WASHINGTON - NASA began early Tuesday the countdown for a test flight of a new rocket to replace the space shuttle fleet, readying for a four-hour launch window opening at 8:00 am (1200 GMT), barring bad weather.

After the US space agency's "call to stations," some 30 team members began their work at two NASA control hubs at Cape Canaveral, Florida.

The main obstacle to a launch remains the weather, the agency said in a statement.

"Currently, there only is a 40 percent chance of favorable weather during the window," it said.