NASA has narrowed the choices for its next unmanned space mission down to three potential expeditions: one aimed at Venus and the others promising to return samples of an asteroid or the moon.

But only one of those contenders will get the green light for $650 million in funding (which excludes rocket costs) and a launch sometime before Dec. 30, 2018. The competition is part of NASA's New Frontiers program to develop medium-class missions to explore the solar system.

"These are projects that inspire and excite young scientists, engineers and the public," said Ed Weiler, NASA's associate administrator for the Science Mission Directorate, in a statement.

NASA has set aside $3.3 million in seed money for each of the three potential missions in 2010 to flesh out their project concepts, feasibility, costs and management plans. The proposals were submitted in July 2009 and a final selection will be made in mid-2011.

"These three proposals provide the best science value among eight submitted to NASA this year," Weiler said.

Here's a look at the top contenders vying for NASA's next New Frontiers mission slot:

Target: Venus — The Surface and Atmosphere Geochemical Explorer (SAGE) mission to Venus would send a probe plunging through the planet's atmosphere to land on its harsh surface. The spacecraft would perform extensive measurements of the Venusian atmosphere and weather on the way down, and then dig into the ground to study surface composition and mineralogy.

Researchers hope the mission will help understand the origins of Venus and why it is so different from Earth. Larry Esposito of the University of Colorado in Boulder is the project's principal investigator.

Target: Asteroid — The proposed Origins Spectral Interpretation Resource Identification Security Regolith Explorer (Osiris-Rex) spacecraft would send a probe to orbit and rendezvous with an asteroid. It would collect more than 2 ounces of samples from the space rock's surface.

The samples would be returned to Earth to help scientists understand the composition of asteroids and shed light on how the solar system formed, as well as trace the origin of complex molecules required for life. Michael Drake, of the University of Arizona in Tucson, is the project's principal investigator.

Target: Moon — The MoonRise: Lunar South Pole-Aitken Basin Sample Return Mission is a proposal aimed at landing a spacecraft in a broad basin near the moon's south pole to collect up to 2 pounds of rock samples for return to Earth.

Researchers believe the rocks on the basin floor were excavated from the moon's mantle and hope the samples collected would fine-tune their understanding of the early history of the Earth-moon system. Bradley Jolliff, of Washington University in St. Louis, is the project's principal investigator.

NASA's New Frontiers program has already launched one mission into deep space with a second in development. The first mission — New Horizons — blasted off in 2006 and is headed to the dwarf planet Pluto for a planned July 2015 flyby. The second mission will send the Juno spacecraft to Jupiter in August 2011 to conduct an in-depth study of the gas giant planet's atmosphere and interior.

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