Mars has intense radiation, furious dust storms, and temperatures that plunge as low as -200 degrees Fahrenheit (-129 degrees Celsius) — and NASA's Mars rovers usually handle them all with aplomb. But Curiosity had a little hiccup last month. As to what slowed the rover down, though? Well, a rock that just wouldn't let go. A still from a gif showing the rover's drill stuck in a rock. (Image credit: NASA/JPL-Caltech) Now, NASA engineers have dealt with their fair share of challenges during deep-space missions. But a whole rock riding out of the ground and refusing to let go? That's a first in Curiosity's more than 13 years on the Red Planet. The team's first move to free the rover was vibrating the drill to shake the rock loose. And that did exactly squat. Four days later, the operators reoriented the arm and tried vibrations again. Atacama shed some sand, but the rock stayed attached. On May 1, engineers pushed harder. They tilted the drill more, rotated it, vibrated it and spun the bit.…