Watch NASA's historic Moon mission rocket test at 4PM ET (update: cut short)

NASA is edging closer to sending a spacecraft around the Moon, and it’s about to conduct a crucial test to ensure that mission goes forward. As Reuters reports, the agency is conducting a hot fire test for the Space Launch System’s rocket core stage at 4PM Eastern, with coverage available on NASA TV starting at 3:20PM. You can also expect a press conference roughly two hours later to discuss the outcome.

This is the first test firing all four of the rocket’s RS-25 engines at once, and the last of the Green Run series ensuring the core stage is ready for the Artemis I mission sending an Orion spacecraft to the Moon. NASA will be much closer to launch if the hot fire goes well.

This isn’t the most powerful heavy-duty rocket in the near term. SpaceX’s Falcon Heavy generates slightly more thrust at 1.7 million pounds at sea level versus SLS’ 1.67 million, and both are well below the 7.89 million pounds of Saturn V’s first stage. It’s more expensive than some offerings, too, at $2 billion per launch versus $90 million for the highly reusable Falcon Heavy and $350 million for ULA’s Delta IV Heavy.

However, SLS is also poised to be one of the most capable platforms. One mission can carry payloads that might require two or more launches even for SpaceX’s upcoming Starship, let alone existing options. That could save NASA money, and greatly simplifies some missions. It’s just a question of how often you’ll see SLS in action — with the incoming Biden administration expected to delay the Artemis program, it might not get as much use as planned.

Update 1/16/2021 at 2:45PM: NASA has moved the test up to 4PM Eastern from 5PM after preparation ran ahead of schedule. Coverage now starts an hour earlier as well, at 3:20PM. We’ve edited the post accordingly.

Update 1/16/2021 at 5:30PM: NASA fired the rockets, but cut the test short of the planned eight-minute burn after what might be a component failure.