Deep Impact gets to chase comet again

A ‘recycled’ satellite, Deep Impact, is hurtling past the Hartley 2 comet today – clocking up 5.15 billion kilometres since its last mission

A CLOSE ENCOUNTER of an extraordinary kind takes place at about 3pm Irish time today when a spacecraft passes within 700km of a comet. It will take dozens of images, despite hurtling past at about 12.5km per second.

The cameras have been rolling since yesterday and the imaging will continue through tomorrow, giving scientists their most comprehensive view yet of a comet as it passes through the inner solar system.

The achievement is all the more interesting given that the spacecraft has been “recycled” from a previous mission to visit another comet several years ago.

The comet under scrutiny today is Hartley 2, a fairly active comet that is just visible to amateur skywatchers with binoculars or a small telescope. It is producing the typical cometary tail and the satellite – Deep Impact – will have a chance to take pictures but also use its onboard spectrograph to analyse the comet and its tail.

The US National Aeronautics and Space Administration’s Deep Impact spacecraft has already given exceptional service. Its original mission was to reach an encounter with the Tempel 1 comet and then send a smaller craft to crash into the comet’s surface. Instruments on Deep Impact recorded the event and studied the material ejected from the surface.

That was back in July 2005 and since then the spacecraft has been co-opted into a new mission to visit Hartley 2, the Epoxi mission. It has travelled a spectacular 5.15 billion kilometres since its last cometary encounter and yet continues to respond to commands from its controllers at the Jet Propulsion Laboratory in California who manage the Epoxi mission for Nasa. The mission’s principal investigator is Prof Michael A’Hearn of the University of Maryland.

This will be the fifth time that American and European satellites have targeted comets, still far too rare an event to feel commonplace. “Hartley 2 has already put on a great show with more than a few surprises for the mission’s science team. We expect more of the unexpected during the encounter,” Prof A’Hearn said prior to Deep Impact’s arrival.

The spacecraft was not designed for this mission but was “recycled” to achieve the Hartley 2 visit. It used gravity boosts via Earth fly-bys to reach the comet, but the downloading of images and data afterwards will be hampered given its limitations.

It can’t track the comet and also keep its transmitting antenna pointed towards Earth during approach, stated Dr Tim Larson, Epoxi project manager at the Jet Propulsion Labs. “When the encounter phase begins, all images the spacecraft takes will be stored aboard its two computers. Soon after we fly past the comet . . . we will be able to re-orient the spacecraft so that we maintain imaging lock on the comet nucleus while pointing our big high-gain antenna at Earth.”

The images and data will allow scientists to compare Hartley 2 and Tempel 1, both made up of leftovers from when the solar system formed.

More information: nasa.gov/epoxi

epoxi.umd.edu

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