Working through dismal weather, SpaceX engineers pressed ahead Monday with preparations to launch two astronauts aboard a Crew Dragon spacecraft Wednesday, the first piloted flight to orbit from U.S. soil in nearly nine years.
SpaceX and NASA held a launch readiness review to verify the Falcon 9 booster and spacecraft are ready for flight while NASA and its international partners went over preparations to welcome two new crew members to the lab complex. Both meetings concluded with an official "go" for launch.
Keeping tabs on the weather, Crew Dragon commander Douglas Hurley and joint operations commander Robert Behnken plan to don their pressure suits and head for launch pad 39A around 1:15 p.m. ET Wednesday. Blastoff is targeted for 4:33:33 p.m., roughly the moment Earth's rotation carries the pad into position for a flight to the station.
No major technical issues of any significance were under discussion Monday, but the weather could be a factor. Forecasters initially predicted a 60% chance of a weather-related launch rule violation, but Mike McAleenan, launch weather officer with the 45th Weather Squadron, said conditions appeared to be improving somewhat.
"If I was to issue the forecast today, right now we would probably be down to 40% chance of violation," he said. "We have a bit more rain to go here and maybe another round of afternoon thunderstorms tomorrow, but ... it looks like much less (cloud) coverage. So we have some hope for launch day."
But McAleenan's forecast does not include downrange conditions in the Atlantic Ocean along the Crew Dragon's trajectory where Hurley and Behnken could be forced to ditch in the unlikely event of aduring the climb to space.
SpaceX managers will assess a complicated mix of weather models, high-altitude balloon data and actual wind, rain and wave data from multiple buoys along the ground track to determine whether conditions, on average, are acceptable for launch.
Hoping for the best, Hurley and Behnken are expected to begin strapping into the Crew Dragon around 2 p.m. Wednesday. The astronauts will arm the spacecraft's emergency abort system around the T-minus 40-minute mark, a few minutes before propellant loading begins.
Hans Koenigsmann, SpaceX vice president of build and flight reliability, said a final assessment of the weather will be made shortly before the abort system is armed. If mission managers are not confident conditions at the launch site and along the trajectory are acceptable, the launch will be scrubbed for the day before fueling begins.
"Usually when we have a satellite to launch we go sometimes all the way down to the wire, to the last minute and then Mike says no, and then we don't go," Koenigsmann said. "In this case, we don't want to do that because we would expose the crew to risk that would be unnecessary."
"So six hours before (launch), four hours before, and then I guess the final call comes at the end, at 45 minutes when we're about to arm the escape system. By that time, we have come to a conclusion whether we go or no-go."
Backup launch opportunities, based on the space station's orbit and the Crew Dragon's ability to catch up with the lab complex, are available Saturday, at 3:22:41 p.m., and Sunday, at 3:00:07 p.m.
"It's getting a little bit far out to have a lot of confidence, but it certainly looks like the guidance is shaping up that the 30th and 31st look much less dynamic than what we have with the tropical low development across Florida," McAleenan said. "So overall, those look like a better probability of launching and lower risk numbers across the Atlantic."