If we were capable of interstellar flight, how would we navigate?

[u/Shynosaur]

Here on earth, even if something like GPS didn't exist, we could still orient ourselves using the stars, for they are so far away and the distances we travel on earth are so insignificant in comparison to that that their positions in the night sky are essentially fixed points.

Once we travel distances relevant in comparison to the distances in between stars, obviously, this would no longer work. So, assuming we wanted to get from star A to star B, how would we find our way?

[u/starcraftre]

There is a system currently installed on the ISS called SEXTANT (Station Explorer for X-Ray Timing and Navigation).

They look at pulsars and use the timing of them as a sort of GPS. The system is hypothetically accurate to within about 5 km anywhere in the galaxy (and they've actually already reached that hypothetical limit in actual practice, if only around Earth).

It's pretty small, tested, and just requires you to have a database of pulsar spin rates, since each one should be unique.

5 km is good enough for pretty much anything you'd want to do on large scales like interstellar.

[u/GeorgeDukesh → u/starcraftre]

The Trident missiles don’t use GPS,they use Inertial navigation, but as they go out of the atmosphere in their ballistic trajectory they use a star shot system that corrects velocity and position system errors (to an accuracy That is classified, but is in the order of a few metres) to recalibrate the inertial system and calculate the final trajectory and impact points. Hubble and the Webb telescopes use a star shot system to accurately position themselves within fractions of a degree. Apollo used star navigation around the moon the check position and speed. Lots of satellites position themselves within fractions astral observations. As do the Mars missions

[u/steppingrazor555 → u/starcraftre]

ive heard of using pulsars for navigation in space before. my understanding of pulsars is that they emit a pulse along an axis of rotation, and that we observe a pulsar at earth is due to being in line, or approximately, with the axis along which the pulse travels. if that is true, can pulsars we observe from earth be used to navigate around the galaxy when the observer may not be able to the observe the same pulsars observed from earth. or is it a multiple of observed pulsars concept, where a few of the thousands of pulsars observed from earth will be observable from anywhere in the milky way, with new observed pulsars being added while traveling through the galaxy.

[u/physicalphysics314 → u/starcraftre]

I’d imagine it’s a lot more accurate that 5km tbh

[u/starcraftre → u/physicalphysics314]

It's a tradeoff. If they use the radio signals of the pulsars, then they project a meter-scale uncertainty. However, the amount of processing time and the size of the antenna go way up.

X-rays have larger uncertainty, but the whole sensor array is the size of a refrigerator and the processing time is minutes for 100 km uncertainty and they only took 2 days to get to 7 km uncertainty in actual use as the side experiment for a piece of equipment designed for something else.

[u/physicalphysics314 → u/starcraftre]

How do you know this info? It’s not super well known. I looked at the 2019 conference proceedings introducing SEXTANT and I work with like 3 of the authors

I still find your X-ray figures a little strange. It definitely shouldn’t take 2 days to triangulate anything.

[u/Shevcharles → u/starcraftre]

How awesome is it that the galaxy is essentially equipped with perfect GPS and it would be impossible to be lost?

[u/taiwanluthiers → u/starcraftre]

Unless you want to jump within 500 meters of a ship from about a light minute away...

[u/Shynosaur → u/starcraftre]

That's pretty cool! But what exactly did we develop it for (minding we are currently not capable of interstellar flight)?

[u/ClownMorty → u/Shynosaur]

Probably just to answer your exact question and see what methods work work for navigating the galaxy. I mean, you'd definitely want to answer that before leaving.

[u/foobar93 → u/Shynosaur]

Interstellar navigation is just a site effect. If I recall correctly, they use it to study gravitational waves and other space anomalies. Basically the pulsars are very precise clocks. If the signal then suddenly does arrive a bit late, that is not because the pulsar was slower for a microsecond, it was because something altered the path of the signal.

[u/physicalphysics314 → u/Shynosaur]

Astronomy.

[u/iamiam123 → u/Shynosaur]

I reckon it has more to do with the paradox that if we were to send humans to say, Proxima Centauri with current technology, and suppose it takes 70000 years, we're looking at several hundred generations that will live and die on a spaceship. If we hypothetically add Cryo sleep (Like in Interstellar movie), there is still a possibility, that within the next thousand years, we invent a technology that will make us reach Proxima Centauri system in 2000 years. So, when the original spaceship reaches there in 70000 years, they might find an already thriving civilization, tens of thousands of years old, with no memory of earth. {Read it somewhere, can't remember where}

[u/John_Hasler]

If we were capable of interstellar flight, how would we navigate?

By the stars.

[u/Muroid]

One major difference between traveling in space and traveling on Earth is that there is very, very little “stuff” in space, which means that if your destination is a big, bright ball of light like a star, it’s exceptionally unlikely that you are ever going to lose sight of it. 

[u/alrogim]

Interesting question. For one we can just use the stars. It would be easy to calculate the changing directions of them.

But there is an even bigger problem. Interstellar travel won't work in straight lines, but in Orbits. If gravity and time is in effect, the travel would work the same way as it does in our solar system. You carefully plan your routes taking into account relevant gravitational fields.

[u/diemos09]

um, point yourself at star B and fire up the engines?

[u/db0606]

There's distances between stars and then there's distances between stars. Even if we can travel to nearby stars there's plenty of stars that are significantly further away and won't really look different from Alpha Centauri than from Sol. Even then, you still have galaxies that are orders of magnitude further away and definitely won't move.

[u/Sad_Leg1091]

Knowing the 3D positions of stars allows you to find out where you are, with a rough initial guess. If you are starting with zero knowledge then it would take a little longer, keying off known stellar objects. If you don’t know when you are, all bets are off.

[u/kcl97]

According to GR, the fact that the residents living on A can see B implies that there is a geodesic path that connects A and B in order for light to go from B to A. So this is what a spaceship on A needs to do to reach B:

1. The ship sends out small probes in all directions. Just a brief impulse to gain some speed but keep them tethered for recovery.

2. Determine which probe is drifting toward B and identify an average direction of the drift. That's the direction you need to head towards.

3. Apply a brief impulse in that direction til some speed and jist drift until you notice you are no longer heading towards B.

4. Repeat 1 to course correct.

e: If your ship is big enough, step 1 can be done inside.

[u/No_Drummer4801]

It would still work. Some stars are very distant and most stars once you start paying attention to them are very individually distinct and locatable in 3d space.

At near light speed travel its wouldn’t be a problem to map as you went and you would know precisely where you were at all times.

[u/SeaInside2292]

everybody here talking about pulsars and fancy x-ray timing systems like we're gonna have perfect maps of every neutron star in the galaxy lol

the real nightmare scenario nobody wants to talk about is what happens when you're 10 light years out and your pulsar database is corrupted. or better yet, you realize halfway through the journey that the spin rates you measured from earth were affected by some gravitational lensing nobody accounted for. good luck explaining to the crew why you're now 0.3 light years off course and burning through fuel reserves

saw this happen with a mars mission simulation i worked on. team spent 6 months prepping their navigation system, triple checked everything. day 47 of the sim they realize their reference frame calculations were off by a tiny fraction because someone forgot to account for jupiter's gravitational influence over the entire trajectory. in space that's basically nothing. over interstellar distances that fraction becomes light months of error

honestly the whole "just point at star b and go" crowd has never dealt with relativistic navigation. your destination star is moving, you're moving, space itself is expanding, and oh yeah time dilation means your onboard clocks are gonna disagree with your reference pulsars anyway. have fun with that math while half your crew is in cryo and the other half is losing their minds from isolation

best bet is probably redundant systems all the way down. pulsars plus local star triangulation plus dead reckoning plus whatever quantum entanglement bullshit we invent by then. still gonna end up lost though

[u/Glittering-Heart6762]

In space there are always stars to navigate by….

If you have a detailed map of the galaxy… or even just a hand full of bright stars… you can use the position of those stars to triangulate your position.

In fact, measuring your distance to 1 star, confines your position to the surface of a sphere centered on that star.

If you know your distance to 2 stars, this confines your position onto a circle, who’s points are equidistant to both stars.

If you know your distance to 3 stars, there can be at most 2 points that satisfy these distance.

At 4 stars or above, you can uniquely identify your location.

If measuring your distance to stars is too difficult, you can achieve the same by measuring the direction of a hand full of stars… the distance variant is just easier to explain.

[u/llDieselll]

By orienting using quasars for example

[u/Shufflepants → u/llDieselll]

Pulsars, not quasars. Pulsars are rapidly spinning neutron stars that rapidly blink in intensity due to that rotation and magnetic fields. Quasars are black holes which are shining brightly due to the friction and fusion in their accretion disks.

[u/Citizen999999]

Well since we're playing make believe and pretending interstellar travel will be possible some day, we could navigate by waving a magic wand that tells us exactly where to go in our Interstellar Make Believe Imagination Ships, (I.M.B.IS. for short) Gandalf will be the Admiral, and Harry Potter will be the co-pilot and we will set off to find a a dwarf planet for the Hobbits to live on in peace. Until Gimli arrives with the rest of the dwarfs claiming that it's their planet, because it's a dwarf planet. In the end they work it all out and agreed to share custody on the weekends

[u/Herb-Alpert]

Probably using distant blazars, this would be the same idea as using stars on earth.