IT WAS a flying visit all right. NASA’s New Horizons probe took nine and a half years to reach Pluto, and yet it could only observe the dwarf planet for three weeks as it zipped past and out into the farthest reaches of the solar system. The images it has beamed back are nothing short of breathtaking, revealing a complex world of enormous floating ice mountains, smooth plains and reddish patches reminiscent of Mars. But just imagine what we could be learning about Pluto had we been able to hang around a little longer.
A decade ago, when New Horizons launched, we couldn’t plot a course accurately enough to get the craft into Pluto’s orbit. If a little-known mission launching this year goes to plan, however, NASA will showcase a new form of celestial navigation that promises far superior precision. Instead of relying on Earthly clocks as we do now, this system relies on the universe’s most reliable timepieces – rapidly rotating stellar corpses many hundreds of thousands of light years from home.
Harnessing the exquisite regularity of their pulses won’t just help us achieve ever closer encounters of the dwarf planet kind, it would also enable crewed missions to reach Mars without relying on constant contact with Earth. In the long run, it could even help our descendants plot a course in interstellar space.
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Currently, spacecraft in low Earth orbit, including the International Space Station (ISS), use the familiar Global Positioning System (GPS) to tell us where they are. This is a network of satellites orbiting our planet at an elevation of 20,000 kilometres. By intercepting signals sent by at least three of those satellites, a GPS receiver can calculate how far away each one is based on how long the signals take to arrive.
Going further from Earth is a bit trickier. Although research published this year by a team in Switzerland showed that spacecraft could use GPS for directions to the moon, beyond that it doesn’t work: signals from our Earth-facing satellites can’t reach that far.
Instead, NASA uses the Deep Space Network (DSN), a system of giant radio antennas in California, Spain and Australia. These tracking stations send out radio signals to a probe, then measure how long it takes for them to bounce back. When a spacecraft is visible from two tracking stations, you can determine its angular position on the sky.
“We need a new GPS – a galactic positioning system that works on board spacecraft“
The trouble is that each of the DSN’s tracking stations covers a third of the sky, so for the most part we only know a probe’s distance and speed along a straight line from one antenna. “At any given time, you can only usually track a probe from one station,” says , whose Outer Planet Navigation Group at NASA’s Jet Propulsion Lab in Pasadena, California, is currently navigating the Cassini, Dawn and Juno missions.
Interpreting the information generated by the radio telescopes and weeding out glitches caused by the signals’ passage through our atmosphere keeps a team of up to 20 astronomers busy. “We call it as much of an art as a science,” says Bhaskaran. And although the DSN has helped astronomers guide probes close to all of our planetary neighbours, it is far from perfect. At Pluto’s distance, roughly 7.5 billion kilometres from Earth, we have navigational accuracy down to the nearest 200 km. At the location of the Voyager 1 probe, which launched in 1977 and is now flirting with leaving the solar system some 20 billion kilometres from Earth, it falls to 500 km.
“Having a more sophisticated 3D-navigation system would allow us the possibility of entering the orbit of distant bodies – the moons and planets – and doing so with less fuel and more mass for instruments,” says at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. In other words, come up with a better way to chart our position in space and we could learn much more about the outer solar system, never mind the universe beyond.
So we need a new GPS – a galactic positioning system – and preferably one that works autonomously on board spacecraft. A fleet of satellites dotted around the cosmos would be ideal, but that is a long way off. Happily, the universe has supplied the brightest of alternatives. Pulsars are the highly magnetised, fast-spinning corpses of stars that have exploded as supernovae. They are incredibly dense bodies, packing in around 1.4 times the mass of our sun into a ball just 20 kilometres across.
“Pulsars spin incredibly fast. One class can rotate up to 700 times per second“
Because they have shrunk so much, pulsars spin incredibly fast. Most of them rotate a few times per second but one class, known as “millisecond pulsars”, can spin up to 700 times a second. As they spin, pulsars emit an intense beam of electromagnetic radiation across several wavelengths, including radio and X-ray bands, from each of their two poles. Those beams sweep across space, much like the beams from a lighthouse, and pulse with exquisite regularity. They are so regular, in fact, that pulsars have been proposed as an alternative to the atomic clock for standardising time – and even as a way to catch the infinitesimal disturbances in space-time caused by passing gravitational waves.
The upshot is that pulsars, and particularly millisecond pulsars, are pretty much the perfect alternative to GPS satellites. And because there are hundreds of thousands of them scattered across our galaxy, they could provide reference points everywhere.
Astronomers have dreamed about navigating with pulsars since they were discovered in the 1960s. Here’s how it would work. If you have an on-board system that measures the arrival times of individual pulses from a known pulsar and compares them with expected arrival times for a fixed reference location, you can calculate how much closer or further away the spacecraft is from the pulsar than that fixed point, albeit only in one direction. By combining measurements from at least three pulsars, you can calculate a precise three-dimensional location.
In 2013, at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany, calculated that pulsar navigation could be accurate to the nearest 5 kilometres (Acta Futura, vol 7, p 11). Others suggest it could be even more precise. “We feel that on a deep-space mission, we could maybe get down to a 1 km solution and maybe a bit better,” says , also at the Goddard Space Flight Center. It’s all down to the amazing regularity of pulsar pulses, which enable precise distance measurements, and the fact that a spacecraft could be surrounded by pulsars in all directions.
Pulsar navigation would also overcome another problem with the DSN. “One of the biggest constraints on deep-space navigation is that deep-space antennas are a very heavily used resource,” says Bhaskaran. “With our current missions, it’s manageable. But in the future, if we have a lot more missions, it’s going to be very difficult.”
So what’s kept us from charting a course by these spinning stars? The problem is that we are still largely in the dark about what makes them tick. But as we get to know more pulsars, we are getting a better idea. What’s more, we now know the location of over 2000 of them, including more than 200 millisecond pulsars, giving us an ever-growing reference map. Perhaps the most important jump, though, has been the advance of compact, lightweight telescopes to detect pulsars’ X-ray emissions (It is possible to detect their radio emissions but radio telescopes tend to be huge).
The first test comes in October this year, when 56 X-ray telescopes, each the size of a poster tube and packed into a device the size of a large washing machine, will be hauled into space aboard a SpaceX Dragon cargo craft bound for the ISS. This hardware will play a central role in NASA’s NICER/SEXTANT mission, which stands for Neutron-star Interior Composition Explorer/Station Explorer for X-ray Timing and Navigation Technology. The mission’s primary goal is to investigate the innards of neutron stars, where densities and pressures outstrip even those found in atomic nuclei, by measuring their X-ray emissions. But it will also serve as the first real test of X-ray-based pulsar navigation.
Flight of the navigator
It should be a stern examination. The ISS sits in low Earth orbit, so the telescopes on board will repeatedly lose contact with any pulsars as the station darts around the planet. The telescopes on a craft travelling long distances through space could keep receiving signals from the same pulsars over long periods, whereas the ISS orbits Earth every 90 minutes, severely limiting the time a pulsar is in a telescope’s field of view. In addition, the ISS needs to occasionally boost itself to overcome atmospheric drag, so its speed varies much more than a probe in deep space.
This fast and bumpy ride will complicate the task of deriving positions and speeds. “We are intentionally putting ourselves in a challenging environment, but if it works here it will work anywhere,” says Arzoumanian, who is on the mission team.
Becker, who is working on designs for both X-ray and radio-based pulsar navigation systems, isn’t convinced that this new project is entirely necessary. He points out that earlier X-ray probes have already collected relevant data. But Gendreau says that NICER/SEXTANT will test an “autonomous navigation algorithm” designed to crunch the numbers on the relatively puny processors aboard the ISS rather than the hefty computers on Earth – something that is vital for future deep-space navigation. The modular telescope design will also allow astronomers to see if navigation can be achieved with minimal components.
“The result will be an autonomous navigation system that is of a practical size and is flight proven,” says Gendreau.
So the future looks bright for pulsar navigation, particularly given that the next generation of radio observatories, such as the Square Kilometre Array in Australia and South Africa, will increase the number of mapped pulsars to between 20,000 and 30,000.
Plotting a course by these zombie stars would transform the way we explore space. For Gendreau, then, this mission is a chance to “demonstrate the technology that humankind will ultimately use to navigate our way out of the solar system and into the galaxy”.
This article appeared in print under the headline “GPS goes galactic”
