Los Angeles — Imagine if scientists found out that a massive asteroid was on a collision course with Earth and would strike somewhere near Los Angeles by September 2020.

What could humanity do?

Not much. At least, that was the result of a day-long tabletop exercise coordinated by NASA and FEMA late last month. In their hypothetical scenario, the space agency concluded that the 330-foot space rock was approaching too quickly to mount a deflection mission. The team from FEMA was left to figure out how to evacuate millions of people from Southern California.

This was a purely fictional exercise. NASA has discovered some 17,000 potentially hazardous near-Earth objects, but none of them is projected to come close to Earth in the next hundred years. No human that we know of has been killed by a meteorite or the effects of an impact, and the likelihood that this could happen to any of us is very, very slim. The chance of an impact big enough to destroy our planet is even smaller. Earth has suffered only one mass extinction-inducing impact that we know of in its 4.6 billion-year history.

Still, researchers don’t want to simply wait around and see what happens. This week, 100 planetary scientists, physicists and engineers published an open letter in support of a joint European Space Agency and NASA mission to survey a near-Earth asteroid and attempt to deflect it.

“It is crucial to our knowledge and understanding of asteroids to determine whether a kinetic impactor is able to deflect the orbit of such a small body, in case Earth is threatened. This is what AIDA [an Asteroid Impact and Deflection Assessment mission] will help us assess.”

The joint ESA-NASA mission would launch a spacecraft in 2020 to slam into an asteroid 10 million miles from Earth.

The mission would provide proof-of-concept for the idea that a “kinetic impactor” can change the trajectory of a rock in space — that is, if it works. In the remote possibility that it becomes necessary, NASA might one day use this technique to shift an asteroid away from Earth’s orbit.

paratively small cost and effort, test technologies that we need for future missions that are farther away.”

At 250 million euros (about $265 million), AIM is considered a low-cost mission. But it faces some technical challenges. For one thing, scientists barely know anything about Didymoon’s shape or composition, because its small size makes it difficult to image. The (relatively) tiny rock also has barely any gravity – according to Space.com, if you jumped on it you wouldn’t come back down, but instead float off into space – making it hard to ensure a lander stays put on its surface.

But ESA’s recent Rosetta mission, which ended earlier this year, demonstrated that the space agency could plop a lander down on a very small body. Granted, the comet visited by Rosetta was about two miles across, not 550 feet. But the authors of the open letter say Rosetta was a big step toward what AIM hopes to achieve.

Beyond demonstrating the potential for deflection, the joint ESA-NASA mission will also help scientists better understand asteroids – ancient chunks of material leftover from the earliest days of the solar system.

“We want to make sure that the heritage of Rosetta, in terms of technology and expertise, continues – leading to new missions and further innovation,” the letter said. “As citizens of our solar system, we need to expand our body of knowledge of the universe in which we live.”