Navigating by the (dead) starsPosted: November 25, 2010
Until the last century, astronomy had one very practical purpose, navigation. Ancient mariners used stars such as the North Star and the Southern Cross to work out where in the ocean they were. With the advent of modern methods (the most up to date of which is GPS) navigating by the stars fell by the wayside. Now a new method that combines the ancient idea of stellar aids to navigation with some of the principles of GPS has been suggested to accurately determine the position of spacecraft, and it uses dead stars.
We all remember from school being told how the North Star can be used to estimate how far north you are. It’s been used for millennia to estimate the latitude of ancient mariners. Conversely to the south, the lack of a bright pole star led the great Polynesian navigators to use constellations such as the southern cross to navigate (I recently saw a nice planetarium show at the Bishop Museum in Honolulu on this subject). Later longitude could determined thanks to the discoveries of Jupiter’s moons, accurate clocks and naval almanacs in the last few hundred years. Recently technology has allowed us to do away with navigating by the stars (although intercontinental ballistic missiles of a previous generation did use celestial navigation). The most common method now used is GPS which uses satellites to determine a person’s position on the Earth.
GPS uses a series of orbiting satellites as reference points to calculate positions. Each satellite transmits its location and the time the signal was sent. These can then be used to estimate the position of the receiver. But what about space-based navigation? The distances to interplanetary spacecraft can be accurately determined from Earth, but their position perpendicular to this relies on measurements of their position on the sky as measured from Earth. However a small error in the measured sky position of the spacecraft can turn into a bigger positional error the further away the craft gets (errors of 4km for every Earth-Sun distance the spacecraft is from Earth). Hence having another way to measure position would be extremely useful.
And this brings me to this paper by astronomers based in Germany. It analyses the idea of using dead, rapidly rotating stars called pulsars to pinpoint a craft’s position. I’ve blogged a bit about pulsars before, they are the remains of huge stellar explosions (supernovae) and are extremely dense (one inch of neutron star material covering Andorra would have the same mass as the Earth!!!). They also spin rapidly and have high magnetic fields. This means that energetic particles, like those that cause the Northern Lights on the Earth, give out huge amounts of energy. As the star is rotating, the emission from each pole is seen briefly as it crosses our line of sight. Hence it appears these objects give off pulses of radiation.
The paper uses these pulsars like GPS satellites. Each pulse takes time to travel through the solar system so if you know when the pulse should arrive at the centre of the solar system and you detect a pulse at a different time, you know you are in a different position. Combine three or more pulsars and you can get your space position. Unfortunately each pulse is not unique so you only know you are a set number of pulses plus the fraction of a pulse you have measured away from the Sun. However you can use other methods to determine your rough position. This sort of pulsar timing array is used on the Earth to search for tiny movements caused by gravitational waves.
The authors suggest using a number of X-ray telescopes to monitor a set of pulsars they have identified. This would allow the spacecraft to measure its position to within a few kilometres, a vast improvement for probes in the outer solar system. It seems like we can’t quite get away from navigating by the stars.
Mike Georg Bernhardt, Tobias Prinz, Werner Becker, & Ulrich Walter (2010). Timing X-ray Pulsars with Application to Spacecraft Navigation Proceedings of Science arXiv: 1011.5095v1