NASA wants to send humans to Mars one day, and it's building a gigantic rocket called the Space Launch System to get them there. Today, the space agency will test one of the rocket's engines — the RS-25, which the agency calls "the Ferrari of rocket engines" — monitoring its performance and looking for problems along the way. It's the sixth of seven planned tests for the engine, and it's scheduled to last for 535 seconds. (That's the amount of time the engine would burn during a regular launch). NASA will live stream the whole test at 5PM ET, and coverage starts at 4:30PM ET.
It's a tuned-up version of the Space Shuttle engine
If the RS-25 looks familiar, that's because it's the same engine that was used by NASA's Space Shuttle program. But while the Space Shuttle famously used three of these RS-25 engines, the SLS will use four to push it to a top speed of more than 22,000 miles per hour (about 35,000 kilometers per hour). In its SLS configuration, the engine is more powerful than the Shuttle version — it generates 512,000 pounds of thrust and can handle temperatures as low as -400 degrees Fahrenheit (-250 degrees Celsius) and as high as to 6,000 degrees Fahrenheit (3,300 degrees Celsius). Unfortunately, we won't see a full test flight until at least 2018, and the first crewed flight isn't scheduled until the 2020s.
If you missed any of the previous tests, The Planetary Society made this handy video that shows you every single one at the same time: