NASA's Galileo spacecraft will obtain its first gravity assist on the long road to Jupiter on the night of February 9-10 when it flies by the planet Venus at a distance of some 10,000 miles (16,000 kilometers) above the cloudtops.
Although the sole objective of this flyby is to pump up Galileo's orbit so that it can eventually reach Jupiter (two Earth flybys will also be required), scientists will use the opportunity to turn the spacecraft's planetary sensors on the cloud-shrouded planet, in order to study its atmosphere and environment.
The scientific data will have to be stored on the spacecraft tape recorder until late October, when Galileo is close enough to Earth to play them back over its low-gain antenna.
The Venus observations were selected by Galileo scientists and mission controllers at the Caltech Jet Propulsion Laboratory on the basis of (1) not risking Jupiter observations in the 1995-1997 Jovian orbital phase, (2) not exceeding the capacity of the tape, and (3) getting the best new scientific information about Venus.
The nearest planet to Earth in space, Venus is also closest to our planet in size, with a diameter 5 percent smaller, and a mass almost 20 percent less than Earth. A massive atmosphere topped by thick clouds blankets the surface, imposing a pressure about 100 times that at sea level here, as well as trapping the Sun's heat and raising the temperature above the melting point of lead.
Venus has been observed from Earth with ever improving technology and by nearly 20 spacecraft operated by NASA and the Soviet space program, which have flown by, orbited and sent probes into the atmosphere and to the surface. Radar systems (the only way to see through the atmosphere) have mapped much of the surface at gradually improving resolution. Later this year, NASA's Magellan mission will orbit Venus for high-resolution global radar mapping. Previous missions have raised new questions about the atmosphere's structure and behavior; Galileo's instruments could provide some answers.
Galileo was launched October 18, 1989, aboard space shuttle Atlantis; an Inertial Upper Stage rocket boosted it out of Earth orbit toward Venus. This system could not give the spacecraft enough velocity to reach Jupiter directly, as originally designed; thus the Venus-Earth-Earth swingby scheme was devised.
The Venus flyby geometry was designed to swing the spacecraft back to Earth on December 8, 1990, for a 600-mile flyby angled to put Galileo in a two-year elliptical orbit around the Sun, bringing it back again on December 8, 1992. This third planetary swingby will boost the spacecraft into an ellipse long enough to reach Jupiter in December 1995 to begin its two-year scientific study of the giant planet, including putting an atmospheric probe into the Jovian clouds.
The Galileo project was developed and is operated by the Jet Propulsion Laboratory for NASA's Office of Space Science and Applications. JPL also designed and built the main part of the spacecraft, which will orbit Jupiter for about 22 months. The atmospheric probe was managed by NASA Ames Research Center. Communications with the spacecraft are maintained by the JPL-managed Deep Space Network, with tracking stations in California, Australia, and Spain.
GALILEO VENUS FLYBY DETAILS
Previous trajectory correction maneuver: Dec. 22, 1989
Venus closest approach: about 10,000 miles (16,000 km) above cloudtops, at 9:59 p.m. PST Feb. 9, 1990
(just before 1 a.m. EST Feb. 10)
Speed change: 4,990 mph (2.2 km/sec), from 83,750 to 88,740 mph in solar orbit
Perihelion: 2 a.m. PST Feb. 25, 65 million miles (0.699 a.u.) from Sun
Next maneuver: Spring 1990
Imaging: day-side (except lightning search), mostly violet filter, a little clear, some near-infrared. No color images. First images acquired, about 9 p.m. Feb 9; last images 7 days later.
Infrared: night-side lower cloud mapping and atmospheric studies; two partial-disk mosaics, plus scans for water and other gas distribution.
First mosaic: 6 p.m. Feb. 9; last mosaic, 10:30 p.m.; additional spectra taken with imaging for 7 days.
Ultraviolet: scans for upper atmosphere composition and distribution.
First scan: 9 p.m. Feb. 9; last scan: midnight or later.
Photopolarimeter/radiometer: scans and maps of cloud-top temperature and other physical properties, perhaps lightning.
First map: 5 a.m. Feb. 9; last scan: midnight or later.
Fields and particles (dust detector, magnetometers, charged particle detectors, plasma-wave): dust and mag, as near continuous as possible through the period around the flyby. Magnetometer has special co-observations with Pioneer Venus Orbiter. Charged-particle observations particularly intense, 7 p.m.-midnight Feb. 9.
No real-time science return; all data recorded on 3 tracks of 4-track tape recorder for playback near Earth in late October.
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