This story was updated at 3:10 p.m. ET on Nov. 4.
NASA's Deep Impact spacecraft may have flown within 435 miles (700 km) of Comet Hartley 2 today (Nov. 4), but it wasn't the first time a robotic emissary from Earth cozied up to an icy wanderer. In fact, Deep Impact is the fifth probe to image a comet up-close, with a sixth mission yet to come.
Astronomers hope Deep Impact's rendezvous — which took place at 10:01 a.m. EDT (1401 GMT) today (Nov. 4) — will teach them more about comet structure and behavior, and perhaps yield clues about the early days of the solar system.
Researchers are eager to compare the Hartley 2 data with information gleaned from the previous flybys. And they'll doubtless learn much more when the European Space Agency's Rosetta probe drops a lander on Comet 67P/Churyumov-Gerasimenko in 2014.
The first spacecraft to visit a comet was the International Cometary Explorer, 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 17.4 million miles (28 million km).
After ICE and before Deep Impact's latest encounter, four missions managed to take up-close photos of comet nuclei. Here's a brief rundown of those four, along with Deep Impact's mission:
The famed Halley's Comet was the first comet a spacecraft imaged up close. In 1986, the ESA's Giotto probe zoomed to within about 372 miles (600 km) 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.
Halley is about 9 miles (15 km) long by 5 miles (8 km) wide or so. It completes a circuit around the sun every 75 or 76 years. It should return to the inner solar system around 2051.
NASA's Deep Space 1 probe flew to within 1,364 miles (2,200 km) 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 Borrelly is about 5 miles (8 km) long and makes a complete trip around the sun once every 6.9 years.
Comet Wild 2
Astronomers gained more insight into comet composition and behavior when NASA's Stardust spacecraft swung within 186 miles (300 km) of Comet Wild 2 (pronounced "Vilt 2") in 2004.
Stardust observed lots of cliffs and hills on the comet's surface, as well as active, gas-spewing vents. The probe also collected some dust from Wild 2's coma — the cloud surrounding the comet's nucleus — and brought the stuff back to Earth.
Wild 2 is a small comet, measuring just 3.1 miles (5 km) across. It makes a full lap around the sun every 6.4 years or so.
Comet Tempel 1
The Deep Impact spacecraft served as mothership for NASA's mission to Comet Tempel 1, which crashed an 820-pound (371-kilogram) probe into the ice ball in 2005. [Best Deep Impact Comet Crash Photos]
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.
Tempel 1 is about 4.3 miles (7 km) across and has an orbital period of 5.5 years.
Comet Hartley 2
Deep Impact has been chasing Hartley 2 for months. Today (Nov. 4), the spacecraft approached to within 435 miles (700 km) of the comet to take data with its three instruments — two telescopes with digital color cameras and an infrared spectrometer.
Researchers hope the flyby gives them a good idea of the composition of the comet's icy nucleus. They are also eager to compare Hartley 2 to the four other comets spacecraft have visited, to generate a general sense of what makes comets tick.
Hartley 2, while just under a mile (1.5 km) across, is incredibly active, spewing lots of dust and gas and coughing up poisonous cyanide. [Video of Comet Hartley 2 jets.]
The comet completes a solar-system circuit every 6 1/2 years and was discovered in 1986 by astronomer Malcolm Hartley.
This story been updated to include the International Cometary Explorer mission, which was the first craft to ever rendezvous with a comet in 1985.
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Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.