NASA just found a spacecraft that’s been lost for two years

By Rachel Feltman, The Washington Post

NASA is reporting that one of the Solar Terrestrial Relations Observatories, known as the STEREO-B spacecraft, has made contact after two years of silence.

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.

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.

And the silence wasn’t likely to be a sign of a total system failure, either – mission control had been testing the spacecraft’s command loss timer, a hard reset that would be triggered if STEREO-B ever stopped communicating with Earth for 72 hours. Ironically, this reset resulted in a weak signal followed by dead air. And the timing of the test – right before STEREO-B’s solar conjunction, when its signals would be blocked by the sun – meant that NASA couldn’t be sure for months whether the spacecraft was still healthy.

“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 reemergence 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.”

STEREO wasn’t designed for such a long estrangement – its period of solar interference was the result of its orbit, which has drifted out of place during its many years of operation.

“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.

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. The DSN 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.