SAN FRANCISCO, California (Reuters) -- For sale: what might be one of the Soviet Sputnik satellites that ushered in the Space Age and sparked a frenzied competition that soon led to the launch of the first man into orbit.

As with so many things Russian, the sale, offered online by Sovietski.com and eBay Inc. for a starting auction price of $25,000, presents a riddle wrapped in a mystery, for no one is able to say how many authentic Sputniks actually exist.

The Soviet Union launched the original Sputnik, a shiny metallic orb with four antennas streaking from the side, in 1957, setting off a panic in the United States, which feared it was falling behind its Cold War rival.

Amid the secrecy that prevailed at the time, Moscow made several back-up models, and since the fall of the Soviet Union, some of these have landed in the hands of their former adversary on very capitalist terms.

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Cathleen Lewis, curator of the Russian and Soviet Space collection at the Smithsonian in Washington D.C., said experts believe Moscow originally made four back-up Sputniks. Yet far more than four Sputniks are now in circulation.

The original 183-pound (83 kilo) Sputnik burned up when it re-entered the Earth's atmosphere.

"There is so little documentation and accountability. We just don't know what it is," she said of the latest sale.

If real, the Sputnik now on offer by Sovietski.com, a firm specializing in Soviet collectibles, could be a relative bargain. They are running an eBay auction through June 5 starting at $25,000 with a outright buy price of $29,500.

Sovietski advertises that their "genuine" Sputnik came from the Ukrainian Science Institute outside Kiev.

The Smithsonian's Air and Space Museum displays one Sputnik; several Russian museums also claim original Sputniks. The Museum of Flight in Seattle spent more than $100,000 two years ago to buy a Sputnik with documentation from Moscow's Museum of Cosmonautics.

"One really has to look at the provenance of the piece they are getting," said Museum of Flight president Ralph Bufano. "That's why I was so wanting this one because I could verify that it was one of the original ones."

"They're trying to build money out there," he said. "I've been told that if you can't get a hold of an original space uniform or something like that, (somebody) will build you one."

George Stauffer, a vintage car dealer in Wisconsin, says he has two original Sputniks, making him the largest collector of the distinct space memorabilia. In an interview, he said he has documentation from Moscow's Museum of Technical Achievements.

"It is always possible that anything you buy is fake," he said. "I'm in the collector and high-performance automobile business and there are some really, really fantastic copies that are made today that you cannot tell whether it's a copy."

Stauffer said he bought his shiny Sputniks three years ago through a NASA official in Florida who knew a Russian with the right contacts. He is now offering one of the two for sale on his Web site -- www.staufferclassics.com -- for $39,000.

Collector Stauffer said even if real, his Sputnik causes some confusion at his dealership.