Traveling from my house to Downtown Los Angeles can be a mission, to put it mildly. So, before setting off, I input the destination into my iPhone. Apple Maps will quickly download GPS data, pinpoint my location and then give me a few route options. Assuming the 101 freeway isn’t a parking lot, I’ll set off, with Apple Maps’ soothing female voice guiding the way.
In this scenario, repeated millions of times around the world, we often have seamless access to GPS satellites (providing location information) and data networks that overlay traffic, weather and other information services that, usually, make our road trips more intelligent. (Having said that, I always have a good old fashioned paper map in the glove compartment should I lose network connection or a solar flare knock out a GPS satellite. You never know.)
But say if you’re planning an interplanetary road trip? How would your spaceship know what route to take? How would you orientate yourself in three-dimensional space when you’re millions of miles from the nearest rest stop?
Currently, our armada of robotic space missions depend on Earth to tell them where they are in relation to our planet, but the uncertainties in solar system location increases with distance from the launch pad. We are able to accurately deduce the distance of a spacecraft from Earth (it’s radial distance) by analyzing the radio signals that we beam to and from the spacecraft to an accuracy of a meter or so. But what about tracking the spacecraft’s angular position in the sky?
Unfortunately, the angular position of a spacecraft can only be known as accurately as the angular resolution of Earth-based radio antennae. The uncertainty in angular position of any given spacecraft, using radio antennae, increases by 4 kilometers per astronomical unit (AU). This may not be a serious issue for spacecraft traveling through interplanetary space within a couple of AU from Earth. But what about Voyager 1? The uncertainty in that spacecraft’s position is over 500 kilometers.
In research published on the arXiv preprint service and reported on by Technology Review’s arXiv blog, a solution to this position uncertainty could lie in rapidly-spinning stellar husks called pulsars.
Werner Becker, Mike Bernhardt and Axel Jessner, of the Max Planck Institute for Radio Astronomy in Germany, have detailed interplanetary navigation by using known X-ray pulsars in our galaxy as fixed points in the sky, providing a spacecraft with a means to find its way in the dark. Like sailors used the stars to navigate their way around the globe, this hi-tech navigation system could be carried by spacecraft to pinpoint their location in three-dimensions during interplanetary sojourns — and it wouldn’t differ too much from the GPS system we use to navigate the 101 during rush hour.
What makes this pulsar navigation system attractive is the convergence of two key factors. First, X-ray optics are shrinking and, in the not too distant future, an array of X-ray detectors could be assembled to create a portable navigation system. This is attractive as the smaller the optics, the less it will cost to launch into space as a component of a spacecraft. Secondly, astronomers know of 2,000 pulsars and that number is expected to rise significantly over the coming years. This factor allows many “positioning pulsars” as possible to remain visible to the positioning system.
A pulsar is a rapidly spinning neutron star that emits powerful radiation from energetic jets emitted from its poles. Like a lighthouse, these beams of intense radiation can sweep in our direction, creating a rapid, predictable blinking many times a second. It’s this predictability that could be used to a navigation system’s benefit. Although these objects are usually detected via their radio wave emissions, they also generate X-rays. X-ray optics are far smaller than radio antennae, therefore making X-rays a more attractive emission to use in a pulsar positioning system. The system would use on board software to analyze the pulsar signals — it would measure the pulses’ arrival time and compare that with their predicted arrival time. The difference in these timings would then allow the position of the spacecraft to be resolved relative to three (or more) target pulsars. It’s position can therefore be found to an accuracy of a few kilometers, say the researchers.
“Using the X-ray signals from millisecond pulsars we estimated that navigation would be possible with an accuracy of ±5 km in the solar system and beyond,” the researchers write.
When discussing interplanetary missions like NASA’s New Horizons, or even Voyager 1, uncertainties in solar system location of dozens to hundreds of kilometers isn’t currently a practical issue.
But in the future, should mankind really become interplanetary, very exact space positioning would be desirable. Finding the nearest asteroid refueling depot in the asteroid belt between Mars and Jupiter, say, could be a matter of life or death; I’m pretty sure our future astronauts wouldn’t want their PPS (Pulsar Positioning System) to be off by a few hundred meters, let alone a few hundred kilometers.