An interstellar navigation technique that taps into the signals from X-ray pulsars is being developed by a team of scientists from the National Physical Laboratory (NPL) and the University of Leicester. The team is using a small x-ray telescope on a spacecraft to determine its position in space extremely accurately. This system is referred to as XNAV and uses careful timing of pulsars to triangulate a craft’s position relative to a standardized location. As these pulsars, or magnetized neutron stars, spin they emit radiation. Some pulsars grow hot enough to admit x-rays, which can be used for stellar navigation.
By 2013, theoretical work describing XNAV techniques had developed to the point where the European Space Agency commissioned a team, led by Setnam Shemar at NPL, to lead a feasibility study, with an eye to one day using it on their spacecraft.
The team analyzed two techniques: delta correction, which would not be automated and would rely on communication with Earth, and absolute navigation, which would require more x-ray detectors. However, there are technology limits to absolute navigation. According to Shemar, it’s a trade-off between accuracy ad the practical limits of technology and cost.
While the engineering behind absolute navigation is complex, “it’s not miles out of the scope of existing technology,” says Adrian Martindale of the University of Leicester. In terms of the cost, complexity and size of X-ray detector required for XNAV, the team cites the example of the Mercury Imaging and X-ray Spectrometer instrument that will launch to the innermost planet on the upcoming Bepi-Colombo mission in 2018.
“We’ve shown that we think it is feasible to achieve,” Shemar said, adding the caveat that some of the technology needs to catch up with the theoretical work. “Reducing the mass of the detector as far as possible, reducing the observation time for each pulsar and having a suitable steering mechanism are all significant challenges to be overcome.”
In February 2017, NASA plans to launch the Neutron star Interior Composition Explorer (NICER), to the International Space Station. Although primarily for X-ray astronomy, NICER will also perform a demonstration of XNAV. As this idea of pulsar-based navigation continues to grow, “space agencies may begin to take a more proactive role and start developing strategies for how an XNAV system could be implemented on a space mission,” says Shemar.
Becker is a little more sceptical about how soon XNAV will be ushered in for use on spacecraft. “The technology will become available when there is a need for it,” he says. “Autonomous pulsar navigation becomes attractive for deep-space missions but there are none planned for many years.”
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