‘Interplanetary spacecraft navigation using pulsars’ might sound like the title of a Star Trek novel but is actually a 2013 scientific paper by a team of CSIRO scientists in Australia and the National Space Science Center in Beijing. The researchers have written software, with the current rather unexciting label ‘TEMPO2’, that could one day guide future spacecraft to distant planets.
Pulsars used for accurate navigation
The research is to be published in the journalAdvances in Space Research and outlines how observations of pulsars – small spinning stars that deliver regular ‘blips’ or ‘pulses’ of radio waves and, sometimes, X-rays – can be used to navigate a spacecraft travelling in the Solar System. In addition, the researchers comment that the method can be used beyond into the wider galaxy.
“We make use of archival observations of millisecond pulsars from the Parkes radio telescope in order to demonstrate the effectiveness of the method and highlight issues, such as pulsar spin irregularities, which need to be accounted for. We show that observations of four millisecond pulsars every 7 days using a realistic X-ray telescope on the spacecraft throughout a journey from Earth to Mars can lead to position determinations better than [approximately] 20 km and velocity measurements with a precision of [approximately] 0.1 m/s [10 cm per second],” the research team write in their paper.
One of the lead researchers, Dr George Hobbs from the CSIRO, said in a press release, “To our knowledge, this is the best accuracy anyone has ever been able to demonstrate.”
“Each pulsar would have to be observed for about an hour. Whether you can do them all at the same time or have to do them one after the other depends on where they are and exactly what kind of detector you use.”
Inaccuracy of navigation from the ground
CSIRO’s Canberra Deep Space Communication Complex and other similar centres throughout the world usually track and guide spacecraft within the Solar System from the ground. However, the further out the craft goes, the less accurately they can measure the craft’s location.
The CSIRO writes that, for voyages beyond the Solar System, spacecraft would need an on-board (autonomous) system for navigation. “Gyroscopes and accelerometers are useful tools but the position information they give becomes less accurate over time.”
“Navigating with pulsars avoids these problems,” says Deng Xinping, a PhD student at the National Space Science Center in Beijing and lead author on the paper describing the navigation system.
Lightweight X-ray detectors
Although the idea of using pulsars to navigate has been around for some decades, it is only with the recent development of small, lightweight X-ray detectors that could receive the X-ray pulses that certain pulsars emit that the idea has been able to progress to a functioning system.
“For deep-space navigation, we would use pulsars that had been observed for many years with radio telescopes such as Parkes, so that the timing of their pulses is very well measured,” say CSIRO’s Dr Dick Manchester. “Then on board the spacecraft you’d use an X-ray telescope, which is much smaller and lighter.”
Dr Hobbs and his colleagues have made a very detailed simulation of a spacecraft navigating autonomously to Mars using this combination of technologies and their TEMPO2 software.
“Unlike previous work, we’ve taken into account that real pulsars are not quite perfect, they have timing glitches and so on. We’ve allowed for that.”
The same pulsar software can be used to work out the masses of objects in the Solar System.
In 2010, Dr Hobbs and his colleagues used an earlier version of the software to ‘weigh’ the planets out as far as Saturn — to 6 decimal places.
The recently developed version of the software lets the astronomers rule out unseen masses, including any undiscovered planets.
“Even if a planet is hard to see, there’s no way to disguise its gravitational pull,” says Dr Hobbs. “If we don’t detect the gravitational pull, then there’s no planet there.”