How Could Rarefied Space Matter Affect a Starship's Speed Near a Star?

[RodionGork]

TL;DR Summary

spacecraft won't fall onto star or planet unless aimed very accurately - I'm curious about calculating effect of space gas and dust.

In the "Andromeda Nebula" novel the author (Yefremov, 1957) describes dramatic story of a starship being inadvertently directed into the neighborhood of the "Infrared (Iron) Star" - so it was not discovered by navigators until too late. The ship is on the return route and doesn't have enough fuel to both speed-up from the danger and complete the route later.

As a child I was impressed by the fragment, but even then I soon become suspicious - if starship isn't directed into the star itself, it should be "fly-by" even with very high speed when close to the star. Author was paleontologist so it is not about criticizing his great work.

But now I wonder - perhaps "the issue" could be solved by suggestion that there is some rarefied space matter - either gas or dust. I would like to create a school exercise about calculating the effect (probably not in analytical form but as a simulation, numeric integration).

For this I need to come up with some feasible model about how this rarefied matter affects the ship's speed.

My first guess would be that, say, these are hydrogen atoms, certain amount N of them per cubic meter of the space. We can calculate the
"column" of space swept by starship per second (given area of its "front profile" and speed) - and this gives us amount of atoms disturbed. If we further suggest they all get some average speed V (directed perhaps sideways though it probably is not important) we get the energy the ship
lost to them - and hence ship's speed reduction.

What points I may be missing here?

- probably we need to suggest atoms are still or relatively slow before collision
- they collide in "perfectly-elastic" manner
- this intergalactic matter has equal density over the region.

Which of those requirements may be too severe? Any other suggestions or corrections?

 

[Baluncore]

Welcome to PF.

Assume the material, swept up by the section of the vessel, will be accelerated to the speed of the vessel. That material then becomes part of a cloud of dust travelling with the vessel.
The energy loss will be proportional to; E = ½·m·v² .

There will be parts of the vessel ablated by the many particle collisions, assume that loss is cancelled by the material embedded in the skin of the vessel. The vessel mass can be held constant.

If the vessel is in a long elliptical orbit about the attractor star, the orbital velocity of the vessel will be changing significantly during the close pass.

It is likely that the density of material in space will vary as a function of the proximity of the attractor star.

 

[votingmachine]

I'm not familiar with the book, but it sounds like it is stuck in some orbit.

My thought is that you exchange one problem for another. Any matter you suggest as present to reduce the speed so it doesn't fall in and then flyout ... that same matter will also KEEP slowing the orbiting ship until it spirals into the star.

I suppose an extreme orbit like a comets might work. A close fly-by with a LONG slow aphelion. That way you can hypothesize the energy reduction is limited to small amount of time, and the orbit could survive for a few cycles.

If you wanted to arrive at a circular orbit, then you have to recognize that the gas you posit to get to that set of conditions will still be a problem with maintaining that set of conditions.

I don't think you have to make an assumption of elastic collisions. Just assume an average energy transfer per H2 molecule slightly lower than elastic and you get a slightly net higher necessary mass of hydrogen.

It seems like an acceleration and drag calculation. The ship is accelerated by the gravity as it falls, and there is a drag from space gas. You want to specify some non-escape velocity condition on the exit path. 1st approximation would be to assume a uniform density, and then calculate the speed via gravity math, and drag based on that speed. Pick a simple path length within that uniform density. Then convert the drag you decided you need to a density

https://courses.lumenlearning.com/s...nal to the square of,the density of the fluid.

But I still think that the hypothetical drag means the ship is never in a stable orbit but eventually falls into the star.

 

[RodionGork]

hypothetical drag means the ship is never in a stable orbit

Ah, sorry for I failed to make myself clear :) I think of creating a task which requires to figure out whether ship will be able to fly by (in a somewhat modified hyperbola) or it shall lose so much energy that its speed is below the escape value and it will spiral down by and by, just as you explain.

(in a book as I vaguely remember they land on some mysterious planet obriting the star and later found some lucky means to leave - thus it is not about any stable orbit, that's true)

Assume the material, swept up by the section of the vessel, will be accelerated to the speed of the vessel. That material then becomes part of a cloud of dust travelling with the vessel.

this would be "inelastic" collision, right? something different to what I thought of, but for the sake of the problem it probably doesn't make much difference, which variant to chose - your version should work equally fine :) though I'm unsure what may make the gas atoms stick to the ship

 

[Baluncore]

though I'm unsure what may make the gas atoms stick to the ship

When atoms impact the ship at that velocity, they will probably melt into the surface of the ship.

To quote previous posts, highlight part of the post, then click on the button to add to quotes, or to reply.

 

[DaveC426913]

in a book as I vaguely remember they land on some mysterious planet obriting the star

If you plan to have this as part of your scenario, you're going to have the further task of explaining why the planet hasn't spiraled into the sun. (though I guess the time scale will solve that.)

 

[RodionGork]

Thanks for the hint, after some manipulations I was able to edit the post :)

Went reading about atoms melting/fusing into the surface of high-speed objects, sounds curious!

If you plan to have this as part of your scenario, you're going to have the further task of explaining why the planet hasn't spiraled into the sun. (though I guess the time scale will solve that.)

That's true :) it would be interesting side-task to make comparison of cases planet / starship. though we may expect that due to planet's size its mass to surface ration is immensely greater so that while it is actually spiraling down, this will take aeons

(but I don't think planet is going to be included into the task)

 

[Tazerfish]

I was pretty incredulous that there could be enough matter "in the vacuum" to slow a ship down.
After all, the density of the interplanetary medium is measured in a few particles per cubic centimeter.
Next you may think that nebulae are dense enough, but according to this Physics Stack exchange post, those only have 100 to 10'000 particles per cubic centimeter, still far less than we'd need to get an appreciable drag.

Back of the envelope calculation:
If our spacecraft has a ratio of mass per frontal area of 1000kg/m^2 and is travelling at 30km/s (earth's orbital speed) then the expected deceleration $a=\rho v^{2} \cdot \frac {A}{2m}$ is somewhere between $a=10^{-17} \frac {m}{s^{2}}$ and $a=10^{-13} \frac {m}{s^{2}}$ depending on the particle densities of 1 to 10'000 per cubic centimeter.
Even on the upper end, this would take more than 1000 years to change the velocity by 1m/s.

We'd need at least 100,000 times more density than to really mess with the orbit, which would be more than a million times more density in the current interplanetary medium.

The only plausible scenario that I can find for that is a very early solar system like a protoplanetary disk.
There, you quite plausibly get high enough densities.

Also consider that any proto-planets within this soup would not in fact have to decay and fall into the star. The gas flows around the sun in a sort of ring/disk, gravitationally bound. Assuming our spacecraft or a planet was moving along with the flow, it would experience little to no drag; by contrast, "entering the roundabout" the wrong way would quickly scrub off enough speed to make one fall into the star, or if the medium is dense enough, change directions and enter the stable orbit co-moving with the gas and dust.

Oh, speaking about dust—if my knowledge of the early solar system is anything to go by, then you'd better have a very solid "windshield"

 

[votingmachine]

For the purpose of making the story scientifically valid, I would introduce some kind of random path crossing with the planets, and a gravity brake effect. Rather than the usual gravity assist, for instance, have the path be the reverse of the Voyager path.

But I think as a class exercise, modeling the velocity in a spreadsheet doesn't have to end up with a realistic answer. It's a sci-fi story and the point is to take a flaw in the science and review one possible way to make it valid.

I'm not bothered too much by the occasional science miss-steps. If the story is good, that is what matters. And no "deus ex machina" science breakthru to get out of a jam.

EDIT: A science slip that bothered me was the ending of "The Martian" when he uses a poorly controlled airleak as a rocket. The two ships are at rest (relative to each other) in space. All he has to do is jump, and coast across. The other person can move to intercept if he is at all inaccurate.

 

[Nik_2213]

Situation may be different if the star-system is still forming, has a substantial accretion disk.

( Until star reaches fusion threshold, difficult to detect optically ? Consider 'Brown Dwarf' T & Y types.. )

As I understand it, such a 'dense' disk may, via 'differential momentum exchange', cause objects with masses ranging from planetismals unto full-on gas-giants to in-spiral.

IIRC, mini-me effect may be seen in Saturn's rings, where complex eddies form near the rings' moonlets...