In May, a group of international scientists assembled near Washington, D.C., to tackle an alarming problem: what to do about an asteroid hurtling toward Earth.

Astronomers at a mountaintop observatory in Hawaii had spotted an 800-foot-wide asteroid, dubbed 2019 PDC, when it was 35 million miles away. By asteroid standards, it was relatively small—not even close to the six-mile-wide piece of space rock believed to have wiped out the dinosaurs 65 million years ago. Still, this asteroid was traveling at 31,000 miles an hour,...