NASA said Tuesday it is recycling two used spacecraft to lead new robotic missions to study comets and planets around other stars.

The encore performances of the Deep Impact and Stardust probes will allow the space agency to further its solar system exploration for a fraction of the cost it would take to start a mission from scratch.

Both spacecraft successfully completed their primary missions to two different comets and their discoveries have helped scientists understand how the solar system formed.

In 2005, Deep Impact released a copper impactor that smashed into comet Tempel 1. The collision carved a crater and spilled a plume of debris from its interior into space. The surviving mothership has since been put in safe mode to conserve energy.

Stardust flew close to the comet Wild 2 and used a mitt to collect minute samples of cometary and interstellar dust. A capsule carrying the particles parachuted to Earth last year while the probe remained in space.

Plan new missions

Scientists plan to activate Deep Impact later this year for a two-part mission that includes collecting data on extrasolar planets to determine whether they have rings, moons or other features. Deep Impact will become an observatory looking at distant stars already known to be orbited by giant planets.

After that, Deep Impact will pass the comet 85P/Boethin in December 2008. It will be the first spacecraft to explore Boethin, a small comet discovered in 1975 that orbits the sun every 11 years. Researchers hope information gathered from Boethin will shed light on how comets evolved and if they played a role in the emergence of life on Earth.

NASA plans to send Stardust to Tempel 1 to examine the crater created by the 2005 impact, making it the first comet to be revisited. Scientists failed to image the crater after the collision because the plume blocked the view, but they hope to get a second chance with Stardust when it flies by the comet in 2011.

NASA did not disclose the price tags of the followup projects, but the costs were expected to be significantly lower than the main missions. Deep Impact cost $333 million US, while Stardust was $212 million.

The Deep Impact team proposed $40 million for the encore mission, but NASA only allotted $30 million, said principal investigator Michael A'Hearn of the University of Maryland. While A'Hearn was disappointed with the budget, he did not want to pass up a chance to reuse the Deep Impact spacecraft.

"Clearly, I still want to fly the mission," he said.