Planetary defene goes on the offense

摘要

In December, a small asteroid exploded in the Earth's atmosphere in the most powerful event since the February 2013 explosion above Chelyabinsk, Russia. And no one noticed, at least at first. The event only recently came to light when it ap -peared on a list of fireballs maintained by the Center for Near Earth Object Studies at the Jet Propulsion Laboratory. The fireball was detected by sensors on U.S. military satellites used for missile warning, and added to the list when it became clear it was natural in origin.