Status: Operational 2006. First Launch: 2006-10-26. Last Launch: 2006-10-26. Number: 2 . Gross mass: 620 kg (1,360 lb).

Twin satellites 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.

• Stereo Ahead - . Mass: 1,240 kg (2,730 lb). Nation: USA. Agency: Seal Beach. Class: Astronomy. Type: Solar satellite. Spacecraft: Stereo. USAF Sat Cat: 29510 . COSPAR: 2006-047A.

  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.

• Stereo Behind - . Nation: USA. Agency: Seal Beach. Class: Astronomy. Type: Solar satellite. Spacecraft: Stereo. USAF Sat Cat: 29511 . COSPAR: 2006-047B.

  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.