With GPS it's harder than ever to get lost here on Earth, but deep space is a different story. Mind-boggling distances, three-dimensional travel and a lack of obvious landmarks all complicate spacecraft navigation. Pulsars have been proposed as a solution in the past, and now a NASA team has demonstrated the viability of the idea with an experiment showing that a spacecraft can constantly and automatically calculate its position by tracking the perfectly-predictable X-ray signals from an array of pulsars.
When stars die, they collapse in on themselves, often becoming black holes. But not all stars follow the same fate. Those with a mass between 10 and 29 times that of our Sun tend to turn into a small, dense objects known as neutron stars. With very strong magnetic fields and very fast rotations, some neutron stars blast beams of electromagnetic radiation from their poles, and if Earth is in the path of those beams we can detect the signals as regular "pulses" – hence the name pulsars.
Since pulsars spin at a constant rate, their signals can be predicted with astonishing accuracy, and for the most precise examples –known as millisecond pulsars – they can be predicted years into the future, down to a scale of microseconds. That extreme regularity makes them perfect navigation tools, on the same level as the atomic clocks used to keep GPS satellites on track.
The idea of navigating by these natural beacons has been kicking around since at least 2012, and in 2016 the European Space Agency released a detailed feasibility study that outlined just how pulsar navigation might work. Now, NASA has put it to the test in the real world in an experiment it calls the Station Explorer for X-ray Timing and Navigation Technology (SEXTANT).
The agency used an observatory known as Neutron-star Interior Composition Explorer (NICER), which is currently studying neutron stars and pulsars from its perch on the outside of the International Space Station. The observatory is equipped with 52 X-ray telescopes, allowing it to easily identify neutron stars across the universe by their X-ray emissions.
Running over two days in November 2017, the experiment homed in on four specific millisecond pulsars, and took 78 timing measurements. An onboard algorithm then stitched these together to pinpoint NICER's location in space, and compared the results to the satellite's GPS data gathered over the same time.
"For the onboard measurements to be meaningful, we needed to develop a model that predicted the arrival times using ground-based observations provided by our collaborators at radio telescopes around the world," says Paul Ray, co-investigator on the SEXTANT project. "The difference between the measurement and the model prediction is what gives us our navigation information."
Even though the craft was speeding around the Earth at more than 17,500 mph (28,000 km/h), it took just eight hours for the technique to narrow down its location to a 10-mile (16-km) radius. At its best, NICER was able to be located within a distance of 3 miles (5 km).
"This was much faster than the two weeks we allotted for the experiment," says Luke Winternitz, System Architect on SEXTANT. "We had indications that our system would work, but the weekend experiment finally demonstrated the system's ability to work autonomously."
Here on Earth, a 3-mile margin of error would make finding a house almost impossible, but out in space where distances are measured on the scale of millions of miles, that's pretty damn accurate. In the long run, scientists hope to narrow the window in deep space down to several hundred feet.
If a pulsar-based navigation system passes muster, future spacecraft could precisely keep track of their location in 3D space, and autonomously adjust their path to stay on target, without needing to communicate with Earth. After some software fine-tuning, a second experiment is due to take place later this year.
"This successful demonstration firmly establishes the viability of X-ray pulsar navigation as a new autonomous navigation capability," says Jason Mitchell, project manager of SEXTANT. "We have shown that a mature version of this technology could enhance deep-space exploration anywhere within the solar system and beyond. It is an awesome technology first."
The findings were presented at the American Astronomical Society meeting last week.
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