After being missing for two years on the far side of the sun, a lost spacecraft is sending messages to Earth again

John-Michael Schneider 08.23.2016


After floating aimlessly through the vacuum of space somewhere on the far side of the sun for nearly two years, a lost spacecraft is now finally sending messages back to Earth.

On Sunday evening, the NASA team responsible for the STEREO-B solar observatory announced that they were able to reestablish contact with the missing probe that was launched in 2006 to capture data from the sun. Using NASA’s Deep Space Network (DSN) — a global array of large antennae which tracks and communicates with space missions — the team finally heard a signal from the lost spacecraft after it first went silent on October 1, 2014.

“Over 22 months, the STEREO team has worked to attempt contact with the spacecraft,” NASA’s Karen Fox said on Monday.

“The DSN established a lock on the STEREO-B downlink carrier…the downlink signal was monitored by the Mission Operations team over several hours to characterize the attitude of the spacecraft,” she explained.

STEREO-B and twin spacecraft STEREO-A, which are together part of the Solar Terrestrial Relations Observatory, were launched in opposite directions from Earth in 2006. STEREO-A was sent ahead of Earth, while STEREO-B was propelled behind. Like many NASA missions, the flights were only supposed to last two years, but have gone on much longer than expected. Information from the two probes have helped scientists predict “space weather” events such as solar storms — bursts of solar radiation that can affect Earth’s magnetic fields.

Prior to the mission, astronomers could only observe one side of the sun at a given time. But on February 6, 2011, the two spacecraft were exactly 180 degrees apart from each other on opposite sides of the sun, allowing the entire star to be seen from 360 degrees at once for the first time.

But there was one problem: as the two spacecraft continued their orbit around the sun, they would eventually pass behind the sun opposite Earth — a time known as solar conjunction. Because the sun produces large amounts of radio interference, the NASA team knew that there would be a roughly 2-month window when communication with the spacecraft would be limited.

According to Joseph Gurman, an astrophysicist at NASA’s Goddard Space Flight Centre, the STEREO team programmed both satellites to enter reset and power back on after 72 hours without contact.

“After about three days, there’s a timer on the spacecraft will go off and put it into safe-mode. It will also turn off power to the instrument. That’s a period of about 15 weeks on the ahead spacecraft, and 9 weeks on the behind spacecraft,” he explained.

STEREO-A powered off and back on as expected. But with the sibling, STEREO-B, something went wrong. Initially, the reboot went according to plan, and NASA received a message after the first 72 hours.

It was the last time that the team would hear from the spacecraft, until recently.

While scientists are still uncertain about what went wrong, data sent back during STEREO-B’s first reboot gives a clue about the moments before the probe went missing.

“The telemetry showed that the Inertial Measurement Unit, which tells the spacecraft if and how fast it’s rotating, failed in a way we didn’t expect,” Dan Ossing, mission operations manager for the STEREO mission, said in a 2015 report. “Rather than cutting out altogether, it was feeding incorrect information into the guidance and control computer.”

In other words, STEREO-B was still in orbit, but the on-board computer thought it was spinning, and was telling the thrusters to adjust for rotation that didn’t even exist. The thruster corrections were a problem for the spacecraft because the outward facing solar panels, which have to face the sun consistently to keep the battery powered, were pointing away from their main energy source.

By late 2015, the NASA team had set aside several hours each week devoted to finding STEREO-B. Most of that time has been spent trying to reprogram the spacecraft to turn off any unnecessary power-usage, including the 72-hour reboot sequence.

Because STEREO-B is roughly 189-million miles away from Earth, it takes about 30 minutes for a signal to travel back and forth in each direction. To make locating the lost spacecraft less time-consuming, the STEREO researchers used a technique called constructive interference to amplify any faint signals that could be coming from so far away.

By combining signals from three 34-metre antennae as well as data from several of the largest radio telescope arrays in the world, including the Arecibo Observatory in Puerto Rico, the STEREO team waited for a signal from the missing satellite.

At 6:27 p.m. Eastern Time on Sunday, after over 700 days of trying to find the lost spacecraft, the technique paid off.

The team plans further recovery processes to assess the health of the probe. STEREO-A is working normally.

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