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Updated at 6:15 p.m. EST (2315 GMT) with details.

SpaceX founder and chief executive Elon Musk announced Monday plans to send two paying “private individuals” on a week-long flight around the moon and back to Earth by the end of next year.

Musk said the would-be space tourists approached SpaceX to fly on a mission beyond the moon, launching aboard the company’s Falcon Heavy rocket from pad 39A at NASA’s Kennedy Space Center in Florida and returning to Earth approximately a week later.

“They’re very serious about it,” Musk said, declining to identify the passengers in a conference call with reporters Monday afternoon. He later added the clients include “nobody from Hollywood,” but would not provide details on their backgrounds.

The two-person crew will be trained for emergencies, but the Dragon spaceship carrying them will fly on autopilot, loop around the far side of the moon on a “free-return” trajectory, then speed back to Earth. Musk said SpaceX aims to launch the circumlunar flight in the fourth quarter of 2018.

“This would do a long leap around the moon,” Musk said. “We’re working out the exact parameters, but this would be approximately a week-long mission, and it would skim the surface of the moon, go quite a bit farther out into deep space, and then loop back to Earth. I’m guessing probably distance-wise, maybe 300,000 or 400,000 miles.”

He acknowledged the trip will be risky.

“I think they are entering into this with their eyes open, knowing that there is some risk here,” Musk said of the passengers. “They’re certainly not naive. We’ll do everything we can to minimize that risk, but it’s not zero.”

He declined to say how much the space tourists will pay, but SpaceX said it has already received a “significant deposit” for the moon mission.

“They do know each other,” Musk said of the prospective space tourists. “I don’t want to comment too much on their background, but they certainly will have extensive training before going on the mission, and I also can’t say the exact amount (they will pay).”

The tourists will ride inside SpaceX’s Dragon 2 capsule, or “Crew Dragon,” in development under a $2.6 billion contract with NASA to ferry astronauts to and from the International Space Station. The spaceship is scheduled to launch on its first uncrewed test flight to the station in November, followed around six months later with a demonstration mission to the outpost with two NASA astronauts on-board.

SpaceX and NASA plan at least one Crew Dragon flight to the station per year once the spacecraft proves itself on the test flights, rotating in and out four-person crews on six-month expeditions.

NASA contracted with SpaceX and Boeing to develop, test and fly commercially-operated spaceships to transport crews to the station, replacing services currently provided by Russian Soyuz spacecraft.

Russia charged the U.S. government $81 million per seat in the latest crew transportation deal. Boeing and SpaceX have not disclosed their prices for an astronaut ticket to the space station, but NASA says the average cost between the two providers runs about $58 million per person for a round-trip flight.

Musk said the price SpaceX’s moon mission fliers will pay is confidential, “but it would be comparable to a little more than what the cost of a crewed mission to the space station would be.”

Space Adventures, a Virginia-based firm, is the only company to date to arrange for paying tourists to fly into space, brokering eight flights by seven clients to the International Space Station in the 2000s. Other companies, like Virgin Galactic and Blue Origin, offer suborbital trips for paying tourists and researchers, but they have not commenced commercial service.

In 2011, Space Adventures announced a venture in partnership with Russia to send two tourists on a trip around the moon for $150 million per person inside a modified Soyuz capsule, but the mission never materialized.

The Crew Dragon spacecraft is a major upgrade from the cargo-carrying version of Dragon currently flying resupply missions to the space station.

It is already being designed to deep space journeys, Musk said, with an ablative carbon heat shield capable of withstanding re-entry speeds from the moon, which are much faster than descent velocities from low Earth orbit missions.

The robust design of the Crew Dragon supports SpaceX’s long-term objective of Mars missions, with an eye toward setting up a permanent base there and making humanity a “multi-planet species.”

But the Dragon capsule will need new systems for the trip to the moon and back — its first time to fly higher than low Earth orbit — such as deep space communications equipment.

“This would be communication at several hundred thousand kilometers (more than 250,000 miles), as opposed to around 400 kilometers (250 miles at the space station’s orbit),” Musk said. “It’s mostly with respect to the communications system, but Dragon is designed to be hardy with regard to … space radiation and have triple-redundant systems. We feel that the modifications necessary will be quite limited.”

The Dragon spacecraft’s lunar flight will come after it completes test flights to the space station, Musk said.

The orbital test flights will wring out many of the Crew Dragon’s life support, navigation and computer systems, and Musk said NASA will certify the capsule as “human-rated” once those demo missions are successfully completed.

But the space station crews will blast off on SpaceX’s Falcon 9 booster, not the Falcon Heavy, which is required for the round-trip voyage around the moon. The Falcon Heavy is expected to launch for the first time some time this summer, Musk said, several years later than originally envisioned.

“Dragon 2 is going through NASA human-rating, (and it) will fly with NASA astronauts to the space station before it flies this mission,” Musk said. “The same is true of the Falcon 9 vehicle. The difference is that we’re adding two additional Falcon 9 first stages as strap-on boosters to increase the performance of Falcon 9, which is what we’re calling Falcon Heavy.”

The Falcon Heavy will lift off on the power of 27 Merlin engines, three times the number that power the Falcon 9’s core stage.

“All the pieces of that system will be human-rated by NASA,” Musk added. “One could say that there’s some incremental complexity with the integration of those systems.”

SpaceX will seek a license from the Federal Aviation Administration for the purely commercial tourist trip around the moon, Musk said.

“We should have quite a bit of Falcon Heavy flight history (by the time the lunar mission flies), and, of course, a tremendous amount of heritage coming from Falcon 9 going into Falcon Heavy,” Musk said. “Dragon 2 has been designed to very high standards, so it’s capable of taking a re-entry from lunar entry velocity, with significant margin, so we’re confident that this will be a good vehicle to fly on.”

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SpaceX said NASA is a “key enabler” for the commercial circumlunar mission, which would be the first flight by humans to the moon’s distance since the Apollo 17 lunar landing in December 1972, and perhaps the most distant space expedition in history.

NASA said in a statement it “commends its industry partners for reaching higher.”

“We will work closely with SpaceX to ensure it safely meets the contractual obligations to return the launch of astronauts to U.S. soil and continue to successfully deliver supplies to the International Space Station,” the agency said. “For more than a decade, NASA has invested in private industry to develop capabilities for the American people and seed commercial innovation to advance humanity’s future in space.”

While NASA has tapped Boeing and SpaceX to pick up launch and landing services for space station crews, the space agency is working on government-owned vehicles to send astronauts to deep space.

The Space Launch System super-booster and Orion capsule are scheduled to take off on their first combined test flight to lunar orbit late next year, without astronauts. NASA targets launch of a crewed mission conducting a similar lunar flyby to the maneuver planned by SpaceX some time between 2021 and 2023.

SLS/Orion missions to the region around the moon, called “cislunar space,” will validate new propulsion thrusters, power systems, robotics and other technologies required for eventual human journeys to Mars.

The Government Accountability Office reported last year that the first SLS flight in 2018, the first two Orion flights on SLS, and the associated ground systems will cost almost $23 billion. In a separate report last year, NASA’s inspector general said the Orion program will have received $17 billion from its start in 2006 through the first crewed mission in the early 2020s.

But NASA may be about to shake up its human spaceflight program.

The agency is conducting a feasibility and risk analysis, under orders from the Trump administration, to determine the technical hurdles and costs of putting two astronauts on the first SLS/Orion launch, named Exploration Mission-1.

If NASA decides to switch to a piloted mission, EM-1 will likely be delayed to allow engineers to finish development of key life support systems that were not originally intended to fly on the uncrewed test flight. If approved, the EM-1 crew would fly around the moon and back, following roughly the same trajectory targeted by SpaceX’s privately-funded mission.

Musk did not say whether he hopes to fly to the moon before NASA, but he said the space agency will have priority if it wants to take part in the circumlunar flight.

“This will be a private mission with two paying customers, (but) NASA always has first priority,” Musk said. “If NASA decides to have the first mission of this nature to be a NASA mission, then of course NASA would take priority.”

“We’re generally encouraging of anything that advances the cause of space exploration,” Musk said. “I think an SLS/Orion mission would be exciting as well. I don’t know what they’re timetable is, and I’m not sure if we will be before or after, but I don’t think that’s really the important thing. What matters is the advancement of space exploration and exceeding the high-water mark that was set in 1969 with the Apollo program, and just having a really exciting future in space that inspires the world.

“That’s what we care about, and we think that there should more companies and organizations doing this than SpaceX. The more the better.”

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Follow Stephen Clark on Twitter: @StephenClark1.