laboratory built the Galileo Spacecraft and managed 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.

At launch, the spacecraft and probe together had a mass of almost 2,700 kilograms and was seven meters tall. One section of the spacecraft rotated at 3 rpm, keeping Galileo stable and holding six instruments that gathered data from many different directions, including the fields and particles instruments. The other section of the spacecraft held steady for cameras and the four instruments that had to point accurately while Galileo was flying through space.

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, were mounted on the spinning section. The instruments included magnetometer sensors, mounted on an 11-meter 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 carried the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged-particle environments the spacecraft flew through, and an added Extreme Ultraviolet detector associated with the UV spectrometer on the scan platform.

The despun section's instruments included 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 was designed to obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best, because Galileo flew closer closer to the planet and its inner moons and because the CCD sensor in Galileo's camera was more sensitive and had a broader color detection band than the vidicons of Voyager.

Galileo's atmospheric entry probe

The 320 kilogram atmospheric probe measured about 1.3 meters 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 172,200 kilometers 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 150 kilometers 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 either melted and vaporized by the intense heat of an atmospheric "hot spot"; or, it was crushed by the atmospheric pressure.

Main antenna failure

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 were 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 were 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.

Future of Jupiter Exploration

After the end of the Galileo mission and in the light of the discoveries Galileo made, NASA is planning a future Jupiter mission called JIMO: Jupiter Icy Moons Orbiter. The JIMO mission is in its early planning stage and liftoff is not to be expected before 2012.

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