Days ago, in a move that many people doubted was even possible, they made two-way contact with the almost-36-year-old spacecraft, and they’ve declared themselves “in control” of it as it rapidly approaches its home. The moment is undeniably cool: Using old-fashioned archival sleuth work, citizen scientists hack their way into an abandoned space object. The team, led by journalist Keith Cowing and space engineer Dennis Wingo, hope to make almost all the data from the ISEE-3 public, and to create a scientific community around the decades-old probe.

But the latest chapter in the life of the probe—usually referred to by its original name, the International Sun/Earth Explorer-3 (or ISEE-3)—is just beginning. Now that the reboot mission is in control, they must successfully navigate the craft around space hazards before they can normalize its operation. Once they figure out how to stabilize the probe, they'll focus on what they want to do with it.

Now that the reboot mission is in control of the ISEE-3, here’s what they must do to keep it alive.

A Fast-Rising Graph

By June 17, the reboot team needs to fire the probe’s rockets.

Engineers talk about the fuel available to spacecraft in terms of “delta-v,” or the amount of energy that needs to be expended for a spacecraft to change velocity. The team believes that ISEE-3 has about 150 meters per second of delta-v remaining: The probe can change its velocity a net 150 meters per second before it runs out of hydrazine to shoot out of its small jets.

As the ISEE-3 gets closer to the Earth and moon—and the two orbs’ gravitational pull strengthens—it will need more and more energy to change its orbital trajectory. The same amount of delta-v lets the spacecraft change its orbit less and less.

When I talked to Cowing, one of the directors of the reboot mission, he said that the graph that describes how much fuel the ISEE-3 would need to change direction resembles a world-population graph. There’s a long, slow upward increase followed by a huge up-swing (which, for world population, comes after the Industrial Revolution).

That inflection point—the moment when it takes huge amounts of fuel to make any tiny movement at all—arrives on June 17. After June 17, in other words, it will be hard to adjust the probe’s trajectory at all. And the ISEE-3 scientists still need to figure out how to tell the probe what to do, a task that—if possible—will require lots of tedious work. You can’t program a flight path into the ISEE-3 because it has no computer.

“Someone literally has to sit there and hit return for a period of time,” Cowing said, and tell the craft to have its tiny rockets fire “itty-bitty pulses.”

A Moonshot

August 10 might be the last day of the ISEE-3’s life. That’s the day it will make its lunar pass, and the ISEE-3 reboot team will gather at McMoons, an abandoned McDonald’s on the grounds of NASA’s Ames Research Center, to watch. (Many of the members of the ISEE-3 reboot team first worked together as part of the Lunar Orbiter Image Recovery Project, which digitized the film of five lunar orbiters from 1966 and 1967.)

According to Farquhar’s original programmed orbit, ISEE-3 will pass about 50 miles over the surface of the moon on August 10. The probe’s not quite where it was programmed to be, though, and—without changes to its trajectory—it might crash into the moon.