The second time was a charm. NASA completed a critical hot-fire test of its Space Launch System core stage on Thursday, a follow-up to a previous test that didn't go as planned. The event lasted about eight minutes and created a massive billowing cloud that stretched into a nearby forest at the Stennis Space Center in Mississippi. 

NASA wants to escort humans back to the moon. The agency has been aiming for an uncrewed Artemis I around-the-moon mission later in 2021, but it needed to put its moon rocket through its paces here on Earth first. 

NASA refers to the core stage as "the backbone of the SLS rocket." Hot-fires are dramatic affairs that are meant to simulate the rigor of launch conditions. Today's event was set to be the final part of the "Green Run" series of tests designed to check out the core stage of SLS before it actually launches from Earth. 

Here's NASA's description of what happens during the SLS hot-fire: "On test day, engineers will power up all the core stage systems, load more than 700,000 gallons of cryogenic, or supercold, propellant into the tanks, and fire the rocket's four RS-25 engines at the same time to simulate the stage's operation during launch, generating 1.6 million pounds of thrust."  

The first attempt at the hot-fire in mid-January didn't go smoothly, as the rocket shut down just over a minute into what should have been an eight-minute test.   

In late January, NASA said it planned to conduct a second hot-fire. The do-over test was originally targeted for late February, but got delayed after engineers discovered a valve -- part of a system that supplies liquid oxygen to an engine -- wasn't working properly. The SLS team troubleshot the issue and repaired the valve.

"After analyzing initial data, the team determined that the shutdown after firing the engines for 67.2-seconds on Jan. 16 was triggered by test parameters that were intentionally conservative to ensure the safety of the core stage during the test," NASA said in a statement Jan. 19. The agency will aim to make it to eight minutes during the do-over. 

NASA reported the core stage, its engines and the test stand were all in "excellent condition" with no major repairs needed before the next attempt. The second test seemed to work out as expected and prompted applause from the NASA team monitoring the exercise. 

While Artemis I won't have humans on board, later SLS missions will be responsible for safely escorting astronauts into space. "All SLS rockets use the same core stage design, NASA said, "so a second Green Run hot-fire will reduce risk for not only Artemis I, but also for all future SLS missions."

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