BEIJING, July 4 (Xinhuanet) -- Although they've completed their initial missions, NASA has decided to recruit two robotic spacecraft -- Deep Impact and Stardust -- that were just kind of hanging around the solar system and send them off chasing comets.

    Deep Impact, a spacecraft that flew by Comet Tempel 1 after sending an impactor in its path in 2005, is due to fly past yet another comet in 2008 and observe stars known to have planets circling them.

    Stardust, which dropped off a sample capsule containing comet dust and interstellar samples as it flew past Earth last year has been in a holding pattern since, will be sent to pay its own visit to Comet Tempel 1 in 2011.

    "These mission extensions are as exciting as it gets," Alan Stern, associate administrator for science at NASA Headquarters, said in a written announcement on the missions. "They will allow us to revisit a comet for the first time, add another to the list of comets explored and make a search for small planets around stars with known large planets.

    "And by using existing spacecraft in flight, we can accomplish all of this for only about 15 percent of the cost of starting a new mission from scratch," he added.

    The new assignments come after months of deliberation over what to do with the two spacecraft, which came away from their comet encounters with their observing instruments and navigation systems intact.

    The costs for the past missions were 212 million U.S. dollars for Stardust, and 333 million dollars for Deep Impact. The extensions were approved under NASA's Discovery program for missions of opportunity, which carries a cost cap of 35 million dollars per mission. Stern told MSNBC.com via e-mail that the combined cost for the two new missions comes to 55 million dollars.

    Along with the new assignments come new names: EPOXI for the Deep Impact probe, and NExT for the Stardust probe.

    (Agencies)