A swarm of spacecraft, designed to fly through the space storms that cause aurora, has been chosen as the next mission in NASA's Medium-class Explorer (MIDEX) program.

"The Explorer program allows the science community to identify the most compelling science questions and then design the most effective mission to answer those questions," said Edward Weiler, Associate Administrator for Space Science, NASA Headquarters, Washington. "The mission we've selected will directly address the science goals of the NASA strategic plan within a focused, moderate sized project," he said.

The mission, to be launched in 2007, is the Time History of Events and Macroscale Interactions during Substorms (THEMIS). THEMIS is a five-satellite mission with the job of determining the causes of the global reconfigurations of the Earth's magnetosphere that are evidenced in auroral activity. THEMIS consists of 5 small satellites, carrying identical suites of electric, magnetic, and particle detectors, that will be put in carefully coordinated orbits. Every four days the satellites will line up along the Earth's magnetic tail, allowing them to track disturbances. The satellite data will be combined with observations of the aurora from a network of observatories across the Arctic Circle. Dr. Vassilis Angelopoulos of the University of California, Berkeley, Calif., will lead THEMIS at a total mission cost to NASA of $173 million.

NASA also selected, as a mission-of-opportunity, an instrument for the Extreme Universe Space Observatory (EUSO) mission of the European Space Agency (ESA). EUSO will study the most energetic particles in the universe. Little is known about the explosive events that create these particles throughout the universe.

From its location on the International Space Station, EUSO will look down on the Earth's atmosphere to observe the characteristic blue light that high-energy cosmic rays generate after hitting the Earth's atmosphere. NASA will provide the largest Fresnel lens ever built for the EUSO telescope. Dr. James Adams Jr. of NASA's Marshall Space Flight Center, Huntsville, Ala., will lead the agency's contribution to EUSO at a total project cost to NASA of $36 million.

NASA has decided to continue studying the Widefield Infrared Survey Explorer (WISE), a four-channel, super-cooled infrared telescope designed to survey the entire sky with 1,000 times more sensitivity than previous infrared missions. A decision on proceeding to flight development with WISE will be made in 2004. Dr. Edward Wright of the University of California, Los Angeles, is the Principal Investigator for WISE.

The Explorer Program is designed to provide frequent, low-cost access to space for physics and astronomy missions with small to mid-sized spacecraft. The first two MIDEX missions are the Imager for Magnetopause-to-Aurora Global Exploration ( (IMAGE), launched in 2000, and the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001. The third MIDEX mission, the Swift Gamma Ray Burst Explorer is scheduled for launch in December 2003. Swift will study the origins of black holes in gamma ray bursts, the most energetic explosions in the universe.

The selected proposals were among 31 MIDEX and 11 mission-of- opportunity proposals originally submitted to NASA in October 2001 in response to an Explorer Program Announcement of Opportunity issued in July 2001. NASA selected five proposals in April 2002 for detailed feasibility studies. Funded by NASA at $450,000 each, these studies focused on cost, management, and technical plans, including small business involvement and educational outreach. NASA's Goddard Space Flight Center, Greenbelt, Md., manages the Explorer Program for the Office of Space Science, Washington.