8 meters will get us down to asteroids around 300 meters diameter reliably - but it will not be reliable for those impactors smaller than 300 meters, in other words those impactors which hit the Earth much more often.

When you look at the cost of Hubble, not only do you have to take into account its construction and launch costs, you also have to add in the cost of the maintanence flights.

A big difference between Hubble and any space based NEO telescope is the data processing requirements - the requirements of a NEO space telescope will be much higher than those of Hubble.

You can either transmit the data to Earth for processing, or you can process it onboard the telescope. Either you have a larger antenna and transmitter, or a larger computer, onboard the telescope, and in addition to this you also have to have larger solar arrays to power either.

The pointing problems go up, and pointing is essential here. And with more mass your fuel requirements go up, and your lifetime goes down. Then you have replacement cost -

And this is just in the optical bands, not taking into account the use of radar frequencies, which in the end might be the only thing that works to solve the problem -

I originally thought that whatever option was chosen, whether the Moon was used or free space scopes, they could be done between the completion of ISS 2006 and the first manned Mars flights in the 'teens. Right now I don't think I'll see manned Mars flight within my lifetime, or even the first steps towards it -

EP