Hartley 2: What has the Deep Impact flyby shown us?

NASA’s Deep Impact probe flew past comet Hartley 2 and revealed a odd-looking peanut-shaped nucleus. What are the first impressions from these exciting images?


Hartley 2 seen by Deep Impact (Image credit: NASA/JPL-Caltech/UMD)

Only about a mile (1.6 km) from end to end, the nucleus of Comet Hartley2 is hardly the most impressive member of the Solar System. Never the less, scientists were fascinated by the sequence of images taken by the Deep Impact probe of this little nugget of rock and ice on 4 November 2010.


Deep Impact: little-known to the general public but hugely important. (Image credit: NASA)

Deep Impact is no stranger to comets, having visited (and bombarded) Comet Tempel 1 in 2005. Since then it has participated in a longer mission called EPOXI which has involved the probe observing extrasolar planets.

On 4 November 2010, the people carrier-sized probe passed within about 700 km (435 miles) of the tiny comet nucleus, observing it with two telescopes and an infrared spectrometer. The encounter, 23 million km from Earth, was accurate to two seconds in time and 3 km in distance showing just how clever humans can be. Five close-up images were returned.


A montage of images taken during the encounter. The top two images were taken 82 and 16 seconds before closest approach, and the bottom three 18, 57, and 117 seconds after closest approach. In the images taken after closest approach under more dramatic lighting, one can see comet jets that originate on the night side of the comet and rise into sunlight. Image credit: NASA/JPL/UMD/ montage by Emily Lakdawalla

Like several previously visited asteroids, Hartley 2 is shaped like a peanut. The ‘waist’ connecting the lobes (which may be two entirely independent bodies) is remarkably smooth, and is probably composed of loose dust and rubble. Like most small Solar System bodies, the nucleus is very dark and only appears light in these processed images. A multitude of jets are blasting into space from each lobe. Unexpectedly, this is happening  even in regions facing away from the Sun (a day on Hartley 2 lasts about 18 hours).

This was a day to remember for one man in particular. Hartley 2 is named after the Australian astronomer Malcolm Hartley who discovered the comet in a photographic plate from a sky survey undertaken in 1986. Hartley was present at the Jet Propulsion Laboratory (JPL) in California to see the images of his comet as they arrived from deep space.