It’s the stuff of Hollywood movies, but the threat to Earth by hazardous asteroids is rooted in fact, not fiction. “We look at the moon, we look at the Earth, we see that it’s full of craters. More on the Moon than on the Earth, which is, of course, telling us that impacts of small bodies on Earth on geologic time scales are normal,” says Thomas Zurbuchen, associate administrator for science at NASA. If all goes well, the boxy, 1,200-pound craft will slam head-on into Dimorphos, an asteroid 525 feet (160 meters) across, at 15,000 mph next September. “The DART spacecraft has to hit the asteroid, then DART has to measure the amount of deflection and then we want to understand why that deflection came about, how it works,” explains DART investigation team lead Andy Cheng. “So, it’s all about measuring the momentum transfer. How much momentum do we put …