Solar Thermal
the astronomical possibilities with Moon based telescopes are fascinating.
In the radio frequnecies SETI should be possible.
The big problem for optics is going to be electrostatic dust control systems.
I think that perhaps solar thermal propulsion may be the best choice for many applications. Solar sails stike me as being too low thrust for some payloads.
As far as interception goes, Mazanek's nuclear powered interceptors with laser ablators are the only thing likely to work. I don't think the use of stable parking orbits will be necessary for these, they'll probably be laaunch and go.