The three finalists focus on the Moon, Venus, or an asteroid
The US space agency Nasa has selected three projects as finalists for its next celestial mission.
The projects aim to either probe the atmosphere and surface of Venus, return an asteroid fragment to Earth, or send back rocks from the Moon's south pole.
The proposals are part of the New Frontiers programme, designed to carry out frequent, low-cost missions.
Nasa has provided funding for a fuller analysis of the projects, with a winner to be selected in mid-2011.
The cost of the winning project must not exceed $650m (£410m) and must be ready to launch by the end of 2018.
These limits are in keeping with the New Frontiers programme's principles of funding focused, short-term, and comparatively cheap space science missions.
The three proposals are:
- The Surface and Atmosphere Geochemical Explorer, or Sage, would gather information about Venus' atmosphere during the descent of a lander, which would then scratch at the planet's surface to determine its chemical and mineral composition in detail.
- The Origins Spectral Interpretation Resource Identification Security Regolith Explorer, or Osiris-Rex, would initially orbit an asteroid, landing on it to collect about 60g of material that would be returned to Earth.
- The Lunar South Pole-Aitken Basin Sample Return Mission would land near the Moon's southern pole, returning about a kilogram of material that scientists believe has risen from the moon's interior to the surface.
Each team has been given $3.3m (£2.1m) to further flesh out the details of their proposals over the coming year.
"These are projects that inspire and excite young scientists, engineers and the public," said Ed Weiler, associate administrator for Nasa's Science Mission Directorate.
"These three proposals provide the best science value among eight submitted to Nasa this year."
The proposal that is eventually chosen will form the third mission in the New Frontiers programme.
The first, New Horizons, was launched in 2006 and is bound for a Pluto fly-by in 2015. The second, dubbed Juno, will be the first craft to orbit Jupiter from pole to pole after it launches in August 2011.