You remember SpaceShipOne, right? It's the passenger-carrying rocket ship that's boosted a big chunk of the way to space by that crazy airplane that's really two airplanes stuck together. A company called Stratolaunch is going to take this concept and supersize it to launch cargo rockets (and eventually passengers) to orbit.
Stratolaunch comes from the space-age minds of Paul Allen and Burt Rutan. The concept is very similar to the way Virgin Galactic's SpaceShipOne/Two (also a Rutan design) launches: a carrier aircraft hauls a rocket to 50,000 feet or so and drops it off, and the rocket then fires up its engine and powers itself the rest of the way up into space.
The difference between Stratolaunch and Virgin Galactic can be summed up in one word: scale. Let's start with the carrier aircraft: Virgin's White Knight Two has a wingspan of 140 feet and can lift 37,000 pounds. Stratolaunch's aircraft has a wingspan of 380 feet and will be able to carry a whopping 490,000 pounds. That's a lot more, if you're counting. In operation, that half million pound payload will be taken up by a multi-stage SpaceX booster that should be able to deliver 13,000 pounds to low-Earth orbit. With a sextet of 747 engines, when this aircraft takes off for the first time, it'll be the largest airplane ever flown ever.
Launching rockets like this is cheaper, safer, more efficient and more versatile than using a ground-based launch platform. Oh, and did we mention that it's cheaper? When Stratolaunch gets up and running, it'll be cheaper for NASA to pay them to shuttle people to the ISS than they're currently paying the Russians to use Soyuz capsules. It's going to take a little bit to get to this point, though: the first test flight of the aircraft is slated for 2015ish, with the first rocket launch happening the next year.
A concept vid from Stratolaunch takes you through the whole process, below.