BEIJING (Reuters) — Chinese researchers want to send more than 20 of China's largest rockets to practice turning away a sizable asteroid — a technique that may eventually be crucial if a killer rock is on a collision course with Earth. The idea is more than science fiction. Sometime between late 2021 to early 2022, the United States will launch a robotic spacecraft to intercept two asteroids relatively close to Earth. When it arrives a year later, the NASA spacecraft will crash-land on the smaller of the two rocky bodies to see how much the asteroid's trajectory changes. It will be humanity's first try at changing the course of a celestial body.