Along with its sister STEREO-A, the prodigal probe is tasked with observing the sun. When the two were launched back in 2006, the STEREO spacecraft entered an orbit around the sun just ahead of and just behind the Earth, allowing them to observe solar phenomena such as coronal mass ejections from multiple angles.
During a coronal mass ejection, the sun expels tremendous ropes of plasma and magnetic field into space at speeds of around 200 miles per second (and sometimes nearly 10 times faster than that). When these high-energy particles interact with the Earth's magnetic sphere, the clash can cause major problems on the ground. A coronal mass ejection doesn't directly threaten human health, but these stunning solar storms have been known to knock out electrical grids, damage satellites and shut down communication systems. The unique perspective of the STEREO spacecraft allowed them to created 3D images of these ejections, which has helped scientists understand and predict them more accurately.
When STEREO-B went quiet in October 2014, it was hardly considered a failure: Its mission was only supposed to last for two years. Like many NASA spacecraft, the STEREO probes have long exceeded their shelf life.
Radio silence from a spacecraft is never fun, but the mission team never had reason to think that the spacecraft was gone for good. STEREO-B went silent just after mission control tested its "command loss timer," a hard reset that would be automatically triggered if STEREO-B ever stopped communicating with Earth for 72 hours. During STEREO-B's primary mission, the spacecraft was never meant to go incommunicado. But because it lived so long, it drifted into a slightly different orbit, setting itself up to pass behind the sun. The mission team was just trying to be prepared for a period when the sun would get in their way.
"The sun emits strongly in nearly every wavelength, making it the biggest source of noise in the sky," Dan Ossing, mission operations manager for the STEREO mission at the Johns Hopkins University Applied Physics Laboratory, said in a statement when his team began waiting for STEREO-B's re-emergence in late 2015. "Most deep space missions only have to deal with sun interference for a day or so, but for each of the STEREO spacecraft, this period lasted nearly four months."
"We had to take a spacecraft that was meant to talk to Earth every day and get it ready for over three months of radio silence," Ossing said.
For some reason - the team still isn't sure exactly why - the spacecraft reacted poorly to the hard reset that its sister probe breezed right through. STEREO-B let out a single weak signal, then went quiet.
Because of the timing of the test, done in anticipation of the period when STEREO-B's communications would be blocked by the sun anyway, the team was left waiting for a window when they could hope to reach STEREO-B once again. Meanwhile, their analysis of that last, weakened signal suggested that the spacecraft had sent itself spinning, drifting in space with incorrect information about what direction it was pointed in. So in addition to waiting for STEREO-B to come back into range, the team had to wait for the spacecraft's uncontrollable trajectory to place its solar panels in the right position to charge.
It sounds like a long shot, but they had reason to hope. After all, it had been done before. The Solar and Heliospheric Observatory (SOHO) lost contact with NASA when it spun out of its proper position in 1998, but eventually received enough solar power for its mission team to take control and set it straight. SOHO still provides valuable weather data today. With this precedent in mind, the STEREO team spent months sending signals meant to correct STEREO-B's position in the hopes of catching it with enough power to respond.
Now, 22 months after the silent treatment began, STEREO-B is back in communication with the Deep Space Network, the antennae system that NASA uses to pick up signals from its missions. After periodic check-ins from the team ever since December, the DSN finally locked on to STEREO-B's signal at 6:27 p.m. EDT on Sunday. Further communications will be required to determine the health of the spacecraft and whether it's capable of resuming its scientific duties with STEREO-A, which continues to function normally.