Organizers say it would be the greatest sailing expedition since Columbus. A racing fleet of gossamer spacecraft, propelled only by the gentle nudge of the sun's rays, would set off for Mars in 1992 to commemorate the 500th anniversary of the Italian explorer's voyage to the New World.

The celestial vessels would consist of huge, kite-like sails made of reflective material no thicker than sandwich wrap. Some proposals envision huge sunflower shapes, others square sails the size of several football fields. All would rely on a bombardment of photons from the sun to accelerate them to Mars.Called the "Columbus 500 Space Sail Cup," the project is long on enthusiasm and short on money for now. Engineers in a half dozen nations are working on spacecraft designs and are hoping to raise money from non-governmental sources. Even some of the enthusiasts, however, are skeptical about the prospects for sending the imaginative craft on their way.

"I'm enthusiastic about it, but I'm not too optimistic," said Andreas von Flotow, head of a design team at the Massachusetts Institute of Technology. "I think the biggest hurdle is to get the thing funded." The MIT craft, kept purposely bare-boned, would cost about $3 million.

Still, the sail race has been sanctioned by the Columbus Quincentenary Jubilee Commission, the body established by Congress to oversee events for the Columbus anniversary. Klaus Heiss, a space technology consultant, sold the commission on the idea and heads the effort to mount the competition. He said last week he hopes to have an agreement by the end of June for an expendable rocket to launch up to four sails.

At an altitude of about 1,200 miles, the sails would unfurl and spend three months or more building to an Earth-escape velocity of 67,000 miles an hour - tacking like sailboats to gain maximum push from the sun's photons. The voyage to Mars could take anywhere from one to five years.

Heiss declined to discuss his rocket negotiations, but members of several sail teams said the booking schedules, insurance costs and preparation time for U.S. rockets and the European Ariane rocket present substantial hurdles. The cost for a single sail launch could run $15 million to $25 million, according to one estimate. But Heiss also has been exploring the use of a large Soviet "Proton" rocket or a Chinese "Long March" vehicle.

Despite the financial uncertainties, engineers and scientists say the solar sailing ships are eminently feasible.

The concept was first proposed in the 1920s and was seriously considered by NASA in the 1970s as a means for sending a spacecraft to rendezvous with Halley's comet. It is based on the principle that light - in the form of packets of energy called photons - exerts a slight pressure when it is absorbed by or reflected from matter. Given a large enough area and a small enough weight, a solar sail will be gradually but steadily accelerated in the vacuum of space. The trickiest challenge, engineers said, is successfully unfurling the huge sails in earth orbit.

If a solar sail were successful, it would help demonstrate a relatively low-cost technology for sending small science payloads to nearby planets and asteroids, experts said. Heiss said he is confident that - at a minimum - at least one sail will be launched for the Columbus celebration. The successful group could well come from abroad, he said. Teams in Britain, Italy, China, the Soviet Union and Japan are working on sail concepts.

The Columbus quincentenary commission would like at least three competitors: from the Americas, where Columbus landed; from Europe, where his journey began; and from Asia, where he thought he was heading. The three flagship solar sails would be named Nina, Pinta and Santa Maria.

Finalists for the "Americas" flagship are:

- A $15 million aluminized, sunflower-shaped craft designed by a group of Washington-area engineers led by Harold Fox of the Johns Hopkins University Applied Physics Laboratory. The craft would be 560 feet in diameter and consist of 480 "petals." It would weigh about 400 pounds - including a payload suspended above the center of the sunflower by a boom and guy wires.

- A square sail, 180 feet on a side, designed by the California-based World Space Foundation at an estimated cost of $6.5 million. It would have a rotating, triangular vane at each corner to help guide the craft. Some of the foundation team members were involved in the original NASA studies on solar sailing.

- An extremely lightweight sail designed by the MIT group. It would weigh about 44 pounds, be about the size of a city block and carry very little instrumentation.

- A Canadian Space Society sail, hexagonal in shape and about the size of three football fields.

Robert Staehle, president of the World Space Foundation, said the projected October 1992 launch date may already be out of reach, given the lead time to build such craft. He said a late 1992 or early 1993 launch may be feasible "if the money can start flowing in six to eight weeks."

Staehle said his group is encouraged by its efforts to raise corporate and private donations. The team already has built a 50-foot test section of sail. Staehle said his team is determined to carry on its work, even if the 1992 race does not pan out. "We are going to learn lots of useful things out of flying our solar sail," he said.