Q&A with Keith Gendreau, principal investigator of the first X-ray space communications experiment (members only)

The U.S. military’s next GPS navigation satellite was ready for launch from Cape Canaveral at 9:43 p.m. EDT Friday (0143 GMT Saturday) aboard a SpaceX Falcon 9 rocket. But SpaceX aborted the countdown just two seconds prior to liftoff.

SpaceX has set the first launch of a “flight-proven” Falcon 9 rocket for Thursday evening after running the booster through a hold-down engine firing Monday at NASA’s Kennedy Space Center in Florida.

A hefty new satellite to beam SiriusXM radio programming across North America rode a SpaceX Falcon 9 rocket into orbit Sunday from Cape Canaveral on course to replace an aging broadcast station launched more than 15 years ago.