They hope to learn the best strategies for responding to a potential strike, starting from the moment a threatening asteroid is first detected by astronomers. NASA’s Planetary Defense Coordination Office will team up with other partners to perform a “tabletop exercise” on how they’d handle the news of a (fictional) asteroid on a collision course with Earth. On March 26, 2019, astronomers discover an asteroid in the night sky, far dimmer than Pluto to their telescopes. They name it 2019 PDC. Initially, it appears that the asteroid’s eccentric orbit bring it within approximately 18 times the Moon’s distance from the...