An early morning launch of a SpaceX Falcon 9 rocket did not get off the ground Sunday at NASA’s Kennedy Space Center in Florida. At around 7:15 a.m. the launch was cancelled due to a sensor issue and rescheduled for Monday morning.

With the sky overhead an early-morning mix of pink and blue, the weather looked perfect for a take-off, as a shroud of liquid oxygen encircled the rocket in the growing light. Despite that, technical difficulties won out the day.

“Due out of an abundance of caution, we have decided to scrub today’s launch,” said John Federspiel, a lead mechanical design engineer and host of SpaceX’s launch live stream.

The Falcon 9 was set to deliver a classified payload dubbed “NROL-76” into orbit on behalf of the U.S. National Reconnaissance Office. Although Elon Musk’s private spaceflight company has been taking military contracts for almost a year, this is its first exclusive launch for the NRO. Given the clandestine nature of the launch, and considering the NRO is a member of the U.S. Intelligence community, many believe Falcon 9’s cargo is a spy satellite.

Live coverage of the NROL-76 mission will resume Monday morning at 7 a.m. Eastern. You can watch this morning’s action — or lack thereof — here: