Two NASA missions to explore the boundaries of Earth's atmosphere with space are scheduled for launch in 2006. Both have recently completed preliminary design phases and are ready to proceed with hardware fabrication, integration and testing.

The Aeronomy of Ice in the Mesosphere (AIM) Small Explorer will determine the causes of Earth's highest-altitude clouds, which occur on the very edge of space. These clouds form in the coldest part of the atmosphere, about 50 miles above the polar-regions, every summer. Recorded sightings of these silvery-blue, noctilucent or "night-shining" clouds began in the late 1800's at high latitudes. They have been increasing in frequency and extending to lower latitudes over the past four decades.

Scientists have hypothesized the more frequent occurrences may be an indicator of global warming, but until now they have not been able to test this idea. Since similar thin high altitude clouds have been observed at Mars, what AIM teaches us about Earth's noctilucent clouds should help us understand the similarities and differences between the martian and terrestrial atmospheres.

AIM will measure all the parameters important to understanding noctilucent cloud formation. This will help determine the connection between the clouds and their environment and serve as a baseline for the study of long- term changes in the upper atmosphere. Dr. James Russell III of Hampton University in Hampton, Va., leads AIM as Principal Investigator.

The second mission is the Time History of Events and Macroscale Interactions during Substorms mission (THEMIS). A Medium Explorer mission, it will fly five small spacecraft through explosive geomagnetic disturbances to solve the mystery of what triggers the colorful eruptions of the Northern and Southern lights. These violent "substorms" reflect major reconfigurations of near-Earth space and have significant implications for space weather, affecting satellites and terrestrial communications.

Over the years several different hypotheses have been proposed to explain this phenomenon. THEMIS will use five probes, strategically placed in different regions of the magnetosphere, to determine which explanation is correct. THEMIS is led by Dr. Vassilis Angelopoulos of the University of California, Berkeley.

The Explorer Program is designed to provide frequent, low- cost access to space for physics and astronomy missions with small to mid-sized spacecraft. NASA's Goddard Space Flight Center, Greenbelt, Md., manages the Explorer Program for the Office of Space Science, Washington.