(FloridaToday.com) - SpaceX will try again Friday to launch an International Space Station resupply mission from Cape Canaveral, while station astronauts prepare for a spacewalk soon to replace a failed component.
Hoping weather cooperates, a Falcon 9 rocket and Dragon capsule are targeting a 3:25 p.m. Friday liftoff from Launch Complex 40 at Cape Canaveral Air Force Station.
Air Force meteorologists expect thick clouds, rain showers and thunderstorms to combine for only a 40 percent chance of conditions acceptable for launch when the instantaneous window arrives.
There's another opportunity at 3:02 p.m. Saturday, if necessary, when the forecast is better.
But if the mission can't get off the ground by then, it will be put on hold until June.
Other events scheduled on the station and a period when it can't welcome new vehicles because of temperature and power limits mean the next launch opportunity would slip to early May, when Orbital Sciences is readying a cargo flight from Virginia.
"I'm kind of double-booking two launches at once here for a little while until we see what actually occurs," Bill Gerstenmaier, head of NASA's human spaceflight programs, told an advisory panel Wednesday. "We're getting low enough on supplies on board station, we've got to get something to station in the next couple of months."
The timing of SpaceX's launch also will determine when Expedition 39 astronauts Rick Mastracchio and Steve Swanson venture outside the orbiting research complex to replace a backup computer box that failed last week.
A Friday launch would put the Dragon on course to reach the station Sunday morning, and their planned two-and-a-half hour spacewalk would follow next Wednesday.
If there's no launch Friday, the spacewalk would move up to Sunday. The Dragon would arrive Tuesday if launched on Saturday.
Mastracchio and Swanson will swap out a 51-pound metal box housing computer cards that give ground teams a backup ability to command various external systems, including coolant loops and joints that rotate eight solar array wings.
Control of those systems would be lost if the primary computer failed without a backupavailable.
"It's pretty straightforward in terms of (spacewalking) jobs," astronaut Chris Cassidy told told NASA TV on Wednesday.
Cassidy is helping to plan the spacewalk from the ground, and will perform a practice run Friday in the giant swimming pool at Johnson Space Center in Houston.
SpaceX, meanwhile, reported Wednesday that a helium valve in the Falcon 9 rocket's pneumatic stage separation system was to blame for Monday's scrubbed first launch attempt. The valve was not holding the right pressure.
The valve will be replaced and the entire system inspected before Friday's countdown.