We're all grateful for GPS. With the touch of a button, you can find out where you are. But what if your GPS was really slow? Every time you wanted to know your location, you'd have to stand still . . . and wait.
That's how spaceships work. In your car, your GPS listens for information from several global positioning satellites at once. These satellites are constantly sending out signals about where they are and what time it is. Based on that info , your receiver can quickly pinpoint where you're standing.
Spacecraft, however, have to rely on roundtrip information. Stations on Earth send up a signal. The waiting spaceship hears that signal and then shoots off a reply. Earth-bound scientists track the time between when they send a signal and when they get the rocket's response. That time, tracked by atomic clocks, reveals the craft's location, and allows the ground-based scientists to start steering.
If all that sounds like it might take a while, you're right. Roundtrip tracking at Mars, for instance, can take 40 minutes. For Jupiter it takes 90 minutes. That's why researchers at NASA recently launched the Deep Space Atomic Clock, or DSAC, on a test mission. A compact 35 pounds, about the size of a small microwave, DSAC should work like our usual GPS. It should listen fo the Earth signals without needing to send a response.
DSAC should allow spacecraft to chart their own trajectories, essentially becoming self-driving spaceships. It could even allow for GPS on other worlds. Satellites with DSAC orbiting Mars would let astronauts track their locations as they wander the red planet.
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