NASA's STEREO mission is a solar probe mission studying coronal mass ejections. The name, Solar Terrestrial Relations Observatory (STEREO), is a contrived acronym, referring to the fact that the mission is actually being carried out by two spacecraft, one leading the Earth in its orbit around the Sun, and the other trailing behind the planet.
- Spacecraft -
Both probes launched together, in October 2006, on a Delta II rocket. The launcher placed them into an elliptical orbit, from which they swung past the Moon, allowing the gravitational slingshot to eject them from Earth's orbit at different points. The result, as intended, was that both entered orbits around the Sun at slightly different points. STEREO A's orbit is slightly inside Earth's and lasts 347 days; STEREO B's, slightly outside, and lasting 387 days.
- Solar Mission and Current Status -
The two STEREO probes each have a mass of 1350 pounds (620 kg) and transmits data at 720 kbps, a little below the speed of today's home Internet broadband connections. They carry cameras as well as four purpose-built scientific instruments: a five-camera package for studying coronal mass ejections as they leave the Sun and enter the interplanetary medium, a radio emission tracker, a plasma analyzer, and a package studying the solar wind. The coronal mass ejection package, known as the Sun-Earth Connection Coronal/Heliospheric Investigation (SECCHI), is designed to track the ejections in visual and ultraviolet light as they leave the Sun and then travel toward Earth.
Coronal mass ejections are emissions of plasma from the Sun's surface layer, or corona, which is believed to result from magnetic fields within the Sun. The resulting mass of material is shot out into the solar system, including toward Earth, where it can disrupt Earth's own magnetic field and cause strong aurora effects (the so-called northern and southern lights). Effects of serious impacts, like those of large solar flares, include disruptions to telecommunications and electrical systems.
The orbits of the STEREO spacecraft were initially precisely timed to create stereoscopic imagery, essentially the same process as the brain's processing of vision from both eyes in humans (and other animals). However, within a couple of years, their different speeds had brought them too far apart for this to work. The spacecraft are now on their way to being diametrically opposed in their orbital positions (180 degrees apart), at which time NASA will be able to take images of the entire Sun simultaneously. This should occur in early 2011. After that, their positions will begin to grow closer again, until stereoscopic imagery again becomes possible. The spacecraft are solar-powered, and therefore can theoretically continue to function indefinitely during this process, barring equipment failures.
- More Information -
NASA. "STEREO" (official website).