Waterside view of Delta 4 GPS 2F-6 Launch

United Launch Alliance engineers filled a Delta 4-Heavy rocket with super-cold liquid hydrogen and liquid oxygen propellants Friday at Cape Canaveral Air Force Station during a practice countdown before the heavy-lifter’s scheduled liftoff in June with a top secret U.S. government spy satellite.

The 16th and last launch of the Delta 4 Heavy configuration also marks the end of Delta line of launch vehicles. NROL-70 with be the 389th Delta launch since 1960 and 45th Delta 4 rocket.

Space missions dispatched into the solar system often have journeys lasting years before reaching a scientific payoff, but NASA’s Parker Solar Probe launched last weekend on a speedy departure from planet Earth is already getting ready to sweep closer to the sun than any spacecraft in history during a flyby later this year.