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Comet Crash, Asteroid Moon on Galileo's '94 Agenda


From the "JPL Universe"
January 13, 1995

Galileo began 1994 (as it would end the year) with the spacecraft on the far side of the sun from Earth. At that distance the communication rate was at its lowest, and the plan was to wait a few weeks till declining distance increased the rate before playing back most of the pictures of asteroid Ida, target of Galileo's second asteroid flyby in August 1993.

And then in the first image processed--a group of narrow horizontal strips of picture called "jail bars"--Ann Harch of the imaging team spotted something next to the asteroid.

The rest of the picture, as well as matching infrared scans, confirmed that the spacecraft had photographed the first known moon of an asteroid, a one-mile orbiting rock later named Dactyl.

Before the scientists had finished telling reporters and the public of Galileo's discovery, the team began to design the spacecraft's program to observe 1994's biggest event in the solar system: The newly discovered, shattered comet named Shoemaker-Levy 9 was going to hit the far side of Jupiter in July.

Only Galileo could view the far side of Jupiter, though most of Earth's observatories would be looking hard, and, as it turned out, seeing most of the show except the opening moments.

Because this was an addition to Galileo's already busy schedule, the scientists, engineers and programmers had little time and no extra resources to set up and carry out what would be the spacecraft's sixth encounter on the way to Jupiter. They knew that the schedule of the 20 expected events would be uncertain to 10 minutes or more, and that the spacecraft would have to transmit its observations at its lowest rate, 10 bits per second--the one that had seemed too slow for asteroid pictures a few months before.

They programmed Galileo to observe about half the impact events, using various combinations of its remote sensors, and to tape-record most of the data, especially images and infrared spectra (showing temperature and gas composition), for much later playback, lasting into early 1995.

When the events came, in the third week of July, they were much more prominent than expected, and Hubble Space Telescope and ground-based observers had a field day. Galileo's photopolarimeter radiometer returned brightness histories of three of the impacts within about a day of the arrivals, and the first actual time-lapse pictures of a comet fragment arriving (it was the last one) came out in August.

By October, three Galileo instrument teams could get together and give the scientific community a brief but coherent history of one impact, in ultraviolet, visible and infrared light: For a moment, they said, the fireball was hotter than the surface of the sun.

The tape playback continued in December and into January, but already the Galileo team had turned back to its main job--getting ready for December 1995, when Galileo's probe plunges into Jupiter's clouds, and the Galileo orbiter begins its long-term mission around Jupiter.

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