The Tomographic Experiment using Radiative Recombinative Ionospheric EUV and Radio Sources (TERRIERS) mission was a very small "University-class" Explorer intended to study a number of ionospheric and thermospheric phenomena, and test the utility of long term solar EUV (extreme ultraviolet) irradiance measurements. Launched 1999 May 17, the spacecraft apparently failed to properly orient its solar panels towards the Sun, and ran out of battery power shortly after launch.
The TERRIERS satellite mission was designed to study the ionosphere - the electrically charged portion of the Earth's upper atmosphere. This highly ionized area, beginning at an altitude of about 30 miles and extending to about 250 miles, is one of the least understood parts of Earth's atmosphere.
Understanding this area is becoming increasingly crucial as conditions in the ionosphere affect the quality of transmissions from communication satellites - impacting devices like cell phones, beepers, and global positioning systems. Short wave radio signals also reflect off this region and manned missions, like the space shuttle and the planned space station, take place in this region.
Conditions in the ionosphere are complex and turbulent, responding to forces originating on the Sun that move toward Earth and interact with Earth's magnetic field. Scientists have coined the phrase "space weather" to describe the changing conditions in this area, including the massive radiation storms that can damage communications satellites, interfere with power grids on Earth, and pose a significant threat to astronauts who might be caught unawares in a storm while working outside their spacecraft.
TERRIERS would have created the first three-dimensional images of the ionosphere. The mission would have used a variety of instruments on the satellite as well as on Earth, to measure ultraviolet, radio, and visible light. It would generate images similar to CAT scan or MRI images of the human body. Medical tomographic images are synthesized from images taken at various levels, or slices, through the body. The TERRIERS orbit and observing strategy similarly slices the ionosphere, allowing it to collect and synthesize multiple images to create a three-dimensional view. Like the radar weather images that we see on the nightly news, these "space weather" images may some day be used in conjunction with modeling programs to begin to forecast weather systems in space.