The United Launch Alliance Atlas V rocket carrying Boeing's Starliner capsule sits on the pad at Cape Canaveral Air Force Station's Space Launch Complex 41 on Dec. 18, 2019.

Boeing's Starliner astronaut taxi is on the launch pad.

The United Launch Alliance (ULA) Atlas V rocket carrying Starliner rolled out to the pad at Cape Canaveral Air Force Station's Space Launch Complex-41 early this morning (Dec. 18), riding a mobile launch platform 1,800 feet (550 meters) from ULA's Vertical Integration Facility.

Starliner remains on track to launch toward the International Space Station (ISS) Friday morning (Dec. 20) on its debut test flight, an uncrewed mission called Orbital Flight Test (OFT).

"Atlas and MLP are hard down at the pad. Successful roll #OFT is very, very, very near," ULA president and CEO Tory Bruno tweeted this morning .

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More: How to Watch Boeing's 1st Starliner Test Flight Online

Boeing has been developing Starliner under a multibillion-dollar NASA commercial crew contract. SpaceX got a similar deal for work on its Crew Dragon capsule. The space agency wants these two private vehicles to fill the astronaut-ferrying shoes of the space shuttle fleet, which retired in July 2011. Ever since then, Russian Soyuz spacecraft have been the only means of crew transport to and from the ISS.

The Atlas V prepares to leave the Vertical Integration Facility for the launch pad on the morning of Dec. 18, 2019. (Image credit: ULA)

The eight-day OFT mission is a crucial step toward bringing Starliner fully online, proving out the capsule's capabilities during a trip to and from the ISS. If OFT is a success, a crewed demonstration mission to the orbiting lab will follow, likely sometime next year.

The Atlas V and Starliner make the 1,800-foot (550 meters) journey from ULA's Vertical Integration Facility at Cape Canaveral Air Force Station to the pad at Space Launch Complex-41. (Image credit: ULA)

SpaceX conducted its version of the OFT, an uncrewed ISS mission called Demo-1 , this past March. The California-based company is gearing up for an in-flight test of Crew Dragon's emergency-escape system in January, which will clear the way for a crewed demonstration flight to the ISS for the SpaceX capsule.

Starliner and the Atlas V are scheduled to lift off Friday at 6:36 a.m. EST (1136 GMT). There's just a 20% chance of a weather-related delay, Air Force officials have said.

Visit Space.com for complete coverage of Boeing's OFT Starliner test flight.

Mike Wall's book about the search for alien life, " Out There " (Grand Central Publishing, 2018; illustrated by Karl Tate ), is out now. Follow him on Twitter @michaeldwall . Follow us on Twitter @Spacedotcom or Facebook .