After a 24-hour delay due to poor weather, an air-launched Northrop Grumman Pegasus XL rocket fired into orbit at 9:59 p.m. EDT Thursday (0159 GMT Friday) after release from a carrier jet at an altitude of 39,000 feet over the Atlantic Ocean northeast of Cape Canaveral. The Pegasus rocket carried NASA’s ICON satellite into orbit to collect measurements of the ionosphere.

The countdown began Saturday evening and will continue through the start of a simulated launch window opening at 9 p.m. EST Monday. The test started two days late because of predicted arctic weather along Florida’s Space Coast.

SpaceX tested astronaut slidewire escape baskets and fire suppression capabilities Friday at launch pad 39A in Florida, demonstrating systems that would be activated in the event of an emergency before liftoff of a crewed mission.