RS-25s aren't exactly new -- they were known as the main Space Shuttle engines back in the day. However, they need to be re-tested, since the SLS is much bigger and needs more power. Plus, the data the agency collects will help it design a new engine controller and software for the rocket. The SLS was created specifically to launch the Orion spacecraft for future deep-space missions, including both unmanned and manned trips to Mars. If all goes as planned, it will ferry an unmanned capsule to space for the first time as soon as 2018.

[Image credit: NASA/MSFC