Illustration of Galileo at Jupiter

NASA's Galileo spacecraft will unfurl its 16-foot-diameter main communications antenna Thursday, setting the stage for transmission of high volumes of science data when Galileo flies by the asteroid Gaspra later this year.

The umbrella-like high-gain antenna, made of metal mesh, has been stowed behind a sun shield since Galileo's launch in October 1989, to avoid heat damage while the spacecraft flew closer to the sun than the orbit of Earth.

Deployment of the antenna will allow Galileo to send data to Earth at much higher rates over greater distances than it can with the low-gain antennas it has used since launch.

Using one of its low-gain antennas, Galileo generally transmits data at up to 1,200 bits per second (bps). With the high-gain antenna, Galileo will be able to transmit at up to 134,000 bps (the equivalent of about one television picture each minute) across hundreds of millions of miles of space.

Commands to unfurl the antenna will be issued by Galileo's computers at about 12:50 p.m. Pacific Daylight Time on Thursday, April 11. In the unfurling action -- which takes less than 10 minutes -- redundant motors drive a worm gear, pushing levers which spread the antenna's ribs, much as an umbrella is opened.

Engineering monitors onboard Galileo will confirm immediately when the unfurling is completed. The first radio transmission over the antenna will be sent May 6.

The antenna -- a modified version of the design used in NASA's Earth-orbiting Tracking & Data Relay Satellites -- has a surface made of gold-plated molybdenum wire woven into a mesh. The mesh is stretched across 18 graphite-epoxy ribs and connected with elastic epoxy bands.

Galileo will use the high-gain antenna during its flyby of the asteroid Gaspra, at a distance of some 255 million miles from Earth, on October 29. The spacecraft will be some 580 million miles from Earth when it arrives at its final destination, the giant planet Jupiter, in December 1995.

On Tuesday, April 9, flight controllers returned Galileo to its normal "dual-spin" configuration, in which part of the spacecraft spins and part remains fixed in relation to space. Galileo will be fully configured for normal operations by the time the next major sequence of commands is sent to the spacecraft April 25.

The Galileo Project is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science and Applications.

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