It’s the stuff of science fiction: Scientists discover that an asteroid is heading for Earth, and we don’t have any measures in place to prevent the space rock from hitting us. But this scenario is actually more plausible than many people realize.

There are about 15,000 asteroids in our immediate galactic neighborhood that have the potential to collide with Earth at some point. Of those, nearly 900 are large enough to concern scientists. On March 2, astronomers at the Catalina Sky Survey in Arizona spotted a 10-foot-wide space rock that passed Earth, “diving in closer than many communications and weather satellites,” Space.com reports. The asteroid came within 9,000 miles of Earth, according to NASA’s Center for Near Earth Object Studies. The moon, by comparison, is about 239,000 miles from Earth.

NASA has a website devoted to the possibility of an asteroid hitting the planet. And it’s involved in an international collaboration with the European Space Agency, known as the Asteroid Impact and Deflection Assessment mission, to address the issue. AIDA is in the planning stages of a proposal to somehow alter the course of asteroids in outer space ― especially any that are in danger of hitting Earth.

Ed Beshore, an engineer who helped build the Catalina Sky Survey software technology that detects asteroids, said we’re still several years away from even the first stages of the AIDA experimental asteroid deflection system, and the project needs a lot of funding.

“NASA can only respond to funded mandates from Congress, so Congress has to be convinced to spend money,” Beshore told The Huffington Post. “These missions are not cheap ― they’re hundreds of millions of dollars ― so Congress has to explicitly allocate money for those sorts of things. And it really gets down to them deciding where the money is supposed to be spent.”

Yet the Trump administration has indicated it may cut NASA’s funding.

“I think, to some degree, the response is pretty much commensurate with the risk,” Beshore said. There’s no immediate, looming threat of an asteroid collision that scientists can point to, and so Congress and policymakers are focusing on the problems they can see.

“Risk is the product of two things: the likelihood that something’s going to happen and the consequences if it does,” he said. “Consequence is dictated by the size of these objects. How big are they and what’s the likelihood that we’re going to get hit by a big enough object to do some damage?”

Beshore is on the review panel for NASA’s half of the combined AIDA mission. He said a 65-foot-wide asteroid that streaked through the sky in 2013 and exploded over Chelyabinsk, Russia, showed up with no warning. Its sonic blasts released 30 times more power than the atom bomb dropped on Hiroshima, Japan, according to Forbes. It injured about 1,000 people and damaged many buildings, as seen in the video below.

“It was one of those objects that was on an orbital path that was difficult to find,” Beshore said. “An object that size could conceivably do more damage if it was on a different trajectory... [It] could do some serious damage if you didn’t have a way to deflect it or at least to find it and let people know it’s coming.”

NASA acknowledges daily near-Earth objects that have approached our planet.

Two of those asteroids brushed by Earth in January, CBS News reported at the time. They were farther away than 9,000 miles, but one of them, 2017 AG13, came as close as 126,000 miles on Jan. 9. The other cosmic rock, 2017 BX (10 to 40 feet in diameter), got within 162,252 miles of Earth on Jan. 24, as illustrated in the video below. In each case, scientists only learned of the existence of the asteroid a few days before the close encounter.

“The objective of missions, like the Catalina Sky Survey, is to inventory those objects, to find ones whose orbits might represent a future threat to the Earth,” Beshore said. “There are 876 1-kilometer and larger asteroids, as of February 21. And then, objects down to the 140-meter level ― there are about 7,500 of those that have been counted. And then, objects smaller than that increase that number to over 15,000.”

“Those are all near-Earth objects,” he went on. “They come within a certain distance to the Earth and, therefore, they represent a future collision threat. But it’s really these larger ones that we worry about.”

Beshore believes it’s important to form a policy response to such threats, whether it happens in the near future or in the longer term.

“At some level, if a 140-meter [400-foot diameter] asteroid hit, the damage it could cause is on the order of the kind of damage that something like [Hurricane] Katrina or a large earthquake could cause,” he said.

It seems almost inconceivable that there’s no planetary defense system already in place ― no matter what it would cost ― to deal with such a cataclysmic possibility.

According to NASA:

In the unlikely case where a particular potential impact event persists until the [asteroid’s] orbit is relatively well constrained, the impact probability and associated risk will tend to increase as observations are added. If an asteroid is indeed going to come very near the Earth then a collision cannot be ruled out early on. The impact probability will tend to grow as the orbit is refined and alternative and safer trajectories are eliminated. Eventually, the impact probability will drop (usually quite abruptly) to zero or, if the asteroid is really on a collision trajectory, it will continue to grow until it reaches 100 percent.

This article has been updated with further comments from Beshore and further information about the number of asteroids in proximity to Earth.