 FAIR: A nuclear weapon to be kept in a low polar orbit could be  designed with only a small retrorocket and a heat shield, but since  the satellite would pass near a particular target only twice a day,  it might take many hours before it could be launched. Large rocket  motors like an ICBMs, as depicted in the film, would be militarily  sensible since targeting would be much more flexible and the  weapons travel time after launch would also be shorter. And with  the weapon in a high orbit, as indicated in the film, large rockets  would be required to reach the surface at all.