It’s 2021 and we finally don’t have to worry so much about the loss of our spacecraft in interstellar space.
Using the changing positions and 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, in the last decade human-created 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 polls in the future. As these spacecraft travel farther and farther from their home planet, communication with Earth takes longer and longer.
New Horizons is currently almost 14 hours light 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 system.
At ever-increasing distances, however, this will no longer be reliable.
“When you travel to the nearest stars, the signals will be too weak and the light travel times will be years of order,” Bailer-Jones wrote in his article, which is currently available on the arXiv prepress server, where he expects the peer review of the astronomy community.
“Therefore, an interstellar spacecraft will have to navigate autonomously and use this information to decide when to make course corrections or turn on instruments. This spacecraft should 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 so. NASA has been working on pulsar navigation, using regular 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 signal distortion by the interstellar medium.
With a catalog of stars, Bailer-Jones was able to show that it is possible to draw the coordinates of a spaceship in six dimensions (three in space and three in speed) with high accuracy, based on the way the positions of these stars change. 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 relative to those in an Earth-based catalog due to parallax, aberration, and 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 process of direct modeling.”
Parallel 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 a star’s light depending on whether it appears to be approaching or moving 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.
It’s actually quite difficult to determine distances to the stars, but we’re 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 with nearby stars from the Hipparcos catalog compiled in 1997, at relativistic spacecraft speeds. While this is not as accurate as Gaia, this is not too 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 at 3 astronomical units and 2 kilometers per second (1.24 miles per second). This accuracy can be improved inversely 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 issues that need to be resolved. The system has not taken into account the stellar binaries nor has it taken into account the instrumentation. The goal was to show that it could be done, as a first step to updating it.
It is even possible that it can be used in tandem with pulsar navigation so that both systems can minimize the defects of the other. And then the sky is literally the limit.
The paper is available at arXiv.