Credit: ESO/L. Calcada
For humanity to become a space-faring species, our technology needs to match our ambitions. However, even our most advanced spacecrafts are more like rafts – they’re slow and can’t navigate away from the ‘shore’ that is our solar system. In order to sail away into interstellar space, we’ll need some sort of ‘galactic compass’.
The Lighthouses in the Sky
Pulsars are fast-spinning neutron stars or white dwarfs whose radio emission appears to be blinking on and off. Pulsars are extremely dense, and they also spin incredibly fast: up to 40,000 times a minute which relates to roughly 700 times per second. The combination of rapid spin and magnetic fields generates powerful beams of electromagnetic radiation, and as the pulsar rotates, these beams sweep the sky like a lighthouse. To a distant observer, a pulsar will appear to blink on and off at fixed time intervals.
The first pulsar was discovered by accident by Jocelyn Bell Burnell and Anthony Hewish in 1967 while they were studying distant galaxies. They observed periodic pulses of radiation which were generated at a particular position in the sky. This odd behaviour seemed to surface again and again like a clockwork. These pulses where so precise that for a time, scientists thought that an alien civilisation might be generating the signals. It was later named PSR B1919+21, after Bell and Hewish unraveled the true nature of the pulsating source. This is what a pulsar sounds like when you convert a radio telescope’s output into an audible signal.
Following Galactic Beacons
Research on these exotic objects has resulted in two Nobel Prizes. The first one was awarded in 1974 to Hewish, Bell Burnell’s supervisor, for the discovery of the exotic cosmic objects. Hewish shared the prize with fellow radio astronomer Martin Ryle, for pioneering research in astrophysics. Unfortunately, Burnell, who was the first to point out the pulsar signal in the chart data, was left out. In recognition for her landmark work on pulsars Burnell was awarded the Special Breakthrough Prize in Fundamental Physics, which is worth 3 million USD, which she endowed for a scholarship program to help underrepresented students in physics.
In 1993, Russell Hulse and Joseph H. Taylor Jr. were awarded the Nobel Prize in Physics for the discovery of the first binary pulsar – a double pulsar system that provides a unique laboratory in which several aspects of Albert Einstein’s general theory of relativity could be tested.
But perhaps the most intriguing use of pulsars is celestial navigation. It’s not a new idea: in 1977, both Voyager spacecrafts were equipped with phonograph records. The Golden Voyager Record, as it’s called, is a sort of time capsule describing humanity’s culture, biology, and position – in relation to 14 pulsars.
However, pulsar research has advanced dramatically since the 1970s. In 2013, a study published by George Hobbs, the lead researcher of the Parkes Pulsar Timing Array project at CSIRO, Australia, and colleagues, analysed how observations of pulsars, that are observable in both radio and X-ray, can be used to navigate spacecrafts. The team found that spacecraft-based X-ray detectors would have to observe at least four-millisecond pulsars, every seven days, in order to determine its position. These detectors would be cumbersome but could work.
Several space missions have already experimented with pulsar navigation. In 2016, China launched the XPNAV 1 satellite, equipped with two sensors tuned to pick up incoming X-rays from pulsars. The primary goal of the mission is to detect signals from 26 nearby pulsars and to create a “pulsar navigation database.”
Meanwhile, NASA’s (Station Explorer for X-ray Timing and Navigation Technology SEXTANT (Station Explorer for X-ray Timing and Navigation Technology) used NICER, an observatory about the size of a washing machine, to detect X-rays from four millisecond pulsars. When compared to a location determined by the GPS receiver, it was found to be accurate within a ten-mile radius – promising, but not quite there yet.
There are numerous challenges that need to be overcome explains Hobbs: “Pulsars are weak, and we need better detectors to observe them. They also aren’t perfect clocks. They glitch or exhibit noise-like properties (known as timing noise). This will restrict precision of any positional determination without careful thought and potentially require ground-based stations to monitor the pulsars.”
Finally, telescopes are relatively bulky and heavy objects. “It’s non-trivial to stick one on a spacecraft or satellite,” Hobbs added. “As for a navigator used in cars today it would not make much sense,” Werner Becker, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, elaborates. “Resources on spacecrafts are usually limited, so is weight and power – the weight of a pulsar-based navigator should not be more than perhaps 10% of the weight of the spacecraft it is used for.”
“As pulsars have to be located in different regions in space, you need either a sufficiently broad field of view to be able to observe them, or moveable independent detectors. A parabola antenna is not suitable as it simply would be too large for the sensitivity required.”, Becker states. Nevertheless, pulsar navigation promises to provide a huge autonomy boost. It could allow spacecrafts to perform manoeuvres that are currently unnavigable, such as behind the sun or in real-time beyond the Oort Cloud.
Space flight has always been about pushing the limits and having a compass that can reliably take you farther away than ever before will become indispensable in the future.