STEREO Behind Loss of Contact Simulation

The following movie illustrates one possible model of the loss of contact with the STEREO Behind spacecraft on October 1, 2014. Since only three packets were received after the planned spacecraft reset, we have very little knowledge of what actually happened on the spacecraft. Instead, we have to make deductions of what might have happened based on the little information that we have, combined with knowledge of how the spacecraft is expected to behave under various conditions.

The movie below is a simulation of a possible series of events that might have led to the loss of contact with Behind. A lot of assumptions and guesses go into this model, and the movie is only intended to give a sense of how the spacecraft reacted to various events. Even if the basic timeline of events is correct, the exact details are still subject to some variation. For example, the final direction of tumbling at the end of movie may not match the real tumbling mode of the spacecraft.

One should also be aware that the time stamps in the simulation are not the same as the time of the actual events on the spacecraft. In the discussion below, both the real times and the "movie times" will be given.

The movie consists of three panels. The two smaller panels on the left show the orientation of Ahead (upper left) and Behind (lower left) as seen from Earth. The much larger panel on the right shows the orientation of Behind as seen from the Sun, which is important for knowing how the solar panels are being illuminated. (Since both spacecraft are well on the far side of the Sun at this point, the Earth and Sun views don't actually differ very much.) Red arrows for Ahead and blue arrows for Behind show the directions of the spacecraft body X, Y, and Z axes, as well as the -Z axes for Behind as a purple arrow. Blue cones show the direction of the main lobe of the radio signal from the high gain antenna.

The timeline of events leading up to the loss of contact with Behind starts with the spacecraft in it solar conjunction "safe mode", with the spacecraft slowly rolling about the direction toward the Sun (the X axis). At 17:49 UT, the spacecraft was commanded to reset as part of the processing of coming out of "safe mode" back into normal operating mode. Except for just three packets a few minutes later, this was the last we heard from the spacecraft. The movie actually starts one minute after the reset, at 17:50 UT, which is labelled 15:30 UT in the movie.

At 17:53 the Inertial Measurement Unit (IMU) failed, providing false data to the spacecraft. We know this from the three packets that were received a few minutes later. This event shows up in the movie at the 15:33 timestamp with the spacecraft starting to roll and slowly drift in pointing. The spacecraft is getting bad data about its rotation rate from the IMU, and trying to correct for it actually makes things worse.

The next event which is expected to occur is an autonomous momentum dump at 18:14 UT (15:54 in the movie). We don't actually know that this happened, but this is what is expected to have happened if the spacecraft was still trying to respond to bad data from the IMU. In this scenario, the spacecraft would fire its thrusters to try to stop it from rotating. However, because the data it was getting from the IMU about its rotation rate was wrong, this thruster firing would have simply sped up its rotation even more. Onboard parameters in the spacecraft guidance and control system would have limited the thruster firing to seven minutes, ending at 18:21 UT (16:01 in the movie), but by that time the spacecraft would most likely be rotating very rapidly. Eventually, because the solar panels would be getting very little illumination to keep the spacecraft powered up, the backup battery would slowly drain to the point where the spacecraft shuts down. Depending on exactly how the spacecraft is tumbling (which may not be quite the same as in the movie) there can be times when the solar panels recieves enough sunlight for the battery to recharge to the point where the spacecraft can start powering back up for a short while before the battery is drained again. Attempts to recover Behind are based on this possibility. The amount of sunlight that the panels receive may also depend on where it is in its orbit, so that recovery may need to wait for the right time of year, though this is only a conjecture at this time.

STEREO Behind loss of contact simulation movie (MP4 format, caution: 51 MB).