I've periodically reported on the status of the International Sun-Earth Explorer (ISEE-3), a spacecraft that was launched in 1978 to study Earth's magnetosphere and repurposed in 1983 to study two comets. Renamed the International Cometary Explorer (ICE), it has been in a heliocentric orbit since then, traveling just slightly faster than Earth. It's finally catching up to us from behind, and will return to Earth in August. It's still functioning, broadcasting a carrier signal that the Deep Space Network successfully detected in 2008. Twelve of its 13 instruments were working when we last checked on its condition, sometime prior to 1999.
When I last reported on ISEE-3, I wrote:
A big question is whether we even still know how to communicate with the spacecraft. It was built in the 1970s, at the same time as the Voyagers. But we've been in continuous communication with the Voyagers since their launch; the same isn't true of ICE. So the first step is for a team at Goddard Space Flight Center to research that question. Can we figure out how to talk to ICE? What will those communications cost?
It's with great sadness that I report today that the Goddard Space Flight Center team has determined that we cannot, in fact, communicate with this spacecraft. Two days, ago, the following was posted on the ISEE3returns Facebook page:
Communication involves speaking, listening and understanding what we hear. One of the main technical challenges the ISEE-3/ICE project has faced is determining whether we can speak, listen, and understand the spacecraft and whether the spacecraft can do the same for us. Several months of digging through old technical documents has led a group of NASA engineers to believe they will indeed be able to understand the stream of data coming from the spacecraft. NASA's Deep Space Network (DSN) can listen to the spacecraft, a test in 2008 proved that it was possible to pick up the transmitter carrier signal, but can we speak to the spacecraft? Can we tell the spacecraft to turn back on its thrusters and science instruments after decades of silence and perform the intricate ballet needed to send it back to where it can again monitor the Sun? The answer to that question appears to be no.
The transmitters of the Deep Space Network, the hardware to send signals out to the fleet of NASA spacecraft in deep space, no longer includes the equipment needed to talk to ISEE-3. These old-fashioned transmitters were removed in 1999. Could new transmitters be built? Yes, but it would be at a price no one is willing to spend. And we need to use the DSN because no other network of antennas in the US has the sensitivity to detect and transmit signals to the spacecraft at such a distance.
This effort has always been risky with a low probability of success and a near-zero budget. It is thanks to a small and dedicated group of scientists and engineers that we were able to get as far as we have. Thank you all very much.
I followed up with Leonard Garcia, who has been one of the leaders of the attempt to regain control of ISEE-3, to ask what, in fact, the cost was. He told me that when the Deep Space Network realized what was going to be involved in regaining this capability, they did not even proceed as far as developing a cost estimate; "they decided this wasn't going to be possible."
How could this happen? Well, the fact that ISEE-3 is still broadcasting a carrier signal was actually an error; it should have been shut down. If they had planned for it to still be functioning at this point, they would have maintained the capability to communicate with it. I don't comprehend the intricacies of deep-space communications well enough to understand the obstacles here, and I don't question their conclusion, but that doesn't make me any less sad.
So ISEE-3 will pass by us, ready to talk with us, but in the 30 years since it departed Earth we've lost the ability to speak its language. I wonder if ham radio operators will be able to pick up its carrier signal -- it's meaningless, I guess, but it feels like an honorable thing to do, a kind of salute to the venerable ship as it passes by.