Here is a great site on space travel. It's geared to the engineering aspect of space travel for science fiction authors. The link is from Milblog.
I take exception to the author's assumptions about spaceports. He assumes they will be on the ground, but that is highly unlikely. Sea ports are at the interface between sea and land, airports are at the interface between air and ground, and spaceports will be at the interface between space and planet. That is, they will be in orbit --probably very low orbit.
There is a very good reason that ports are at interfaces. The whole purpose of a port is to transfer cargo from one form of vehicle to another. Sea ports transfer cargo between ships and ground transportation (like railroad and trucks). Airports transfer cargo between airplanes and ground transportation. Space ports will transfer cargo between space vehicles and planetary vehicles (air-to-orbit shuttles).
By the time there is any substantial planetary travel, air-to-orbit will probably be accomplished from normal airports. There are various schemes to do this and with any serious economic incentive, one or more of them would soon be made to work. There will be no need for anything called a "spaceport" on the ground (except that of course administrators will want to call their airports a "spaceport" for the prestige).
No spaceship is going to travel commercially from Mars to Earth and then land on Earth (or on Mars, for that matter). That would make as much sense as designing an ocean-going cargo ship that gets out of the water at San Diego and drives overland to deliver its cargo directly to Phoenix. Possible, yes. Economical, no.
The name "spaceport" should be used only for facilities that real spaceships can dock with, facilities that serve as the cargo transfer point between spaceship and planetary shuttle. This leaves us without a word for the ports that serve landing craft on planets without air. "Airport" doesn't seem quite appropriate. I propose that an "orbitport" serves as the interface between a planet and orbital facilities. And prestige-grasping administrators can call their airports orbitports if they want.