My answer to How powerful will a nuke dropped from space be?
Answer by Vishu Menon:
Your expectation that the bomb would get extreme speed is sadly (rather happily) wrong, even if one were to assume that the nuke would ever hit the ground where you needed it dropped.
Space, with respect to earth, you may consider to begin at some 400 kilometers away from earth since that is where the ISS (international Space Station) orbits. Gravity there is miniscule (which is why they call it micro-gravity) and the atmospheric pressure outside is virtually nil.
ISS flies at an average speed of 27,576 kilometers per hour. A spacecraft on geo-stationary orbit would be flying at 13,320 kilometers per hour. Let us say while dropping the bomb, you are flying at one of these speeds or somewhere in between.
The moment you let go off the bomb, it would not drop down in a straight line. (It won’t, even if you drop it from a jet plane.) Released from a flying spacecraft, the bomb will orbit along with it for several days, or weeks, or even months depending on your radial distance from earth and hence the gravitational decay on the orbit earth would have on your bomb. One alternative is that you set up controls and retro-rockets on your bomb, along with a GPS and monitoring system. Not entirely impossible; they are doing it on miniature drones. Even if you have equipped the bomb with all that, you would still need to do some orbiting before you make a re-entry to earth.
Now that re-entry is not like a pair of glass doors in the mall that part automatically at the sight of you. The nuke would hit a massive screen, generating temperatures between 1600 to 2000 degrees centigrade. I suppose that could be high enough to set off your bomb. You remedy that possibility by fitting a suitably shaped heat shield on the nose of the bomb and ensuring that it meets earth’s atmosphere nose down.
If your bomb is on a free fall, it would continue to fall in a parabolic trajectory until the resistance of the atmosphere owing to the rising viscosity of air slows it down to what is known as the terminal velocity for the mass of your bomb. There is a formula for working out the TV given the weight of the bomb, but I shall not venture to work out that. Unless the bomb is powered to fly like an aircraft (or the now abandoned space shuttle), the progress of the bomb would be now without acceleration. Since F=ma (surely you know what that means), the bomb would have lost all force derived from speed.
Don’t let that disappoint you. When the nuke hits the ground it encounters a massive deceleration – from a high velocity to zero velocity in virtually zero time. (F=-ma). Hence there will be an impact, but with far less a force than if it were dropped from a jet plane flying at, say, 40,000 feet with little chance of attaining terminal velocity, and the nuke still in the mode of acceleration.
If you were to guide the nuke all the way in a powered craft, you might as well as use a huge remote-controlled drone to do the job and to hit the ground with the force required to set off the fusion, or fission, whatever.
So don’t try it, even if you dislike your mother-in-law or are in a hurry to destroy the Islamic State.