Earl Colby Pottinger wrote:
Anthony Frost :

In message
Sander Vesik wrote:

Keith F. Lynch wrote:

By the time it's noticed that a ship is approaching earth at 90% the
speed of light, it's far too late to do anything about it. And when
it hits, it will devastate a continent.

You need an early warning system that will spot any such 1-2 light
minutes
out and then you shoot a stream of protons at 99.9999+% of lightspeed at

it and watch it blow up.

And hope it takes less than 12 seconds to get the cannon aimed and
firing. Much, much less than 12 seconds in fact because, even if you've
managed to destroy it, you've still got a cloud of lumpy plasma heading
for you at nearly 90% of lightspeed which isn't going to do your
atmosphere any good.

You seemed to make very limited range for detection. If the SS is detected
one light year out then you have just over a months warning. If you fire
soon after that warning you show intercept the SS about 15-18 light days out.

That's a moderately dense net.
A light year out, a months warning.

Assuming a tenth of that is light travel time to the sensor, and assuming
it's passive, that's around a thousand detectors.

Then there is the interesting issue of how you detect a non-emitting
body being as stealthy as it can going past a light week away.

How brightly would such a body make interstellar plasma glow?