NASA ‘hears’ from lost spacecraft after nearly two years


Credit: NASA
Credit: NASA

NASA re-established contact with a wayward sun-watching science satellite Sunday nearly two years after the spacecraft suddenly dropped off line during a test, the agency said in a statement Monday.

NASA’s Deep Space Network, or DSN, “established a lock on the STEREO-B (spacecraft’s) downlink carrier at 6:27 p.m. EDT,” NASA said in a statement. “The downlink signal was monitored by the Mission Operations team over several hours to characterize the attitude of the spacecraft and then transmitter high voltage was powered down to save battery power.

“The STEREO Missions Operations team plans further recovery processes to assess observatory health, re-establish attitude control and evaluate all subsystems and instruments.”

Launched in 2006, the STEREO mission featured two spacecraft — STEREO-A and STEREO-B — designed to monitor solar activity from different locations, one “ahead” in its orbit and one “behind,” allowing scientists to see the entire star, not just the side facing Earth. The spacecraft were built and are managed by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

Years beyond the original two-year mission duration, the spacecraft reached positions in their orbit relative to the sun and Earth where they would be on the far side of the star and out of direct contact with Earth for up to three months.

Both spacecraft featured a “command loss timer” designed to force the flight computer to reboot if it didn’t hear from Earth over a three-day period. On the far side of the sun, the command loss timer would trigger repeated computer resets during the three months the spacecraft were out of contact with Earth.

This graphic shows the positions of the two STEREO spacecraft and their orbits in relation to Earth, Venus, Mercury and the sun. Credit: NASA
This graphic shows the positions of the two STEREO spacecraft and their orbits in relation to Earth, Venus, Mercury and the sun.
Credit: NASA

To make sure the system was working properly, flight controllers tested both spacecraft by withholding commands and forcing the command loss time to trigger a reboot. The STEREO-A spacecraft responded normally. But STEREO-B had major problems.

Based on fragmentary data received before contact was lost, engineers concluded the timer triggered a restart as expected, but STEREO-B’s inertial measurement unit and star trackers, which tell the flight computer how the spacecraft is oriented and moving through space, suffered malfunctions.

“The bad IMU told STEREO-B that it was spinning, even though it was stationary,” Dan Ossing, the mission operations manager, said last December. “The spacecraft would have automatically taken steps to correct the supposed spin.”

But firing thrusters or adjusting spinning reaction wheels to counteract a non-existent spin would cause a stabilized spacecraft to do the opposite, imparting an unwanted spin. Engineers believe STEREO-B is doing just that, limiting the amount of sunlight that falls on its solar arrays.

That, in turn, probably prevents the on-board battery from fully charging and during multiple computer resets, the battery likely is exhausted before enough power is available for the spacecraft’s radio transmitter, NASA said on the mission website.

Using the DSN, flight controllers began sending commands “in the blind” last year, telling the spacecraft to turn off some of the systems that power up automatically after a reboot in a bid to improve battery charging. Additional commands were sent to turn on STEREO-B’s transmitter when enough power was available.

The work apparently paid off Sunday when the DSN picked up a “carrier” signal from STEREO-B.