This semester I'm teaching General Relativity, and as part of discussing gravitational waves, this week I briefly discussed pulsars. It was quite timely therefore when I learned of a new proposal that pulsars may ultimately provide a perfect navigation system for spacecraft far from Earth.
Here on Earth, the Global Positioning System (GPS) gives us a highly accurate way of determining position, and many of us now use hand-held devices every day to help with directions. These work because GPS satellites provide a set of clocks, the relative timings of the signals from which can be translated into positions. This is, by the way, another place where both special and general relativity are crucial to how the system works. Out in deep space, of course, our clocks are unfortunately useless for this purpose, and the best we currently can do is by comparing the timing of signals as they are measured back on Earth by different detectors. But the accuracy of this method is limited, since the Earth is a finite size, and our terrestrial detectors can therefore only be separated by a relatively small amount. The further away a spacecraft is, the worse this method is. What Werner Becker of the Max-Planck Institute for Extraterrestrial Physics in Garching has realized (and announced yesterday at the UK-Germany National Astronomy Meeting in Manchester), is that the universe comes equipped with its own set of exquisite clocks - pulsars - the timing of which can, in principle, be used to guide spacecraft in a similar way to how GPS is used here on Earth. Of course, it isn't quite as simple as all that. A significant obstacle to making this work today is that detecting signals from the pulsars requires X-ray detectors that are compact enough to be easily carried on spacecraft. However, it turns out the relevant technology is also needed by the next generation of X-ray telescopes, and should be ready in twenty years or so. Perhaps one day our spacecraft will map their routes through the cosmos thanks to yet another spinoff from basic research.
