An illustration of ISEE-3 and the Giacobini-Zinner comet (Image: NASA Goddard Spaceflight Center)

On September 11, 1985, a NASA satellite dubbed International Sun-Earth Explorer 3 (ISEE-3) made history when it became the first spacecraft to intercept a comet. But that wasn't its intended mission. ISEE-3 was stationed between the Earth and the sun, and was supposed to study phenomena like solar winds — that is, until a NASA mission design specialist named Robert Farquhar "borrowed" it to chase the Giacobini-Zinner comet, and sent it on a long journey to the far side of the sun.

Now, as NPR reports, the satellite is heading back toward Earth, and Farquhar is hoping to knock it into its old orbit so it can get back to work.

Farquhar first got the idea of chasing the comet after NASA announced that it wasn't joining the European Space Agency and others in the race to attempt a flyby of the more famous Halley's comet — since the mission was too expensive.

So Farquar, a specialist in devising unusual orbits, decided instead to divert ISEE-3, which met up with the Giacobini-Zinner comet nearly six months before the other agencies reached Halley's comet. For his efforts, Farquar was lauded at the time by no less than president Ronald Reagan, who sent him a congratulatory letter. 

But not everyone was so happy, including the scientists who were using the data from ISEE-3.

"They thought that — it was in the newspapers, even — that we stole their spacecraft," Farquhar told NPR. "We didn't steal it; we just borrowed it for a while! That's what I tried to tell them."

The catch was: "a while" was about 31 years. Now that ISEE-3 is heading back toward Earth, Farquar, now 81, and others are trying to figure out how to get it to fire its thrusters and return to the spot it used to occupy between our planet and the sun.

"It's not often that something that you've sent off supposedly into oblivion sort of comes back to you," Daniel Baker, director of the Laboratory for Atmospheric and Space Physics at the University of Color, told NPR. "It really, to me, is a fascinating thing that we can even dream of reassembling the puzzle here and put it back the way, sort of, it was — before Bob stole the spacecraft." Baker said he believes ISEE-3 could still provide useful readings if it were returned to its old spot.

But it won't be so easy to make that happen: NASA scientists aren't even sure how they might communicate with the aging spacecraft, since much of the equipment from that era is no longer around. The agency, meanwhile, hasn't given a final ruling on whether it will proceed with the effort.

If it does, it'll have to move fast: ISEE-3 will approach the Earth and moon in August, and a command will have to be sent by May or June to change its flight path. After that, according to one researcher, it may be another 200 years before it comes close enough again.

Via NPR