Interstellar navigation to Proxima B with an unmanned probe

[BWV]

Curious about the difficulties of this - everything I can find on, say reaching Proxima B with an unmanned probe deals with propulsion- but say we have something that can go 0.2C and want to put a satellite in orbit around Proxima B, can we navigate that accurately? Is the position of the planet ~20+ years from now calculable or are there chaotic dynamics due to insufficient precision in our measurement abilities?

 

[FactChecker]

Proxima B is 4.2 light years away. So there would be an 8.4 year round-trip communication delay by the time it got close. The probe would undoubtedly need its own automatic guidance system that does not rely on our measurements or corrective inputs.

 

[Vanadium 50]

want to put a satellite in orbit around Proxima B

Uh, how do you plan on slowing it down?

 

[russ_watters]

We don't fly to the moon without course corrections so I don't see why it would be a problem for interstellar travel either. These are trivial adjustments compared to the massive delta-v of getting a ship there and then stopping it.

 

[BWV]

We don't fly to the moon without course corrections so I don't see why it would be a problem for interstellar travel either. These are trivial adjustments compared to the massive delta-v of getting a ship there and then stopping it.

But you have up to an 8+ year communication lag

 

[Baluncore]

Star sights provide the information needed to find the position and orientation of spaceships. The star sights are usually taken by computers without human intervention. The course corrections can be calculated by the same computers. We do not need to be there. They will call home, not for guidance or permission, but just to let us know how they are going.

 

[Vanadium 50]

The question of stopping is relevant.

If you stop it by magic, you can steer it by magic.

If you stop it with some kind of engine, you need a LOT of fuel. The weight of a lot of fuel plus a telescope, a camera and a computer is approximately the same as a lot of fuel.

So where's the problem?

 

[FactChecker]

Star sights provide the information needed to find the position and orientation of spaceships. The star sights are usually taken by computers without human intervention. The course corrections can be calculated by the same computers. We do not need to be there. They will call home, not for guidance or permission, but just to let us know how they are going.

An automated system could do it. It was interesting that the Boeing Starliner used an automated system to dock with the ISS even though there were two excellent pilots onboard.

 

[Janus]

Curious about the difficulties of this - everything I can find on, say reaching Proxima B with an unmanned probe deals with propulsion- but say we have something that can go 0.2C and want to put a satellite in orbit around Proxima B, can we navigate that accurately? Is the position of the planet ~20+ years from now calculable or are there chaotic dynamics due to insufficient precision in our measurement abilities?

A trip would be likely done in stages. First stage: Reach the Proxima system. Second stage: assume an orbit around Proxima. Third stage: locate Proxima B, and observe it long enough to get a good fix on its orbit. Forth stage: alter orbit to one intersecting with Proxima B. Fifth stage: orbital insertion around Proxima B upon arrival.

 

[russ_watters]

But you have up to an 8+ year communication lag

I'm not sure what problem you think that presents? The ship/probe can navigate itself.

 

[BWV]

The question of stopping is relevant.

If you stop it by magic, you can steer it by magic.

If you stop it with some kind of engine, you need a LOT of fuel. The weight of a lot of fuel plus a telescope, a camera and a computer is approximately the same as a lot of fuel.

So where's the problem?

Using online calculators, the stopping fuel is about 25% of the mass with 1G acceleration to 0.2C, and that ignores what you maybe could do with gravity assists . You could accelerate the probe with a light sail or some other stationary energy source so only need fuel to stop

 

[BWV]

I'm not sure what problem you think that presents?

None, as long as you are confident in your code

 

[glappkaeft]

Using online calculators, the stopping fuel is about 25% of the mass with 1G acceleration to 0.2C,

Nowhere near that with any kind of current or near future rocket system. Current systems use 90+% of their mass in fuel to reach say 9 km/s or a little more if going to the moon (~13 km/s) or interplanetary. That is 0.0003c.

 

[BWV]

Nowhere near that with any kind of current or near future rocket system. Current systems use 90+% of their mass in fuel to reach say 9 km/s or a little more if going to the moon (~13 km/s) or interplanetary. That is 0.0003c.

Certainly not current, but accelerating a very tiny probe to 0.2c with a light sail is not an unreasonable 'near' future. That is Starshot's goal, for example. Admittedly it does not involve slowing the craft down

https://breakthroughinitiatives.org/research/3

 

[Vanadium 50]

what you maybe could do with gravity assists

You can't do anything substantial with gravity assists. You would need to find something moving near your 0.2c, and there isn't anything.

he stopping fuel is about 25% of the mass

Not even close. At 0.2c, you need to convert 2% of your mass to energy. Minimum. That's 20 MeV per proton. Even fusion provides 6-7 MeV. Your vessel needs to weigh thousands of tons just to carry the fuel to slow itself down. What's another 5 pounds for a computer?

 

[glappkaeft]

Certainly not current, but accelerating a very tiny probe to 0.2c with a light sail is not an unreasonable 'near' future. That is Starshot's goal, for example. Admittedly it does not involve slowing the craft down

https://breakthroughinitiatives.org/research/3

Breakthrough Starshot, while not unphysical, is a very long chain of very optimistic extrapolations of current technological development, that many really hard problems can be solved and the construction of a 100 GW laser. I not very optimistic that they will amount to anything, especially launching by 2036.

Apparently they have also run out of money
https://interestingengineering.com/innovation/mark-zuckerbergs-100m-interstellar-space

 

[Vanadium 50]

Breakthrough Starshot was a bunch of billionaires playing "let's pretend". The idea that you can go 100 million times farther than Apollo for 0.05% as much money was, frankly, just plain silly. But nobody wants to tell billionaires "no" when they have their checkbooks out.

 

[berkeman]

You can't do anything substantial with gravity assists. You would need to find something moving near your 0.2c, and there isn't anything.

But Proxima B will be moving at 0.2c when the probe gets there...

 

[DaveC426913]

But Proxima B will be moving at 0.2c when the probe gets there...

Har har. He means co-moving with you - relative to the system. Otherwise it's gone in a flash.
But you knew that...

 

[BWV]

Breakthrough Starshot, while not unphysical, is a very long chain of very optimistic extrapolations of current technological development, that many really hard problems can be solved and the construction of a 100 GW laser. I not very optimistic that they will amount to anything, especially launching by 2036.

Apparently they have also run out of money
https://interestingengineering.com/innovation/mark-zuckerbergs-100m-interstellar-space

Sure, but something like this is the only conceivable way to send something that distance. It also depends whether eventually economic reasons exist to build the orbital manufacturing infrastructure necessary to make something like this - on that I am skeptical - outside of Earth's orbit there is likely nothing in the solar system of any value to us.

 

[sonhousen]

Uh, how do you plan on slowing it down?

To get to 0.2c you have to accelerate so you accelerate halfway and then decelerate the rest of the journey so you end up with close to zero velocity when you get there.
Just like the idea of being able to accelerate at 1G for extended periods, of course impossible with today's propulsion systems but if we get fusion thrusters or even better antimatter thrusters, with a decent amount of fuel, we may be able to get to 0.2c or about 60,000 Km/sec, so acel for half way, turn the thruster to produce thrust slowing the craft down and you arrive at the destination with close to zero relative velocity, but endgame thrust would still be needed to maneuver to proper orbits and such.

 

[sonhousen]

To get to 0.2c you have to accelerate so you accelerate halfway and then decelerate the rest of the journey so you end up with close to zero velocity when you get there.
Just like the idea of being able to accelerate at 1G for extended periods, of course impossible with today's propulsion systems but if we get fusion thrusters or even better antimatter thrusters, with a decent amount of fuel, we may be able to get to 0.2c or about 60,000 Km/sec, so acel for half way, turn the thruster to produce thrust slowing the craft down and you arrive at the destination with close to zero relative velocity, but endgame thrust would still be needed to maneuver to proper orbits and such.

Roughly speaking if you accelerate at 1G for one year you will be close to 1c, so 0.1G of accel for 2 years gets you to 0.2c. If you could imagine a sling that would get your to 0.2 G in say one second or so, the trip time would be some 20 years, so 0.1G for 2 years means you are at 0.2c fairly early in your trip so you would have to coast for some 16 odd years to allow you to decel at that same rate, 0.1G for two years does the job of getting you to Alpha Centauri with more or less zero velocity.
To me BTW, a trip to Alpha Centauri would be a GREAT first interstellar trip because you get a 3 for one deal, three stars to examine only about 0.2 light year apart, a much easier trip to examine all three up close.

 

[Vanadium 50]

As I said, if you propel it by magic, you can steer it by magic.

 

[DaveC426913]

... with a decent amount of fuel ...

There's the rub.

 

[sonhousen]

Every star has a focus line leading away from the sun and that focused energy could be used to power solar sails. Gravitational lensing starts that focal line around 80 billion Km away from the sun so if you have a star whose focal line leads to a wanted destination, you get free energy the whole way there, with caveats of course, too long a subject to discuss fully here.

 

[DaveC426913]

Every star has a focus line leading away from the sun and that focused energy could be used to power solar sails. Gravitational lensing starts that focal line around 80 billion Km away from the sun so if you have a star whose focal line leads to a wanted destination, you get free energy the whole way there, with caveats of course, too long a subject to discuss fully here.

There is no focus "line". For any star in the right position, there is a point:

And since you are moving, you would pass through that point quickly. Lensing won't work before or after it:

And even at that, the amount of energy gained from lensing is vanishingly small. If you were outbound from Sol, you are effectively trying to get a solar boost from some star many light years away, such as Barnard's star, which is just a point in our sky. Lensing it makes it as bright as several points, but the gain is negligible. (And short-lived.)

 

[mfb]

A star is not like a conventional lens. Its relation between distance and deflection is reversed. It can be used as lens along a line, which is interesting for telescopes. Your animation has a spacecraft crossing the line, not staying on it.

But you are right that the power is negligible.

 

[DaveC426913]

Is this addressing my post?

Your animation has a spacecraft crossing the line, not staying on it.

Yes.

And since you are moving, you would pass through that point quickly. Lensing won't work before or after it:

 

[mfb]

Is this addressing my post?

Yes.
If you fly straight away from a star then you get a ring image for a long time (in principle forever, but with significant effect for hundreds of AU at least). It's not a point.

And since you are moving, you would pass through that point quickly. Lensing won't work before or after it:

That's wrong, that's why I replied.