If we could capture an asteroid or meteor, can we use it as a vehicle for space travel? From what I understand, propulsion is a result of mass flow, so we could use a rail gun configuration to propel the rock mass from the asteroid or meteor. The asteroid could also provide some protection from flying into micro meteors. Is this a possibility or am I a dreamer? (I am an absolute novice.)
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1$\begingroup$ Yes, that exact approach has been considered, and would definitely work. See Is Railgun propulsion being researched?. The approach would not be competitive with other kinds of propulsion unless bits of rock that can't be processed into any kind of fuel was all you had to work with, and you had lots and lots of that, and lots of energy from sunlight or nuclear power. $\endgroup$– kim holderJan 22, 2018 at 1:34
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$\begingroup$ thank you for the link @kimholder, there seems to be a fair amount of "usless bits of rock" available. Maybe a slice of moon would even work. $\endgroup$– tnt-roxJan 22, 2018 at 6:42
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2 Answers
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There have been many discussions about using asteroids for reaction mass. One obvious way would be to process material from icy asteroids into rocket fuel. Much of them are largely water, and that gives hydrogen and oxygen. If a carbon source were also accessed (like a carbonations contrite) then methane could be made. Or some may contain frozen methane. An obvious way to do this would be to transport small asteroids to L5 in the earth / moon system and process them there. Another way would be to use any solids as reaction mass, fired from a mass driver (magnetic rail gun). Even if large reusable rockets become common, the cost of lifting fuel will continue to encourage people to find sources that are already in orbit.
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$\begingroup$ Why use methane and not use lox and hydrogen in the engien directly? $\endgroup$– lijatJan 22, 2018 at 9:23
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$\begingroup$ Hydrogen is very problematic to store and handle. It has an incredibly low boiling point. It is hard to store for long, it boils off. Being the smallest element it leaks through the finest cracks. When it burns the flame is invisible. It causes metals to become brittle. (hydrogen embrittlement) On the other hand we store methane in our daily life all over town. It's easy. Elon Musk said methane "avoids the pain in the ass factor with hydrogen" $\endgroup$ Jan 24, 2018 at 5:20
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$\begingroup$ Why store it then, if the asteroid is the propellant source a low thrust truster could burn of the hydrogen in the same rate it is created, as long as ISP is good why would low thrust matter when already in space. (I guess that a case can be made for orbital insertion and the like) $\endgroup$– lijatJan 24, 2018 at 6:39
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The main problem with using the asteroid as reaction mass is that you need a large power source. Science fiction stories usually assume that a nuclear fusion (or even anti-matter) reactor is available to heat up the asteroid material to a high enough temperature so that its momentum can propel the asteroid.
In reality it will be quite a long time before such power sources become available - if ever. Until then, the only possible power source is solar power. Near earth, approximately $1 kW$ is available per square metre. As the asteroid belt is more than twice as far away from the sun as the earth is, the available power there is less than a quarter, or less than $250W/m^2$.
Now let's do some calculations. Assume an ice-based asteroid of 50 m diameter. It would have a mass of about 500,000 tons. To accelerate this at $1m/s^2 $ we need a thrust of 500,000 tons. The Saturn first stage engines generated a total of about 750 tons thrust or 40 MW each.
Allowing for some inefficiency, let's assume we need 50 MW to generate 500 tons of thrust, or 40 GW for to achieve our $1m/s^2$ acceleration. We would then need a solar panel with a total area of $160,000,000 m^2$, a square of more than 12 km on a side. Reducing the acceleration to $1mm/s^2$ still leaves us with a square of 400 m.
Then we need to ship all this to the asteroid. At $100kg/m^2$ for the solar panel, we are talking about sending 1,600 tons to the asteroid.
All this may become feasible in the distant future, but don't hold your breath.
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$\begingroup$ You seem to be assuming 1 m/s^2 acceleration and an asteroid in the main belt. I suggest reading the Keck Report. $\endgroup$– HopDavidJan 22, 2018 at 1:25
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$\begingroup$ I don't think acceleration is the issue as much as ∆V is. To refuel vehicles already in orbit, especially if it's not LEO, (say, around the moon or interplanetary) one needs to think of what is closest in terms of velocity. The velocity change that is required from the surface of a planet is going to be much greater than what is required to reach something in a similar orbit. Near earth orbit asteroids are in a much closer neighborhood to high earth lunar / orbit, or even mars transfer orbit than the surface of earth mars or the moon. Once you are out of that deep gravity well, stay out. $\endgroup$ Jan 22, 2018 at 3:06
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$\begingroup$ You forgot to mention the efficency of the solar panel. You don't get from 1 kW or 250W/m2 of radiation power the same electrical power, its only about 20 %. But it is very important to think about the huge energies and gigantic solar panels as you did. $\endgroup$– UweJan 22, 2018 at 12:13