Recent ground-based telescope observations by Planetary Science Institute researchers have offered new insights into the nucleus of a comet that will be the target of a NASA spacecraft flyby this week.

Analysis of images of a jet of cyanide gas emerging from comet Hartley 2 will be combined with data derived from a spacecraft flyby scheduled for Thursday morning to offer researchers a clearer view of the comet's nucleus, said Beatrice Mueller, senior scientist at PSI.

NASA's EPOXI project will see a spacecraft pass within 700 km of Hartley 2's nucleus Thursday. The craft will image the comet as the two move past each other at a relative velocity of more than 12 km per second, or 27,000 mph.

While EPOXI offers great scientific opportunities because of its closeness to Hartley 2, the quick nature of the flyby encounter means not all details will be revealed, Mueller said.

Numerous nights of observation from Earth can offer data that a quick encounter with a passing spacecraft would be unable to gather, she said.

"You cannot get a complete picture, or as complete a picture. The spacecraft will only be able to image part of the comet. It flies by pretty fast. It only sees part of the nucleus, so they get a tiny little snapshot," she said. "But it is important. A flyby is really exciting. You do not get this resolution from the ground. You can get really close and get really good details."

Utilizing images taken from September to October using a 2.1-meter telescope at Kitt Peak National Observatory near Tucson, Mueller and Nalin Samarasinha, senior scientist at PSI, were able to analyze activity in the comet's coma and study the cyanide (CN) jet that was revealed after images were enhanced by Samarasinha to carefully remove background contribution and focus on the CN jet activity.

"We would like to put the spacecraft images into context with what we see from the ground. So we should be able to tell where on the nucleus a jet originates. The spacecraft sees less area due to the high resolution and proximity, and cannot see the other side of the comet and get a full view of what is going on with the nucleus," she said.

"If I only see the back of your head, I would not know there are the eyes and the nose. It's kind of the same thing."

Samarasinha likened the CN jet to a garden hose gushing water, which places a torque on the comet's nucleus and causes it to change its spin.

Visit to see a jet of CN gas, shown red, emerging from the nucleus of comet Hartley 2 in an enhanced color image 23,000 miles across taken recently by PSI researchers.

Earth-based telescope observations have indeed detected a changing rotations period for Hartley 2, he said.

"That is a physical property which would help to interpret other observations, from the ground as wall as from the spacecraft," he said.

"This kind of observations put some of the spacecraft observations in context. You have to have the whole, big picture."

Observations and analysis by Mueller and Samarasinha were funded by a grant from the NASA Planetary Astronomy Program to PSI. The principal investigator of the observing proposal was University of Maryland professor and EPOXI principal investigator Michael F. A'Hearn.