Australasian Science: Australia's authority on science since 1938

Cosmic GPS

By Stephen Luntz

Pulsars can be used to accurately establish one’s place in the universe, or at least the galaxy, according to research published in Advances in Space Research.

Pulsars are exceptionally accurate time-keeping devices because pulses arrive regularly enough that changes reveal the influence of nearby stars. “We can also work backwards,” says Dr George Hobbs of CSIRO Astronomy and Space Science. “We can use information from pulsars to very precisely determine the position of our telescopes.”

By tracking the movements of the Parkes Radio Telescope in relation to several pulsars over the course of a year, Hobbs jokingly demonstrated that the Earth goes around the Sun. “This was nailed a couple of hundred years ago,” he admits. “But if you still need proof, we’ve got it.”

The idea could be more useful for spacecraft. Earth tracking becomes less precise with distance, and gyroscopes and accelerometers provide less accurate information over the course of a space flight.

Hobbs has demonstrated that once the behaviour of particular pulsars has been documented with radio telescopes, an X-ray telescope allows a spacecraft to use four pulsars to identify its location. “We could do it with three if they were at 90° to each other,” Hobbs says, “but nature has not provided us with pulsars in the right formation”.

Hobbs admits that X-ray telescopes weighing many tonnes would be problematic additions to interstellar space voyages, but adds: “They’re much lighter than The Dish”. Their inclusion would enable a spacecraft to know its location within 20 km and its velocity to 10 cm/s, although Hobbs says the craft would need either multiple telescopes to track each pulsar, or a way of turning so that each could be checked in turn.

A third application is to measure the mass of planets. The Earth orbits around the centre of mass of the solar system, which lies within, but not at the centre of, the Sun. The centre of mass shifts according to the orbits and mass of the planets, and its location affects the timing of pulse arrivals. “If the pulsar signals appear to be coming in at the wrong time, we know that the masses of the planets that we are using in the equations must be wrong, and we can correct for this,” Hobbs says.

Hobbs used these observations to rule out the existence of Nibiru, a hypothetical planet that pseudoscience websites predict will make a close approach early this century. More significantly, pulsar observations were used to measure Saturn’s mass to six decimal places.