WASHINGTON A futuristic "rocket ship" the length of a football field was launched from the banks of the Pascagoula River in Mississippi on Thursday morning as a ferry vessel for a new class of big boosters.

The ship, named the M/V Delta Mariner, is hauling Boeings new Delta 4 rockets from their production plants in Decatur, Ala., to launch pads at Cape Canaveral, Fla., and Vandenberg Air Force Base, Calif.

With a maiden launch of the rocket on track for April 2001 from the Cape, program managers were relieved to get a reliable ferry system going for the rockets before years end.

The ship is the only way to transport the 15-foot diameter Delta 4 about the size of a commercial airplane fuselage -- to the launch sites.

"You cant move them by truck like you can other rockets. These are too big," said Walt Rice, a Boeing spokesman in Huntington Beach, Calif.

The Delta Mariner is only the third ship in the world used to transport rockets -- and the first rocket carrier to traverse the Panama Canal.

The French government uses ships to moves its Ariane rockets across the Atlantic to launch pads in Kourou, French Guiana, and Boeing uses ships to move its SeaLaunch rockets from Long Beach, Calif., to the Pacific where they are launched.

At a top speed of 15 knots, it will take the ship about three weeks to reach California from Alabama via the Panama Canal and about a week to reach Florida.

The ship will move space-bound hardware from Decatur, down the Tennessee Tombigbee waterway through Mobile Bay and into the Gulf of Mexico.

The Delta Mariner is 312 feet long, 82 feet wide and can carry 4,620 tons of cargo. It is roomy enough to haul two fully assembled Delta 4 rockets.

"No other boat in the world can do this," Rice said. "This is specifically designed to transfer the Delta 4."

The ships activation is "the perfect milestone with which to end the year" for the Delta 4 program, said Mike Kennedy, Boeing vice president of Evolved Expendable Launch Vehicle/Delta 4.

Technicians already have completed test firings of the rockets engines and finished assembling the booster cores.

Construction on the Capes Delta 4 launch pad, which was once used by 1960s-era Saturn rockets, should be finished around the first of the year.

Launches at 'Slick Six'

At Vandenberg, Boeing will send the Delta 4s off from the storied Shuttle Launch Complex (SLC) 6 pad, known as "Slick Six."

That pad, which Boeing will begin leasing in January, was built by the U.S. Air Force in the 1970s for polar orbits by NASAs space shuttle fleet but never used for that purpose.

Boeing has signed up the Air Force, Paris-based Skynet and Loral for launches with the Delta 4. The rocket is a follow-up to the Delta 3, which flew twice, in August 1998 and May 1999. Both of those launches failed.