The European Space Agency (ESA) wants to know if it’s possible to use dead stars as a navigational aid for traveling in deep space. To answer that question, ESA has contracted Britain’s National Physical Laboratory (NPL) and the University of Leicester to investigate whether pulsars can serve as navigational beacons in the far-flung reaches of the outer Solar System or interstellar space.

With GPS, smartphones and online maps, navigation has become so simple that we hardly give it a thought anymore – unless a software glitch has the satnav telling us to drive into a lake. For probes heading into deep space, it’s another matter. Currently, spacecraft are guided by radio signals from ground stations on Earth or other spacecraft, but the farther out one goes, the less reliable radio signals become. Radio beams can take hours within the Solar System and days, months or years outside of it. Also, the power needed for ground stations to punch a signal so far soon becomes uneconomical.

The alternative is to use navigation beacons much as terrestrial sailors use lighthouses, buoys and RDF stations. Unfortunately, space is insanely large and installing manmade beacons would be expensive, impractical and involve problems of its own. NPL and the University of Leicester have a different approach. They plan to use natural beacons that can be used by spacecraft to fix their positions.

In this case, the substitute is X-ray pulsars. A pulsar is a kind of neutron star. That is, a star that blasted its outer shell of gas away in a supernova explosion. These dead star remnants have collapsed in on themselves so tightly that gravity has squeezed the electrons and protons in their atoms together to form neutrons. This creates a unique substance called neutronium, which consists only of neutrons and no atoms. This neutronium is so dense that a teaspoon of it would weigh as much as Mount Everest. Not surprisingly, the gravitational pull of a neutron star is so great that a mountain located on one would be shorter than a paper crease.

Because the star has shrunk dramatically, the law of the conservation of angular momentum means that it spins very fast. It’s exactly like if you spin around in a chair holding a couple of dumbbells in your hands. Pull them in toward you and you spin faster. In the case of a neutron star, it can spin up to 40,000 times per minute.

Pulsars are neutron stars that shoot a very powerful beam of radio energy through their magnetic poles, which are the only spots where the radiation can easily escape the stars' extremely powerful magnetic field. An observer can only detect this beam when it is pointed straight at him, so the rapidly-spinning pulsar seems to flash on and off with great regularity like a lighthouse. In fact, when the first pulsar was detected in 1967 by Jocelyn Bell Burnell and Antony Hewish, they called it LGM-1 or “Little Green Men 1” because at first they thought it was a signal from an alien civilization.

An X-ray pulsar gets the power to send out beams of X-rays from gases that it draws from a nearby companion star that it orbits. As the gases fall to the surface of the pulsar, they accelerate to nearly the speed of light, resulting in a tremendous burst of energy.

Pulsars are very easy to detect with their high energy, and to differentiate because they each have peculiar pulse-frequencies and radio signatures. NPL and the University of Leicester believe that pulsars can work like GPS does on Earth. In fact, with the extremely precise timing that pulsars provide, a cluster of them can – like GPS satellites – be used in the same way that a ship at sea takes multiple fixes on landmarks to establish its position precisely.

"Using on-board X-ray detectors, spacecraft could measure the times of pulses received from pulsars to determine the position and motion of the craft," said Setnam Shemar, leading the project for NPL's Time and Frequency Team. "The University of Leicester will use their experience in X-ray astronomy to come up with potential designs of the device and NPL will develop timing and navigation algorithms to determine the potential accuracy of this technique. Funding received from ESA will allow us to investigate the feasibility of using these dead stars and the potential navigation performance that could be derived."

Another benefit is that since these pulsars broadcast their presence like navigation beacons or GPS satellites, an indefinite number of spacecraft can use them, as opposed to ground stations that must beam their signals directly at a particular spacecraft.

This is not the first time pulsars have been suggested as a means of navigation. The plaque bolted to NASA’s Pioneer 10 Jupiter probe launched in 1972 was etched with, among other symbols, a radial pattern of 15 lines all meeting at the same point. The intersection was the location of Earth and the lines represented its distance and position relative to the galactic plane of 14 pulsars. The 15th line represented the distance of Earth from the galactic center.

On the pulsar lines were hash marks that represented the frequency of each pulsar. The hash marks were in binary code based on the flip-transition of a hydrogen atom – that is, the time it takes for an electron in a hydrogen atom to flip from one energy state to another, which was also explained in a diagram on the plaque.

By use of this diagram, NASA hoped that the finder would be able to calculate where the probe came from and when it was launched. The ESA pulsar beacon system is a bit more sophisticated than this, but the general principle is the same.

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