Last month, NASA performed a simulation of what might happen if an asteroid was on a collision course with Earth. (NASA)

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. Remember that Earth has suffered only one mass extinction-inducing impact that we know of in its 4.6 billion-year history, and even that asteroid didn't end life entirely. Our planet is pretty resilient.

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

“Unlike other natural disasters, this is one we know how to predict and potentially prevent with early discovery,” the letter reads. “As such, 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.”

In early December, ESA's council of ministers will meet to decide on funding for the first half of the AIDA mission.

ESA's half of the joint project with NASA is called the Asteroid Impact Mission (AIM). It would launch a spacecraft in 2020 to the Didymos asteroid and its “moonlet,” known as Didymoon. The pair of space rocks is expected to pass within 10 million miles of Earth around 2022. From an interplanetary perspective, that's a hair's breadth.

The smaller of the two bodies — 550-foot wide Didymoon — is AIM's target. The spacecraft would release a probe to land on the rock's surface, as well as two satellites to orbit around it.

Four months later, a NASA spacecraft would complete the second part of the mission with a Double Asteroid Redirection Test (DART). With the ESA satellites watching, the DART spacecraft is scheduled to slam into Didymoon and attempt to knock it off course.

Aside from demonstrating NASA's over-the-top love for acronyms, the two-part 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.

“New NEOs [near-Earth objects] are now being discovered at the rate of some four per day,” Alan Harris, a letter signatory and senior scientist at German Aerospace Center, noted at a news conference. “We need a coordinated international strategy for near-Earth object impact mitigation.”

Many asteroid researchers say that the Chelyabinsk meteor strike, which injured at least 1,200 people in the Ural Mountains in Russia in 2013, was something of a wake up call. It's estimated that the rock weighed about 10 tons and was about 65 feet across when it broke up in the sky above the city of Chelyabinsk. That no one died is largely a product of the remote location and incredible luck.

An analysis of the meteor published later that year suggested that dangerous strikes could be more common than many of us realized.

Fourteen images taken by the Arecibo radio telescope of near-Earth asteroid (65803) Didymos and its moonlet, taken on Nov. 24, 2003. (NASA)

The timing of the mission is important, AIM supporters say, because the Didymos pair's orbit won't bring it this close to Earth's again for another 100 years.

“It is a very unique opportunity,” Cornelius Schalinski, deputy head of business development for OHB, a German private space company helping to implement AIM, told “It has to go in 2020. Otherwise, the opportunity is lost. It is an asteroid that gets pretty close to Earth, so we can, with comparatively 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, 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 left over 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.”

Read more:

At the ‘Crater of Doom,’ geologists dig up rocks from the day the dinosaurs died

Ancient Swedish space rock may be a whole new kind of meteorite

Asteroids splashed into the young moon — and they may be the source of its water

This strange, tailless comet could teach us about Earth’s origins

This asteroid almost certainly isn't going to crash into Earth and kill us all