Communications with NASA's STEREO Behind spacecraft were interrupted on October 1, 2014 immediately after a planned reset of the spacecraft performed as part of a test of solar conjunction operations. There have been no successful communications since then, though attempts to recover the spacecraft continue. The spacecraft telemetry indicated an anomaly in the guidance and control system, but this is still being evaluated. Initial indications are that a series of problems in the guidance and control system led to the anomaly. Because both STEREO 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.


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 conjuction period, which will continue in its various phases until early 2016.


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. The purpose of this is to correct any problems that might be preventing the spacecraft from receiving commands from the ground. While the spacecraft is out of contact on the far side of the Sun, this reset will occur every 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. The only information that NASA has is from the limited telemetry extracted from the radio signal just before and just after the reset.


Actual solar conjunction operations will take place between March 22 and July 14, 2015. STEREO Ahead continues to operate nominally, and is currently providing our only views of the far side of the sun.


Over the past few weeks, astronomers have been borrowing the giant, 100-meter-wide Green Bank Telescope to try to detect the spacecraft's radio signal, so far without success. The 70-m dishes of NASA's Deep Space Network are also trying to reestablish contact.


Even if the STEREO Behind spacecraft is currently drifting without power, not all hope is lost. A similar situation occured with the SOHO mission in 1998, where the spacecraft ended up in spin with the solar panels pointed away from the Sun. As SOHO continued to move along in its orbit, it eventually got to a point where sunlight started falling on the solar panels again. The operations team (with a considerable amount of effort) was able to recover the spacecraft into operating mode. Now, sixteen years later, SOHO is still providing valuable data to the solar and space weather communities.


Once communications are restored and the anomaly resolved, the operational plan for exiting the solar conjunction testing will continue to return the Behind observatory back to nominal science data collection as soon as safely possible.


Launched in 2006, the STEREO mission achieved its prime science goals within the two-year prime mission, but continues to explore solar and heliospheric activity through the current solar maxmimum and beyond.