It’s 2021 and we finally don’t have to worry so much about our spaceship 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 navigation on the fly for spacecraft traveling far beyond the Solar System.
Interstellar space navigation may not seem like an immediate problem. However, already in the last decade, human-made instruments have entered interstellar space, as 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 ships travel farther and farther from their home planet, communication with Earth becomes longer and longer.
New Horizons is currently nearly 14 light hours from Earth, which means it takes 28 hours to send a signal and receive a response; It is not an impossible navigation and tracking system, but a lanky one.
However, at increasing distances this will no longer be reliable.
“When traveling to the nearest stars, the signals will be too weak and the travel times of the light will be on the order of years,” wrote Bailer-Jones in his article, which is currently available on the preprint server arXiv, where he hopes peer review from the astronomy community.
“Therefore, an interstellar spacecraft will have to navigate autonomously and use this information to decide when to make heading corrections or turn on instruments. Such a spacecraft must be able to determine its position and speed using only onboard measurements.”
Bailer-Jones, who works at the Max Planck Institute for Astronomy in Germany, is not the first to think of this. NASA has been working on pulsars navigation, using the regular pulsations of dead stars as the basis for a galactic GPS. This method sounds pretty good, but it can be subject to errors at greater distances, due to signal distortion by the interstellar medium.
Using a catalog of stars, Bailer-Jones was able to show that it is possible to calculate the coordinates of a spacecraft in six dimensions (three in space and three in velocity) with great precision, based on how the positions of those stars change. from the point of view of the spaceship.
“As a spacecraft moves away from the Sun, the observed positions and speeds of the stars will change relative to those in a terrestrial catalog due to parallax, aberration and the Doppler effect,” he wrote.
“By measuring only the angular distances between pairs of stars and comparing them to the catalog, we can infer the coordinates of the spacecraft through an iterative forward modeling process.”
Parallax and aberration refer to the apparent change in the positions of the stars due to the movement 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 or further away from the observer.
Because all of 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.
Actually, it is quite difficult to determine the distances to the stars, but we are improving a lot. The Gaia satellite is conducting an ongoing mission to map the Milky Way in three dimensions and has provided us with 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, at relativistic spacecraft speeds. Although this is not as accurate as Gaia, that is not very important: the goal was to prove that the navigation system can work.
With just 20 stars, the system can determine the position and speed of a spacecraft with an accuracy of 3 astronomical units and 2 kilometers per second (1.24 miles per second). This precision can be improved by the inverse of the square root of the number of stars; with 100 stars, the precision dropped to 1.3 astronomical units and 0.7 kilometers per second.
There are some problems that should be solved. The system has not taken stellar binaries into account, nor has it considered instrumentation. The aim was to show that it could be done, as a first step towards updating it.
It is even possible that it can be used in conjunction with pulsars navigation so that the two systems can minimize the failures of the other. And then the sky, literally, is the limit.
The document is available in arXiv.