STEREO is an acronym for Solar Terrestrial Relations Observatory. It consists of two satellites placed in orbit around the Sun in the same approximate orbit as Earth. One satellite is in orbit ahead of Earth (known as STEREO Ahead) while the other is behind Earth (STEREO Behind). It is possible to see the entire sphere of the Sun at one time using this unique position of the two satellites. Using two satellites like this allows scientists to look at the Sun and other bodies in our Solar System from a new point of view.
Studying the Sun
The Sun makes a rotation about once every two Earth weeks. For the Earth, this has meant not seeing events taking place on the other side of the Sun. Events we could see often rotated out of view before they completed, limiting what we know to what we can see from Earth. Using the STEREO satellites, we can watch events we might never see and know about events that are happening on the other side of the Sun that will come into view. For the first time, we can track events on the surface of the Sun or in its atmosphere throughout the life of the event, even if it occurring on the side away from Earth.
With two satellites opposite each other in orbit around the Sun it is possible to take 3D images of it. This will give the formally flat images a perspective that was previously lacking. Having aN images with a 3D perspective will be especially important to the study of coronal mass ejections and space weather.
Coronal Mass Ejections
The main purpose of STEREO is to observe and gain a better understanding about the solar storms caused by Coronal Mass Ejections (CMEs). CMEs blast billions of tons of the Sun's atmosphere into space at about one million miles an hour. The resulting solar storm can cause problems on Earth with radio and television transmissions and are extremely dangerous to astronauts working outside the protection of a spacecraft. They can also damage satellites and disrupt our electrical power grids. CMEs are poorly understood and scientists will gain a better understanding of them by observing and studying them using the observatory.
Solar flares are similar to coronal mass ejections but have a different cause. A coronal mass ejection takes place when the stress between magnetic fields on the Sun causes the magnetic fields to suddenly snap, hurling matter into space. A solar flare happens when a buildup in a magnetic field on the Sun's surface reaches a peak. Solar flares are fairly localized on the Sun's surface while a CME covers a much larger area.
Predicting the solar storms, like flares and CMEs is another benefit of the observatory. Solar activity reaches a maximum level (called a solar maximum) about every 11 years and scientists expect the years 2012–2013 to be a solar maximum. We'll learn much during this period using STEREO images of the Sun from this NASA program.
Credits, Resources, References
STEREO Image of the Sun by NASA STEREO (annotated by MJ Logan)