A long-lost, pioneering spaceship—still functional thanks only to chance and human error—is coming home for the first time in three decades. It wants to explore new worlds. But we've forgotten how to talk to it.
ISEE-3—short for International Sun-Earth Explorer—was launched in 1978, executing a pair of scientific missions and a groundbreaking in-space maneuver that's still used to this day. "It's definitely a special spacecraft in the history of planetary exploration," the Planetary Society's Emily Lakdawalla said. The satellite comes in at just over 850 pounds, but its contributions to science have been significant.
As it nears Earth—it will arrive in August—we'll have our best opportunity in 31 years to redirect it on another mission. But that won't happen. The spacecraft was long ago given up for dead, and as a result, the '70s-era technology we used to talk to it was scrapped. The Deep Space Network lost the proper transmitters in 1999.
That should have been the end of the story. When we stopped talking to the ISEE-3, it was supposed to stop talking to us. But in 2008, someone remembered they'd neglected to tell the ship to turn off its transmitters. A quick signal search revealed the craft was still in its precise orbit with 12 of 13 scientific instruments still operational.
"Ordinarily when NASA is done with a mission, they send a command to turn off the transmitter," Lakdawalla said. "This was kind of a mistake, the fact that this transmitter is still operating." More impressive is the fact that its 30-plus-year-old instruments still seem to be in working order. "As far as we know, this thing is working extremely well given its age—and it has a lot of useful scientific instruments." The problem is that we can't tell ISEE-3 to give us a complete picture of its health "because we can't currently speak its language."
If we could talk to ISEE-3, we could use its Earth approach this summer as a jumping-off point to send it comet-hunting (it became the first spacecraft to fly past a comet in 1985). It's equipped to measure plasmas, energetic particles, waves, and fields.
Some want to crack the history books and figure out how to rebuild the transmitter necessary to once again talk to our lonely spaceship. Why waste a resource that we've already put in space? But NASA says that would prove too expensive. "They thought it would [cost] so much that it wasn't worth spending the staff time to get the estimate," Lakdawalla said. "NASA has limited resources, and they have to focus on the ones that are producing the best science right now."
So ISEE-3's contributions will be limited to its past. They're not insignificant.
The ship's most notable breakthrough was using a Lagrangian point to change its trajectory, a move that had never been done before. What's a Lagrangian point? Lakdawalla compares it to the midpoint of a dumbbell, a spot in which gravitational pull is nearly balanced in both directions. Five such locations exist in the Earth and sun's orbital configuration, allowing spacecraft to "park" and conserve energy. ISEE-3 was the first to use L1, orbiting continuously between Earth and the sun to monitor solar wind.
In 1982, when ISEE-3 was reassigned to comet-hunting duty, it was able to use L1 to easily reposition itself into orbit around the sun. "If you navigate a spacecraft to one of those [Lagrangian] points, you get an opportunity to change the spacecraft's path a great deal with very little energy," Lakdawalla said. "This particular technique was pioneered by ISEE-3."
It studied Comet Giacobini-Zinner's plasma tail in 1985, then joined the "Comet Halley Armada" in 1986 when earthbound scientists became fixated on the approaching comet. It's remained in heliocentric orbit since then, gradually getting closer to Earth as it maintains a slightly faster orbit. It's completed 31 orbits of the sun during the same time Earth has completed 30.
Come August, ISEE-3 will finally near its terrestrial origins again, causing its orbit to change—but we won't be able to tell it where to go next.