NASA may have less than 2 minutes to rescue its long-lost spacecraft
The long silence ended on Sunday, when the probe - some 189 million miles away from Earth - finally said "hello" again to the space agency.
However, scientists in charge of the STEREO sun-observing mission are not celebrating just yet.
"The very hard and scary work is just beginning," Joe Gurman, a STEREO project scientist, told Business Insider.
"This spacecraft was designed to be as autonomous as possible when it ran into trouble," Gurman said. "If we turn on the computer, which is the only way we can get insight into what is wrong with the spacecraft ... what got us into this mess in the first place could turn back on again."
That means that even though NASA has re-established contact, scientists will have only about two minutes to rescue STEREO-B before they lose the spacecraft to the void all over again.
A race against time
NASA launched two STEREO spacecraft in 2006 to monitor our backyard star from all angles.
One is named "A" for "ahead," and the other is named "B" for "behind," since their Earth-like orbits are slightly staggered.
Though the $550 million mission was supposed to wrap up in 2008, it was a big success and NASA kept it going.
But trouble struck on Oct. 1, 2014, when STEREO-B went into a hard reset and lost touch with Earth.
As NASA explained in a December 2015 article about the recovery effort, the space agency was preparing STEREO-B for a 3-month period when it'd slip behind the sun and lose contact. (The sun is "the biggest source of noise in the sky," Dan Ossing, STEREO's mission operations manager at the Johns Hopkins University Applied Physics Laboratory, told NASA.)
The 72-hour and 20-minute test succeeded. But then STEREO-B stopped talking.
By poring over the last bits of data STEREO-B sent out, technicians learned that the instrument which tells the spacecraft whether it's rotating or not, and how fast, had failed. That's a big problem for a robot that needs to aim its solar panels at the sun - without that instrument, it won't point in the right direction, and its batteries will drain.
NASA tried every week, and then later every month, to use the Deep Space Network to regain contact over the course of 22 months. It finally succeeded on Aug. 21, 2016, after the sun had trickle-charged the probe from a glancing angle.
The challenge now, said Gurman, is to upload some kind of fix to STEREO-B before it drains its feebly charged batteries.
"We have something like 2 minutes between when STEREO-B receives the command to boot up one of its computers and when it starts doing what we don't want it to do," Gurman said.
Making matters worse, it takes about 20 seconds to send commands to the spacecraft, at a data rate that makes a dial-up modem seem lightning-fast.
As a result, Gurman said engineers are taking their time to hammer out a set of brief rescue instructions while they know the spacecraft is still responsive.
If their efforts to point STEREO-B's solar panels back toward the sun fail, he said they'll have another chance to send instructions in six months: when the spacecraft has partially charged up its batteries again by slowly orbiting the sun.
"This is the third solar probe I've had to rescue in three decades," Gurman said, noting his involvement in the Solar Maximum and SOHO missions. "I could do with a little less excitement."