The twin solar observing spacecraft STEREO-A (“Ahead”) and STEREO-B (“Behind) were launched into heliocentric orbit in 2006, in carefully planned orbits that would cause them both to gradually recede from the Earth, one falling behind and one moving ahead, until eventually both would cross paths on the other side of the Sun, with the objective of achieving stereoscopic observations of coronal mass ejections, and 360 imagery of the solar surface.
On January 24, 2009, they reached quadrature — 90 degrees separation. Over the course of that year, they passed through the Sun-Earth L4 and L5 points, becoming the first spacecraft to do so; in addition to their normal duties, they spent time looking for Lagrangian companions of Earth, finding none. On February 6, 2011, they reached 180 degrees separation — each spacecraft was looking at the opposite hemisphere of the Sun, with the Sun blocking any view of one another (not that they could see one another anyway, being so tiny and so distant from one another). From that point on, we had 360 imagery of the Sun, and NASA launched their 3D Sun app so that the general public could hold nearly-live 360 imagery of the Sun in the palm of their hands.
Then, on October 4, 2014, NASA lost contact with STEREO-B. It was expected to go into solar conjunction shortly, and controllers were testing a procedure to help it recover if it lost contact during conjunction. (The twin STEREO spacecraft wouldn’t actually be *directly* behind the Sun as seen from Earth, but close enough for tremendous radio interference.) It was believed that it had gone into a spin, which would tend to be a deadly condition since it would be difficult to recover before it had drained its batteries or its propellant reserves attempting to correct this condition. However, that may not be what happened. Mission controllers never gave up on STEREO B, and since the mission was still funded in order to continue working with STEREO A, they kept trying to contact STEREO B. They sent commands blindly in its direction, hoping it would eventually receive and respond, uplinking commands that, if executed, would at least help it to conserve its batteries and propellant until mission controllers could sort things out, searching for its carrier signal as the spacecraft’s position was no longer known with sufficient precision.
At 6:27 PM EDT yesterday, mission controllers’ long patience finally paid off. After nearly two years, the DSN achieved a lock on the signal. STEREO-B has been found, and it is still alive! The team will now concentrate on conserving its resources and evaluating its health before deciding what to do next.