SAN FRANCISCO—In a beige room on Monday, in the center of a sprawling corporate conference center, lifelong government researchers described one relatively quick and sudden way that the world as we know it could come to an end.

Luckily, they also had a plan to stop it.

For most of human history, “comets were portents of impending catastrophe, not the catastrophe itself,” said Joseph Nuth, a career geochemist at NASA. “That was until [Walter] Alvarez, in 1980, suggested that a comet or asteroid collision could wipe out the various species on the surface of the earth.”

In response to that paper, NASA realized that it was woefully unprepared to fend off a possible asteroid collision. It began tracking 5,000 “potentially hazardous objects,” or PHOs, mostly asteroids. Since more space rocks are discovered every night, the number of “PHOs” now exceeds 700,000, said Nuth.

But NASA—and Earth—remain utterly vulnerable to a comet collision. Unlike asteroids, comets can approach Earth at unusually high speeds as they depart the Oort Cloud, the sphere of icy objects that surrounds the farthest reaches of the solar system.

This means that while NASA might have years or decades to plan for a close-call with an asteroid, a comet could side-swipe Earth only 18 months after it was detected.

So Nuth had a recommendation: The United States should build two comet-destroying spacecraft ahead of time. The first, an “observer” craft, could be launched as soon as a threat is identified. The second, an “interceptor” craft,  could dislodge the asteroid or comet from its orbit or blow it up.

Both spacecraft would sit in storage until such a time as they were needed. Doing this advanced work, he said, would reduce the U.S. government’s “response time” to any hazardous object from about five years to less than 12 months.

“The recommendation is for anyone who will listen,” said Nuth at the press conference. “I’m a NASA scientist. I’m not a NASA policymaker. I’m not even in the administration of NASA.”

The two advanced craft are not official NASA plans yet, nor is the agency formally requesting them from Congress. Nuth said Congressional approval would be the biggest obstacle to getting a mission ready.

Nuth was joined on stage by three scientists from the Los Alamos National Laboratory, all of whom study planetary defense. They research, in other words, what the “interceptor” craft would actually do. Catherine Plesko, an applied physicist at Los Alamos, showed geometrically simple illustrations on-screen, demonstrating overlapping orbits.

“Here, the PHO intercepts the Earth’s orbit,” she said. “And if we’re having a very bad day, then it’s going to intercept with the Earth’s orbit on a day when the Earth is there.”

The Los Alamos team described two different methods to fend off a rogue space rock. It could use a kinetic impactor, “basically a giant cannonball,” said Plesko. This has the best chance of working if the planet has enough forewarning of the approaching rock.

How long would “enough” be? “Decades to centuries,” said Plesko. “Cannonball technology is very good technology because you’re deflecting an object at high speed.”

“But if the lead time is extremely short, there is really only one option left, and that’s a nuclear explosive,” said Robert Weaver, who leads planetary defense at the national laboratory. “They pack the largest amount of energy in the smallest amount of deliverable mass.”

With a nuclear bomb, researchers would not aim to explode the asteroid or comet in its totality. Rather, they would aim to initiate a “standoff burst” just to the side of the PHO. This would heat the side of the asteroid or comet. As this warmed side off-gassed and expelled matter, the rock would push itself out of orbit.

Plesko uses supercomputers at Los Alamos to simulate standoff bursts like these. “The best thing about this work is that when I’m done, I reach down to my keyboard and press delete,” she said. “For nuclear deflections, it really is best to contemplate the computer simulations before we have to do anything real.”

The final member of the team, Galen Gisler, examines what would happen if the world failed to intercept a perilous PHO in time. Most of the Earth is water, and an asteroid or comet smaller than about 300 meters could break up above the ocean in an “airburst” without significant harm to human civilization.

“An airburst over a populated shore will be highly dangerous,” said Gisler, a senior researcher at the University of Oslo. “Things could be very disastrous if you happen to live near an impact site.”

“An airburst or an impact that occurs far from shore—which is, of course, most of the ocean—they would be spectacular, and they may produce detectable waves, but probably not tsunamis,” he added. The larger an asteroid is, the more harmful it would be, as well. A 100-meter asteroid could break up in the atmosphere with little threat to anyone.

The Los Alamos team, who are led by Weaver, described their work as a kind of advanced planning. Plesko said the press conference showed a “summary of the results that we hope to someday provide to NASA and our national policymakers.”

“We are very carefully doing our homework before finals week, so to speak,” she added. “If you look at life as an academic class, you need to do the homework before we don’t have time to study anymore and have to make some decision.”