Comet Hartley 2,103P is a small periodic comet with an orbital period of 6.46 years.
It was discovered by Malcolm Hartley in 1986 at the Schmidt Telescope Unit, Siding Spring Observatory, Australia. Its diameter is estimated to be 1.2 to 1.6 kilometres (0.75 to 0.99 mi).
Hartley 2 was the target of a flyby of the Deep Impact spacecraft, as part of the EPOXI mission, on 4 November 2010, which was able to approach within 700 kilometers (430 mi) of Hartley 2 as part of its extended mission.
As of November 2010 Hartley 2 is the smallest comet which has been visited. It is the fifth comet visited by spacecraft, and the second comet visited by the Deep Impact spacecraft, which first visited comet Tempel 1 on 4 July 2005.
The comet passed within 0.12 AU (18,000,000 km; 11,000,000 mi) of Earth on 20 October 2010, only eight days before coming to perihelion (closest approach to the Sun) on 28 October 2010. From northern latitudes, during early November 2010, the comet was visible around midnight without interference from the Moon.
Despite its current close passage by Earth's orbit, the comet is not yet a known source of meteor showers. However, that could change. Dust trails from the recent returns of 103P/Hartley 2 move in and out of Earth's orbit, and the 1979-dust trail is expected to hit in 2062 and 2068
The DEEP IMPACT spacecraft, which had previously photographed Comet Tempel 1,was reused by NASA to study Hartley 2.
The initial plan was for a flyby of Comet Boethin. However, Boethin had not been observed since 1986, and its orbit could not be calculated with sufficient precision to permit a flyby, so NASA re-targeted the spacecraft toward Hartley 2 instead.
The spacecraft came within 435 miles (700 km) while moving at 27,500 miles per hour (44,300 km/h) on 4 November 2010. The data from the flyby were transmitted back to Earth through NASA's Deep Space Network.
The flyby was able to show that the comet is 2.25 kilometers (1.40 mi) long, and "peanut shaped". Some jets of material are being ejected from the dark side of the comet, rather than the sunlit side.
Scientists involved in the EPOXI mission describe the comet as being unusually active, with mission scientist Don Yeomans stating that "It's hyperactive, small and feisty."
NASA's scientists reported that the rays coming off the rough ends consist of hundreds of tons of fluffy ice and dust chunks – the largest particles are of golf ball to basketball-size – and they are ejected by jets of carbon dioxide.
The scientists also said that this is the first time that comet activity powered by sublimation of frozen carbon dioxide is observed as the comet nears the sun; the CO2 ice within the comet must be primordial, dating from the beginnings of the solar system.