When Elon Musk announced plans last month that SpaceX wants to send two people on a commercial mission around the moon as soon as late 2018, it was a surprise, but also hardly unique.

Musk, in a conference call with reporters Feb. 27, said that SpaceX had been approached by two individuals interested flying around the moon. SpaceX has proposed flying them on a version of its Dragon 2 spacecraft under development, launched on a Falcon Heavy. The mission would launch in the fourth quarter of 2018, pending the development schedules of both the rocket and the spacecraft.

“This is, I think, a really exciting thing that’s happened, that we’ve been approached to do a crewed mission beyond the moon,” Musk, the founder and chief executive of SpaceX, said. He declined to identify the customers, but said they had already paid a “significant” deposit on a ticket price comparable or slightly more expensive than a trip to the International Space Station. “They’re very serious about it.”

As proposed, the Falcon Heavy would launch the Dragon 2 spacecraft onto a “free return” trajectory, making a close flyby of the moon before looping back, returning about a week after launch. The mission, Musk said, would require few changes to the Dragon 2 under development beyond an improved communications system for deep space operations.

SpaceX, though, is not the first company to propose using vehicles either already in operation or under development to fly people around the moon. More than a decade ago, Space Adventures, the space tourism company that has arranged flights to the ISS by several private citizens, announced plans to send people to the moon as well.

The Space Adventures concept involved launching a Soyuz with an upgraded heat shield first to the International Space Station. After a 10-day stay, the Soyuz would depart the ISS and dock with a habitation module and departure stage launched on another rocket, such as a Proton. The vehicle would then fly a six-day free-return trajectory around the moon.

Space Adventures originally offered two seats at $100 million a ticket — the Soyuz’s third seat was reserved for the professional cosmonaut who would command the mission — and later increased that price to $150 million. The company said in 2012 that it had sold one ticket and was in the process of selling the second.

“Because we’ve sold the first seat, and the second seat is very close to being sold, we plan to have it launch before the 50th anniversary of the start of the Apollo program,” or February 2017, Eric Anderson, chairman of Space Adventures, said in a February 2012 video.

Anderson said at the time that he saw the circumlunar mission as a key milestone of commercial human spaceflight in general. “If two people are ready to pay $150 million each to go around the moon, that has broad and very positive implications for the overall marketplace,” he said. “That is a fantastic validation of the marketplace for private spaceflight.”

That estimated launch date for the mission has now passed, with little sign of progress by Space Adventures towards carrying out the mission, including identifying the customers it has signed up. The company remains tightlipped about who its customers are and when, or if, they might fly.

“It is not our practice to comment on any particular Space Adventures’ client’s prospective or planned mission before the client has announced it personally, regardless of vehicle, destination or mission timeframe,” company spokeswoman Stacey Tearne said Feb. 28.

Russian company RSC Energia, which has worked with Space Adventures in the past on space tourist flights to the ISS, has shown signs of developing its own lunar tourism plans. The company suggested in recent years it was in talks with a number of potential customers for a flight.

Vladimir Solntsev, head of Energia, told the Russian news service Sputnik Feb. 22 that such a mission was in development, but not until the early 2020s. “I think that RSC Energia will be ready to be the first to offer this service on the international market by 2021–2022,” he said.

Even before Space Adventures, one other company considered adapting Soyuz spacecraft for a circumlunar flight. In 2004, Constellation Services International (CSI), a startup company involved in early concepts for commercial servicing of the ISS, proposed a mission concept called Lunar Express that also involved an upgraded Soyuz flying around the moon with a habitat module and upper stage.

At the time, CSI considered seeking a national space agency as one potential customer for the mission, along with private citizens. “Would someone be willing to pay for a premier space adventure: first an initial week-long stay at ISS, and then a week-long trip around the moon like Apollo 8 or 10?” said Charles Miller, chief executive of CSI, when the concept was unveiled at a July 2004 conference. “Would another nation be willing to pay for the second seat, and send their first citizen ever around the moon?”

CSI did not make progress on the Lunar Express concept, and the company later disbanded. Miller went on to becoming a senior adviser for commercial space at NASA and briefly returned to the agency after Donald Trump won the 2016 presidential election to serve on the transition team.

While most previous efforts for commercial human lunar missions have focused on lunar flybys, one company attempted to develop a commercial human landing system. In December 2012, Golden Spike Company unveiled its plans to send humans to the surface of the moon and back.

“By adopting what we call a maximally pragmatic strategy, we found a way, a suite of lunar exploration architectures, that can enable our company to do its first lunar mission for a cost of between seven and eight billion dollars,” Alan Stern, the president and chief executive of Golden Spike and who is perhaps best known as the principal investigator for the New Horizons Pluto mission, said at the Washington press conference announcing its plans.

That strategy relied on using existing vehicles — including SpaceX’s Dragon — for getting to the moon, focusing its development on a two-person lunar lander and other unique systems. Stern’s cost covered development and the first mission, with subsequent missions slated to cost $1.5 billion each.

Golden Spike planned to focus its business on national space agencies, but didn’t rule out also flying tourists. “One individual who could be in a position to arrange such a mission has approached us, very seriously,” he said at the press conference, declining to name that person.

The company, Stern said at the announcement, was going to use a financing model similar to that used by Airbus and Boeing for new airliners, which relies on advance sales. Those sales, though, never publicly materialized, and the company faded away after a couple of years. Today, its website consists solely of its logo and the words “Under Construction.”

SpaceX arguably is in a better position than others to attempt such a mission. It has both all the key hardware under development and the financial resources to prepare them for such a mission, which previous efforts have lacked. Staying on schedule, though, is another matter: both Falcon Heavy and Dragon 2 have suffered significant delays, and even the announcement of the mission last month was a half-hour late.

Space Adventures’ Tearne said the company was supportive of SpaceX’s plans, even if they are competitive with its own. “Our goal is to bring the experience of spaceflight to private citizens around the world, and so we would like to congratulate SpaceX and see this news as an exciting development for the industry,” she said.

And, despite the poor track record of previous companies unable to fly even one mission to the moon, Musk suggested the flight might not be a one-off mission for SpaceX. “I think it could be a significant driver of revenue,” he said, suggesting circumlunar flights could later account for 10 to 20 percent of company revenues. “I think there’s likely a market for at least one or two of these per year.”