CAPE CANAVERAL, Fla. - Countdown clocks were ticking down on Sunday for the launch of a SpaceX Falcon 9 rocket from a historic launchpad leased from NASA at the Kennedy Space Center in Florida.Blastoff of Space Exploration Technology Corp's Falcon 9 rocket is targeted for 9:38 a.m. local time on a mission to fly supplies and science experiments to the International Space Station.SpaceX scrubbed its first launch attempt on Saturday seconds before liftoff due to concerns about the steering system in the rocket's upper stage, the company said.SpaceX founder and Chief Executive Elon Musk wrote on Twitter after the delay, "99% likely to be fine ... but that 1% chance isn't worth rolling the dice. Better to wait a day."The National Aeronautics and Space Administration, which hired SpaceX to fly cargo to the station after the shuttle program ended, will closely monitor Sunday's launch to learn more about SpaceX's operations before it clears the company to fly NASA astronauts on SpaceX rockets."We're going in and listening to their launches and getting smart so we can have intelligent discussions with them and offer feedback about how things might be different if you're launching people," Stephen Payne, NASA's launch integration manager for the commercial space taxi program, said in an interview.SpaceX and Boeing are scheduled to begin flying crew to the station by the end of 2018. But a Government Accountability Office report last week said both firms face technical hurdles that likely will delay their programs.This is the first time SpaceX is launching a rocket from Kennedy Space Center's historic Launch Complex 39A, which was originally built for the 1960s-era Apollo moon program and later repurposed for the space shuttles.SpaceX leased the pad from NASA in 2014 and is spending upwards of $100 million to ready it for a variety of NASA, commercial and military launches, SpaceX President Gwynne Shotwell said."It means a lot to see the pad just not sit and waste away," Kennedy Space Center director Bob Cabana told reporters.SpaceX hopes to have its second Florida launchpad, located at the nearby Cape Canaveral Air Force Station, back in operation this summer. That pad was heavily damaged in a Sept. 1 rocket explosion.