Comet Hartley 2, a small periodic comet

 
Comet Hartley 2, a small periodic comet

Image Credit: NASA/JPL-Caltech/UMD

Comet Hartley 2 (officially designated as 103P/Hartley) is a small periodic comet of about 1.2 to 1.6 kilometres (0.75 to 0.99 mi) in diameter, with an orbital period of 6.46 years. Its perihelion is near the Earth’s orbit at 1.05 AU from the Sun. The comet spins around one axis, but also tumbles around a different axis. It was discovered by Malcolm Hartley on March 15, 1986.

The comet passed within 0.12 AU (18 million km; 11 million mi) of Earth on 20 October 2010, only eight days before coming to perihelion (closest approach to the Sun) on 28 October 2010. Hartley 2 is estimated to come back to perihelion around 20 April 2017.

Hartley 2 is 2.25 kilometers (1.40 mi) long, and “peanut shaped”. Its nucleus is highly elongated and rotates over an 18-hour period. The nucleus has a radius of about 0.57 kilometers (0.35 mi) and a low albedo. The mass of the comet is estimated to be about 300 megatonnes (300,000,000,000 kg).

The comet has a smooth, relatively inactive middle region, or waist. On its rougher ends, the comet’s surface contains glittering, blocky objects that are about 50 meters (165 feet) high and 80 meters (260 feet) wide. These objects appear to be two to three times more reflective than the surface average.

Hartley 2 is a hyperactive comet, spewing out more water than other comets its size. When warmed by the Sun, dry ice (frozen carbon dioxide) deep in the comet’s body turns to gas jetting off the comet and dragging water ice with it. The comet should only be able to survive up to another 100 apparitions (around 700 years) at its current rate of mass loss.

Several jets of material are being ejected from the dark side of the comet, rather than the sunlit side. 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 CO2 ice within the comet must be primordial, dating from the beginnings of the Solar System.

The strong activity in water release and carbon dioxide-powered jets didn’t occur equally in the different regions of the comet. The jets were seen at the ends of the comet, with most occurring at the small end. In the waist of the comet, water was released as vapor with very little carbon dioxide or ice.

Some of the dust, icy chunks, and other material coming off the ends of the comet are moving slowly enough to be captured by even the weak gravity of the comet. This material then falls back into the lowest point—the waist.

Despite its 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.

Hartley 2 was the target of a flyby of the Deep Impact spacecraft, as part of the EPOXI mission, on 4 November 2010, with closest approach of 694 kilometers (431 mi) of Hartley 2. This encounter was the fifth time a spacecraft from planet Earth has imaged a comet close-up.

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