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NASA's Deep Impact Mission and the History of Comet Missions

As some of you may already know, NASA's Deep Impact mission is scheduled for its impact with Comet Tempel 1 at 1:52 a.m. EDT tomorrow morning. (I'll probably be asleep when it happens.)

The first comet to be visited by a spacecraft was Comet Giacobini-Zimmer, which was observed by the International Cometary Explorer (ICE) on September 11, 1985. I remember following the comet mission when I was a teenager, because Comet Halley (pronounced HAL-ley, not HALL-ley) was coming back to our part of the solar system in 1986.


My family took a special trip to see Comet Halley, as living in the middle of New York City made it hard to see the stars in the sky. Fortunately, at the time I was a member of the Amateur Astronomers Association of New York CIty. They arranged for a special van to take a group of people from Manhattan to a park in Far Rockaway, Queens, where the lights and pollution from the city would not taint our view of the night sky. It turned out that the only people who signed up for the event were my family, and we were taken there by the head of AAA.

Comet Halley itself was a disappointment, a tiny shmear of light against the sky, barely visible. What impressed me much more was the grand majesty of the rest of the night sky; I had never seen so many stars at once. Even the Moon's light did nothing to wash away the image of all those stars, a view I could never get from Forest Hills. (I also remember how cold it was, and that the building we used to get warm was hosting an Alcoholics Anonymous meeting. The AA members shared their cookies with us, and joined the AAA outside to see the comet. They also shared their cigarette smoke, which did not thrill me as much.)

But I digress. (TM Peter David.) ESA launched the Giotto probe on July 2, 1985 (twenty years ago yesterday) with the goal of getting as close to Comet Halley as possible. They also harbored the hope of being the first space agency ever to have a remote spacecraft visit a comet.

However, the simple existence of such a mission led to a benign type of space race. Every space agency wanted to "scoop" Giotto, as it were, to be the first agency to send a spacecraft to a comet. So NASA reactivated the ISEE-3 (International Sun-Earth Explorer), and sent it through the tail of Comet Giacobini-Zinner, thus winning the race and becoming the first probe to visit a comet. About half a year later, it also managed to observe Comet Halley from a distance of 28 million kilometers.

A few other missions scooped Giotto, including Russia's Vega-1 and Vega-2 spacecraft, which left landers on the surface of Venus and then flew on to photograph Halley, making their closest approaches on March 6, 1986, and March 9, 1986, respectively. Japan got into the act, too, with the Suisei probe, which made its closest approach to Halley on March 8, 1986, and Sakigake, which made its closest approach on March 11, 1986.

But until today, Giotto still held the record of being the spacecraft to make the closest approach ever to a comet. On March 13, 1986, Giotto came within 600 kilometers of Halley's nucleus, giving us our sharpest photos ever of a comet. Then, on July 10, 1992, Giotto passed within 200 kilometers of Comet Grigg-Skjellerup, making it the first spacecraft to visit two comets. And Giotto is still out there, orbiting the Sun about six times every seven years; it may have a chance to make history again.

But not tomorrow.


Tomorrow belongs to NASA's Deep Impact Mission, a spacecraft about the size of a Volkswagon Beetle, which has already released an impactor spacecraft about the size of a living room coffee table. Eighty-three million miles away from us, the 820-pound impactor will hit Tempel 1's nucleus at a speed of roughly 23,000 miles per hour and bore into the surface. Back in 1986, Giotto gave a chance to see what the exterior of a comet's nucleus looked like; now we're about to find out what it's like inside.

Hang on. It's going to be a bumpy ride.


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