Figure 1: ISEE-3 Post Flyby Trajectory (Courtesy of Mike Loucks www.see.com)

Since the lunar flyby on August 10th the ISEE-3 Reboot team has continued to work with Google Creative Lab to bring to full fruition the spacecraftforall.com website to provide real time data from ISEE-3. We have been working with the various dishes that have supported us until now, including Arecibo, Bochum, the SETI Institute, Morehead State and others. We now have a problem.

The ground stations listening to ISEE-3 have not been able to obtain a signal since Tuesday the 16th. Arecibo, Morehead, Bochum, SETI, as well as the Usuda 64 meter dish in Japan and the Algonquin 45 meter dish in Canada have all pointed at the spacecraft with no positive results. So, at this time we are assuming that the spacecraft has gone into safe mode.

What This Means

Safe mode on ISEE three can basically only occur from one problem, loss of power. Before the lunar flyby ISEE-3 orbited closer to the sun than the Earth. This resulted in a very good power profile for the spacecraft. However, as seen in the figure 1 here, since the flyby the spacecraft is traveling much farther away from the sun than it has been before:

We have not had many opportunities to get data from the spacecraft since the flyby as the antenna configuration has also been much worse from an attitude perspective. Also, we no longer have propellant to change the attitude of the spacecraft to improve this configuration. We can change the antenna pointing a bit but the first time we tried it, it did not work.

Next Steps

When ISEE-3 goes into safe mode it turns off all of the experiments and it turns off both transmitters and waits for help. Due to some uncertainty in the trajectory this may end up being a bit more of a problem than otherwise. We are working now to put together the commands to turn the transponders back on and obtain engineering telemetry. The last telemetry we have looked ok, but the spacecraft is still traveling farther away from the sun, and thus it is probable that last week the voltage on the power bus dropped enough to trigger the safe mode event. There is no functioning battery on the spacecraft now as it failed in 1981.

So, stay tuned for more information.

Dennis Wingo

ISEE-3 Project Co-Lead