At least not right away.

NASA's next-generation Crew Exploration Vehicle could begin carrying up to six astronauts to low-Earth orbit by 2014 and to the moon by 2020, but the gap between the shuttle and its successor could seriously hinder the U.S. space program, some experts say.

Lacking a new vehicle, NASA will be left in limbo, said Peter Bond, former space science adviser for the Royal Astronomical Society in England and author of ``Guide to Space.''

``It remains to be seen how quickly the Crew Exploration Vehicle can be completed and made operational,'' he said. ``Past experience doesn't suggest it will be ready.''

The National Aeronautics and Space Administration has considered a number of concepts, but none panned out. The agency had hoped to fly its original fleet of shuttles 100 times each. It has completed 112 missions with five orbiters and had two failures.

After the Columbia accident in 2003, President Bush mandated that the three remaining shuttles would be retired by 2010. NASA officials had no choice but to accelerate plans for a future-generation craft.

We're trying to reduce the gap between shuttles and the next generation, said Michael Braukus, a spokesman at NASA headquarters in Washington.

The replacement most likely will be a capsule that rides into orbit atop a rocket. It also will be considerably smaller than a shuttle. Additional components, such as a power stage or fuel, could be added in orbit.

Last month, the agency selected two aerospace companies to compete for the new design and prototype. Lockheed Martin Corp. and the team of Northrop Grumman Corp. and Boeing Co. will create concepts, then demonstrate their ability to manage the cost, schedule, and risk to astronauts.

Tuesday, both teams were awarded $28 million contracts to move ahead on the designs, and one will be chosen for the final concept as early as next year.

The vehicle could carry up to six astronauts into low- Earth orbit as well as to the moon and back. Most importantly, the craft has to be far safer and more reliable than a shuttle.

``We plan on giving them a system that's an order of magnitude safer than the shuttle,'' said Joan Underwood, a spokeswoman for Lockheed Martin in Denver. ``Safety is a priority.''

Lockheed Martin released its design concept in June. The Northrop-Boeing team has yet to go public. Lockheed's craft has a titanium inner shell to better protect the crew and rides atop a rocket. In the event of a pad explosion, the crew vehicle ``escapes'' from the area and deploys a parachute down-range. A similar concept was used in the Mercury, Gemini and Apollo programs.

The design differs from the shuttle in another way: It requires no landing strip, which eliminates the need for a precise re-entry, flight path and good weather.

``A drogue parachute allows you to come in just about anywhere,'' Underwood said. ``It can be land or it can be water.''

Reporter Kurt Loft can be reached at (813) 259-7570.