Navigation in outer space

Factual Questions
21

Although the locations of the closests stars in 3D space are well known thanks to the Hipparchos satellite, the systematic errors are pretty bad the further away you get. By the time you get out to the distance of Deneb, a few thousand light years away, the error bars are counted in hundreds of light years. That’s why the pulsar/GPS concept is so attractive; it should allow you to locate yourself to within a light year or less even at quite respectable distances from Sol.

Assuming that you can actually detect enough familiar pulsars from that location, that is - and assuming that the pulsar hasn’t changed its rotation pattern in the intermediate time. Pulsars sometimes undergo starquakes, which redistribute the matter and change the rotation slightly; I suspect that after a voyage of hundreds or thousands, or tens of thousands of years, several pulsars in our catalogs would have changed their caharacteristics in some way.

22

Won’t pulsars suffer from the same amount of inaccuracy in their distance as stars do? Why are they any better than using stars?

23

Reading this pdf about the idea,

hmmm… it seems that they are assuming that the pulsars are at spacial infinity and fixed - the navigation method uses phase-differences between the emissions.

Apparently they hope that the fact that real pulsars are neither fixed or at spatial infinity can be accomodated for by ‘making adjustments’ to the readings; that really makes the process not much more accurate than making a map of the galaxy as you travel. Oh, well…

It seems very likely to me that, when and if we start to explore the galaxy, we’ll have very good maps to navigate by already. The mere act of travelling though space towards a distant star will allow us to cross-check all the locations of stars (and pulsars) en route, allowing us to build up a detailed map over time.

24

I think there is no option but to make the map as you explore.

The OP’s question is interesting to contrast to earthbound navigation. Dead reckoning navigation on ocean and in the air don’t work well because there are disturbing forces that can’t be easily predicted - ocean currents, wind. As pointed out a few times above, dead reckoning can work reasonably well in space. However it isn’t perfect. As the Apollo missions found, imperfect knowledge of the distribution of mass in the moon, and errors in estimation of velocity changes from rocket burns, means you need to keep track of where you are, and make corrections.

So, for interstellar travel I’m assuming similar issues. You don’t know the precise gravitational field you are travelling though, and won’t know exactly what your initial velocity is. So dead reckoning errors will accumulate. Thus being able to monitor your position whilst in flight will be important. For any real distances we should assume that we are not travelling in free flight, but are in continuous acceleration (or deceleration). So we need further continual monitoring. Thus the idea of using pulsars, and phase measurements in the same manner as ships and planes used Decca /Loran.

The issues of directionality of pulsars is important. I have assumed that pulsars are ubiquitous enough that there will always be a reasonable number in view. That seems reasonable as we can currently see quite a few, and there is no reason to think we are anywhere special. However we would need to continue to look for new ones as we explore, and to build a 3D map of their locations. Proper motion of all the constituents of the galaxy is going to be a problem for very long journeys. It isn’t just that the pulsars that move, your destination will be moving too, so you have better have a good idea of what is going on before you set out.

25

I was feeling pretty good about my next space trip due to this pulsar navigation, but after reading about this pulsar I’m nervous about getting lost out there.

26

But if you were in a spaceship and you had star readings at every step of that 1000 light-year journey, you’d have a history of parallax readings that you could never get from Earth. Instead of a base of 180 million miles, you’d have a base of 6 quadrillion miles.

27

Well, pulsars pulsate (hence the name) so if you spot a pulsar with a period of 6.37859 seconds, you can say “Oh, that’s PSR 0065-81C,” whereas if you’re trying to navigate by conventional stars, after a while the red giants start to all look alike.

28

Bumped.
Apparently, a proof of concept of Pulsar navigation was done last year on the ISS.

29

Well, if you’re talking about a Star Wars Hyperdrive system, those Astromech Droids become really necessary. Because you’d have to start with a large database of space objects, stars and planets, along with all of their positions and relative motions. The time of the calculations for the jump being plotting out how everything is moving and very precisely you need to end up to be in orbit or in a precise location relative to other ships when you emerge from hyperspace. Otherwise, as they say in the movies, you could end up in the middle of a planet or space rock.

Obviously that would require a lot of surveying, a lot of data storage and constant updating of the astronomical database. When you jumped into a new system, you would literally be flying blind, trying for a spot unlikely to have orbiting rocks and planets. Then you’d have to survey the planets and rocks and if further transit was needed through the system or a useful planet found, you’d need to provide the exact movement patterns around the star and the safe spots to jump in relative to the planet.

So ultimately, your jump comes down to "enter hyperspace at this precise angle, be in hyperspace for exactly n.n to many decimal places seconds, then hope you come out in the right place. At least to start.

But this is science fiction. Something someone made up. We have no evidence of “Hyperspace”.

30

I thought you maybe posted this from Alpha Centauri after trying some of the suggestions in the thread and the post was just now reaching us.

31

It’s certainly not like dusting crops.

By the way, does anyone really believe anyone but R2D2, BB8 or the other astromech droids are the ones flying the starfighters? It would certainly explain how an 8 year old or a farmboy with some experience flying space Cessnas can jump in the cockpit of a top of the line combat spacecraft and not kill themselves.
Up thread, someone pointed out that all you need to do is point your starship and keep track off your thrust and no random forces would alter your course. That isn’t quite true. All those objects you pointed out have gravity. Even light years away, there will be some small influence on your ship and over time the ship will drift like a boat being pushed by the wind and tides. So unless you are able to calculate the effects of gravity of all those objects, you will probably need to course adjust.

32

If you can see the star you’re going to from your starting point, I’d think you just aim at it and go. You can make minor adjustments as you go keeping the star in the cross-hairs so to speak. When you get close to the solar system then you can decide what plants you want to swing by to slowdown or visit.

If you can’t see the star you’re aiming for, you presumably do know what general direction to start in. You’ll see it before too long.

If you need a ballistic course, of course, you need to be a heck of a lot more precise.

33

More like bulls-eying womp rats?

34

The basic question is - what are you doing?

We have a pretty good idea of the position of the nearby stars, especially the bright ones. (eg. Sirius)
We also have a good idea of the location of the center of the galaxy, something that would be visible from a lot of local space.
Plus, we know the location of a large number of globular clusters orbiting our galaxy center, not to mention local galaxies like Andromeda and the Magellanic clouds in our local group.
Presumably you could get a pretty good “triangulation” from any of those, taking into account the drift or angle change due to precession (everything moves).

First, understand the scale.
The nearest star system is Alpha/Proxima Centauri, 4.3ly away.
There are a few dozen known stars with pretty good measurements in the nearby 50ly or so.
If you travel within that 50ly, as Dracoi points out, you stop every so often and re-measure the parallax of at least the main stars (really bright ones like Sirius and Deneb) and more likely, every star you can find.
Eventually, as more and more people do this, we will have a map of stars and their relative locations.

The galaxy (Milky Way) is approximately 100,000ly across. Andromeda, about 1Mly away.
Unless you are proposing sublight voyages of tens of thousands of years, the movement of the individual stars is only a measurement adjustment not a problem.
Alpha Centauri will move from 4.3ly to 3.11 ly from Earth in about 27,000 years.
Presumably any computerized star catalog to navigate from will include adjustments for proper motion.

See this: Orion Arm - Wikipedia for our understanding of the general shape of our Galactic neighbourhood.
Add to this details like pulsar location relative to other stars, the galaxy center, etc. -
So the first intrepid explorers will not be heading out into the fog with no idea where they are headed or how to get back, or how to locate themselves.
We have a pretty good idea of the neighbourhood, and like most of science, we will build on that as we progress. It’s not like Magellan setting out on a voyage with only hazy idea of what was where…

(Unless they are Hollywood screenwriters. They seem to have no clue of the scale of planets and stars, they do stupid stuff like a moon blown out of orbit will pass a different star system every week… sort of like “I’m going to run out for a loaf of bread and some milk; oops, I’m lost, that looks like the Taj Mahal, and if I keep going for a minute, look, there’s the Eiffel Tower and I’ll catch a taxi to Big Ben and then hike over to Easter Island, and look, I walked home and the jug of milk is still cold…”)

35

Use triangulation by quasars to get a first idea of which galaxy you’re heading into. Then, we might discuss other options…

36

I guess one of my points was that we don’t know where every rock is around every star. Even if we could calculate a precise jump to land at, say, 1 AU from a star, you have no idea if you’re coming out of hyperspace in the middle of an uncharted planet or directly in front of a 100 meter iron asteroid moving at 25,000 meters per second.

Unless you’re assuming that, from inside this theoretical hyperspace, you can see and discern objects in normal space from great distance.

Barnard’s Star is moving toward us, closing from 5.93 light years to 3.8 ly in the next 9500 light years. So it is moving toward us at 67 kilometers per second. A ship launching toward that system three days after the previous ship will find that it has moved 174,000 km closer in that short period. The Earth will have moved 1.23 million miles in it’s orbit. If you know the planet you’re aiming at, it will have moved however far in it’s orbit. Now, how long will it take you to get there and how far will it have moved in that time. Ok, so you’re aiming at that spot from where you started, while avoiding any known rocks between where you start and where you want to end up.

That’s the astromech’s calculation. That precise angle of jump, possibly with expected course corrections along the way. You want to be precise, because that’s time, money and fuel. And not accidentally popping out of hyperspace in the middle of the moon, because you miscalculated the movement of the sun.

37

I haven’t done the calculations, but my gut instinct is that when coming out of hyperspace, the chance of hitting something really massive (like, say, more than a milligram) is pretty much zero.

38

There’s a word for spots like that. It’s called “everywhere”. If you jump into a random spot in space, even a random spot close to a star, even a random spot close to a star on that system’s ecliptic plane, your chances are going to be far higher of dying from your reactor spontaneously overloading or the galley systems starting a fire or just having an aneurysm than from randomly jumping right into an orbiting rock.

39

riding along with this thread, I am afraid my planned spaceship isn’t as advanced as the OP. I have to use hibernation techniques for the crew and I am afraid my sublight engines aren’t completely stable. Safe but the thrust isn’t completely predictable, so I don’t know the exact speed of the ship at any given time. That is what happens in a low budget project… sigh

That out of the way, the OP discusses the challenges of determining the position of a point in space. I am interested in determining the time. Of course I have clocks so I know the time on my ship, but I want to know the time back home. Without calculating the time dilation due to the ship’s motion (which I postulate is not known with sufficient accuracy), what external phenomena might be used to determine time? I know I can use the CBR redshift to determine the absolute age of the universe, but I am looking for a technique accurate to better than ±100,000,000 years. I first thought of pulsars, but I don’t see how they can give me an absolute time-though the slowing of the rotation might be a clue. Kind of hard though.

40

Well pardon me if I’d prefer to travel on a ship that has considered the options and taken precautions rather than one that says “eh, the odds are that nothing will happen”.

Because over time, those odds become a certainty, especially as the number of ships grows.

“Eh, the odds of winning the lottery are so high that statistically, no one ever wins.” Except that people do.

“Statistically, your odds of dying in a plane crash are…” And yet it still happens.