News of ISEE-3’s demise may be a bit premature. A months-long bid to bring the 35-year-old International Sun-Earth Explorer 3 back into the vicinity of Earth seemed to have met its end this week, when the citizen group attempting to “reboot” the mission failed to get the thrusters to produce much more than a burp.
Tests done on Wednesday during a communications session with the Arecibo telescope seemed to suggest that the culprit was a lack of “pressurant." The spacecraft appears to have run out of the nitrogen gas it used to force hydrazine fuel from the tanks and into the thrusters.
But the reboot team, led by editor Keith Cowing and entrepreneur Dennis Wingo, CEO of California-based Skycorp Incorporated, isn’t quite ready to give up. One of the project volunteers has suggested that perhaps the nitrogen isn’t actually gone. It may in fact still be there, but dissolved in with the hydrazine.
If that’s the case, Wingo says, ISEE-3 could potentially repressurize the propellant by powering up the tank heaters, raising the temperature up perhaps 10 degrees from the roughly 25 °C where it stands now. "If [the idea] has any merit, then we could turn the heaters on and drive at least some of the nitrogen out of solution. That would give us more pressure that just heating the tanks themselves," Wingo says. "It’s not desperation," he adds. "There is some good physics behind this."
Wingo says more research will be needed to understand whether this is a possibility. Some aspects of the spacecraft design, for example, are still unclear, such as whether the nitrogen and hydrazine are actually stored together or separately in the tanks. The team is asking volunteers for help researching the problem. "The bottom line is we haven't given up hope yet," Wingo says.
ISEE-3, which launched in 1978, was redirected from its original orbit near Earth in order to fly by comets Giacobini-Zinner and Halley (see Nell Greenfieldboyce's excellent story at NPR for the background). The spacecraft’s thrusters were last fired in 1987, a maneuver designed to ensure the spacecraft would come close to Earth this year. But this long homecoming almost went unmarked. NASA no longer had the capability to communicate with ISEE-3 and had no plans to try. So Cowing and Wingo stepped in, raising nearly $160,000 on the crowdfunding site RocketHub to finance the effort and replicating much of what was originally hardware with software-defined radio.
The ISEE-3 team established 2-way communication with the spacecraft in May and pushed to get ready for a set of thruster firings that would put the spacecraft back in an orbit that would keep pace with the Earth.
Everything seemed to be going well last week, when the team managed to fire a pair of thrusters to spin up the spacecraft quite close to its original mission-specified rotation rate of 19.75 rotations per minute.
On Tuesday, the team attempted to fire the spacecraft’s main thrusters to adjust the spacecraft’s orbit. The first set of pulses was partially successful, but the next two failed.
The next day, the team set out to troubleshoot what might be wrong. Wingo says it took the team about an hour and a half to make its way through the fault tree, working through all the different permutations of tanks and latch valves and thrusters. They confirmed that the latch valves, which sit between the tanks and the thrusters, worked, as expected; they vibrate when opened, and the shudder can be picked up by the spacecraft’s accelerometers.
In the end, the team concluded that the likeliest explanation was that the nitrogen pressurant had leaked away in the 27 years since the thrusters were last fired. ISEE-3’s successful firings could be explained by a small bit of pressurized fuel still left in the lines.
Wingo says the team should be able to test this new hypothesis by commanding the spacecraft to turn on its tank heaters next week.
Time will be of the essence. ISEE-3 is set to make a flyby of the moon on 10 August. As a matter of trigonometry, the earlier the spacecraft can adjust its trajectory, the less fuel it will need to do so.
Should the team not be able to redirect ISEE-3, Wingo says the spacecraft should still be able to return useful science data. Signals from the spacecraft should be relatively easy to receive for the next three months. After that, picking up those signals will likely have to be done intermittently, since it will require large antennas that are expensive to reserve.
Rachel Courtland can be found on Twitter at @rcourt.