A civilian test pilot flew a stubby rocket plane to the outer edge of the Earth's atmosphere over California's Mojave desert this morning, apparently making him the first astronaut to fly a privately financed spacecraft.

But difficulties controlling the rocket plane led pilot Michael W. Melvill to briefly consider aborting the rocket flight, which instruments showed reached a few hundred feet beyond the boundary of space.

Independent confirmation of the accomplishment will come later from the nearby Edwards Air Force Base. But the flight's corporate organizers were confident they had reached their target altitude, from which Melvill said he could see the Earth's horizon curving from coastal islands north of Los Angeles to San Diego.

"The colors were pretty staggering," said pilot Michael W. Melvill at a news conference immediately after landing. "Looking at the Earth from up there is almost a religious experience."

Melvill, 62, was chosen from a group of four experienced commercial test pilots to fly the bug-eyed rocket plane, called SpaceShipOne, to an altitude of about 62 miles -- the official marker for space.

Thousands of spectators began gathering before dawn at the Mojave Civilian Flight Test Center, about 100 miles north of Los Angeles, to watch the historic flight, which was conceived by a small locally based aviation firm and approved by the Federal Aviation Administration.

Shortly before 8 a.m. Pacific time (11 a.m. EDT) the rocket plane disconnected from its gawky custom-made, twin-engine carrier jet at its launch point, nearly 50,000 feet above the desert.

Melvill then fired SpaceShipOne's rocket to power his steep climb to today's target altitude: 328,084 feet. On board, instruments showed that the rocket plane reached an altitude of 328,491.

On the ground, a trail from the rocket was visible to spectators far below for more than a minute.

After a few minutes of weightlessness, Melvill's space plane began its return flight, reconfiguring its wings to increase drag in the upper-atmosphere and decrease its speed. As the rocket plane reached denser parts of the atmosphere, the wings returned to their normal configuration and the unpowered rocket plane began to glide back to the commercial airport runway where the mission began. It landed safely at 8:15 a.m.

Melvill said at a news conference immediately after the flight that the spacecraft rode smoothly at supersonic speeds and described hearing "incredible rushing wind" outside the sealed cabin. He also described hearing a loud bang, which he speculated might have come from the rear of the craft, where part of the structure buckled.

At a later briefing, the pilot described rolls of as much as 90 degrees in either direction during the rocket plane's ascent and troubles controlling the rocket plane's trim during its return flight.

The rocket plane's designers were still reviewing data from today's flight to determine if any of the difficulties Melvill encountered would delay their plans for subsequent flights -- including the pursuit of a multi-million dollar prize for the private group that builds the first successful reusable suborbital spacecraft.

Private citizens have flown in space many times in the past two decades aboard U.S. and Russian space missions, but every manned spacecraft ever flown before today's flight was paid for by a governmental agency.

The private space plane was designed by aviation pioneer Burt Rutan and paid for by Microsoft co-founder Paul G. Allen, who has said he spent more than $20 million on the project. Melvill is the general manager and a vice president at Rutan's California-based aviation firm, Scaled Composites LLC, which built SpaceShipOne.

"I had my heart in my throat while I was watching this," Allen, now a Seattle investor and philanthropist, said after the flight.

Rutan said the team that oversaw the mission from a control center at the airport were "extremely emotional" at key points in the flight, which Rutan conceived more than five years ago.

"It was amazing," Rutan said. "It looked like what it did in my mind in 1999."

Rutan proudly described the rocket plane's supersonic wing reconfiguration, which he said made his spacecraft the first vehicle that can have a "care-free reentry" into the Earth's atmosphere by making the return easier and safer.

An earlier Rutan-designed aircraft, the Voyager, captured the world's attention in 1986 when it became the first plane to circumnavigate the globe without refueling. (The historic two-person aircraft is now displayed near the main entrance of the Smithsonian's National Air and Space Museum in Washington.)

Melvill has flown SpaceShipOne in eight of its previous test flights, including its first rocket-powered test flight, which took it past the sound barrier in December -- another first for an entirely private aircraft.

Melvill also piloted SpaceShipOne in its third powered test flight a month ago, flying more than twice the speed of sound (Mach 2.5) and reaching an altitude of 211,400 feet -- a little less than two-thirds of the way to space.

Today's test flight was an important milestone for the rocket plane, which was designed to win the Ansari X Prize. Inspired by the $25,000 award that Charles Lindbergh won for crossing the Atlantic nonstop in 1927, the $10 million X Prize will go to the first private group that can fly three passengers on a suborbital flight aboard the same spacecraft twice within two weeks. More than two dozen groups in seven countries are competing for the prize, although no other group has yet flown a manned rocket like SpaceShipOne.

The ultimate goal is to foster a commercial manned spaceflight industry.

"In 12 to 15 years, we'll have suborbital space tourism that costs as much as a luxury cruise," Rutan told The Washington Post in a recent interview, "and very soon after that, you'll be able to spend your vacation in orbit."

Rutan said today that if all had gone as planned today SpaceShipOne's next mission would have been the first of its three-person flights toward winning the X Prize, but because of the control difficulties Melvill experienced he might consider another test flight first.

Today's suborbital flight was similar to NASA's first two manned Mercury space missions in 1961. It did not reach Earth orbit, where spacecraft such as the Space Shuttle can circle the Earth every 90 minutes. But Melvill's rocket plane was designed to return to Earth like a glider and land on a conventional runway and then fly again, much like NASA's winged, reusable shuttles, which have been grounded since the Columbia was destroyed as it reentered the atmosphere at the end of a mission more than 16 months ago.

Today's flight began at about 6:47 a.m. Pacific time, when SpaceShipOne's carrier aircraft -- a twin turbojet called White Knight, which also was designed and built by Rutan's company -- carried the rocket plane aloft to begin an hour-long flight to the spacecraft's mid-air launching point.

SpaceShipOne's hybrid rocket engine combines elements of solid- and liquid-fueled rockets, burning a combination of Nitrous Oxide (more commonly known to dental patients as laughing gas) and hydroxy-terminated polybutadiene (rubber) to propel the space plane to about three times the speed of sound.

Back on the ground, Melvill climbed atop his spacecraft and held his fists in the air.

Melvill said at points in the flight he was subjected to forces equal to about five times the gravity of the Earth. He rode without a spacesuit in a pressurized cockpit with a view through more than a dozen small portals designed to minimize structural loads and reduce the spacecraft's weight.

Booth reported from Mojave, Calif.