NASA announced yesterday it has selected four missions to be considered for the Explorer Program, which emphasizes relatively low cost spacecraft with narrowly focused goals.

The agency also said it would explore further the possibility of participating in a European Space Agency observatory on the International Space Station that would investigate mysterious cosmic rays.

Explorer Program candidates will be reviewed further and two missions will be selected by early 2003 for development as Medium-class Explorer, or MIDEX, flights. The two spacecraft would then be launched in 2007 and 2008.

The selected MIDEX proposals

• The Astrobiology Explorer (ABE): a cryogenic telescope to determine the abundance, distribution and identities of the chemical building blocks of life. ABE would measure interstellar organic compounds and would be led by Scott Sandford of NASA's Ames Research Center in Moffett Field, Calif., at a total mission cost to NASA of $180 million.

• The Next Generation Sky Survey (NGSS): an infrared telescope designed to survey the entire sky with 1,000 times more sensitivity than previous missions. NGSS would be led by Edward L. Wright of the University of California, Los Angeles, at a total mission cost to NASA of $180 million.

• The Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission: a study of the onset of magnetic storms within the tail of the Earth's magnetosphere. THEMIS would fly five microsatellite probes through different regions of the magnetosphere and observe the onset and evolution of storms. THEMIS would be led by Vassilis Angelopoulos of the University of California, Berkeley, at a total mission cost to NASA of $150 million.

• The Advanced Spectroscopic and Coronagraphic Explorer (ASCE): solar telescopes that would study the physical processes in the outer atmosphere of the Sun leading to the solar wind and explosive coronal mass ejections. ASCE would carry three solar instruments 100 times better than previous coronal telescopes and would be led by John L. Kohl of the Smithsonian Astrophysical Observatory, Cambridge, Mass., at a total mission cost to NASA of $177 million.

In a separate process, NASA announced yesterday it had also selected an investigation to be flown on the International Space Station in partnership with the European Space Agency. At the end of the study, NASA will make a final decision on participating in the ESA-led mission.

The Extreme Universe Space Observatory (EUSO) would detect the highest-energy cosmic rays known by using the entire Earth as a particle detector. As extremely energetic particles pass through the Earth's atmosphere they emit a form of blue light that that would be observed by EUSO's large telescope from its vantage point on the ISS.

EUSO is under study by ESA for flight on the Columbus module of the ISS, and NASA would provide the large Fresnal lens for the telescope. NASA's contribution to EUSO would be led by James H. Adams Jr. of NASA's Marshall Space Flight Center, at a total mission cost to NASA of $21 million.

NASA also selected a proposed mission for technology-development funding of the proposed instrument. Stephan S. Meyer of the University of Chicago will develop a frequency-selective bolometer to study dusty galaxies in the early universe from a balloon-borne telescope over Antarctica. Meyer will receive $500,000 over the next two years for his study.

The current MIDEX missions are the Imager for Magnetopause- to-Aurora Global Exploration (IMAGE), launched in March 2000, and the Microwave Anisotropy Probe (MAP), launched in June 2001. The Swift Gamma-Ray Burst Explorer, a third MIDEX mission, will launch in September 2003.

The Explorer Program is managed by NASA's Goddard Space Flight Center.