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Mission Operations

Halley's Comet will be of greatest scientific interest from Jan. 20 to Feb. 22; perihelion is on Feb. 9. At that time Halley will be 139.5 million miles from Earth and 59.5 million mi. from the sun. The Shuttle will go into an orbit 176 miles high and inclined 28.5 degrees to the equator. This will have Halley visible for more than 3,000 seconds per orbit (about 56 percent of the orbit), including more than 90 seconds with the sun occulted by the earth.

After a predeployment health check of Spartan voltages and currents, the Shuttle robot arm will pick up the spacecraft and hold it over the side. Upon release, Spartan will perform a 90-second "pirouette" to confirm that it is working and the Shuttle will back away to at least 5 miles so light reflected from the Shuttle does not confuse Spartan's sensors. After two orbits of preparation, the 40-hour science mission will begin. A backup timer will ensure that the spectrometer doors open 70 minutes after release.

Spartan-Halley will conduct 20 orbits of science observations interspersed with five orbits of attitude control updates. A typical science orbit will start with four 100-second calibration scans of Earth's atmosphere, followed by a 900-second tail scan. Observing will be interrupted for 15 minutes of pointing updates and housekeeping. It then resumes with four 200-second scans of the coma, followed by sunset and four coma scans while the sun is occulted. At the end of the mission, Spartan-Halley will be retrieved by the Shuttle robot arm and placed in the payload bay.

After the mission, the processed film and data tapes will be returned to the University of Colorado team for scientific analysis.

The Science

Current theories hold that comets are "dirty snowballs" made up largely of water ice and lightweight elements and compounds left over from the creation of the solar system. Remote sensing of the chemistry of Halley's Comet, by measuring how sunlight is reflected, will help in assaying the comet. The "dirt" in the snowball is detectable in visible light, and the "snow" (water ice) and other gases are detectable, indirectly, in ultraviolet.

The most important objective of the Spartan-Halley mission is to obtain ultraviolet spectra of comet Halley when it is less than 67 million miles from the sun. As Halley nears the sun, temperatures rise, releasing ices and clathrates, compounds trapped in ice crystals.

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