It’s 2021 and we finally don’t have to worry so much about our spacecraft getting lost in interstellar space.
Using the positions and changing light of stars, both near and far, astronomer Coryn AL Bailer-Jones has demonstrated the feasibility of autonomous, on-the-fly navigation for spacecraft traveling far beyond the solar system.
Interstellar space navigation may not seem like an immediate problem. However, human-made instruments have penetrated interstellar space as early as the past decade, when the first Voyager 1 (in 2012) and Voyager 2 (in 2018) crossed the boundary of the solar system 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 travel further and further from their home planet, communication with Earth takes longer and longer.
New Horizons is currently nearly 14 hours of light from Earth, meaning it takes 28 hours to send a signal and receive a response; not an impossible tracking and navigation system, but a bulky one.
However, this will no longer be reliable at ever greater distances.
“When you travel to the nearest stars, the signals will be far too weak and the light travel times will be of order years,” wrote Bailer-Jones in his paper, which is currently available on the preprint server arXiv, where it is waiting for 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 turn on instruments. Such a spacecraft must be able to determine its position and speed with 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 has been working on navigation through pulsars, using the dead stars’ regular pulses as the basis for a galactic GPS. This method sounds quite good, but can be subject to errors from a greater distance due to distortion of the signal from the interstellar medium.
Using a star catalog, Bailer-Jones was able to demonstrate that it is possible to work out the coordinates of a spacecraft in six dimensions – three in space and three in speed – with high accuracy, based on the way the positions of those 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 from those in an Earth-based catalog due to parallax, aberration and the Doppler effect,” he wrote.
“By just measuring the angular distances between pairs of stars and comparing them to the catalog, we can derive the spacecraft coordinates through an iterative forward modeling process.”
Parallax and aberration both refer to the apparent change in the position of stars due to the motion of the Earth. 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.
Since all these effects relate to 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 a lot better. The Gaia satellite is on an ongoing mission to map the Milky Way in three dimensions and has given us the most accurate map of the galaxy to date.
Bailer-Jones tested his system using a simulated star catalog and then on nearby stars from the Hipparcos catalog compiled in 1997, using relativistic space travel speeds. While this isn’t as accurate as Gaia, it’s not terribly important – the purpose was to test whether the navigation system can work.
With only 20 stars, the system can determine the position and speed of a spacecraft to within 3 astronomical units and 2 kilometers per second (1.24 miles per second). This accuracy can be improved inversely with respect to the square root of the number of stars; with 100 stars, the accuracy was reduced to 1.3 astronomical units and 0.7 kilometers per second.
There are some kinks that need to be worked out. The system has not taken into account stellar binaries, nor instrumentation. The goal was to show that it could be done, as a first step to make it happen.
It is even possible that it can be used in conjunction with pulse navigation, so that the two systems can minimize each other’s flaws. And then the sky is, literally, the limit.
The paper is available on arXiv.