Space Planes are ground to orbit vehicles that operate pretty much like airplanes except they go really high. The advent of anti-gravity technology (see next article) gave a running start to the technology, but as yet only 10 of these craft have been built.
Reed Richards of the Fantastic Four built the first one and licensed the technology to STAR Labs, which sublicensed it to various corporations. Of the ten built, seven are in normal operation. Three are being used to ferry materials to space stations. Two are being fitted for an extended journey to the Moon, one of these is being fitted to land on the Moon. One is owned by Spaceview Inc. and is used for really high paying tourist runs. One is being used by Doctor Doom/Latveria (this is the original built by Reed Richards, who is suing in international court to get it back). Of the other three built, one was destroyed in a super fight, one was hijacked by the Ultimates, and one just plain disappeared.
The Atlanteans have looked for it on the sea bed and found nothing. Or so they say.
These Space Planes should not be confused with the sub orbital planes being used for long-distance travel these days.