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Planetary Resources is a company speaking publicly about mining near Earth asteroids for resources. I'm interested the physicality of a claim they make in a video here:

http://www.youtube.com/watch?v=dVzR0kzklRE&t=27m55s

They would like a method to reenter resources into the Earth's atmosphere that is more economic than conventional approaches, and they propose making a metal foam that is super low average density. In particular, they propose turning 100kg of Platinum into a 2m diameter ball. They mention that the terminal velocity of this (presumably at sea level) is <60 mph. Then this would ideally avoid the problems that:

  • low masses get burned up in the atmosphere on reentry
  • high masses slam into the Earth's surface

This doesn't make intuitive sense to me. Sure, the drag coefficient is high and the mass is low, but that doesn't change the fact that you have to dissipate orbital kinetic energy over a finite distance. How would this avoid the problem of melting due to the heat and thus breaking up in the atmosphere?

If a higher drag to mass ratio was a panacea, then wouldn't we just use parachutes for reentry, even in the initial stages?

meta note: if it proceeds to beta, I may request to migrate this question to http://area51.stackexchange.com/proposals/43283/space-exploration-and-technology

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you are right. the terminal velocity of the ball falling through air at or near the earth's surface is irrelevant in the context of a re-entry vehicle.

as you point out, the central task of engineering a re-entry is how to get rid of the kinetic energy that the re-entry vehicle picks up as it falls from its orbit above the earth's surface.

all re-entry vehicles do this by converting that kinetic energy into heat by compressing the air in front of them, forming a hypersonic shock wave, as they slam into it at extremely high speed- and then either shedding the heat before it melts the vehicle or cooks the contents, or blocking its conduction into the vehicle.

re-entry temperatures generated by this shock would melt that ball promptly.

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I think that you answered the question yourself. With a smaller mass, the orbital kinetic energy is smaller. With a large drag coefficient (thus large surface area) you can convert that kinetic energy into heat on the spacecraft, and then re-radiate that heat much faster due to the large surface area (power emitted is proportional to surface area). You also have more time to do this since the aircraft is falling more slowly.

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