I wonder, could they be employed in hard SF, if yes, how? If one intends to capture a planet (example: Mars with a few cities, capital on south pole) the defenders only doomed if big ships can reach low orbit and burn them with lasers. Big surface lasers can easily outpower even the biggest ships, but can there be any reason why were they ineffective against surface units? Anti-satellite missiles can be a serious threat to any big ship coming close, while fast agile shuttle craft can be hard to hit, especially if the attackers can kill orbital recon, and cleanse a landing belt from radars. (The defenders could put those missiles to satellites also, but then they would be very vulnerable to small delta-V missiles and orbital fighters launched from high orbit) Extrapolating present day results, how big mass driver would be needed to throw serious amount of projectiles (with a minimal delta-V to track ships) to low orbit from martian surface?