NASA's Commercial Crew Program will likely start test flights this year, six and a half years after STS-135, the last human spaceflight mission to lift off from American soil.

A NASA update, released Jan. 4, outlines what the two participating companies — Boeing and SpaceX — will do to prepare their vehicles for space in the coming year. Under the schedule, both Boeing and SpaceX will fly uncrewed flights in 2018. SpaceX also plans to fly astronauts this year, but neither NASA nor Boeing provided details on when the crewed Boeing flights will occur. Boeing and SpaceX are performing the flights as a part of NASA's Commercial Crew Program under a contract that began in 2014. [The Most Exciting Space Missions to Watch in 2018]

Three crew modules and several service models for Boeing's spacecraft, called the CST-100 Starliner, will be outfitted at the Kennedy Space Center in Florida, NASA officials said in the update. Boeing plans several service-module hot-fire tests in White Sands, New Mexico; environmental testing in El Segundo, California; and tests in 2018 concerning loads, shock and separation in Huntington Beach, California.

Boeing's spacesuit will undergo multiple tests. The company will also test the spacecraft's engines and conduct a pad abort test at White Sands, recovery training in the western United States and parachute testing near Yuma, Arizona. (Boeing has already finished two of the five qualification tests for parachutes.)

The culmination of this work will be an uncrewed orbital flight test in which CST-100 leaves Space Launch Complex 41 at Cape Canaveral Air Force Station and docks to the International Space Station for two weeks. This test flight will demonstrate how the launch vehicle, spacecraft and Boeing team perform during a flight. Boeing will then run the first crewed flight, which will have two astronauts on board and will launch from a United Launch Alliance Atlas V rocket from the same launch complex. (The company did not give an expected date for either the uncrewed or the crewed launch.)

SpaceX's Dragon spacecraft is already used for cargo flights to the space station, but a crewed version is in development. The company expects to wrap up structural qualification tests on the crewed Dragon in the first half of 2018, NASA officials said in the update; SpaceX also will continue hardware and software testing on the environment-control-and-life-support system, as well as preparations for the crew module

The company plans to extensively test its spacesuits, Merlin/SuperDraco engines, parachutes and recovery training procedures. Like Boeing, SpaceX has already completed some of its parachute qualifications. The spacecraft will be recovered off the coast of Florida, so that is also where the recovery training will take place.

SpaceX plans two flight tests in 2018, according to the update. Demo-1 will happen in the second quarter, when an uncrewed Dragon will launch from Space Launch Complex 39A at Kennedy Space Center. This test will encompass the Falcon 9 rocket, the Dragon spacecraft and ground systems. The second test flight, called Demo-2, will happen in the third quarter of 2018. In this mission, two NASA astronauts will fly to and from the International Space Station using Dragon. [Boeing Unveils Spacesuits for Starliner Astronaut Taxi (Photos)]

SpaceX also plans an in-flight abort test at Kennedy Space Center in 2018, in between its two demonstration flights; the company will demonstrate how Dragon can safely pull itself away from a rocket in case of an in-flight emergency.

Once the companies pass their certification tests, both SpaceX and Boeing are expected to fly six crew missions each to the International Space Station between 2019 and 2024.

The last mission to fly from U.S. soil was STS-135, in July 2011; since then, NASA has been buying seats on Russian Soyuz spacecraft to continue ferrying its astronauts from Kazakhstan to space until a replacement spacecraft is ready.

As the shuttle entered its last few years of service, NASA embarked on a Commercial Crew Program with multiple phases. During the first phase of the program, in 2010, NASA awarded a total of $50 million to five American companies. Each phase of the program was competitive, with multiple companies applying for spots. Phase 2 took place between 2011 and 2012, and Phase 3 was from 2012 to 2014.

In 2014, Boeing and SpaceX were selected in the final phase, which is intended to make their spacecraft fully ready for flight. The potential value for the companies in the final phase is $4.2 billion for Boeing and $2.6 billion for SpaceX, depending on what milestones they meet.

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