A spacecraft to conduct a global study of a critical region in Earth's atmosphere is taking shape at the Applied Physics Laboratory. The mission, known as TIMED (Thermosphere, Ionosphere, Mesosphere, Energetics and Dynamics), will study the influences of the sun and humans on the least explored and understood region of Earth's atmosphere--the mesosphere and lower thermosphere/ionosphere, or MLTI. The TIMED spacecraft will be boosted into its 625-kilometer circular orbit aboard a Delta II 7920-10 launch vehicle from Vandenberg Air Force Base, Calif., in May 2000.

The two-year TIMED mission will focus on the portion of Earth's atmosphere located between 60 and 180 kilometers above the surface, where the solar X-ray and ultraviolet radiation is absorbed. "We will examine the entire region as a whole: its basic structure and thermal balance, how the mesosphere is coupled to the thermosphere/ionosphere, how the MLTI region is coupled to space and the lower atmosphere below and how energy is transported from one altitude or latitude to another," says Sam Yee, project scientist for the mission. "We will gain a better understanding of the dynamics of this gateway region and its effects upon communications, satellites and spacecraft reentering Earth's atmosphere," adds Larry Paxton, project scientist for one of the TIMED instruments.

Previous studies of this region have been limited because it is difficult to reach. "This atmospheric region is too high for balloons to reach," says Glen Cameron, mission systems engineer for the TIMED project, "and ground-based instruments can only see a small part of the atmosphere over the observation site. Rockets flown into the region can only provide local snapshots of its activity."

But with technological advances in remote sensing, TIMED will be able to observe this relatively unexplored frontier from space, obtaining an unprecedented set of comprehensive global measurements of the MLTI region's temperature, pressure, winds and chemical composition, along with the energy inputs and outputs.

Integration of TIMED's four-instrument payload has begun with the Global Ultraviolet Imager, a collaborative effort between APL and the Aerospace Corporation in El Segundo, Calif. The GUVI instrument will measure composition and temperature profiles of the MLTI region, as well as the high-latitude auroral energy inputs.

TIMED's three other instruments will be integrated over the course of the next few months. The Solar Extreme Ultraviolet Experiment instrument, being built by the University of Colorado, will measure solar X-ray, ultraviolet and far ultraviolet radiation--the primary energy source of the MLTI region. The Sounding of the Atmosphere using Broadband Emission Radiometry instrument, being developed and built by Hampton University and Utah State University for NASA's Langley Research Center, will measure atmospheric cooling rates, as well as pressure, temperature and chemical constituents. The TIMED Doppler Interferometer instrument, being constructed at the University of Michigan, will measure the region's winds and temperature profiles.

The TIMED spacecraft will be the initial launch in NASA's Solar Terrestrial Probe Program, part of NASA's initiative to lower mission costs and provide more frequent access to space. The TIMED program is cost-capped at approximately $130 million (FY99 dollars) with a maximum three-year development cycle.

TIMED is sponsored by NASA Headquarters, Office of Space Science, Washington, and is managed by NASA's Goddard Space Flight Center, Greenbelt, Md. The mission operations and science data centers, located at APL, will support data analysis for an additional two years after the conclusion of the mission.