Question: When is a space shuttle not a space shuttle? Answer: When it's a space plane.

The space shuttle Atlantis flew the last shuttle mission on July 21, 2011, capping close to 30 years of spectacular missions that began on April 12, 1981 when Columbia took to space. Over the past four years, the U.S. has wondered what's next.

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We now have the answer: a robot space plane that won't be crewed by astronauts. This vehicle won't even have the capability to orbit the Earth like the space shuttle.

The successor to the venerable space shuttle is a reusable space vehicle for launching small payloads into low Earth orbit (LEO). For want of a sexier name, this vehicle has been given the name XS-1 by the geniuses at the Defense Advanced Research Projects Agency (DARPA) that's building it.

DARPA officially refers to XS-1 as a "space plane." It plans to fly a prototype by 2020.

Besides it not being a true space vehicle, XS-1 will be notable because it'll be a drone, a robot space ship.

It will launch itself to the edge of space (basically 100 kilometers up there) and release its payload into LEO. It's being called a plane because it'll take-off and land like a plane on every mission.

DARPA's toy will then be refueled and launched again. DARPA wants its space plane to be so reliable it can fly "10 times in 10 days." DARPA expects the cost of a space plane flight to come to a measly $5 million compared to the $450 million once spent to launch a space shuttle.

DARPA has now entered phase two of its space plane project, which means it'll soon award a contract to competing private firms to build a prototype.

Three industry teams are competing: Boeing and Blue Origin (Jeff Bezos' space transport firm); Northrop Grumman allied with Virgin Galactic and Masten Space Systems with partner XCOR Aerospace.

This winner of phase two will be awarded the phase three contract to conduct flight tests of the space plane prototype.

Tagsspace plane, XS-1, Defense Advanced Research Projects Agency, DARPA, Boeing, blue origin, Northrop Grumman, Virgin Galactic, Masten Space Systems, XCOR Aerospace