Between 1988 and 2017, US sensors observed more than 700 fireballs from asteroids and space debris, most of them pretty small.

Even asteroids that don't make it to Earth's surface can cause damage. In 2013, a small asteroid exploded over Chelyabinsk, Russia. 1,500 people were injured from the resulting shockwave.

NASA is mainly trying to detect the small- and medium-size asteroids that could damage cities.

More than 16,000 of them are considered near-Earth objects because sometimes they come as close as a mere 28 million miles from us — a hair's breadth away in cosmic terms.

In 1998, and then again in 2005, Congress told NASA to find these objects and make plans for diverting ones that posed a risk to Earth.

Sometimes these asteroids collide with each other and break off into smaller chunks called meteoroids.

There are millions of such rocks, called asteroids, in the solar system. An estimated 1 million to 2 million large asteroids exist in the space between Mars and Jupiter.

Billions of years ago, when the solar system formed, it left behind large chunks of space rocks that weren't big enough to become planets.

Today, thanks to Hollywood movies like Armageddon, we might be vaguely aware that life on Earth could end via an asteroid collision. But it wasn’t until the end of the 20th century that scientists really had much of a clue about the frequency of asteroid impacts. In 1980, Luis and Walter Alvarez found the first compelling evidence that the dinosaurs were annihilated by an asteroid: a thin layer of sediment from the impact, spread around the world. In 1990, the crater left by that asteroid was identified off the Yucatan Peninsula. Then in 1994, scientists witnessed a comet smashing into Jupiter; you could see its impact from a backyard telescope. In the bottom left of this infrared video of Jupiter, Shoemaker-Levy 9 makes impact.

Wikimedia Commons

In 1998, Congress asked NASA to find at least 90 percent of the asteroids and comets 1 kilometer wide or larger that would come close to the neighborhood of Earth’s orbit. Those asteroids strike Earth once every half a million years, and could threaten life on our planet as we know it. But it’s the thousands of yet-undiscovered, smaller ones — ones that could destroy towns and cities — that scientists are most uneasy about. What we can’t see coming might hit us. "The small ones are the hardest to find, of course," Donald Yeomans, the former manager of the Center for Near Earth Object Studies at the NASA Jet Propulsion Laboratory, said last year. "And the next impact we have is almost certainly going to be something small."

Scientists study asteroids and comets to predict the future Asteroid and comet chasers like Yeomans are in the business of predicting the future. To date, the center has tagged 18,000 asteroids, comets, and bits of space debris that orbit in the neighborhood around Earth. How NASA detects these objects is pretty simple, Yeomans explains. You take many pictures of the sky over several days, and a computer program searches for moving objects. Based on an object’s speed and brightness, the scientists can start drawing the path of its orbit.

Catalina Sky Survey

Here’s what astronomers are looking for in their search for asteroids. These images — taken on January 1, 2014, by the Catalina Sky Survey — show a very small asteroid (circled faintly in red and purple) streaming past static stars. This 2- to 4-meter asteroid was discovered to be on a collision course with Earth. A day later, it punched through the atmosphere above the Atlantic Ocean and likely broke apart before hitting the water. Typically, NASA scientists can predict the path of the orbit with some certainty for 100 years. But "the uncertainties in the orbit grow with time," Yeomans says. That’s because as the asteroid passes by planets and other objects, its orbit changes slightly. Now the Center for Near Earth Object Studies believe they've tagged more than 90 percent of these kilometer-wide monsters. Why are they so sure? Because the population of near-Earth objects is based on the rate of their discovery. "It’s kind of like fishing in a small lake — if you catch one, tag it, throw it back, when you start catching 90 percent tagged fish, you know you’ve basically tagged 90 percent of the fish," Paul Chodas, the current manager of the center, said in 2017. "That’s basically the way it works."

The destructive power of asteroid collisions is a function of their size and velocity. The larger an asteroid is, the more energy it generates on collision. "A 150-meter object is going to have the energy of 288 megatons — 288 million tons — of TNT explosives," Yeomans explains. (The largest nuclear weapon ever built had a payload of 50 megatons.) "On the other hand, a 10-meter object will have the impact energy of 100 kilotons," he says. "It get nastier very quickly as you get to larger sizes." There’s no hard cutoff for which size asteroid will reach the surface and which will just explode in the sky. "A car-size asteroid is impacting [Earth] a few times per year," Eric Christensen, the director of the Catalina Sky Survey, an Arizona-based effort to discover new asteroids, said last year. But these break up in the sky and will maybe rain down a few meteorite shards. The most likely threat we face is from smaller asteroids — like the 20-meter-wide rock that exploded over Chelyabinsk, Russia, in 2013, with the force of a nuclear explosion.

A portion of the 20-meter-wide Chelyabinsk meteor recovered from Chebarkul Lake, Russia. Alexander Firsov/AP

That's why in 2005, Congress directed NASA to find 90 percent of the near-Earth objects 140 meters or larger. (This search also picks up on smaller asteroids like the one that hit Russia.) And there are many, many more fish in this pond. Chodas says only around 30 percent of these near-Earth objects have been discovered so far. "We’re finding 800 per year at that size range," he says. But even at that rate, it could take three or more decades to find all 25,000 of them. And for each one found, they have to chart its orbit and determine if it is, or could ever possibly be, on a collision course with Earth. NASA has not found an asteroid or comet that poses a realistic threat within the next 100 years. But it does keep an eye on about 2,000 "potentially hazardous" objects that come within 4,647,790 miles of Earth and are large enough to cause damage.

The Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) is one of the primary telescopes NASA uses to spot asteroids. Here, you can see all the objects NEOWISE discovered from 2013 to 2016.

These smaller asteroids are more difficult to find because they shine less brightly in the night sky, and they need to come close to Earth before we can spot them with ground-based telescopes. And our search for these smaller (but still destructive) asteroids is limited by a huge blind spot: the sun. If an asteroid is coming from the direction of the sun, we won’t be able to spot it (that’s what happened in Chelyabinsk). NASA scientists are hoping for a new space-based infrared observatory to speed up the search, but it has yet to be fully funded. How an asteroid will kill you, in one chart In 2017, the journal Geophysical Research Letters published an analysis of the likely source of casualties from an asteroid impact. "Effects such as cratering, seismic shaking and ejecta deposition [i.e., ejected debris] provide only a minor contribution to overall loss," the study concluded. The biggest source of casualties: wind generated from the impact blast.

Are we prepared? If a 140-meter asteroid, which is large enough to destroy a city, were discovered today and the path of its orbit predicted a collision in a few months, there would be nothing we could do about it except evacuate. Though Lindley Johnson, NASA’s planetary defense officer (yes, a real position), assures me "we can determine well in advance — certainly several weeks in advance — the exact time and place it’s going to happen." And evacuations could ensue. If by chance — and it really is chance — we spot one of these space rocks seven to 10 years out from collision, it would be possible to launch a mission to deflect the impact. There are two ways this could be done. One: NASA could launch a spacecraft to act as a battering ram to push the asteroid slightly off course. Or it could go the Hollywood route, and use "a nuclear device that would vaporize part of the surface, and then the recoil from the surface would push the asteroid," Chodas says. (Though as Popular Science points out, the use of nuclear weapons in space is technically banned.) Also keep in mind: NASA doesn’t have a battering ram rocket or an interspace nuclear warhead at the ready. We’d have to design and build these deflectors, possibly in an international collaboration for the sake of speed. "We would certainly like to have 10 years of warning; it would be enough time where we’d stand a chance of being able to pull together and launch the mission," Johnson says. We’d need to speed up or slow down the pace of an asteroid by just a centimeter per second to push it off course. But "you need to do that three years in advance of the impact." Another thing to know: Asteroids have the potential to help human life, not just take it away. "Asteroids are not only threats; they are opportunities," Yeomans explains. "Many of them have platinum-group elements and hydrated minerals that could be used for building structures in space." There are serious efforts underway to figure out how to capture and mine asteroids for their rare metals and other natural resources. NASA has laid groundwork for an eventual "asteroid redirect" mission. The plan: A robot spacecraft will land on an asteroid, grab a boulder, and bring it into orbit around the moon. The mission is not just for the minerals. "Scientifically, [asteroids] represent the least changed objects from the solar system’s origin process," Yeomans says. And that’s why asteroids are so fascinating. They’re a window into the deep past of our solar system — but also a window into the future of life (or lack thereof) on planet Earth.