The US space agency NASA has asked for a second crewed mission to the International Space Station (ISS) from SpaceX, providing the orbital service with the help of its Falcon 9 rocket and Dragon capsule.

As part of the Commercial Crew Transportation Capability (CCtCap) contracts that NASA has signed lately with SpaceX and Boeing, this order is the fourth and last guaranteed one.

However, the space agency has said that it is looking forward to use private space taxis of one or both of these companies in the future. A single company’s deal gives permit for a potential maximum of six crewed flights.

In a statement, Kathy Lueders, manager of NASA's Commercial Crew Program, said, “The order of second crew rotation mission from SpaceX, paired with two ordered from Boeing, will help ensure reliable access to station on American spacecraft and rockets”.

Lueders added that such systems are going to ensure dependable US crew rotation services to the space station, and will work as a lifeboat for ISS for up to seven months.

Boeing is going to use its CST-100 Starliner capsule to take astronauts to and from the space station. It got its two flight orders last year in May and December, while SpaceX got its first one in November last year.

According to representatives of both companies, SpaceX is all set to launch astronauts for the first time ever next year, while Boeing must be ready a year next to it. The US space agency hasn’t make any announcement so far about which company will fly crews to the ISS first.

Since 2011 when the retirement of the space shuttle took place, the US space agency’s astronauts have launched to, and returned from, the station inside Russian Soyuz spacecraft, which can carry three people.

However, the agency officials said that a standard mission aboard Dragon or Starliner is going to fly four astronauts, and 220 lbs. of pressurized cargo.

A report published in Space News revealed, "NASA had already ordered two such missions from Boeing, the other company with such a contract. Those contracts cover the development of commercial crew vehicles and include at least two, and as many as six, post-certification missions."

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According to a report in Space by Mike Wall, "The order is the fourth and last guaranteed one that NASA will make under the Commercial Crew Transportation Capability (CCtCap) contracts the agency recently signed with SpaceX and Boeing."

"The order of a second crew rotation mission from SpaceX, paired with the two ordered from Boeing, will help ensure reliable access to the station on American spacecraft and rockets," Kathy Lueders, manager of NASA's Commercial Crew Program, said in a statement. "These systems will ensure reliable U.S. crew rotation services to the station, and will serve as a lifeboat for the space station for up to seven months."

"With the commercial crew vehicles from Boeing and SpaceX, we will soon add a seventh crewmember to space station missions, which will significantly increase the amount of crew time to conduct research," NASA ISS chief scientist Julie Robinson said in the same statement.