SpaceX successfully launched the U.S. Air Force's reusable robotic space plane on its latest secret mission today.

The X-37B Orbital Test Vehicle (OTV), built by Boeing, is an unmanned spacecraft that has spent up to 700 days orbiting the Earth on previous missions.

This is its fifth mission, but the first aboard a SpaceX rocket.

The space plane was launched aboard a Falcon 9 rocket at Kennedy Space Center at 10 a.m. ET.

Following the launch, the first stage of the Falcon 9 rocket returned to SpaceX's landing zone at Kennedy.

Falcon 9 first stage has landed at Landing Zone 1. <a href="https://t.co/TBDKNm6hbe">pic.twitter.com/TBDKNm6hbe</a> —@SpaceX

The U.S. Air Force has provided few details about this or previous X-37B missions, saying only that the orbiters "perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies." The cost of the program is also classified.

The X-37B first flew in April 2010 and returned after eight months.

A second mission launched in March 2011 and lasted 15 months, while a third took flight in December 2012 and returned after 22 months. The fourth launched in May 2015 and landed this past May after almost two years in orbit.