[noir_lord]

> Most people forget that it costs almost as much delta-V to get down as it does to get up in the first place

Doesn't that depend on how you get it down.

Metal foam dropped from orbit into an ocean would float, you could have a retrieval vessel go pick it up.

[evgen]

Metal anything dropped from orbit at the speeds we are talking about would either burn up in the atmosphere or else hit the ocean surface and become lots of small chunks of something that is now sinking to the ocean floor. You need to shed a huge amount of delta-V just to get to the point where the landing site is described using any term other than 'impact crater.' Most of the mechanisms for dumping this energy tend to be a bit tricky to pull off and I am not sure, but there might be an upper limit after which ablation stops being a viable option.

[noir_lord]

If the foam ball you drop is large enough losses from ablation wouldn't be a big deal.

You can certainly get the speed down enough so that the words impact crater wouldn't be in the report, also you smack them into oceans not land (though smacking them into the desert would be an option I guess if you could shed enough speed).

For a 10 meter sphere of nickel foam with a density of 2000kg/m3 (so about 60% 'air' by volume) it's ~2000 miles per hour[1].

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html#c5

Fast but not insurmountable.

That sphere would be ~8000 tonnes of pure nickel minus ablative losses (which would reduce the impact speed).

Of course you'd want the density to be below 1000KG/m3 or it'd sink.

So a 10m sphere at 850kg/m3 would have a terminal velocity of ~1300 miles per hour - or about mach 2 and you'd be picking up about 3500 tonnes of pure nickel bobbing about.

I'd quite like to see that actually (from a good distance away).

[evgen]

3.4 million kg, traveling at 600 m/sec, and you think you would end up with a big ball of floating nickle? Okay....

Not a physicist, but my thinking would lead me to say that when this ball hits the water (after having already been heated a bit by ablation on the way down) it is going to compress into a pancake. Assuming the energy released at impact does not break the ball into lots of smaller pieces, the energy of the impact is going to vaporize a lot of water and melt a lot of nickle. I think what you are going to end up with after the large impact event is a compressed blob of nickle that is now rapidly sinking to the floor of the ocean.

We already know what happens to big chunks of metal from outer space that impact the surface of the earth, it tends not to be pretty. Oh yeah, and try to convince any country on the planet to let you drop your giant ball of awe-inspiring kinetic energy on a path that happens to cross over them. Not. Going. To. Happen.

[LargoLasskhyfv]

When you have the ability to make it into foam in space, why not use that to make lifting bodies out of it, and let them glide down, instead of crashing like a stone?

Stop thinking in terms of projectiles. Just because the Space Shuttle came down like brick in a controlled crash doesn't mean there are no other ways to do this. Without the need for ablation, btw!

[noir_lord]

The lifting body idea is interesting, shape it so that it's got a flat side that self orientates (basically like an uncontrolled shuttle) and 'glide' (if you can call mach 1 gliding) it in, neat. You'd want to be very sure it was going to come down where you expected or you might affect rental prices.

[LargoLasskhyfv]

I think one could aim for a https://en.wikipedia.org/wiki/Landing_footprint in the seas near the coastline without much trouble.

[evgen]

You can't 'glide down' from orbit. You need to dump your delta-V somewhere. Stop thinking of orbit as though this chunk of metal is suspended from some big balloon out there. _Everything_ in space is moving, 0.0% if it is moving at the right speed and direction to enter the atmosphere and soft land without a lot of external assistance.

[LargoLasskhyfv]

I know that. I've written elsewhere how I imagine that. With modular thrusters which initiate the deorbiting, detaching when it is en route. Could be autonomous, could be 'tugboats' with human crew, I don't care. Doesn't really matter, as long as the lifting body of refined raw materials has the right weight to surface ratio, and therefore rarely even peaks above 400°C for a short time.

[Symmetry]

Meteorites that arrive on the Earth from outer space are most certainly a thing that happens sometimes, that's wear all our existing Mars rocks come from for instance. Whether something survives reentry depends on a lot of factors including the size of the object, the angle of re-entry, its composition, etc.