NASA's Solar TErrestrial RElations Observatory (STEREO) mission is comprised of two identical observatories, one following the other in orbit around the Earth. The satellites are known as STEREO-A , "A" for ahead, and STEREO-B, "B" for behind, with STEREO-A leading STEREO-B by about one million miles. One of the observatories leads the Earth in its orbit and the other follows, which enables the two satellites to gain offset views of the Sun. When researchers combine the two sets of images a three dimensional representation is produced.
STEREO is the third mission in the Solar Terrestrial Probes (STP) program and is expected to last two years. The spacecraft were launched on October 25, 2006 and reached their assigned locations on either side of the planet in January of 2007. The mission is designed to provide the first stereoscopic measurements for the study of the sun and its coronal mass ejections, or CMEs. The instruments will observe the energy and matter traveling from the sun to the Earth and provide a three dimensional structural view of the CMEs and possibly give some insight as to their cause. The system will also enable observers to detect solar ejections directed toward the planet.
Coronal mass ejections are clouds consisting of billions of tons of electrified gas, or plasma, ejected from the Sun into space and moving at one million miles per hour. Lika Guhathakurta, STEREO Program Scientist at NASA headquarters said, "STEREO's ability to see these clouds in 3-dimensions will revolutionize our understanding of CMEs and improve our ability to predict when they will hit Earth." Satellites, radio communications and power grids on the Earth can be disrupted by violent solar weather originating in the Sun's atmosphere, making the prediction of these eruptions extremely important.
The Sun's atmosphere, or corona, has an appearance similar to wisps of smoke moving along the solar magnetic fields, and it is difficult to determine which images are in the foreground and which are in the background. Assessing distance is also problematic due to the two dimensional images previously available. Three dimensional imaging will make it much easier to understand the nature of the solar environment. Also, with the enhanced viewing made possible by the STEREO mission, it will be possible to observe the solar disturbances all the way from the Sun to the Earth, whereas with earlier systems it was possible to view the eruptions as they left the Sun and for only a short distance along the path to the Earth.