SpaceX launched the Air Force’s X-37B robotic space plane on its latest months-long, classified mission today – marking another first for the company, with an oncoming hurricane adding to the pressure.

Evacuations in advance of Hurricane Irma’s Florida landfall have already begun, but SpaceX managed to get its Falcon 9 rocket launched, and its first-stage booster landed, at the end of a smooth countdown.

“Everything proceeded nominally,” SpaceX launch commentator Michael Hammersley said. “Weather was looking potentially a bit tricky with those clouds, but ended up being a ‘go.'”

The Falcon 9 rose from Launch Complex 39A at NASA’s Kennedy Space Center in Florida at 10 a.m. ET (7 a.m. PT), sending the X-37B spaceward for the fifth mission of the test program. The four earlier launches were executed using United Launch Alliance’s Atlas 5 rocket.

In a tweet, Gen. Jay Raymond, who heads the Air Force Space Command, said the launch was a success.

The Air Force has never detailed exactly what the Boeing-built X-37B, also known as the Orbital Test Vehicle or OTV, is designed to do. However, at least some of the mission objectives relate to testing in-space technologies with potential military applications.

One of the experiments aboard the craft for OTV-5 is the Air Force Research Laboratory Advanced Structurally Embedded Thermal Spreader, which is meant to test experimental electronics and heat transfer systems for long-duration spaceflight.

The Air Force says OTV-5 will travel in a higher-inclination orbit than previous missions, to expand the operational envelope for the X-37B.

X-37B flights typically last for months. The fourth test mission, which ended in May, lasted for 718 days and set a record for flying a reusable plane in orbit.

At the end of each X-37B mission, the winged craft glides down from space to a horizontal runway landing.

SpaceX chalked up its own landing today. Minutes after launch, the Falcon 9’s second stage separated to send the X-37B on its way to orbital deployment, and the first-stage booster began a series of rocket burns to put itself on a trajectory heading back to the Florida coast.

The booster made a smooth vertical landing at SpaceX’s Landing Zone 1 in Cape Canaveral, not far from the launch pad. That marked the seventh on-the-ground touchdown for SpaceX’s rocket, with another nine landings made on autonomous drone ships at sea.

Such landings are a big part of SpaceX’s campaign for rocket reusability, which is expected to drive down the cost of access to space and eventually open up the way for flights to Mars and back.

SpaceX’s billionaire founder, Elon Musk, is due to update his long-term vision for Mars settlement later this month during the International Astronautical Congress in Australia.