Hitching to the outer solar system

[nowickisrj]

Does anyone know if hitching a ride on an incoming comet has ever been considered as a means of conserving fuel for missions to the outer solar system? A craft could dock with the comet, use local gases to refuel and use the momentum of the comet to carry it with a healthy outbound velocity until it is advantageous to accelerate under it's own power.

[IsaacKuo]

This won't work because the only way to dock with a comet is to first accelerate to match its velocity. If you can do that, then you can simply use your own momentum to coast to the outer solar system rather than try to dock with the comet (which will generally not be headed to the outer solar system anyway, but rather be coasting far above or below the plane of the solar system).

Comets do not have any mysteriously special "momentum" which is different from the momentum of any other object in space. Whether you "dock" with it or coast alongside it will have no significant effect on how far or fast you will travel.

[speedfreek]

I'm sure that being able to use the comet to refuel would be an advantage, but I don't see any advantage in the craft sitting on the comet as it orbits around the sun, or the craft orbiting by itself. The craft would have to achieve the same orbit as the comet, in order to dock with it.

Having said that, perhaps the comet would serve to shield the craft in some way.

[Jens]

I don't know if this is a realistic option at all, but it would be possible if you had a very strong craft that could just get in the way of the comet and crash into it and then be carried along the path of the comet.

[baric]

I don't know if this is a realistic option at all, but it would be possible if you had a very strong craft that could just get in the way of the comet and crash into it and then be carried along the path of the comet.

The comet has no gravity to speak of. You'd most likely bounce off of it like a billiard ball.

[Trakar]

The comet has no gravity to speak of. You'd most likely bounce off of it like a billiard ball.

Harpoons and long reels of cable perhaps, but even if you could get such a system to function, comets are hardly a convenient or timely means of transport.

[IsaacKuo]

The comet has no gravity to speak of. You'd most likely bounce off of it like a billiard ball.

Bouncing off would be a bonus--you get up to twice as much delta-v out of an elastic bounce as opposed to an inelastic crash.

However, in either case the amount of delta-v which is plausible is pathetically low. If your spacecraft can survive 1000 gees and has a shock absorption layer that can collapse 1m, then you'd be able to survive at most a 141m/s collision. If we assume a perfect bounce, that gives at most 283m/s delta-v. That requires only extra fuel of only 10% spacecraft mass, assuming thrusters with 300s I_sp. In contrast, the mass penalty for building the spacecraft to survive 1000 gees and the mass of the shock absorption would likely be much higher than 10%. And if you don't hit the comet exactly right, then your mission literally crashes in flames.

And the real kicker is that this comet won't be in exactly the ideal location, while the extra fuel can be used in the ideal location. In this case, the ideal location is at LEO altitude, where the effectiveness of the burn is about an order magnitude greater than in deep space. So really, the mass penalty for the "simply carry extra fuel" option is only about 1%.