SpaceX’s Starship, the largest and most powerful rocket ever built, is supposed to land NASA astronauts on the surface of the moon in a couple of years.

Even more ambitiously, Elon Musk, who owns and runs SpaceX, has said that the company would send about five Starships to Mars next year — sans astronauts, but with Optimus robots built by his electric car company, Tesla.

The past two test flights of Starship have ended in explosive failures, and a third failure during the next launch, planned for Tuesday evening, would be embarrassing for SpaceX and Mr. Musk.

Mr. Musk, however, seems to relish taking on doubters. He plans to give a talk to SpaceX employees titled simply “The Road to Making Life Multiplanetary,” at 9 p.m. Eastern time. The talk had been scheduled for 1 p.m., but it was pushed back several times until Mr. Musk announced that it would be postponed until after the launch.

When is the launch and how can I watch it?

The launch window, which lasts for 60 minutes, opens at 7:30 p.m. Eastern time for the ninth test flight of Starship to lift off from SpaceX’s Starbase site in South Texas. SpaceX will provide live coverage on its website beginning about 30 minutes before liftoff.

What happened during the last flights?

SpaceX takes pride in its “fail quickly and fix quickly” approach to rocket design, so it is not surprising that none of the eight test flights so far have worked perfectly. But the last two — flight seven in January and flight eight in March — were more disappointing. They could not repeat the successes of earlier test flights.