How do you test a new kind of rocket engine? Step 1: Bolt it to a trailer in the middle of the Mojave Desert. Step 2: Vroom! In this case, NASA is firing up the 5M15, which runs on compressed liquid methane. The odorless substance has multiple advantages over conventional rocket propellants: It's cheaper, it requires much less insulation, and it exists on several planets NASA hopes to travel to — Mars, here we come. That means astronauts could collect their own fuel for the trip home. Bonus rocket science: Those glowing figure eights in the blast stream are called Mach disks, after the guy who lent his name to the speed of sound. They're shock waves, created as the expanding fuel hits the higher atmospheric pressure outside the nozzle. If part of this blast weren't obscured, you could take the number of Mach disks (we count seven) and multiply by the speed of sound — about 758 mph at the 1,300-foot altitude of this test — to estimate the speed of fuel exiting the engine. Just don't get too close.