After two years of radio silence one of the Solar Terrestrial Relations Observatories, or the STEREO-B spacecraft, has made contact with NASA scientists.

STEREO-B is a probe that observes the sun along with sister craft STEREO-A, which were both launched in 2006. While STEREO-A orbited the sun just ahead of the Earth, STEREO-B orbited just behind. This allows them to observe solar phenomena from different angles, writes Rachel Feltman for The Washington Post.

NASA
NASA by MatHampson, Flickr

Probes exceed expectations after launch in 2006

However STEREO-B lost contact in October 2014. Scientists were still happy with the performance of the probe, given that it was only meant to remain in operation for two years. The STEREO probes are just two of a number of NASA spacecraft that have continued working long beyond their predicted shelf life.

Although radio silence is never welcome, NASA researchers never thought that STEREO-B was lost forever. The craft lost contact after scientists tested its “command loss timer,” a hard reset function that was designed to trigger if the craft didn’t make contact with Earth for 72 hours.

While the craft wasn’t designed to go incommunicado during its primary mission, it’s incredible longevity meant that STEREO-B started to follow a slightly different orbit. From this position there would always be a period of time when the sun would block communications from the probe.

Hard reset causes unexpected reaction from STEREO-B

“The sun emits strongly in nearly every wavelength, making it the biggest source of noise in the sky,” said Dan Ossing, mission operations manager for the STEREO mission at the Johns Hopkins University Applied Physics Laboratory, in 2015, when researchers were still waiting for the probe to make contact again. “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 reasons that remain unclear, STEREO-B didn’t handle the hard reset well at all. In fact the probe sent out a weak signal for one last time before falling quiet.

Chance for scientific research to continue

The fact that the hard reset test was undertaken before STEREO-B was blocked by the sun meant that the researchers had to wait for that period to end before trying to renew contact. At the same time the last signal received on Earth showed that the probe was drifting in space with incorrect data about where it was headed.

This meant that the researchers also had to wait until the solar panels on board the probe were in the right position to charge. Fortunately for the team that time has now elapsed. Some 22 months after losing touch, STEREO-B is back in contact and beaming information to the antennae system known as the Deep Space Network.

The next stage for the researchers is to carry out tests designed to work out whether STEREO-B can resume its scientific duties. STEREO-A is still working fine, awaiting further input from its sister probe, and hopefully STEREO-B will be back online soon. If it does re-enter service, researchers could start observing solar phenomena such as coronal mass injections from different angles. After a brief hiatus, STEREO-B could soon resume its scientific contributions.