NASA will test-fire its 1st SLS megarocket for moon missions today. Here's how to watch.

Update for 9:48 pm: NASA test-fired its first Space Launch System core booster Saturday, but the trial did not go as planned. Read our full story here.

NASA will attempt to fire the engines on its Space Launch System (SLS) megarocket for the first time today and you can watch the fiery action live online.

As part of a critical test before the rocket behemoth  lifts off for the first time, the agency plans to ignite the four main engines on its heavy-lift core booster this at about 5 p.m. EST (2200 GMT) today, Jan. 16. The test, which is designed to simulate the core stage's performance during launch, will take place at the agency’s Stennis Space Center, in Mississippi. 

You can watch the test live here and on the homepage, courtesy of NASA, beginning at 4:20 p.m. EST (1920 GMT). You'll also be able to watch the test directly from NASA here.

Today's engine test is the final step in the agency's "Green Run" series of tests designed to ensure the SLS rocket is ready for its first launch — called Artemis 1 — that will send an uncrewed Orion spacecraft around the moon. That first flight is scheduled to blast off later this year.

Video: How NASA's SLS megarocket engine test works

The SLS is NASA's next-generation heavy-lift rocket that will ferry astronauts to the moon as part of the agency’s Artemis lunar program. Launching by the end of this year, Artemis 1 will be the first in a series of missions that will culminate in NASA's first crewed lunar landing since the Apollo era. That mission,  called Artemis 3, could happen as soon as 2024 if all goes as planned. 

To that end, NASA is putting the massive SLS rocket's four RS-25 engines through their paces prior to launch. The agency has been systematically testing each engine and conducting launch-day procedures such as fueling to ensure all systems are working as expected. 

The upcoming hot-fire engine test, is the final step in the testing process. On Saturday, engineers will load the SLS core booster with over 700,000 gallons of superchilled propellant before igniting all four of its RS-25 engines at once. This will mark the first time that four RS-25 engines will fire at the same time. (The same engines powered the space shuttle but it took only three to make the orbiter fly.) 

Related: These are the space missions to watch in 2021

NASA's first Space Launch System Core booster undergoes a fueling test on the B-2 Test Stand at the Stennis Space Center near Bay St. Louis, Mississippi on Dec. 21, 2020. The rocket's four main engines will be test fired today. (Image credit: NASA)


Burning for approximately 8 minutes — the duration they'll burn during a launch to the moon — the RS-25 quartet will generate a whopping 1.6 million pounds of thrust during the test. 

"When we ignite the engines, the stage actually will think it is flying," Ryan McKibben, NASA's Green Run test conductor at Stennis Space Center, said during a pre-test media conference on Jan. 12. "That's what it's built to do. But of course, it won't go anywhere because the stage is fastened at the same locations where the solid rocket boosters anchored would be anchored."

As part of the agency’s “Green Run” testing schedule, the megarocket underwent two wet dress rehearsals, during which fuel was loaded, and subsequently drained. Officials said that the tests went well; however, they were not without issue. One of the fueling ops ended early, one was delayed due to temperature issues, and the campaign was also affected by multiple tropical storms as well as the global pandemic. As a result, the agency chose to delay the hot fire test

Photos: NASA's 1st SLS megarocket core stage for the moon has its engines

Agency officials explained that the delays proved fruitful as the team was able to revise procedures and update the terminal countdown sequence based on pre-flight testing. 

The test is scheduled to take place late Saturday afternoon, and that morning, the day will start with a go/no-go meeting where the team will decide to begin fueling procedures. Once that's underway, a final poll will be conducted at T-10 minutes to determine if it’s safe to proceed with the hot fire test.

The engines will burn for 485 seconds, or roughly 8 minutes. Once the test is complete, a data review will begin, and is expected to take several days, according to NASA’s Julie Basser, program manager for SLS at Marshall Space Flight Center. 

"This is the first time we fired up this core stage and this is a huge milestone for us," she said. We are doing everything we can to ensure that we get the most out of this hot fire test and we are ready for launch. Testing provides an opportunity to learn and make sure that the rocket is ready to fly astronauts to the moon."

If all goes as expected the core stage will be refurbished and then shipped to Kennedy Space Center to prepare for launch. Its expected arrival is slated for sometime in February, where it will be integrated with the rest of the vehicle already on site. 

Currently, the massive rocket's solid rocket booster segments are being stacked one by one in the Vehicle Assembly Building at NASA’s Kennedy Space Center in Florida. 

Along with the four RS-25 engines, the SLS will be powered by two solid rocket boosters that consist of five segments fitted together. (Each booster is made from recovered segments that were used on NASA's space shuttle program.) 

Once fully assembled, each of the two solid rocket boosters will stand 177-feet-tall (54-meters) and produce more than 3.6 million pounds of thrust at liftoff — the bulk of the power during the first two minutes of launch and flight.

Related: Coronavirus delays key tests of NASA's new SLS megarocket

This first SLS rocket will be used for the  Artemis 1 mission, which is an uncrewed flight that will send  NASA's Orion space capsule on a trip around the moon, helping pave the way for an eventual planned lunar landing near the moon’s south polar region.

Orion is the third vehicle NASA currently has in development that will eventually fly NASA astronauts to low-Earth orbit and beyond. The first, SpaceX’s Crew Dragon capsule entered service in 2020 as it ferried astronauts to the space station in May and November. 

Boeing’s Starliner crew capsule is expected to launch astronauts later this year, following a successful second orbital flight test. Starliner first launched in 2019, on an uncrewed flight to the space station but failed to reach the orbital outpost following a series of software anomalies. It’s next test flight is scheduled for no earlier than March and if all goes well, then it will carry a crew of three astronauts to the space station later this year. 

Having three different astronaut-toting capsules will provide NASA with the flexibility to routinely send astronauts to low-Earth orbit while also exploring the moon and eventually Mars. 

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Amy Thompson
Contributing Writer

Amy Thompson is a Florida-based space and science journalist, who joined as a contributing writer in 2015. She's passionate about all things space and is a huge science and science-fiction geek. Star Wars is her favorite fandom, with that sassy little droid, R2D2 being her favorite. She studied science at the University of Florida, earning a degree in microbiology. Her work has also been published in Newsweek, VICE, Smithsonian, and many more. Now she chases rockets, writing about launches, commercial space, space station science, and everything in between.