Recently, some inside the Trump Administration, the most prominent being Elon Musk, have called for NASA's moon program, called Artemis, and the rocket designed for it, the SLS, to be cancelled. These critics have often said that it is too expensive, and that Musk may persuade Donald Trump to cancel the program entirely, opting to go to Mars instead. What is the SLS, and why does Elon Musk want to cancel it?