Artist's concept of the twin STEREO satellites observing the sun. Photo NASA

NASA has announced selections of four investigations to be flown on the Solar TErrestrial RElations Observatory (STEREO) mission, planned for launch in 2004. The STEREO mission will be a multilateral international collaboration involving participants from France, Germany, the United States, and United Kingdom. STEREO is the third mission selected for NASA's Solar-Terrestrial Probe (STP) Program, under the Agency's Sun-Earth Connections Theme.

STEREO will for the first time unveil the Sun in three dimensions. Its objective is to address the origin, evolution and interplanetary consequences of one the most massive disturbances in our solar system called the coronal mass ejection (CME). This will be achieved by sending two identically instrumented spacecraft, both at 1 AU orbit around the Sun, but one flying well ahead of the Earth and one behind.

The instrument suite for STEREO will characterize the CME plasma all the way from the solar surface to the orbit of the Earth. These instruments will measure physical characteristics of CME's with remote sensing and local sensing instruments, allowing scientists to determine solar origins of CME's, their propagation into the interplanetary medium and ultimately their consequences on Earth's magnetic field.

By viewing CME's in three dimensions, STEREO will be able to pinpoint their speed and distance from Earth, and thus more accurately time the arrival of the plasma cloud. The planned 2004 launch date will enable STEREO to make observations during the simpler, declining phase of the current activity cycle, which is expected to reach solar maximum around the year 2000.

"Existing spacecraft only provide a limited picture of these huge solar eruptions, called CME's, which can hurl up to ten billion tons of electrically charged gas toward Earth at more than one million miles per hour," said STEREO co-investigator Dr. Barbara Thompson of NASA's Goddard Space Flight Center (Greenbelt, MD). "By placing two spacecraft off the Sun-Earth line, STEREO will reveal details about CME structure and dynamics that have been impossible to obtain."

The investigations selected by NASA are:

• Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) has four instruments: an Extreme Ultraviolet Imager, two white-light coronagraphs and a Heliospheric Imager. SECCHI's integrated instruments will study the 3-D evolution of CME's from birth at the Sun's surface through the corona and interplanetary medium to its eventual impact at Earth. Dr. Russell Howard, Naval Research Laboratory, Washington, D.C., will lead this investigation.

• STEREO/WAVES (SWAVES) is an interplanetary radio burst tracker for STEREO that will track the generation and evolution of traveling radio disturbances from the Sun to the orbit of Earth. Dr. Jean Louis H. Bougeret from Centre National de la Recherche Scientifique Observatory of Paris and lead U.S. co-investigator, Dr. Michael Kaiser of Goddard, will lead this investigation.

• In situ Measurements of Particles and CME Transients (IMPACT) investigation will sample the 3-D distribution and provide plasma characteristics of solar energetic particles and the local vector magnetic field. Dr. Janet G. Luhmann, University of California, Berkeley, will lead this investigation.

• PLAsma and SupraThermal Ion and Composition (PLASTIC) experiment will provide plasma characteristics of protons, alpha particles and heavy ions. This experiment will provide key diagnostic measurements of the form of mass and charge state composition of heavy ions and characterize the CME plasma from ambient coronal plasma. Dr. Antoinette Galvin, University of New Hampshire, will lead this investigation.

"This selection forms the final link of a powerful partnership between the international science community, NASA and Johns Hopkins University Applied Physics Laboratory," said STEREO Mission Manager Abby Harper of Goddard. "I am eager to work with such an experienced team, as STEREO will produce out-of-this-world class science and space weather data for a relatively low cost."

The two STEREO spacecraft will be built by Johns Hopkins University Applied Physics Laboratory (Laurel, Md.). STEREO is a $150 million development mission (phase C/D real year cost), which equates to $64 million for instruments and $86 million for the two spacecraft. Mission operations and data analysis cost is about $45 million. Development is scheduled to begin January 2001. Goddard will provide mission management and control of STEREO.