NASA has bestowed four more contracts each to SpaceX and Boeing for ferrying astronauts to the International Space Station in 2018.

The new crew rotation missions to the private firms will take the number of total missions with SpaceX and Boeing to six each. A statement from NASA said the space taxis will transport up to four astronauts and 220 pounds of critical cargo to the space station.

"Awarding these missions now will provide greater stability for the future space station crew rotation schedule, as well as reduce schedule and financial uncertainty for our providers," said Phil McAlister, director of NASA's Commercial Spaceflight Development Division.

Flight Schedules

The debut flight test of Space X without crews is expected in November, to be followed up by a Crew Dragon capsule test in May 2018. Boeing's CST-100 Starliner test with crew will take off in August 2018.

In the crewed missions, a lifeboat capability will be a highlight, allowing the ISS astronauts to return safely to Earth in an emergency.

Despite the publicized schedules, the crewed missions to ISS would take off only after the companies get the flight certification nod from NASA.

For NASA, the fresh contracts make sense, as the arrangements have to be in place for the transfer of astronauts to ISS as the deal with Russian Space agency may not continue beyond 2018. The United States ended its Space Shuttle program in 2011 and was relying on Russia's Soyuz rockets to ferry ISS crew.

For using the services of the Russian spacecraft to ferry astronauts to and from the ISS, the spending by NASA was near $80 million for every seat on the Soyuz rocket.

Russia's space agency Roscosmos stated in 2015 that it will not continue the arrangement of sending U.S. astronauts to the ISS after 2018. This has added a new urgency to NASA.

It means SpaceX or Boeing has to keep the capsules ready by 2018 to manage the U.S. presence on the space station.

NASA inked the space taxi contracts with Boeing and SpaceX in 2014 and offered them two missions each as a starter.

Both Space X and Boeing will be launching their space taxi services from Florida's Space Coast. Boeing may operate from Cape Canaveral Air Force Station, while SpaceX will opt for the Kennedy Space Center of NASA.

ISS Transit

Meanwhile, an opportunity is coming up to see the International Space Station passing in front of the sun on Thursday morning at the Denver Metro area.

A map of the space station's transit across the sun through Denver has been released by the Denver Astronomical Society.

The transit is expected at 11:16 a.m. Thursday.

For watching the ISS solar transit, binoculars or telescopes with an eyepiece and solar filter must be used.

"The sun is way too bright to look at without safe filters. It can actually cause blindness, either temporary, or in some cases, permanent damage to the eye," said Ron Hranac, president of the Denver Astronomical Society.

Since the movement of ISS will be will be faster, it will take only less than a second to pass across the sun.

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