Don’t Panic, But There’s An Asteroid Right Over There We’re still trying to find all the space invaders that could damage Earth.

An asteroid no bigger than a school bus whizzed past Earth at 18,000 miles per hour on Sept. 7, harmlessly continuing its cosmic journey after paying our planet an exceptionally close visit. The rock, dubbed 2016 RB1, made its closest approach just 25,000 miles from the Earth’s surface. That’s just a tad farther out than the orbit of weather satellites — an extremely close shave, in space rock terms. Scientists spotted it just two days before it passed.

There’s no chance 2016 RB1 will hit us, and even if it did, Earth would barely notice. It would be statistically most likely to explode over an unpopulated area, and it’s too small to make a dent; the asteroid is 25 to 50 feet wide, a range that’s hard to pin down because 2016 RB1 is so small and moving so fast. It’s probably at least 10 feet narrower than the rock that exploded over Chelyabinsk, Russia, in February 2013. Still, this wimpy asteroid is notable for two reasons: its close passage and the fact that scientists saw it at all.

The solar system is full of crumbs left over from the birth of the sun and the planets. Some of them hold clues to the formation of the Earth and, potentially, the beginning of life, which is why NASA just launched a spacecraft to a nearby coal-dark rock called Bennu. Some asteroids hold precious metals, which private companies would like to mine someday for use in electronics and catalytic converters on Earth. And some asteroids occasionally make their way to our planet, where they explode in the atmosphere or excavate giant craters and, sometimes, cause global catastrophes.

NASA and other space agencies are trying to find as many asteroids as they can, but most of them remain unknown to us. Many are too small, too dark, too distant or in the wrong place in the sky for our telescopes to catch them. Yet some of them are big enough to pose a threat to people on Earth, so figuring out where they are is a key part of NASA’s mission.

There is good news: Gigantic space rocks like the one that probably killed the dinosaurs will not arrive anytime soon. Using infrared telescopes in space, as well as sophisticated ground-based telescope networks, scientists are confident they’ve found more than 90 percent of the Earth-killer asteroids out there — the ones about half a mile wide or larger, which would cause global extinctions if they crashed into Earth. None of those threaten our livelihoods in the next few centuries.

But there are plenty of small and mid-size asteroids, and they can do a lot of damage, too. Congress mandated that, by 2020, NASA is supposed to have found 90 percent of asteroids measuring 460 feet across, roughly the size of a football stadium, which would cause regional havoc if not global devastation. Scientists think there are about 25,000 such asteroids, and they have located some 20 percent to 25 percent of them. Asteroid-hunting surveys find about 500 a year, and NASA has said it won’t hit the deadline for that 90 percent mark without more funding. Then there are the house-size and bus-size rocks, like Chelyabinsk and 2016 RB1. More than a million asteroids like those are thought to exist, but scientists have details on maybe 1 percent of them.

Researchers build model asteroid populations based on current observations, which roughly follow a power law, said Eric Christensen of the University of Arizona in Tucson, who directs the Catalina Sky Survey, a NASA-funded asteroid-tracking program that found 2016 RB1. Small asteroids are plentiful, and larger asteroids are rarer. All told, surveys turn up about 1,500 previously unknown asteroids each year, ranging from a few feet in diameter to half a mile, he said.

“We think we know about how many objects of a given size are out there,” Christensen said. “The models have to be anchored in something, so they’re typically anchored in observations of objects that we know about. But we only know about objects that we know about because we were able to discover them.”

Put another way, we know what we know, and we know some of what we don’t know, but there are also things we don’t know that we don’t know.

In May, the physicist Nathan Myhrvold — Microsoft’s former chief technologist, a self-taught chef and an all-around statistics gadfly — stirred up a controversy over the latter. He took aim at a NASA mission called NEOWISE, which uses an infrared space telescope to look at near-Earth asteroids, and argued that agency scientists incorrectly estimated the sizes of many newly discovered rocks. Myhrvold argued that the NEOWISE team failed to account for peculiarities of thermal radiation and, as such, their best guesses of many asteroids’ size and mass were wrong. “It’s just not possible with the WISE data to achieve a goal of plus or minus 10 percent accuracy,” he wrote. He said their mistakes meant that widely held assumptions about Earth’s threat level were incorrect.

NASA quickly countered that his math was flawed. But Myhrvold’s claim shone light on a problem in asteroid hunting: Because asteroids are hard to find and even harder to visit, there are still plenty of uncertainties.

Survey missions like Catalina are improving matters, reducing the margin of error between model predictions and asteroid observations. But even an all-sky search can miss a threatening space invader. The Chelyabinsk meteor started out as an asteroid about 56 feet across, but nobody saw it coming because it approached Earth from the same direction as the sun, which blinded telescopes’ view. (An asteroid is considered a meteor when it enters Earth’s atmosphere.)

The Chelyabinsk meteor slammed into Earth’s atmosphere at 42,500 mph, breaking into pieces 19 miles above Earth’s surface. As it fell, the superheated meteor appeared brighter than the sun, even for people more than 60 miles away, and its ultraviolet radiation caused sunburns. The meteor caused an airburst that shattered windows, shook buildings, knocked people off their feet and sent more than 1,000 people to hospitals, though no deaths were reported. In other words, finding objects like this before they hit — even a couple of days before their approach, as in the case of 2016 RB1 — could save lives and property by allowing people to evacuate or take shelter. The problem is knowing where to look, Christensen said.

To increase the odds of finding asteroids, the Catalina Sky Survey uses two telescopes that take pictures of the entire sky every clear night, except during full moons. The survey’s smaller telescope looks for asteroids near the ecliptic, which is the plane in the sky where the planets — and asteroids — orbit the sun. The larger telescope looks at a wider field of the sky. When they spot an asteroid, it just looks like a tiny white dot moving across the background stars. Human observers double-check the pictures.

“It’s very difficult to train software to replicate the human brain. By putting a person in the final step of validation, we can very efficiently sort through real and false detections,” Christensen said.

The Catalina team got a new camera Sept. 7 that will spruce up its larger telescope, allowing it to see an even greater chunk of the sky in a single image. But to really ramp up the asteroid-detection rate, Catalina and other surveys will need more funding, Christensen said.

In January, NASA brought its asteroid-hunting efforts under a new umbrella program called the Planetary Defense Coordination Office. Along with finding and tracking new objects, the office issues notices about close passes, such as 2016 RB1, and will issue warnings of potential impacts. NASA’s budget for the 2016 fiscal year includes $50 million for near-Earth asteroid observations and planetary defense, which is a tenfold increase since the beginning of the Obama administration. There are no definitive ways to deflect a threatening asteroid, but NASA is working on it. The agency’s Asteroid Redirect Mission includes plans to lasso a space rock and drag it closer to the Earth and moon sometime in the 2020s; this would test the so-called “gravity tractor” method of using another object’s mass to change an asteroid’s course. And the joint NASA-European Space Agency Asteroid Impact and Deflection Assessment mission, if it gets funding, would demonstrate how to deflect an asteroid by crashing a spacecraft into it. Even if nobody can do anything about a looming asteroid strike, NASA’s new office will work with the Federal Emergency Management Agency to coordinate response plans.

Still, Christensen said that, despite the ever-increasing number of known near-Earth asteroids, he doesn’t spend too much time worrying about rocks falling from the sky.

“I’m not concerned that humanity is going to be wiped out by an asteroid impact in our lifetimes. That’s just statistically so unlikely,” he said. “We need to worry about it in the appropriate measure. And I think our current level of effort in discovering and tracking near-Earth objects is about right.”

CORRECTION (Sept. 13, 9:45 p.m.): An earlier version of this article misstated the position of asteroid 2016 RB1 relative to that of GPS satellites. The asteroid’s passing distance of 25,000 miles from Earth was about twice the distance of GPS satellites from Earth. Weather and communications satellites, on the other hand, are about 22,000 miles from Earth.

CORRECTION (Sept. 14, 10:45 a.m.): An earlier version of this article misstated the number of asteroids measuring at least 460 feet across that scientists believe are near Earth. There are thought to be about 25,000, not 125,000.