Navigating the Solar System Using Pulsars as GPS

by Markus Hammonds on June 20, 2013

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A spacecraft makes a close pass by Jupiter. Credit: Adrian Mann

A spacecraft makes a close pass by Jupiter. Credit: Adrian Mann

Picture the scene: It’s the not too distant future and humanity has started to construct colonies and habitats all across our solar system. We’re gearing up to take that next big step into the unknown – actually leaving the cozy protection of the Sun’s heliosphere and venturing into interstellar space. Before this future can happen, however, there’s an important thing which is often overlooked in discussions on this subject.

Navigation.

Just as sailors once used the stars to navigate the sea, space travelers may be able to use the stars to navigate the solar system. Except that this time, the stars we’d use will be dead ones. A specific class of neutron stars known as pulsars, defined by the repeated pulses of radiation they emit. The trick, according to a recent paper, may be to use pulsars as a form of interplanetary – and possibly even interstellar – GPS.

Theories and ideas on spacecraft engines are plentiful. Foundations such as Icarus Interstellar keenly advocate the development of new propulsion systems, with some systems such as the VASIMR thrusters appearing rather promising. Meanwhile, fusion rockets are expected to be able to take passengers on a round trip from Earth to Mars in just 30 days, and researchers elsewhere are working on real life warp drives, not unlike the ones we all know and love from the movies.

Interplanetary GPS

For Voyager 2, out on the edge of our Solar system, conventional navigation methods don't work too well. Credit: NASA

For Voyager 2, out on the edge of our Solar system, conventional navigation methods don’t work too well. Credit: NASA

But navigation is just as important. After all, space is mind-meltingly vast and mostly empty. The prospect of getting lost out in the emptiness is, frankly, terrifying.

To date, this hasn’t really been a problem, particularly seeing as we’ve only sent a small handful of craft past Mars. As a result, we currently use a messy mishmash of techniques to keep track of spacecraft from here on Earth – essentially tracking them with telescopes while relying heavily on their planned trajectory. This is also only as accurate as our instruments here on Earth are, meaning that as a craft gets more distant, our idea of where exactly it is becomes increasingly less accurate.

This is all well and good when we only have a few craft to track, but when space travel becomes more easily attainable and human passengers are involved, routing everything through Earth will start to become more and more difficult. This is particularly the case if we’re planning on leaving the confines of our home star – Voyager 2 is presently over 14 light hours away, meaning that Earth-based transmissions take over half a day to reach it.

Navigating Earth with modern technology is quite simple thanks to the array of GPS satellites we have in orbit around our world. Those satellites are constantly transitting signals which are, in turn, received by the GPS unit you may have on your car dashboard or in your pocket. As with all other electromagnetic transmissions, those signals travel at the speed of light, giving a slight delay between when they were transmitted and when they’re received. By using the signals from 4 or more satellites and timing those delays, a GPS unit can pinpoint your location on the surface of Earth with remarkable accuracy.

The Icarus Pathfinder starship passing by Neptune. Credit: Adrian Mann

The Icarus Pathfinder starship passing by Neptune. Credit: Adrian Mann

The pulsar navigation system proposed by Werner Becker, Mike Bernhardt, and Axel Jessner at the Max Planck Institute, works in a very similar way, using the pulses emitted by pulsars. By knowing the initial position and velocity of your spacecraft, recording those pulses, and treating the Sun as a fixed reference point, you can calculate your exact location inside the solar system.

Considering the Sun to be fixed this way is technically referred to as an inertial reference frame, and if you compensate for the motion of the Sun through our galaxy, the system still works perfectly well when leaving the Solar system! All you need is to keep track of a minimum of 3 pulsars (ideally 10, for the most accurate results), and you can pinpoint your location with surprising accuracy!

Interestingly enough, the idea of using pulsars as navigation beacons dates all the way back to 1974, notably not long after Carl Sagan had used pulsars to show Earth’s location on the plaques attached to the Pioneer 10 and 11 space probes. If Project Daedalus had ever been constructed, it might have been equipped with a system not unlike the one described here.

Packing for long haul

Becker and his colleagues looked at the different types of pulsar visible in the sky, and picked out a type known as rotation-powered pulsars as the best type to use for a galactic positioning system. In particular, a sub-type of these known as millisecond pulsars are ideal. Being older than most pulsars they have weak magnetic fields, meaning they take a long time to slow down their spin rates – helpful as strongly magnetised pulsars can sometimes change their rotation speed without warning.

An x-ray image of the Vela pulsar, one of the brightest known millisecond pulsars. Credit: NASA/CXC/PSU/G.Pavlov et al.

An x-ray image of the Vela pulsar, one of the brightest known millisecond pulsars. Credit: NASA/CXC/PSU/G.Pavlov et al.

With countless pulsars to choose from, the question turns to how you might equip your spacecraft to track them. Pulsars are easiest to spot in either x-rays or radio waves, so there’s a little choice as to which may be better to use. Essentially, it all turns out to be a question of how large your spacecraft is.

Smaller vehicles, more akin to modern spacecraft, would be best off using x-rays to track pulsars. X-ray mirrors, like the ones used in certain orbiting space telescopes are compact and lightweight, meaning that a few could be added for a navigation system without increasing the overall mass of the craft all that much. They may have the minor disadvantage that they may be easily damaged by an x-ray source which is too bright, this wouldn’t be a problem except under some unfortunate circumstances.

On the other hand, if you’re piloting a large space ship between planets or even stars, you would likely be better using radio waves. In radio frequencies, we know a lot more about the way in which pulsars work, as well as being able to measure them with a higher degree of accuracy. The only drawback there is that the radio telescopes you’d need to install on your ship would require an area of at least 150 m². But then, if you happened to be flying a starship, that kind of size probably wouldn’t make much difference.

It’s interesting to bear in mind the way that astronomers frequently use the analogy of pulsars being “like lighthouses” when explaining why they appear to pulse. If we someday find ourselves using them as actual navigation aids, that analogy may take on a whole new meaning!

You can read the team’s paper here.

The Icarus Starfinder, shown leaving the Solar system. Ships like this may be equipped with a pulsar navigation system. Credit: Adrian Mann

The Icarus Starfinder, shown leaving the Solar system. Ships like this may be equipped with a pulsar navigation system. Credit: Adrian Mann

Images are used here with kind permission from Adrian Mann of Icarus Interstellar, whose full gallery is viewable online at bisbos.com

About 

Every iron atom in your blood was forged inside the core of a massive star. Our inextricable link to the universe around us has always fascinated me – so much so that it motivated me to study a PhD in molecular astrophysics and to try and share the wonder of the cosmos with others.

Aqua4U June 20, 2013 at 4:08 PM

How about – a future starship, upon reaching a large percentage of the speed of light, travels far enough and fast enough to a place and time where the guide pulsar has stopped pulsing… and other reference pulsars are no longer observable due to angular shifting? Oops… In an infinite universe of possibilities, this has probably happened to somebody somewhere? LOL!

Planemo June 20, 2013 at 8:49 PM

Yes Aqua4U, it was I, Planemo! And I got back with out asking for any directions either. I never ask anyone for directions. Not even Spock on Vulcan. Its a man thing. I can just see Spocks head slant slightly and left eye-brow lift and hear him say, “Fascinating”. ;-) …lol.

Kevin Frushour June 20, 2013 at 5:17 PM

“It’s the not too distant future”… next Sunday A.D…..

Planemo June 20, 2013 at 8:44 PM

More like the 2nd Sunday of next week…lol. ;-).

Dima R June 20, 2013 at 5:33 PM

I wonder if there’s a way to navigate the space using gravity signatures, if there’s such a thing at all…

Planemo June 20, 2013 at 8:43 PM

Navigation? There is one thing I know. This ‘man’ will NEVER ASK for directions. I could careless I be in some galaxy in the middle a cluster of millions of galaxy’s, and I JUST WILL NOT ASK FOR DIRECTIONS! Heavens no! I’ll find earth and Cape Cod. ;-)

Kevin Frushour June 20, 2013 at 9:13 PM

You’d probably just get some six-eyed tentacles thing telling you “GRrrrBbBzzAAAazzzXXxxxXX!”

Planemo June 21, 2013 at 1:40 AM

LOL…Wow, 6 eyes. Hate to be his dad getting eye ware!

Grimbold June 20, 2013 at 10:18 PM

I think it would be good to park some GPS-like satellites at the two Earth-Sun L4 and L5 points. That would give us parallax with a baseline of 1.7 AU.

Baksa Péter June 21, 2013 at 7:14 AM

GPSystem relies on knowing the exact position of the satellites. How do we know where are the pulsars?

EarthlingX June 21, 2013 at 8:50 AM

I guess pattern recognition to be used to locate proper constellations and stars.

lcrowell June 21, 2013 at 4:53 PM

The distances to the pulsars do not appear to be relevant. They are treated as existing on the projective space or “heavenly sphere.” The timing of the pulses is referenced with respect to some coordinate origin, and the phase shift is then a measure of the local distance. That local distance could be within the solar system or further out into interstellar space.

LC

Rick Holcomb June 21, 2013 at 2:05 PM

“you can pinpoint your location with surprising accuracy!”

Why didn’t you go ahead and surprise us? How accurate?

Dave Salt June 22, 2013 at 8:28 PM

You may be interestrd to know that JPL have been working on this idea for more than a decade and that NASA are now planning to test the technology aboard ISS…
http://www.nasa.gov/topics/universe/features/zombie-stars.html

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