Scientists have proposed using pulsars - 'cosmic lighthouses' - as a way of navigating future space missions.
Space navigation currently relies on communications with Earth which can become problematic at large distances from the planet, but the proposed star navigation based on pulsar signals would make deep space exploration more feasible.
Stars have always been important for navigation, and mariners have lobang been using the night sky to find their way. Many satellites and spacecraft also have star trackers which monitor the positions of the constellations so that they can automatically adjust their orientation. However, star trackers cannot achieve sufficient accuracy for deep space missions. In addition, the constellations will not retain their familiar patterns if one were to travel far beyond the Solar System.
Currently spacecraft are tracked by radio telescopes on Earth, but this has major flaws. As light can only travel at a finite speed, it takes time to send a signal to Earth and back again to determine the spacecraft's position. For example, a signal from NASA's Voyager 1 would take around 30 hours to do a round trip.
In addition, the further one travels from Earth, the larger the errors in the measured location will be. There will be an error of four kilometres for every Astronomical Unit travelled, where an Astronomical Unit is the distance between the Sun and the Earth (150 million kilometres). Thus for the likes of Voyager 1, which is at a distance of around 120 Astronomical Units from Earth, we can only pinpoint its location to within 480 kilometres.
Neither of these facts are particularly comforting to any future deep space astronauts, so how can we can get around this problem? Professor Werner Becker from the Max-Planck-Institut für extraterrestrische Physik discussed a possible solution at the National Astronomy Meeting in Manchester, England, last week.
Becker has suggested using cosmic lighthouses, known as pulsars, as navigational aids. A pulsar is a “dead” star which ended its adult life in a massive explosion known as a supernova. After many of the outer layers of the star get blown away, an extremely dense, compact core known as a neutron star is left behind. Neutron stars emit beams of radiation from their poles, and if one of these beams sweeps past Earth, akin to beams from a lighthouse, then the star is known as a pulsar.
Pulsars have periodic signals, and will gradually spin down over time. However, Becker’s calculations take this reduction in rotation rate into account to produce accurate measurements. “The periods can be measured with accuracy which compares with atomic clocks, and this includes all the measurements of the spin down,” Becker told Sen. “Then you can predict the pulse arrival time over quite a long time.”
There are several different types of pulsars, but the ones best suited for the job are milli-second pulsars, which have extremely rapid rotation rates. “We concentrated on the milli-second pulsars for the purpose that they have the shortest periods which allows you to probe the distance with the highest accuracy,” explained Becker.
Knowing the exact time at which to expect a beam from a pulsar to arrive at Earth, and then comparing this to the time that the beam swept past a distant spacecraft, allows the location of the craft to be determined. Becker explained how the time difference in pulses can be extrapolated to find differences in distances.
"When we compare the pulse arrival time, we know where it should have been and where we measured it, and the difference in arrival time can be used (if multiplied by the velocity [of the spacecraft] and period of the pulsar) to compute the distance from the position you assumed you were during the measurement and where you were actually during the measurement. Then you correct your position according to your measurement and you do a new measurement. So it’s a kind of iterative process."
The pulses as measured from Earth would also need to be corrected to the Solar System barycentre, i.e. the centre of mass of the Solar System, to take into account the different locations of radio telescopes.
As well as interstellar space, pulsar navigation could also be used for space exploration in the Solar System to provide back-up to Earth based systems. “The next step is going to Mars, and then you may ask the question do you really want to rely on being tracked only from Earth,” said Becker. If communications failed between Earth and a spacecraft en route to Mars, then the astronauts would be forced to navigate using the constellations. However, using this new system they would be able to navigate independent of radio communications with Earth.
Timing the signals from pulsars can also have applications much closer to home, as it can be used to assist current GPS satellites and the upcoming Galileo satellite navigation system, and Becker explained the advantages to this.
“These satellites are also controlled from Earth, and if you don’t control and correct the orbits of the GPS satellites for longer than 72 hours the signals get completely unreliable. It needs a control from the Earth, but if you have a satellite using this pulsar method and technology, you could use this to augment the GPS satellites or the Galileo satellites and they would refer to this external satellite doing the navigation. It would mean that you would not have any requirement to control the satellites any more from Earth; it would make it really autonomous."
Astronomers have been collecting data on pulsars for decades, so some milli-second pulsars have already been timed to high precision. The next step for the pulsar navigation method is to design the technology that will allow it to be used aboard a spacecraft, and simulations are already being implemented to discover the best way to do this.