Hartley 2 Comet

The Hartley 2 comet has made it to the news recently: first because of the comet’s flyby near Earth and then due to it being visited by the Deep Impact spacecraft. The comet is commonly known as Hartley 2, but it’s official scientific name is 103P/Hartley.

Orbit of comet Hartley 2Image credit: Ethan Siegel.

2010 Earth approach

The comet passed within 0.12 astronomical units (18,000,000 km) of Earth on October 20, 2010 and was visible to the naked eye. Hartley 2 is a small periodic comet with an orbital period of 6.46 years. This means that it’s orbit is very close to the Sun for a comet. Most of them originate in the Oort Cloud and Kuiper Belt beyond the orbit of Neptune, but this one goes only as far as slightly beyond Jupiter’s orbit. It must have been slingshot from the outer solar system into an orbit much closer to the Sun in a gravitational perturbation probably by one of the gas giants.

Comet Hartley 2 collageImage credit: NASA/JPL-Caltech/UMD.

Deep Impact flyby

The Deep Impact spacecraft, which had previously photographed Comet Tempel 1 has been reused by NASA to study Hartley 2. The spacecraft came within 700 km of the comet while moving 44,300 km/h on November 4, 2010. The data from the flyby transmitted back to Earth showed that the comet is 2.25 kilometers long, and “peanut shaped”. Scientists describe this comet as being exceptionally active.