After 22 months of failed communication attempts, NASA's Deep Space Network has finally reestablished communication with STEREO-B solar observatory. The STEREO Missions Operations team plans further recovery processes to assess observatory health, re-establish attitude control, and evaluate all subsystems and instruments.
Communications with the STEREO-B were lost on October 1, 2014, during a test of the spacecraft’s command loss timer, a hard reset that is triggered after the spacecraft goes without communications from Earth for 72 hours. The STEREO team was testing this function in preparation for something known as the solar conjunction when STEREO-B’s line of sight to Earth – and therefore all communication – was blocked by the Sun.
Because both STEREO-A (Ahead) and B (Behind) spacecraft are now on the far side of the Sun, new modes of operations needed to be developed to protect the High Gain Antenna from the elevated temperatures caused by pointing near the Sun, and also to put the spacecraft into an autonomous safe mode for the period when solar radio interference will prevent communication.
The location of STEREO A and B spacecraft on August 23, 2016. Credit: NASA/STEREO
Collectively, these new modes are known as solar conjunction operations. A series of tests were conducted to make sure that the spacecraft perform as planned with these new operations modes. The first spacecraft to be affected was STEREO Ahead, and these tests were all carried out successfully on that spacecraft. On August 20, 2014, Ahead entered the first phase of the solar conjunction period and resumed all science operations in November 2015.
On September 27, 2014, the STEREO Behind spacecraft began one of the test sequences that had already been carried out on Ahead. This sequence was designed to test putting the Behind spacecraft into the safe mode that it will be in during the solar radio interference period, and then to bring it back out again into normal operations. One part of this test was to observe the firing of the spacecraft hard command loss timer, which resets the spacecraft if no commands are received after three days.
On October 1, 2014, the hard command loss timer on the spacecraft was observed to fire at the expected time, causing the spacecraft to reset. However, the radio signal observed on the ground immediately after the reset was very weak, and then quickly faded away. That was the last signal received from the Behind spacecraft until August 21, 2016.
Over 22 months, the STEREO team has worked to attempt contact with the spacecraft. Most recently, they have attempted a monthly recovery operation using NASA's Deep Space Network, or DSN, which tracks and communicates with missions throughout space.
The DSN established a lock on the STEREO-B downlink carrier at 22:27 UTC on Sunday, August 21, NASA said in a statement yesterday. The downlink signal was monitored by the Mission Operations team over several hours to characterize the attitude of the spacecraft and then transmitter's 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.
Meanwhile, STEREO-A continues to work normally. The spacecraft has resumed its science operations and began transmitting data at its full rate on November 17, 2015.
Featured image credit: NASA