It's been a nail-biting few days for the planet-hunting community, after NASA announced late last week that its Kepler space telescope had unexpectedly switched to "emergency mode," and was rapidly burning up its fuel some 75 million miles from Earth.
On April 11, the space agency said that Kepler is now stable, and that the emergency has been lifted. NASA still hasn't clarified exactly what went wrong (although they can't blame the space debris orbiting Earth, as Kepler is just too far away), or whether the telescope can continue its mission.
Kepler's rescue is a testament to the Deep Space Network—a worldwide array of radio antennae that links us to spacecraft beyond the Moon.
In fact, Kepler probably couldn't have been recovered without the Deep Space Network. It's the biggest and most sensitive scientific telecommunications system in the world, according to NASA, and even though lots of people have never heard of it, it's our information highway into space.
"Everything that happens beyond the moon comes through the Deep Space Network," planetary scientist Emily Lakdawalla, senior editor at the non-profit Planetary Society, told Motherboard. Those high-res photos of Pluto, Curiosity's Martian landscapes, Cassini's views of Saturn's moons—it all beamed through space to Earth over the network, and it was that same network that Kepler scientists had to use to figure out what was wrong with their telescope, and get it under control.
More than half a century old, the Deep Space Network consists of three main ground stations, in California, Spain, and Australia. They're each about 120 degrees in longitude apart, "which means that NASA is capable of talking with its spacecraft 24 hours a day, around the clock," Lakdawalla said.
Missions have to compete for access to the network, which allows scientists to talk to distant spacecraft. That made Kepler's rescue even more urgent. As its team, based at the NASA Ames Research Center in California, had to race to regain control, they must have felt extra pressure knowing that other missions were giving over valuable time. As a result of the recovery, others—probably Cassini or New Horizons, Lakdawalla said—could have missed out. A small amount of non-crucial data might even have been lost.
"New Horizons' data downlink is super slow," Lakdawalla said, partly because the spacecraft is so far away. (Kepler is much closer than New Horizons, but it would take a signal a full 13 minutes to reach the telescope and come back to Earth, according to NASA.)
"It takes them five hours to downlink a single image." If they did have to give time over to Kepler, it will take even longer to bring home data.
And Cassini might have lost some non-essential data, Lakdawana said. It's designed to fill up its recorders and "squeeze as much down to Earth as it can," which means that some of the lowest-priority data isn't transmitted home by design. That's not usually a big deal, although sometimes, it's in the low-priority stuff where scientists strike gold.
After Kepler's state of emergency was lifted, mission manager Charles Sobeck expressed his gratitude to the Deep Space Network, and "to other missions that surrendered their scheduled telemetry links."
The team seems hopeful that Kepler will be able to continue with the next phase of its mission, which is designed to use a planet-hunting technique called gravitational microlensing.
To Lakdawalla, the space telescope's recovery goes to show how much we rely on the Deep Space Network. "It's one of those things [people] take for granted, although it's absolutely crucial to every aspect of deep space exploration."
Without it, we might never have seen Pluto or Mars—or recovered Kepler, which has already told scientists a lot about how abundant planets are throughout our universe.