If the weather and Falcon 9 rocket cooperate, the first batch of SpaceX’s Internet satellites will launch from Florida on Wednesday evening. With a mass of 18.5 tons, this will be the company’s heaviest launch to date for either the Falcon 9 or Falcon Heavy rocket.

Wednesday’s rocket will boost 60 Starlink satellites, each weighing 227kg, to an altitude of 440km. This is the first block of Starlink satellites for what should eventually be a much larger constellation, and they will help SpaceX gauge its performance and conduct tests of several key systems. Over the coming months, these first satellites will be joined by six additional launches carrying similarly sized payloads. These launches will bring the constellation to an initial “operational” capability.

There is no guarantee all will go well, SpaceX founder Elon Musk said during a teleconference with reporters on Wednesday evening. “This is very hard,” Musk said. “There is a lot of new technology, so it’s possible that some of these satellites may not work. There’s a small possibility that all of these satellites will not work.”