Astronauts work through practice countdown

Astronauts Douglas Hurley and Robert Behnken were escorted to their SpaceX Falcon 9 rocket and Crew Dragon spacecraft Saturday for a dress rehearsal countdown that sets the stage for launch Wednesday's on a historic test flight to the International Space Station.

Hurley and Behnken walked through the same timeline Saturday they'll follow on launch day. The astronaut's itinerary included a weather briefing, putting on their futuristic SpaceX-designed pressure suits and heading for pad 39A in a white Tesla SUV, escorted by security personnel.

After taking an elevator to the 265-foot-level of the launch pad gantry, Hurley and Behnken planned to walk across a 50-foot-long access arm to a small “white room” where they could climb into their seats in the Crew Dragon. They were assisted by a SpaceX closeout crew responsible for sealing them inside the spacecraft before leaving the tower.

The countdown timeline called for a simulated scrub about an hour before the planned launch time, ending the simulation and giving the crew a final chance to run through launch pad exit procedures.

The astronauts arrived at the Kennedy Space Center launch site last Wednesday, just in time to witness the Falcon 9 rocket's roll out to the pad, NASA's official flight readiness review and a test firing of their booster's first stage engines Friday afternoon.

“Exciting couple of days here at @NASAKennedy!” Behnken tweeted. “Crew arrival in Florida was awesome, seeing our vehicle roll to 39A was epic, and watching our @SpaceX Falcon 9 1st stage fire one more time before our mission still has a smile on my face!”

NASA's first piloted launch from U.S. soil in nearly nine years is targeted for 4:33 p.m. EDT Wednesday, roughly the moment Earth's rotation carries the launch pad into the plane of the station's orbit.

But as usual in early summer along Florida's Space Coast, the weather could be a factor.

Meteorologists with the 45th Space Wing at nearby Patrick Air Force Base predicted a 60% chance of conditions that could violate launch commit criteria due to “flight through precipitation, thick cloud rule, cumulus cloud rule.”

Another wild card for the Crew Dragon mission: weather along the spacecraft's ascent trajectory that could be a factor in the unlikely event of a major booster malfunction triggering an emergency abort. The Crew Dragon is equipped with a sophisticated abort system designed to quickly propel the capsule and its crew to safety at any point from the start of fueling all the way to orbit.

But an in-flight abort could result in an unplanned splashdown at sea anywhere from just off Cape Canaveral all the way to Newfoundland and beyond to the North Atlantic Ocean near Ireland.

The Crew Dragon spacecraft requires relatively benign conditions for a safe splashdown and launch managers will be evaluating actual conditions along the flight path on launch day.

“It's like expecting every light to be green on your way to work in the morning. It's not gonna happen,” said Steve Payne, a veteran NASA engineer who worked with the Air Force to develop the agency's emergency rescue plans. “You can get as many as you can, but you'll always have some patches where it's not as great as you'd like. We understand that, it's a risk we have to accept because we can't guarantee perfect weather.”

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If the Crew Dragon does not get off the ground Wednesday, launch will move to 3:22 p.m. Saturday. A third backup opportunity is available Sunday at 3 p.m.

NASA managers unanimously cleared the Crew Dragon and its Falcon 9 rocket for launch Wednesday after a two-day flight readiness review that concluded Friday, signing a formal certificate of flight readiness, or COFR.

Two-and-and-a-half hours later, SpaceX test fired the Falcon 9's first stage engines in a standard pre-flight exercise to verify all systems are “go” for launch. There were no issues.

SpaceX plans its own launch readiness review Monday and the space station program will hold a final review to make sure NASA and its international partners are ready for the arrival of the Crew Dragon and two more crew members.