NASA Has Turned on a Deep Space GPS

Imagine heading out on a road trip, but instead of taking a GPS device with you, you phone a friend every hour to ask for directions. Now imagine doing that in the void of deep space.

In essence, that’s the strategy currently in use for spacecraft that travel far beyond Earth. These probes rely on radio instructions from ground stations, where large atomic clocks calculate ideal trajectories for their voyages. As a spacecraft wanders into deep space, it can take minutes or even hours to receive updated flight commands from Earth.

Scientists hopes to overcome this inefficient back-and-forth by miniaturizing atomic clocks so that they can fit on space probes. That would enable these robots—and perhaps eventually crewed spacecraft—to be autonomously guided to their destinations with a kind of on-board deep space GPS.

NASA took a major step toward this vision last week by switching on the Deep Space Atomic Clock (DSAC), a toaster-sized clock that was launched into Earth orbit in June. The DSAC is “the first timekeeper stable enough to map a spacecraft's trajectory in deep space while being small enough to fly onboard the spacecraft,” the agency said in a statement on Monday.

Atomic clocks are the most precise timekeeping devices ever invented. Built around the subtle signals emitted by electrons as they change energy levels, these clocks are typically the size of a refrigerator, making them far too big to be bundled into most space missions. (One exception is the constellation of large satellites that brings us GPS capabilities on Earth, which carries atomic clocks )

The DSAC is a mercury ion clock, which means it tells time based on the oscillations of mercury ions in an ultra-cold environment. The clock has been circling Earth for two months on a spacecraft called Orbital Test Bed 1, which contains four other experiments.

Now that it has been activated, scientists at NASA will monitor the clock’s performance in the space environment over the next year.

"The goal of the space experiment is to put the Deep Space Atomic Clock in the context of an operating spacecraft—complete with the things that affect the stability and accuracy of a clock—and see if it performs at the level we think it will: with orders of magnitude more stability than existing space clocks," said Todd Ely, principal investigator of the project at NASA, in a statement.

The DSAC is projected to lose just one second of time over a period of 10 million years, a level of precision that would enable spacecraft to find their way without the help of constant directions from Earth.

Not only will this allow robots to explore the far-flung reaches of the solar system, it will also ensure that astronauts on long duration space missions to Mars, or beyond, will have a reliable GPS system onboard with them.