STEREO

STEREO (Solar TErrestrial RElations Observatory) is a solar observation mission. Two nearly identical spacecraft were launched in 2006 into orbits around the Sun that cause them to respectively pull farther ahead of and fall gradually behind the Earth. This enabled stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections. Contact with STEREO-B was lost in 2014 after it entered an uncontrolled spin preventing its solar panels from generating enough power, but STEREO-A is still operational. Mission profile The two STEREO spacecraft were launched at 00:52 UTC on October 26, 2006, from Launch Pad 17B at the Cape Canaveral Air Force Station in Florida on a Delta II 7925-10L launcher into highly elliptical geocentric orbits. The apogee reached the Moon's orbit. On December 15, 2006, on the fifth orbit, the pair swung by the Moon for a gravity assist. Because the two spacecraft were in slightly different orbits, the "ahead" (A) spacecraft was ejected to a heliocentric orbit inside Earth's orbit, while the "behind" (B) spacecraft remained temporarily in a high Earth orbit. The B spacecraft encountered the Moon again on the same orbital revolution on January 21, 2007, being ejected from Earth orbit in the opposite direction from spacecraft A. Spacecraft B entered a heliocentric orbit outside the Earth's orbit. Spacecraft A took 347 days to complete one revolution of the Sun and Spacecraft B took 387 days. The A spacecraft/Sun/Earth angle will increase at 21.650° per year. The B spacecraft/Sun/Earth angle will change −21.999° per year. Given that the length of Earth's orbit is around 940 million kilometres, both craft have an average speed, in a rotating geocentric frame of reference in which the Sun is always in the same direction, of about 1.8 km/s, but the speed varies considerably depending on how close they are to their respective aphelion or perihelion (as well as on the position of Earth). Their current locations are shown here.