Atlas 5/Cygnus post-launch news conference

United Launch Alliance crews at Cape Canaveral have topped off an Atlas 5 rocket for liftoff Monday with the U.S. military’s next infrared sentinel to join a satellite fleet designed detect missile launches around the world.

On its third flight Monday night, SpaceX’s Falcon Heavy rocket will fly to three different orbits with two dozen spacecraft on a mission set to last more than six hours, prompting SpaceX founder Elon Musk to declare it the company’s “most difficult launch ever.”

Arianespace’s third Ariane 5 launch of the year was set liftoff Friday from Kourou, French Guiana, but an issue with a sensor on the rocket’s first stage liquid hydrogen tank caused officials to scrub the launch attempt. The Ariane 5 is poised to carry three U.S.-built commercial satellites into orbit, while testing a few rocket upgrades, including a modified fairing needed for the launch of the James Webb Space Telescope next year.