Posted on 03/22/2021 7:55:09 AM PDT by Red Badger
It's 2021, and we finally don't have to worry quite so much about our spacecraft getting lost in interstellar space.
Using the positions and shifting light of stars, both near and far, astronomer Coryn A.L. 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, already in the last decade human-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 travel farther and farther from their home planet, communication with Earth takes 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; not an impossible tracking and navigation system, but an ungainly one.
At greater and greater distances, however, this will no longer be reliable.
"When travelling to the nearest stars, signals will be far too weak and light travel times will be of order years," Bailer-Jones wrote in his paper, which is currently available on the preprint server arXiv, where it 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 to switch on instruments. Such a spacecraft needs to be able to determine its position and velocity using only onboard measurements."
Bailer-Jones, who works at the Max Planck Institute for Astronomy in Germany, isn't the first to think of this. NASA has been working on navigation by pulsars, using the dead stars' regular pulsations as the basis for a galactic GPS. This method sounds pretty great, but it may be subject to errors at greater distances, due to distortion of the signal by the interstellar medium.
With a catalog of stars, Bailer-Jones was able to show that it's possible to work out a spacecraft's coordinates in six dimensions - three in space and three in velocity - to a high accuracy, based on the way the positions of those stars changes from the spacecraft's point of view.
"As a spacecraft moves away from the Sun, the observed positions and velocities of the stars will change relative to those in a Earth-based catalog due to parallax, aberration, and the Doppler effect," he wrote.
"By measuring just the angular distances between pairs of stars, and comparing these to the catalog, we can infer the coordinates of the spacecraft via an iterative forward-modelling process."
Parallax and aberration both refer to the apparent change in the positions of stars due to 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 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 pretty difficult to determine the distances to stars, but we're getting a lot better. The Gaia satellite is conducting 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, at relativistic spacecraft speeds. Although this is not as accurate as Gaia, that's not terribly important - the aim was to test that the navigation system can work.
With just 20 stars, the system can determine the position and velocity of a spacecraft to within 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 came down to 1.3 astronomical units and 0.7 kilometers per second.
There are some kinks that would need to be worked out. The system hasn't taken stellar binaries into consideration, nor has it considered the instrumentation. The aim was to show that it could be done, as a first step towards actualizing it.
It's even possible that it could be used in tandem with pulsar navigation so that the two systems might be able to minimize each other's flaws. And then the sky, literally, is the limit.
The paper is available on arXiv.
A 3 dimensional velocity vector would be (speed in x direction, speed in y direction, speed in z direction), if the 3 spatial dimensions are (x,y,z).
More mathematically, instead of speed in x direction, you would say change of location in the x direction with respect to time.
So, velocity would be (dx/dt, dy/dt, dz/dt), where d essentially means ‘change in’.
To summarize, what they are saying as that they know not only your current position in space, they also know how fast you are moving in each of the 3 dimensions, x,y,z, where x is left to right, y is forward-backward, and z is up-down.
Cosmos crashed and burned before getting past Saturn. You can see the debris plume on Europa.
Right, I was focusing on weakness/degredation of signal, which relays can solve.
Still sounds like trajectory to me. Guess I don’t have the brain power to get it.
It is a trajectory, but it includes speed as well as direction.
All blackholes are wormholes but all wormholes are not blackholes.
You just made my point by describing x,y,z dimensions. How fast you're traveling within those known dimensions is called speed, no matter the angle or trajectory. If they want to call velocity by another term, fine, but most believe it to be the speed of a given object.
So, is the speed of light a velocity or just 186,000 miles per second as in speed? Yes, even light can be bent as in a prism or other within our perception and calculated physics. Maybe that's where they get the other 3 dimensions. This is all probably beyond my little brainpan.
Speed in a specified direction is velocity, as velocity always includes direction. In space, which is 3-dimensional, you have to say how fast you are moving in each of those 3 spatial dimensions. That is a 3-dimensional velocity (speed in x, speed in y, speed in z). The ‘speed’ in the direction of travel is the magnitude of the 3-dimensional vector (square root of the sum of the squares of the speed in each direction).
speed in x, speed in y, speed in z is a 3-dimensional velocity vector. You can compute the angle of travel from those values by trigonometry.
Light is another thing altogether and requires relativistic physics. It is just weird.
I think I just read 3 of his "juvenile" novels (when in grade school) so they were tamer: Red Planet, Tunnel in the Sky, and Time for the Stars. The last is the one using the concept of telepathy. In Tunnel in the Sky one of the characters is a Zulu woman--when I read that I don't think I realized that that meant she was black.
It’s kind of a joke in parts of the SciFi readers’ community. After SIASL Heinlein kind of went off the deep end. ‘Time Enough for Love” is a real slog to get through with Lazarus Long very obviously being Heinlein’s alter ego. He became almost as unreadable as Asimov’s later output.
To navigate “a spacecraft needs to be able to determine its position and velocity.”
Technically, speed and velocity are different. Your velocity is your speed plus your direction of travel. In other words, velocity gives you a little more information than your speed alone.
It’s easy to see that when navigating to a specific point on the map, knowing your current position and velocity are more useful than just knowing your position and raw speed... You’re at 9th and Elm and walking at 5 mph... So what? Are you getting closer to your desired destination or farther away? You have no idea without knowing your velocity.
Velocity is even more important in deep space, where things aren’t laid out in a nice grid pattern like city streets (that can give you all kinds of hints about which direction you’re going).
Space dimensions: Sticking to earth’s surface, you could designate your position with two numbers... e.g., you could say you’re one mile east and one mile south of your house and everyone would know exactly where you were, 1.414 miles southeast of your house.
Velocity dimensions: Similarly, you could designate your velocity by saying you were travelling at 3 mph east and 4 mph north and everyone would know you were travelling at 5 mph northeast.
Congrats, you’ve just worked out your coordinates in four dimensions, two in space (1,1), and two in velocity (3,4).
I thought Farnhams Freehold was odd as well.
Oops, not due northeast, but at 53.13° wrt the east-west axis... a little trig mistake there.
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