NASA said that the Deep Space Network (DSN) established a lock on a carrier signal from the STEREO-B spacecraft at 6:27 p.m. Eastern Aug. 21. NASA monitored the signal for several hours, and plans additional contacts with the spacecraft to assess the state of the spacecraft.
The contact was the first time NASA had made contact with STEREO-B since Oct. 1, 2014. At that time ground controllers were testing the spacecraft’s “command loss timer,” which reboots the spacecraft’s computer after three days without contact with the ground, in anticipation of an extended communications outage. At the end of the test, controllers only received a brief, weak signal from the spacecraft before losing contact entirely.
In recent months, NASA had carried out regular efforts to restore contact with STEREO-B, transmitting instructions in the blind in the hopes the spacecraft would be able to receive them and respond. NASA started those efforts late last year once STEREO-B was far enough from the sun, as seen from the Earth, so that radio interference from the sun was no longer an issue.
The initial loss of contact was an unintended consequence of the spacecraft’s extended mission. STEREO-B is one of the twin Solar and Terrestrial Relations Observatory spacecraft, launched with STEREO-A in 2006. STEREO-B, or Behind, was placed in an orbit around the sun that caused it to gradually drift behind the Earth, while STEREO-A, or Ahead, was in a slightly closer orbit that allowed it to drift ahead.
The STEREO spacecraft were intended to operate for only two years, but NASA extended the mission several times given the performance of the spacecraft and the scientific benefit of seeing the sun from different angles. By 2014, the two STEREO spacecraft had drifted to the opposite side of the sun from the Earth, meaning that solar interference would cut off communications for several months.
Prior to the communications cutoff, controllers tested the command loss timers to ensure that they would work as planned, rebooting the spacecraft every three days. Tests with STEREO-A went as expected, but controllers lost contact with STEREO-B after it rebooted.
A subsequent analysis of the limited telemetry STEREO-B returned before losing communications indicated to engineers that the spacecraft lost attitude control. “The telemetry showed that the Inertial Measurement Unit, or IMU — which tells the spacecraft if and how fast it’s rotating — failed in a way we didn’t expect,” said Dan Ossing, STEREO mission operations manager, in a NASA statement last December. “Rather than cutting out altogether, it was feeding incorrect information into the guidance and control computer.”
That incorrect information, engineers believe, led the spacecraft to think it was spinning even though it was not. The spacecraft, in turn, fired thrusters to correct the perceived spin, which instead caused the spacecraft to spin up in reality. That would have limited the ability of the spacecraft’s solar panels to generate power, hindering recovery efforts.
It’s unclear if, or when, STEREO-B will be able to resume normal operations. STEREO-A, unaffected by the problem, has been working normally throughout its mission.