"Our vision is to create a reliable and affordable U.S.-based commercial human lunar transportation system," said former Apollo flight director Gerry Griffin, who serves as chairman of the firm, named Golden Spike.

The expeditions would use existing rockets and spacecraft now under development to fly NASA astronauts to the International Space Station.

Depending on how many customers sign up, the company said it could be ready to fly its first mission by 2020. It did not elaborate on any existing or pending contracts with customers or suppliers.

The first mission would require an investment of $7 billion to $8 billion, said Golden Spike President Alan Stern, NASA's former associate administrator for science. Once established, mission costs would drop to about $1.5 billion to fly two people to the moon for up to two days.

"This is a game-changer," Stern told reporters in Washington and on a conference call. "We can fly human lunar missions for the cost of a robotic mission."

Stern declined to specify how many missions the company would need to sell to turn a profit.

"If we only sell three or four expeditions, it's completely upside down. We need to sell a bunch. But we do not need to sell ridiculous numbers," he said.

A market study shows 15 to 25 nations can afford lunar exploration and may want to do so, he added.

Potential customers include civilian space agencies, corporations, research institutes and some extremely wealthy individuals.

"We can make it affordable for mid-sized countries like a Korea, an Indonesia, or a South Africa to be in the business of lunar exploration, which would cost them a great deal more to invent that capability," Stern said.

In addition to advance ticket sales, the company is counting on advertising and marketing campaigns to raise funds.

Golden Spike is not the first company proposing privately funded missions to the moon. Other firms include Moon Express, a mining outfit, and companies participating in a Google-sponsored competition to land a robotic probe on the satellite.

"If I could find investors to get started with, we would be going back to the moon within 10 or 15 years to harvest its energy resources and use them back here on Earth," former Apollo astronaut Harrison "Jack" Schmitt told Reuters in a separate interview.

"The return of investment has to be fairly high because of the perceived risk - in addition to the actual risk to that investment capital - but nevertheless I believe it's possible that it could be done," Schmitt said.

(Editing by Tom Brown and Xavier Briand)

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