Millionaire entrepreneur Dennis Tito got space enthusiasts excited last month when he announced a project to fly a married couple around Mars in 2018—but NASA may have passed on a similar mission when it was proposed in the late 1990s by a prominent aerospace engineer.

According to Robert Zubrin, president of the Mars Society and a prominent advocate for exploration of the red planet, he had meetings with former NASA administrator Daniel Goldin in the late 1990s to pitch him a nearly identical mission to Tito's that would have launched in 2001 and cost the agency about $2 billion.

Dubbed Athena, the mission would have used technology that existed in 1996 on a two-year Mars flyby mission. Two astronauts would have orbited the planet for about a year, remotely-controlling rovers on the Martian surface with about 100 times less lag time than rovers controlled from Earth. The spaceship would never land on Mars, which Zubrin contends was Goldin's problem with the mission.

"He passed on it—he said if we go to Mars, we want to land, we want to explore," he says. "I contend that this is a lot better than nothing. This would have been an icebreaker mission. It would have killed the dragons that suggest we can't go to Mars."

Zubrin and other commercial space advocates have grown fed up with NASA's timeline for getting back into the manned spaceflight game. Tito said as much in a press conference announcing his plan, which he estimates will cost between $1 and $2 million and is scheduled to happen in 2018.

"We have not sent humans beyond the moon in more than 40 years," he said. "I've been waiting, and a lot of people my age, have been waiting. And I think it's time to put an end to that lapse."

In a proposal paper published by Zubrin in the Journal of the British Interplanetary Society in 1996, he admits that a flyby mission is "not an optimal mission plan for human exploration of Mars," but "is a way to get started," an opinion he still holds.

With several rovers already exploring Mars, astronomers would get little new scientific data from a human flyby of the planet—the next great leap is considered to be a Mars sample return mission aboard an unmanned vessel. Ideally, of course, Zubrin and others would like to land humans on the Martian surface.

A representative for Goldin says he met with Zubrin in the 1990s but the former administrator, who served between 1992 and 2001, has remained largely silent on his time at NASA. Goldin declined to speak with U.S. News for this story and a representative for the Intellisis Corporation, a neuroscience firm where he is now CEO, said he "has maintained his longstanding policy of not commenting on NASA activities."

During his tenure, Goldin was famous for abandoning expensive, multi-decade missions in favor of launching many relatively cheap, robotic missions.

Roger Wiens, a principal investigator on the $2.5 billion Mars Curiosity rover project and designer of one of those missions in the 1990s, says a mission like Tito's or Goldin's doesn't make sense for the agency.

"It's totally a budget question and that gets to our national priorities. If our country wanted to send people to Mars badly enough, we would do it. But NASA isn't going to do it in a highly risky way," he says. "We want to bring people back alive and healthy, but if other people want to do it in a different way, that's their prerogative."

Zubrin says that the threats deep space poses to humans has not changed since the mid-1990s, but that they might be overstated.

According to some estimates, a person who spends two consecutive years in space increases their chances of getting cancer during their lifetimes by about 2 percent. Other studies have suggested that spending long periods in zero gravity can cause other health problems—in Zubrin's Athena mission, he proposed that by spinning the capsule, artificial gravity could be created.

NASA says it will "continue discussions with Inspiration Mars to see how the agency might collaborate on mutually-beneficial activities that could complement NASA's human spaceflight, space technology and Mars exploration plans," but has no plans to do a Mars flyby mission of its own anytime soon.

In 2010, President Obama said he hopes that "by the mid 2030s," NASA will "send humans to orbit Mars and return them safely to Earth."

A representative with NASA said he could not confirm whether or not Zubrin had met with Goldin to discuss Athena, but that Zubrin is "a highly respected scientist."

Wiens says NASA "would not get anything from a simple human flyby mission except to prove we can go there," to which Zubrin says, "exactly."

"Conceivably, a person can say 'What's the point of a flyby mission,' and the point is to say 'Follow me, this can be done,'" Zubrin says. "It's fear holding NASA back. These are risky missions. If you want to be safe, stay on the ground."