Comet Hartley 2 is the fifth comet to be visited by a close-approach space probe.
The first spacecraft to visit a comet was the International Cometary Explorer (ICE), which zipped through the tail of Comet Giacobini-Zinner in September 1985. ICE was originally launched in 1978, as part of the International Sun-Earth Explorer mission to study Earth’s magnetosphere and its interaction with the solar wind.
ICE then turned its instruments on Comet Halley in 1986, observing the ice ball from a distance of 28 million kilometers.
Between ICE and Deep Impact’s Nov. 4 encounter with Comet Hartley 2, four missions managed to take close-up photos of comet nuclei.
- Comet Halley: The famed Halley’s Comet was the first comet a spacecraft imaged up close. In 1986, the European Space Agency’s Giotto probe zoomed to within about 600 kilometers of the icy wanderer’s nucleus. Four other spacecraft also visited Halley that year — two each from the Soviet Union and Japan — but none approached as close as Giotto, according to NASA.
Giotto returned a lot of useful information, finding that the comet’s nucleus is rough, porous, dark and dusty. The probe’s data also helped scientists determine that Halley is made of some of the oldest stuff in the solar system, volatiles that condensed onto dust particles about 4.5 billion years ago.
- Comet Borrelly: NASA’s Deep Space 1 probe flew to within 2,200 kilometers of Comet Borrelly in September 2001. The spacecraft returned dazzling and surprising photos, showing rolling, pitted terrain marked by grand mesas.
Deep Space 1’s pictures of the potato-shaped Borrelly were hailed by scientists as the best yet taken of a comet. These images showed that Borrelly is even darker than Halley, reflecting just half as much light as the surface of the Moon.
- Comet Wild 2: Astronomers gained more insight into comet composition and behavior when NASA’s Stardust swung within 300 kilometers of Comet Wild 2 in 2004.
- Comet Tempel 1: The Deep Impact spacecraft served as mothership for NASA’s mission to Comet Tempel 1, which crashed a 371-kilogram probe into the ice ball in 2005. The impact revealed a great deal of water inside and on the surface of Tempel 1, as well as many organic molecules — the building blocks of life — in its interior. Researchers also got glimpses of layered, primordial material within the comet, yielding clues to how the comet may have formed 4.5 billion years ago.