# Using an Asteroid's Momentum for Spaceship Propulsion

There's a lot of talk on futurist blogs about mining asteroids for propellant and other resources for space travel, but I have something else in mind. Could a spaceship, starting out in geocentric orbit, somehow grab onto an asteroid with a highly elliptical orbit and use that asteroid's momentum to toe the craft out into heliocentric orbit?

The biggest problem with this idea is the enormous difference in relative speed between the spaceship and the asteroid - it's hard to imagine any way the spacecraft could physically interact with the asteroid that wouldn't result in the immediate annihilation of the craft. Perhaps some sort of magnetic device used on a metallic asteroid?

If safety is a concern, would this be possible for an unmanned cargo craft?

• The chairlift ride to the asteroid's apogee doesn't matter much; non-eccentric orbits are a function of velocity, not altitude. Also, if you misjudge the mass of the asteroid, the new spacecraft+asteroid system will not go where you expect. – user535733 Jul 28 at 16:12
• If you use gravity as your tow line then you... erm... invented the slingshot manoeuvre. – Joe Bloggs Jul 28 at 19:39
• @JoeBloggs I guess I'm looking for a less metaphorical slingshot maneuver. – Pink Sweetener Jul 31 at 2:20

Let the asteroid accelerate you slowly.

0: Spot incoming asteroid. Call out "Asteroid ahoy!"

1. Get in position near asteroid's path.

2: Harpoon asteroid with cable. You have a railgun harpoon which you will use for other reasons later in this story.

3: Pay out cable such that you are accelerated at your tolerance and lengthening cable accommodates the difference in relative speeds.

4: You will eventually be going the same speed as the asteroid, at the end of a long cable.

5: Reel yourself in to the asteroid if you like. You are on your way.

• It would have to be an extremely long cable. If the intitial speed difference is (say) 10 km/s, and you can handle an acceleration of 50 m/s/s (5g) for a few minutes. It would take you 3 min 20 s to get up to speed. During that time, you will move 1,000 km, while the asteroid would move 2,000 km. Your cable would hence have to be 1,000 km long. This cable would have to be strong enough to haul your spaceship at 5g acceleration, so it isn't likely to be light. – Klaus Æ. Mogensen Jul 30 at 7:59
• @KlausÆ.Mogensen I was about to write the same thing. Just to finish your train of thought, Kevlar cable is 20 tons/km, and steel is way heavier still. So unless you have something incredibly strong and light to make the cable out of, (maybe carbon nanotubes?) you're far better off exchanging that huge payload mass for a smaller quantity of propellant. In other words, unless we invent way better cables without also improving jet propulsion, this plan definitely requires more fuel than it saves. – Gilad M Jul 30 at 8:23
• If you have a spacelift - you can have such a kind of acceleration – ksbes Jul 30 at 12:50
• @KlausÆ.Mogensen: thanks for the comments. You could detach your cable before you matched the speed of the asteroid. You could reuse your cable later. Supermaterials are fair game for this sort of SF I think. Mostly though I thought the asteroid harpoon was awesome. – Willk Jul 30 at 14:16

Obvious problem: there doesn't seem to be a reliable supply of earth-crossing asteroids that frequently pass sufficiently close to earth that you could make your manoever. This is a good thing for life on earth as a whole.

Secondly, magnets won't help you much; you might get a small boost, but the strength of a magnetic field drops off too quickly with distance and the target is flying too fast to be in range for long. Either you accelerate massively quickly (possibly smashing your ship and liquifying the meat cargo in the process) or you won't get much benefit.

Lastly, close interceptions with massive objects with relative speeds measured in many kilometres per second sounds like a really risky sort of thing to do. Rather you than me.

• You are wrong about there not being a reliable supply of earth-crossing asteroids. As I wrote in my own answer, 93 asteroids crossed the Earth within the Moon's orbit last year. And, sure, close encounters at high speed are risky, but without an atmosphere, there aren't a lot of variables, so it should be perfectly calculable. I wouldn't rely on human steering, though. – Klaus Æ. Mogensen Aug 1 at 7:26

The way to go is to make a close fly-by, the way deep-space probes pick up speed by passing major planets. If you do it right, you can add up to twice the asteroid's orbital speed to your velocity.

This would require making a tight elliptical orbit past the asteroid, seen from the asteroid's frame of reference. This will make your ship leave at the same speed it entered orbit, just in the (nearly) opposite direction - again, seen from the asteroid's frame of reference. Seen from your spaceship's original frame of reference, you will now be whizzing ahead of the asteroid at twice it's speed.

The way to go about it is to go as close to the asteroid as you dare when it passes, allowing you to slingshot yourself around it. This would require very little fuel, but very precise maneuvering.

You would not need a very big asteroid, just one that is a good deal heavier than your spaceship. Too small, and you will slow it down as you speed up and hence not get the full benefit. A solid rock 10 to 20 meter in diameter would probably do the trick. Such rocks often pass close to Earth. In 2018 alone, 93 asteroids were detected passing Earth within the Moon's orbit, though many were only a few meter in diameter.