Spacecraft to capture up close image of comet

LOS ANGELES - Thursday morning at 7:01, a Jet Propulsion Labratory probe will sweep within 434 miles of comet Hartley 2 and take its picture, only the fifth time a spacecraft will have taken an image of a comet up close.

Each of those previous encounters has surprised scientists, painting a diverse picture of the makeup of comets, which were once thought to be little more than "dirty snowballs." Hartley 2 has already startled researchers by spewing cyanide for eight days in early October.

Such "outgassing" events are usually violent and accompanied by massive amounts of dust, but this one was not, said astronomer Mike A'Hearn of the University of Maryland, principal investigator for the mission. "Hartley 2 has already put on a great show, and we expect more of the unexpected during [this] encounter."

The spacecraft is Deep Impact, which launched an 820-pound copper impactor into the core of comet Tempel 1 on the Fourth of July in 2005, revealing that object to be less of a dirty snowball and more of a snowy dirtball, mostly of dust.

That impact was supposed to be the end of the $333-million mission, but NASA officials gave JPL controllers permission to aim the probe at another comet. The team settled on Hartley 2, discovered 24 years ago by British astronomer Malcolm Hartley, working at the Schmidt Telescope in Australia. It orbits the sun every 6.46 years.

On Oct. 20, Hartley 2 passed within 11 million miles of the Earth and was faintly visible to the naked eye. It made its closest approach to the sun last Thursday. It is still close by and can be seen with a telescope or binoculars. It will appear early in the morning today and Thursday, passing to the right of the constellation Gemini's brightest stars, Castor and Pollux.

The new mission, which cost about $30 million, was dubbed Deep Impact Extended Investigation, or DIXI. During its five-year voyage, the probe has trained its camera on distant stars, searching for exoplanets for a project called Extrasolar Planet Observation and Characterization, or EPOCh. Combining the two acronyms, the mission is now known as EPOXI.

The probe was designed to photograph Tempel 1 and simultaneously send those images back to Earth, but the orbital characteristics of Hartley 2 make that impossible this time around.

The Hartley encounter can be monitored live at ustream.tv/user/nasajpl2.