Status: Active. First Launch: 2006-10-26. Last Launch: 2009-03-07. Number: 2 . Gross mass: 231,900 kg (511,200 lb). Height: 38.90 m (127.60 ft). Diameter: 2.44 m (8.00 ft).
The booster was used to put the twin spacecraft in a 182 km x 403,810 km x 28.5 deg lunar transfer orbit. They would use a series of lunar flybys to eventually place themselves in two different solar orbits: Stereo Ahead in a 0.95 AU x 0.97 AU x 0.12 Deg / 344 day orbit around the Sun leading the Earth, and Stereo Behind in a 0.99 AU x 1.09 AU x 0.03 deg / 389 day orbit trailing the Earth. The satellites were equipped with optical, ultraviolet, radio, and particle sensors that would allow them to form a three-dimensional image of the sun's corona using identical sensors from two vantage points at the same moment.
Communications with the STEREO-B science craft in solar orbit had been lost on 2014 Oct 1 for unknown reasons; by 2016 it was assumed the mission had been lost, but on Aug 21 the big DSS-14 dish at Goldstone picked up a signal from it. This was great news for the heliophysics community, but attempts to recover full communications with the tumbling and underpowered spacecraft have met with mixed success, and as of Oct 11 recovery attempts were scaled back until STEREO-B drifts into a more favorable attitude and orbital position.
Used a 0.95-meter aperture differential photometer with a 105 deg2 field of view to constantly view 145,000 main-sequence stars, detecting planets orbiting around those stars when the planets passed in front of them, dimming them during transit. In its first three years of operation, Kepler detected over 2,000 possible planets, and it was determined that 5.4% of all stars host Earth-size planet candidates, and that 17% of all stars had planets.