At What Rate Of Constant Acceleration Is Earth No Longer A Threat?

[natsirt721]

On 6/3/2020 at 6:21 PM, Spacescifi said:

Let's round the number it has off to 50, and as these are scifi missiles (not the human ones), let's say they are only a meter long each.

If these are scifi (read: high-tech) missiles, then they are almost guaranteed to have some sort of closed-loop guidance, and you are probably dead if your best defense is running away (or 'evasive maneuvers', as the pansies like to say). If you do have some sort of point-defense (railguns, cannon, lasers, other missiles, etc.), you stand a better chance of surviving, but then the ball is totally in your court as to whether or not you defeat the missiles - PD performance is almost as highly debated as the laser-missile superiority argument.

On 6/3/2020 at 6:21 PM, Spacescifi said:

It does not even bother trying to evade, as the 500g counter missiles quickly penetrate the atmosphere and detonate close enough to the launched missiles that either an EMP takes them out or the air blast wave does.

At these kinds of speeds, the distance between the incoming missiles is probably very large, to the point where you need at least one PD missile per incoming, and probably three or more if you want to actually survive. Also, any astromilitary worth its salt is going to have multiple silos situated very far from each other, so best case you still need one missile per silo.  You definitely don't need to match their acceleration, even a few gees should put enough distance between you and the missiles at the time of detonation. Making them slower means they're cheaper and lighter, which means you can carry more of them - a quality that PD systems should have, especially missile versus missile systems. 

In general, given approximately equal technology levels, planetary defense systems are going to clobber anything in LEO nine times out of ten, simply because the planet has more of everything than the spacecraft does. More electrical power, more space to put things, more weapons installations, more computing resources, more targeting radars, etc. etc. Your best bet is just to stay the hell away.