So we have managed to invent a propulsion system which allows us to travel between the stars. Now, how do we navigate and get where we want to go. Do we…use the stars? As which star is where in our horizon will keep changing and not predictably a s on earth and GPS (galactic positioning system) is not set up yet, how?
Depends if you mean science fiction “invent” or science engineering “invent.” Or in other words - if you mean invent faster than light travel, all bets are off. But of you mean very high energy but within the bounds of known physics fast, then my bet will be pulsars. Lots around, each with its own signature frequency. Not only can you work out where you are by taking bearings on a few, but you could probably use a from of Decca/Loran to navigate whilst in flight. As you explore you will find more pulsars, and get better and better knowledge of the precise position of the previously known ones, and so build up an exact 3D map, allowing precise navigation. At least precise enough to be able to get close enough the star you want to get to to.
Within the bounds of physics. And re pulsars, please explain.
You can use the stars. We have a good handle on position, including distance, of nearby ones, and also know their motion well enough, to be able to navigate within, say, a couple hundred light years of Earth. You’re not going to shoot off 50,000 light years in some direction without first looking around nearby, are you? As time goes on, you’ll be able to catalog more and more stars with accurate position and motion to be able to navigate farther.
I mean you don’t allow faster than light - we don’t believe there is any way this is possible, so being able to do it, is akin to magic, and any idea of what the passing universe looks like whilst you do travel so fast is fancy only. So ignore it.
Pulsars are rotating neutron starts, and they emit pulses of radio energy at very precise frequencies. So if you see a pulsar, you have a very good idea of which pulsar it is, no matter where in the sky it appears, and no matter how messed up the other starts look. So, if you know the 3D relationship of pulsars, you can work out where you are with simple geometry, really little more than a 3D version of how a plane can navigate with nothing more than bearings to radio beacons.
However because the frequency is so precise, you can look for the phase relationship between the received signals from a number of pulsars. Once you know where you are, you can observe the phase relationship and keep track of it as you move. The relationship allows you to work out the change in relative distance to each pulsar, and so long as you keep a continual track on the relationship, you can keep track of where you are, without needing to maintain a directional fix on the pulsars. It should be reasonable to apply relativistic corrections to the phase relationship as well, so even at very high (but sub-light) speeds, you can maintain a track of where you are.
Interesting. This is sort of analogous to present-day GPS, in that you are using the phase of the pulsar signals as a kind of timestamp: once you’ve established your general location, you no longer particularly care where the pulsars are relative to you, but the distance of the pulsars from you (as determined by the signal phasing) becomes important.
Technically speaking, you don’t need to navigate in space - Isaac Newton will do that for you. All you have to do is keep track of which way you’re pointing, what your mass is and how much energy you’ve applied, and you’ll always know exactly where you are. There are no random factors that will force you to take your bearings.
The one problem with using pulsars is that they’re not omnidirectional. At a random location in space, the pulsars whose beams “sweep” through that point will be completely different from those whose beams hit the Earth. So if I teleport you to a random spot in space, with only the knowledge of the pulsars that are visible from the Earth, there’s no guarantee you could figure out where you were from only that knowledge. (This is why the Pioneer plaque included 14 different pulsars rather than just the minimum of four that would be needed to specify the Sun’s location.)
Agreed: dead reckoning in deep space would probably be pretty easy. It’s possible, though, that there could be small gravitational gradients that would push you “off course”. I’d suggest using the Doppler shift of the spectral lines in various stars to figure out your speed relative to them, and extrapolating your position from there.
Using current astronomical knowledge, from anywhere in the known Universe, you can find your position to within an error of about 1% of your distance from Earth, or less. And with any sort of interstellar propulsion technology, your astronomical knowledge will get significantly better.
The key is to start with larger or more distant structures, and work your way in. For instance, from anywhere within a redshift of 1 or so from us, the cosmic microwave background will look mostly similar to how it does from here, but the differences will allow you to get a first estimate of your position. From this first estimate, you can determine which galaxies are likely to be close to you, and look for them. Find them and compare them to where you expected them to be, and you can refine your position further. If you happen to be in the Milky Way, then you look for the globular clusters that we know orbit our Galaxy, to get you a position within the Galaxy, then to open clusters and nebulae, then to bright stars, and so on.
Current satellites already use star trackers to monitor their position and attitude. They are basically cameras that compare what they are seeing to the catalog of stars compiled by the Naval Observatory and other sites. Seems to this non-expert that the basic technology is here, it just hasn’t been applied to this specific problem yet.
Celestial navigation works from earth because our orbit around the sun allows but minimal changes in perspective. Once you start moving about between stars, the view of anything remotely nearby changes dramatically. Asterisms (constellations to non-astonimers) will completely lose their shape…in the right location some of the stars of the big dipper would be on opposite sides of your sky. Your coordinate system has to go from a 2 dimensional (declination and right ascension) centered on the center of the earth to a 3 dimensional system (distance, for example would need to be added) centered at ???.
I am pretty sure that if you took a skilled astronomer and plunked them down at a random location in the Milky-Way, even with access to all known data, they would not be able to figure out exactly where they were and where earth was. They could probably figure out which side of the milky way they were on by looking at distant galaxies but that is about it. We can’t even see the stars on the other side of the milky way to have mapped them.
Creating a 3-D map of stars is really quite easy even with today’s technology. Then you’d know what local stars look like from other local stars. If you have a computer that’s half as advanced as the engines posited by the OP, then your computer watches as additional stars become visible, maps those as well, and keeps a running map of its location.
It might be a challenge if you had to deal with teleportation to a random location, but using the stars as breadcrumbs to travel out and back would be an afternoon project for a computer science student.
Without doing the math, my gut is tells me that tiny errors in measured thrust would far outweigh any gravitational effects, for constant 1G trips over a year or so (in pilot time). Unless you went close to a star, of course.
I doubt anyone traveling interstellar distances would rely on dead reconing alone. However, if you (say) went to sleep for a couple years and then woke up, it would help to use DR to figure the current location.
If we get transported instantaneously to some unknown location in the galaxy, finding out where we are would be a pretty tricky problem (but IMHO not insoluble). Use of quasars in other galaxies could help a lot.
I’m not sure this is correct. There may be unknown massive objects which will change your course. And might not Brownian motion be relevant?
It be a big challenge since the stars move. The further you “jump” from your starting point, the further away the star will be from where you think it is. ie if you jump to a star 1000 light years away, you need to correct for the fact that you are seeing where it was 1000 years ago, not where it is now.
When I took linear algebra, the professor wanted us to come up with a real-life use of what we were learning. I used it to come up with a way to navigate in space and the teacher dismissed it as being too simple.
“That’s not our system.”
“I know that…!”
Is there an app to make my smartphone a Pulsar Position Navigator yet?
Right - a jump is difficult and that’s where you need things like pulsars.
But as long as you traveled at any sub-light speed to your destination, you watched each star move along the way. A few calculations in the computer and it knows where that star is in 3D space at all times.
Since no one’s mentioned it yet I’ll point out that the Apollo astronauts used stellar observations (i.e. same as sailors do) to navigate from the Earth to the Moon and it was more than adequate. They also used what was essentially the first inertial navigation system (it was initially invented & tested for aircraft but received massive funding and came of age via the Apollo Program). Before GPS INS was relied upon heavily for aircraft navigation (it’s misuse also caused several famous disasters, the Mt. Erebus crash, JAL007 shoot down). Thirdly they relied upon ground-based tracking systems (actually, this was the primary system, the stars & INS were considered the backups).
Obviously once you start getting light years distant any Earth-based tracking becomes impractical (communication takes too long) but both INS and stellar navigation would still work fine. Given the huge increase in microprocessor power a computer could easily calculate gravity protuberances in INS (they’d be minuscule anyway) as well as compensate for known star positions in 3D (for navigating our local stellar neighborhood distant known stars are still more than far enough away).
As someone said Issac Newton did 98% of the work already. Interstellar space travel would be the ultimate ‘as the crow flies’ in that it would consist of little more than perfectly straight, albeit incredibly long, flight paths (actually, for 99.999999% of the voyage they’d just be ‘drift’ paths.)