It’s 2021, and finally, we don’t have to worry so much that our spaceship is lost in interstellar space.
Using the changing positions and light of stars, both near and far, astronomer Coryn AL Bailer-Jones demonstrated the feasibility of autonomous navigation in space for spacecraft traveling far beyond the Solar System.
Interstellar space navigation may not seem like an immediate problem. However, already in the last decade man-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 heliopause.
It’s only a matter of time before New Horizons joins them, followed by more probes in the future. As these spacecraft travel farther and farther away from their home planet, communication with Earth takes longer and longer.
New Horizons is currently almost 14 light-hours from Earth, which means it takes 28 hours to send a signal and receive a response; not an impossible tracking and navigation system, but an unpleasant one.
However, at increasing distances, this will no longer be reliable.
“When you travel to the nearest stars, the signals will be far too weak, and the light travel times will be years of command,” Bailer-Jones wrote in his paper, which is currently available on the arXiv prepress server, where awaits peer review from the astronomy community.
“An interstellar spacecraft will therefore have to navigate autonomously and use this information to decide when to make course corrections or start instruments. Such a spacecraft must be able to determine its position and speed 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 it. NASA worked on pulsar navigation, using the usual pulses of dead stars as the basis for a galactic GPS. This method sounds pretty good, but it can be subject to errors at longer 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 calculate the coordinates of a spacecraft in six dimensions – three in space and three in speed – with high accuracy, based on how the positions of these stars change. from the point of view 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 Doppler effect,” he wrote.
“By measuring only the angular distances between star pairs and comparing them to the catalog, we can deduce the coordinates of the spacecraft through an iterative process of forward modeling.”
Parallax and aberration refer to the apparent change in the positions of the stars due to the motion of the Earth. The Doppler effect is the change in the wavelength of light from a star, depending on whether it appears to be approaching or moving away from the observer.
Because all these effects involve the relative positions of the two bodies, a third body (the spaceship) in a different position will see a different arrangement of the stars.
In fact, it is quite difficult to establish distances from the stars, but we improve a lot. The Gaia satellite is carrying out a continuous 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, then on nearby stars from the Hipparcos catalog compiled in 1997, at the spacecraft’s relativistic speeds. Although this is not as accurate as Gaia, this is not extremely important – the purpose was to test whether the navigation system can work.
With just 20 stars, the system can determine the position and speed of a spacecraft up to 3 astronomical units and 2 kilometers per second (1.24 miles per second). This accuracy can be improved inverse to the square root of the number of stars; with 100 stars, the accuracy dropped to 1.3 astronomical units and 0.7 kilometers per second.
There are some issues that should be addressed. The system did not take into account stellar good, nor did it take into account instruments. The aim was to show that it can be achieved, as a first step towards updating it.
It is even possible that it can be used in tandem with pulsar navigation, so that the two systems can minimize the defects of the other. And then the sky is literally the limit.
The paper is available on arXiv.