Have you ever wondered how satellites stay on course when they're millions of miles from Earth? Recently, a listener named Don reached out to us with that exact question.
It turns out the answer is simpler — and more fascinating — than you might expect.
To help us understand the mechanics of deep-space navigation, NHPR's Patrick McNameeKing sat down with John Gianforte from the University of New Hampshire.
When a spacecraft needs to know its orientation, it doesn’t rely on a terrestrial GPS signal or a compass. Instead, it looks to the oldest navigation system in history: the stars.
The primary tool used for this purpose is called a star tracker. Think of these as high-precision cameras mounted to the chassis of the spacecraft.
“A star tracker will take a picture of a particular place in space and match that to a memorized model of the sky," says Gianforte.
By comparing the patterns of stars it sees against an onboard database of the celestial sphere, the spacecraft can calculate its exact attitude and orientation. This allows the satellite to keep its antenna pointed at Earth, its solar panels facing the sun, and its scientific instruments locked onto their targets.
How accurate is this system?
To illustrate, Gianforte points to the NASA mission to Saturn, Cassini.
Shortly after arriving at the gas giant, Cassini detected massive geysers erupting from the south pole of Enceladus, one of Saturn’s small moons. Scientists made the bold decision to fly the spacecraft directly through those icy plumes to analyze their composition.
The challenge was astronomical in every sense of the word.
"You got to know exactly where your spacecraft is," Gianforte says. "If you want to fly through a plume that's 125 miles high, emanating from a moon that's 300 miles across, orbiting a planet that's orbiting the sun at a billion miles away. So that was really, really challenging and trying for that star tracker."
Despite the immense difficulty, the star tracker system succeeded. Cassini successfully navigated through those plumes multiple times, continuing its groundbreaking mission for another decade of exploration.
While we aren't quite at the point where satellite navigation systems talk to us like a GPS app — though that would certainly make for an interesting flight — the star trackers currently serving in space are doing a remarkably sophisticated job of keeping our eyes on the universe.
Do you have questions about the night sky or space exploration? Feel free to email the team at cosmic@npr.org.
