Engineers April 2 started the shuttle Atlantis's countdown to blastoff on a flight featuring the first post-Challenger spacewalk and the launch of a new astronomy satellite. If all goes well, Atlantis will blast off on the 39th shuttle mission at 9:18 a.m. April 5 to kick off a five-day mission, the first of six planned for 1991. The primary goal of Atlantis's flight is the deployment of the $617 million Gamma Ray Observatory, a massive 35,000-pound astronomy satellite sensitive to extremely high-energy radiation generated by the most violent stars and galaxies in the universe.

With the more famous Hubble Space Telescope already in orbit studying the universe in visible and ultraviolet light, the gamma-ray satellite is the second in a series of four "great observatories" designed to map the heavens across the electromagnetic spectrum.

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