And they have transported only space agency astronauts, in addition to Russian cosmonauts.
Nasa said there would be up to two such trips a year, and stays would be for a maximum of 30 days.
"These missions will be privately funded, dedicated commercial spaceflights".
These travelers would be ferried to the orbiter exclusively by the two United States companies now developing transport vehicles for NASA: SpaceX, with its Crew Dragon capsule, and Boeing, which is building one called Starliner.
The bulk of the bill for would-be space tourists will be an estimated $58 million for a round-ticket seat on a space taxi.
The spaceflight to the ISS will account for a large chunk of the cost.
DeWit noted, however, "it's not going to be a profit-making venture for NASA at all".More news: Cup race at Michigan on hold because of rain
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They will be able to buy 90 hours of "crew time" per year, meaning Nasa astronauts could be used to advertise products. Crews rotating on 4-6 month schedules have lived aboard the ISS since November 2000.
Keeping astronauts on board the ISS is a pricey business. According to Gizmodo, a single night with food and other crucial supplies on the ISS would cost $35,000 at least.
A large bank balance won't be enough either: you'll have to pass Nasa's rigorous health checks and training procedures. Potentially, up to a dozen private astronauts could visit the station each year. In 2011, the Center for the Advancement of Science in Space (CASIS) was established to manage the USA national lab aspects of the ISS. "NASA is providing a forecast of its minimum long-term, low-Earth orbit requirements, representing the type and amount of services that NASA intends to purchase when those services become commercially available".
And Axiom Space, a Houston-based company hoping to organize trips to the ISS, has pledged to charge $55 million (£43.2 million) for a 10-day trip to the ISS.
The main advantage seems to be keeping costs down, as the ISS is very expensive to run.
But it's also about continuing to test space travel - to make it safer and cheaper for everyone. The agency has contracted with SpaceX and Boeing to fly future crewed missions to the space station. The ISS functions under a set of global agreements, namely the USA need the consent of partners for the commercial utilization of the station.
Nasa wants to return astronauts to the moon by 2024 and is looking at new ways of raising money. The agency said this will also reduce the cost to USA taxpayers for this next lunar mission.