Communications with the 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.
Here we explain the events that led up to the loss in communications, and the activities that have been carried out in an attempt to recover the spacecraft. As explained below, 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. (See this page for more information.) 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 we have is from the limited telemetry extracted from the radio signal just before and just after the reset. Here is what is known so far:
- The telemetry from just before the reset shows no problems with the spacecraft.
- Only three packets could be successfully recovered from the very faint telemetry received just after the reset. These data show that the Inertial Measurement Unit (IMU) was powered on. The IMU uses ring laser gyros to measure the rotational rates and orientation of the spacecraft. At this stage of the mission, it’s normally powered off. The IMU being powered on indicates that the Star Tracker, which normally provides this information, had not acquired a set of guide stars in a predetermined length of time.
- This is not unexpected—there have been other occasions when it took the Star Tracker several minutes, or even a few days, to start determining the spacecraft orientation based on star images. In fact, on September 28, as part of the same test sequence, the spacecraft was reset, and it took 12 minutes for the Star Tracker to start providing an attitude solution. When the Star Tracker is offline, the spacecraft will automatically turn on the IMU to provide rotational rate information.
- The lack of data from the Star Tracker would explain why the radio signal after the reset was initially very weak and then faded away. Without the Star Tracker, the high gain antenna cannot be properly pointed at Earth, resulting in the initial faint signal, which then drifted even further so that the signal was lost completely.
- However, the limited telemetry received after the spacecraft reset also indicates that one of the laser gyros in the IMU had failed, and was providing bad data to the attitude control system. Thus, two simultaneous failures had occured in the attitude control system—the Star Tracker and the IMU—and the ability of the spacecraft’s guidance and control electronics to cope with multiple failures is limited.
With only that limited amount of information, it’s unclear what happened next. If the spacecraft managed to recognize that the IMU had an anomaly, it would have disabled it, and fallen back on the only system left for determining orientation, the solar aspect sensors. There are five solar aspect sensors on the spacecraft, among them covering the full sky. With just these sensors operating, the Behind spacecraft should at least have kept itself well enough pointed to keep the solar panels pointed at the Sun, and maintain power on the spacecraft.
However, if the spacecraft did not detect that the IMU was providing bad data, it may have fired its thrusters to stop the roll that it thought it was in. With bad data coming from the IMU, instead of stabilizing the spacecraft, this would have sent it into a spin about its principal axis of inertia. The solar panels may have then stopped getting enough sunlight to keep the spacecraft powered, draining the batteries and shutting the spacecraft down.
Attempts to reestablish communications with the STEREO Behind spacecraft are ongoing. Since the loss of communications on October 1, the operations team has been broadcasting commands designed to correct a number of possible scenarios as to why no communications have been heard. Some of these commands are intended to aid the Star Tracker come back online. Other commands are intended to reactivate the spacecraft transmitter, in case it somehow got deactivated. Commands have also been broadcast to disable the failed IMU. So far, none of these activities have been successful. Attempts were also made to detect the signal from Behind with the Green Bank 100 meter radio telescope, without success.
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. STEREO Ahead continues to operate nominally, and is currently providing our only views of the far side of the Sun.