WASHINGTON - NASA has selected a pair of Mars missions for further study for a 2011 flight opportunity.

The U.S. space agency announced this week that it had selected competing mission proposals from two Boulder, Colo.-based institutions to spend the next nine months and $2 million refining their concepts in advance of selecting one of the missions in late 2007 for full development as NASA's next Mars Scout mission. The chosen mission would have to launch by 2011 at a cost of no more than $475 million. The two finalists were selected from more than two-dozen proposals the agency received last summer.

The Mars Atmosphere and Evolution Mission, or Maven, was proposed by prominent astrobiologist Bruce Jakosky of the University of Colorado, Boulder, to "address key questions about Mars climate and habitability and improve understanding of dynamic processes in the upper Martian atmosphere and ionosphere," according to a NASA news release. NASA's Goddard Space Flight Center, Greenbelt, Md., would manage the project.

The Great Escape mission, proposed by Alan Stern of the Boulder-based Southwest Research Institute would, according to the release, "directly determine the basic processes in Martian atmospheric evolution by measuring the structure and dynamics of the upper atmosphere." The spacecraft would also seek out and measure "potentially biogenic atmospheric constituents such as methane." Stern currently is the principal investigator on NASA's New Horizons mission, which was launched in January 2006 on a nine-year journey to Pluto and the Kuiper Belt.

NASA also announced Jan. 8 that it will spend $800,000 for St. Louis-based Washington University researcher Alicia Wang to participate as a member of the science team for the European Space Agency's 2013 ExoMars mission.