United Launch Alliances's Delta IV Heavy rocket has been undergoing several rounds of testing to get the vehicle ready for launch No Earlier Than August 4th, 2018 from Launch Complex 37. The Wet Dress Rehearsals (WDR) started on July 2nd, where they focused on fueling the three first stage boosters. A second successful WDR was performed a few days later on July 6th, where they fueled the entire rocket and simulated a liftoff sequence. These tests help ensure that the rocket, ground systems, and launch support team are all ready for the launch.
The Delta IV Heavy rocket is launching a probe for NASA which will be closer to the sun than anything has ever been. By using Venus' gravity well, it will shrink its orbit around the sun and begin to fly through the Sun's corona. It will do this 24 times over the period of 7 years and will be the fastest human-made object in the solar system. The spacecraft is currently in Florida at the Astrotech Space Operations facility at NASA and will be transported and mated to the Delta Heavy in Mid-July.