A SpaceX Falcon 9 rocket lifts off on a supply mission to the International Space Station from historic launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida, US, February 19, 2017. [Photo/Agencies]

The launch of the first privately funded tourist flight beyond the orbit of the International Space Station is tentatively targeted for late 2018, Space Exploration Technologies Chief Executive Elon Musk told reporters on a conference call.

Musk declined to identify the customers or say how much they would pay to fly on the weeklong mission, except to say that itis "no body from Hollywood."

He also said the two prospective space tourists, who know each other, have put down a "substantial" deposit and would undergo "extensive training before going on the mission."

"I think there's a market for one or two of these per year," he said, estimating that space tourist fares charged by SpaceX could eventually contribute 10 to 20 percent of the company's revenue.

Plans call for SpaceX's two-person lunar venture to fly some 300,000 to 400,000 miles (480,000 to 640,000 km) from Earth past the moon before Earth's gravity pulls the spacecraft back into the atmosphere for a parachute landing.

That trajectory would be similar to NASA's 1968 Apollo 8 mission beyond the moon and back.

Musk also said that if NASA decides it wants to be first inline for a lunar fly by mission, the US space agency would take priority.

At the behest of the Trump administration, NASA is conducting a study to assess safety risks, costs and potential benefits of letting astronauts fly on the debut test flight ofits heavy-lift Space Launch System rocket and Orion capsule.

That mission is currently planned to be uncrewed and scheduled to launch in late 2018.

Musk said the privately funded moon expedition would take place after his California-based company begins flying crew tothe International Space Station for the National Aeronautics and Space Administration.