The window for launching the SES-9 communications satellite into orbit tonight opens at 6:46 p.m. ET. After sending the payload into orbit, SpaceX will attempt for the fourth time to land its Falcon 9 on a drone ship in the open ocean.
Despite their success returning a Falcon 9 to a landing pad in December, SpaceX doesn't always have the option to land the Falcon 9 on solid ground.
As SpaceX attempted a drone ship landing last month, Elon Musk explained the landings are needed when it is "just not physically possible to return to launch site." The drone ship landings are especially needed for "high velocity missions," which would allow payloads such as satellites to reach a higher orbit, according to Musk.
As mentioned before, ship landings are needed for high velocity missions. Altitude & distance don't mean much for orbit. All about speed.— Elon Musk (@elonmusk) January 17, 2016
SpaceX isn't giving tonight's landing attempt high odds of success due to the unique orbit profile of the SES-9 satellite, however the company is still planning on an "experimental" landing on its drone ship, named "Of Course I Still Love You," which is parked off the coast of Florida. (SpaceX's drone ship on the West Coast is named "Just Read the Instructions.")
If all goes according to plan, the Falcon 9's first stage will separate from its payload once it has left Earth's atmosphere. Cold gas thrusters will help the rocket complete a flip maneuver that will bring it barreling back toward Earth.
A series of engine burns will help control the rocket and as it nears Earth, while grid fins will provide aerodynamic guidance. Engines will ignite one final time before the rocket -- hopefully -- will land upright on the drone ship.
While conventional rockets burn up on re-entry, SpaceX designed the Falcon 9 to be able to withstand the heat and land vertically so the rocket can be used again on a future launch. The Falcon 9 SpaceX returned to Earth in December has been inspected, and Musk said is free of damage and capable of firing again.
SpaceX almost had a perfect landing at sea in January, however one of the four legs didn't lock completely, causing the rocket to tip over and explode as it touched down on the ship.