Loss of Contact with STEREO Behind
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
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
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
- 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
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,
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.
A simulation of one possible model of the events that led up to the loss of
contact with STEREO Behind is available here.