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Crab Nebula
Pulsars, like the one embedded within the Crab Nebula, offer a new way to navigate throughout the solar system. (credit: NASA, ESA, CXC, JPL-Caltech, J. Hester and A. Loll (Arizona State Univ.), R. Gehrz (Univ. Minn.), and STScI)

X-ray pulsar navigation: the deep space solution?

While the GPS system and other satellite navigation systems are proving more and more indispensable on Earth, it is becoming obvious that their uses for deep space operations are extremely limited. Inside the Earth-Moon system the GPS signal may still be of limited usefulness, but beyond the Moon’s orbit it simply cannot be used. Navigating deep space probes is still more of an art than a science and even the best artists sometimes fail, as we saw with Mars Climate Orbiter in 1999.

A pulsar is a rapidly-rotating neutron star with a very strong magnetic field. Neutron stars are the remnants of massive stars that have reached the end of their lives. As they rotate they “pulse” and send out a signal in the x-ray band at a regular rate: 30 times a second is typical. There are enough powerful ones nearby that they could be used as navigation and timing signals. This means that future explorers in the solar system could use them as a sort of universal GPS system, instead of depending on star trackers or on a possible future system-wide radio frequency navigation constellation.

Taken together, pulsar navigation and star trackers should eventually be able to tell a spacecraft where it is to within a few meters or even less.

In October 2005, the Defense Advanced Research Projects Agency (DARPA) established the XNAV project with the John Hopkins University’s Applied Physics Laboratory (APL) to examine the feasibility of this technology. Cooperating with Ball Industries of Boulder Colorado, APL is working on a file cabinet-sized prototype. They hope to fly a device based on this technology either on a spacecraft or on the International Space Station.

The core of the system is an antenna based on the sensor mechanisms used in digital medical x-ray cameras that are found in most doctors’ or dentists’ offices. This technology is well established and the industry has an incentive to continually improve its sensitivity and robustness. Transitioning from the doctor’s office to the exterior of a deep space probe is a big step, but as long as the basic hardware is well designed, it is by no means an impossible one.

Star trackers are small, lightweight, and have proven to be highly effective, so why replace them with x-ray pulsar navigation? It’s always nice to have an alternative technology available, but the greatest role for this technology will be the ultra-precise navigation and positioning it will provide when combined with a star tracker. Taken together, the two systems should eventually be able to tell a spacecraft where it is to within a few meters or even less.

This capability will be needed in the near term in order to map the gravitational fluctuations around in low lunar orbit called “mascons”. This data will be needed in order to allow the next generation of manned and unmanned craft in lunar orbit to maneuver safely and efficiently. With an accurate map of these phenomena they could even be used to create miniature “slingshot effects” to reboost spacecraft with minimum use of onboard fuel.

For the military, ultra-accurate maps have long been a requirement. The Navy spends huge sums every year on oceanographic research that allows submarines to take advantage of every fold and seamount on the ocean floor and to use even the smallest changes in water conditions to hide in or to detect the enemy. XNAV-derived maps of cislunar space should eventually give US military space systems a similar edge.

Any instrument that provides a more accurate and precise way to measure anything is going to be of scientific value. Beyond these applications, x-ray pulsar positioning, navigation, and timing devices are going be used for purposes and in systems which have not yet been imagined, let alone put on the drawing board.

In the medium to long term this technology should find its principal application in robot probes that travel to the asteroid belt and beyond. NASA’s Deep Space Network is already oversubscribed and plans to expand it seem stuck, at least for the moment. Highly autonomous probes using x-ray/star tracker hybrid navigation will be able to confidently travel through the solar system, sending back data based on precise coordinates. This will give future scientists data of far greater accuracy than they now can get from probes that just use star trackers.

X-ray pulsar navigation might some day be used by autonomous rovers on the surface of Mars, Venus, or on the moons of Saturn. Star trackers, which work well enough in space, cannot be expected to work in the presence of even a thin atmosphere such as the one on Mars.

Any instrument that provides a more accurate and precise way to measure anything is going to be of scientific value. Beyond these applications, x-ray pulsar positioning, navigation, and timing devices are going be used for purposes and in systems which have not yet been imagined, let alone put on the drawing board. This technology is in its infancy, but its promise is bright and the space industry should look forward to using it in five or ten years, when it has had a chance to mature.


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