* SpaceX is expected to launch a ninth uncrewed test flight of Starship, the most powerful rocket system ever constructed.

* Liftoff of the vehicle is expected to occur during a one-hour launch window that opens at 7:30 p.m. ET (6:30 p.m. local time).

* SpaceX is attempting to reuse one of its Super Heavy rocket boosters for the first time after successfully recapturing the 60-story-tall vehicles in the arms of a launch tower on three prior test flights.

* This launch comes after two consecutive test missions -- Flight 7 in January and Flight 8 in March -- ended in explosive failure of the upper Starship spacecraft minutes into flight. Those explosions rained debris near islands east of Florida.

* Despite recent setbacks, Starship is considered crucial to NASA's goal of returning astronauts to the moon this decade.

* SpaceX CEO Elon Musk has also long billed Starship as the vehicle that will carry humans to Mars for the first time, an effort for which President Donald Trump has signaled his support.