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Comment by khuey | Hacker News Reader

Getting stuff up into space is hard and expensive. But if you have a big chunk of metal in earth orbit it's not hard to get it down.

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Once you have your rock in a stable Earth orbit you can aerobrake it down into low orbit and from there probably land it into some shallow sea.

Though possibly the asteroid would be more valuable in orbit than on Earth. It's hard to get materials up there!

I'm pretty sure the Tunguska event was an asteroid aero-braking into the ground. Where are you going to put the kinetic energy? Once it hits atmosphere there's not a lot you can do. Maybe attach a really big parachute.

The speed at which energy is released varies by altitude and speed. The Tunguska asteroid managed to reach just 10 km over the Earth's surface before exploding where the air is getting quite thick. What you want to do is rather than dive in head first just skim the atmosphere and drain your energy bit by bit. Once you're going slowly you can afford to go lower without burning up.

When astronauts return from space they have to carefully control how they descend and make sure that they don't get too low while they're traveling too fast. Otherwise they'd burn up.

Gliding/skipping entry is not feasible for such a massive, unpredictable and uncontrolled object. Both due to accuracy and aerothermodynamics. Thermodynamically speaking, entry angle is a trade-off between the peak heat load and the total heat received, and for the prolonged reentry the object has to be engineered to radiate much more heat away than with a steeper angle entry. Also, skipping entry is extremely inaccurate, and must be precisely controlled to achieve the desired result.

This all assumes that for some strange reason you’re dropping the whole asteroid rather than packets of pre-processed material which is what you’d actually do. As a bonus you could use some of the cruft from the original body to make heat shielding for the “parcels” to be used on reentry.

You can shape the thing so it slows down to a lowish terminal velocity. Think of a wide shallow cone shell with rounded tip.

Or even a hollow sphere.

Landing asteroids in the sea is a really bad idea if you care at all about either the ocean ecosystem, or anything you have on the coastline anywhere in the region.

Can you break off smaller pieces and parachute them down?

That must be why landing the space shuttle was such an easy and inexpensive problem.

To be fair, the "getting it down" the gravity well is easy and inexpensive, but "landing" it is a different problem.

There's nothing like the unique joys of spending tens of billions of dollars and decades to have your metallic asteroid burn up 90% of it's mass on reentry, the rest explodes over a wide area or sinks into the sea, and being arrested for use of a WMD.

Heat shielding for reentry is a solved problem, you will not lose 90% of anything, or have it explode.

Easy solution - just send down bigger asteroids, and drop them on whomever might object.

Easier solution - park the asteroids somewhere and have the nations of the world pay you not to.

Landing the space shuttle was hard because it used a crappy design for secret reasons. Technicians at the time tried to propose a sensible design instead, but obviously could not override this.

Also hard because of the requirement to have crew and not kill them, both of which go away when all you’re doing is dropping e.g. one ton cylinders of mildly purified gold wrapped in a steel tube.

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