It’s 2021, and we finally do not have to worry so much that our spacecraft gets lost in interstellar space.
Astronomer Coryn AL Bailer-Jones uses the positions and shifting light of stars, both near and far, and has shown that the feasibility of autonomous, on-the-fly navigation for spacecraft moves far beyond the solar system.
Interstellar spatial navigation may not seem like an immediate problem. Already in the last decade, man-made instruments have entered interstellar space, as first Voyager 1 (in 2012) and Voyager 2 (in 2018) crossed the solar system boundary, known as the heliopause.
It’s only a matter of time before New Horizons joins them, followed by more probes in the future. As these spacecraft move further and further away from their home planet, communication with Earth takes longer and longer.
New Horizons is currently nearly 14 ligures from Earth, meaning it takes 28 hours to send a signal and receive a response; not an impossible detection and navigation system, but a bad one.
At greater and greater distances, however, it will no longer be reliable.
“If you travel to the nearest stars, the signals will be far too weak and light travel times will be years of order,” Bailer-Jones wrote in his paper, currently available on preprint server arXiv, where it is waiting peer review of the astronomy community.
“An interstellar spacecraft will therefore have to navigate autonomously and use this information to decide when to make course corrections or to turn on instruments. Such a spacecraft must be able to determine its position and velocity using only measurements on board.”
Bailer-Jones, who works at the Max Planck Institute for Astronomy in Germany, is not the first to think about this. NASA worked on navigating through pulses, using the dead stars’ regular pulsations as the basis for a galactic GPS. This method sounds pretty good, but it can be subject to errors at greater distances due to the distortion of the signal by the interstellar medium.
With a catalog of stars, Bailer-Jones was able to show that it is possible to work out the coordinates of a spacecraft in six dimensions – three in space and three in velocity – with high accuracy, based on the way the positions of the stars change from the position of the spacecraft.
“As a spacecraft moves away from the sun, the observed positions and velocities of the stars will change compared to those in an Earth-based catalog due to parallax, aberration and the Doppler effect,” he wrote.
“By simply measuring the angular distances between star pairs and comparing them with the catalog, we can deduce the coordinates of the spacecraft by means of an iterative process for forward modeling.”
Parallax and aberration both refer to the apparent change in the positions of stars due to the earth’s motion. The Doppler effect is the change in the wavelength of light from a star based on whether it appears to be moving closer to or away from the observer.
Because all these effects involve the relative positions of the two bodies, a third body (the spacecraft) in a different position will see a different arrangement of the stars.
It is actually quite difficult to determine the distances to stars, but we are getting much better. The Gaia satellite carries out an ongoing mission to map the Milky Way in three dimensions and has given us the most accurate map of the Milky Way to date.
Bailer-Jones tested his system using a simulated star catalog and then on nearby stars from the Hipparcos catalog compiled in 1997, at relativistic spacecraft speeds. Although not as accurate as Gaia, it is not very important – the purpose was to test whether the navigation system can work.
With only 20 stars, the system can determine the position and velocity of a spacecraft within three astronomical units and 2 kilometers per second (1.24 miles per second). This accuracy can be inversely improved as the square root of the number of stars; with 100 stars, the accuracy came down to 1.3 astronomical units and 0.7 kilometers per second.
There are a few twists you need to work out. The system did not take into account star binaries, nor did the instrumentation. The aim was to show that this could be done, as a first step towards it.
It is even possible that it can be used in conjunction with pulse navigation, so that the two systems can reduce each other’s errors. And then the sky is, literally, the limit.
The paper is available on arXiv.