NASA one step closer to replacing shuttle

A Lockheed Martin concept for a reusable launch vehicle

(CNN) -- For decades, NASA has relied on the space shuttle to send astronauts and critical satellites into orbit. This week, the agency took a major step forward in selecting a successor to the aging workhorse.

Hoping to replace the shuttle with a safer and cheaper reusable launch vehicle, NASA has explored hundreds of design concepts in recent years.

From those, the number of potential successors had been whittled down to about 15 or so, the agency announced Tuesday.

"It may be a smaller vehicle and not as impressive or loud as the shuttle. [But] it could be able to launch and land within a few minutes," said Dennis Smith, director of NASA's Space Launch Initiative.

The planned budget for the program totals $4.8 billion through fiscal year 2006. NASA hopes to select the final design that year and place the new spacecraft into service by 2012.

While the field has narrowed, the remaining shuttle replacement candidates represent a wide range of engineering designs and launch systems.

One of numerous designs from Northrop Grumman-Orbital Sciences

Some burn kerosene. Others hydrogen. Some lift off like airplanes. Others like rockets. Most but not all are smaller than the shuttle.

There are some common specifications: They must take both humans and satellites into orbit. They must carry both government and commercial payloads. They must be developed and operated by private industry. They must launch with a fraction of the risk -- and cost -- of the shuttle.

Four aerospace companies are responsible for the remaining designs: Lockheed Martin, the Boeing Company, Northrop Grumman and Orbital Sciences Corp. The last two are working on joint concepts.