The biggest asteroids that have ever hit Earth

The reality about menacing asteroids coming to destroy Earth in the modern-day is, perhaps surprisingly, not too scary.

Yes, the doomsayers thrive online, but the prognosis is encouraging:

- Astronomers have already found over 90 percent of the half-mile-plus "planet-killer" asteroids that at times pass near Earth's neighborhood, and there's no known threat of collision from these giant rocks for the next century; meanwhile, the likelihood of an impact in the next thousand years is exceedingly low.

- Using specialized telescopes, researchers are now discovering around 500 sizable space rocks (over 460 feet across) in our solar system neighborhood each year. None are threats, so far.

- NASA has successfully tested the first-ever endeavor to intentionally move an asteroid — a skill that may be necessary should we ever need to deflect an incoming object.

Yet, past cosmic violence is preserved in Earth's crust. Geologists have confirmed nearly 190 ancient impact craters on Earth — though our planet's evolving surface has certainly erased many of the earliest bombardments. The craters we know about tell a tale of a starkly different time in our planetary past, when fiery rocks plummeting through the sky were common.

"The solar system used to be a lot more violent than it is now," Sally Dodson-Robinson, a planetary scientist at the University of Delaware, told Mashable.

Early in our solar system, small grains of rock and ice began clumping together, creating miles-wide objects called planetesimals. They would collide and at times merge, eventually forming the familiar planets we see today. But many planetesimals weren't fated to become planets. Some flew around the sun, smashing into planets. This evidence is written in the well-cratered moon, Mars, and beyond.

"Crater evidence shows that during the first billion years or so of solar system history, asteroids were regularly bombarding planetary bodies at a devastating rate," Dodson-Robinson explained.

SEE ALSO: If a scary asteroid will actually strike Earth, here's how you'll know

Today, the leftover planetesimals are the rocky asteroids and icy comets in our much more tranquil solar system. (Of course, it's not completely tranquil.)

"The solar system used to be a lot more violent than it is now." Radar images of the 1,100-foot-wide asteroid Apophis. It will pass so close to Earth in 2029 that it'll be visible in the sky from certain locations. Credit: NASA

The preserved, or in some cases partially preserved, impact craters on Earth remain poignant reminders of our chaotic cosmic past. Here are some of the most significant known craters.

The largest impact crater on Earth

The Vredefort Crater in South Africa as viewed from above. Credit: USGS / Landsat / NASA

An asteroid some six miles (10 kilometers) wide or bigger smashed into Earth and created the Vredefort Crater, in present-day South Africa, some 2 billion years ago, long before even the dinosaurs evolved.

At the time, researchers estimate the impact crater was a whopping 112 to 186 miles (180 to 300 km) wide. "The world’s oldest and largest known impact structure was formed," NASA said.

Eons later, the south portion of the crater is no longer visible, having been blanketed in younger rock.

Sudbury basin

A view of the topography of the ancient Sudbury impact crater. Credit: NASA Shuttle Radar Topography Mission

Some 1.8 billion years ago, a large comet — which is an ancient "dirty snowball" composed of icy and rocky grains — slammed into modern-day Canada. The impact basin is largely eroded today, though with aerial views and radar, one can make out parts of the impact crater.

The original crater was likely some 120 miles (200 km) wide.

Today, the region is home to nickel and copper mines. That's because the powerful impact, by cracking the crust and allowing parts of Earth's mantle to rise up from below, ultimately generated a great nickel deposit. Nickel is a vital element in electrical wiring, engines, batteries, and beyond.

"There have been positives from some of these impacts," Simon Jowitt, a geochemist at the University of Nevada, Las Vegas, told Mashable.

"But obviously we don't want something the size of Sudbury hitting right now," he added. (A roughly six-mile-wide behemoth wiped out the dinosaurs, and the Sudbury comet was probably similar in size.)

The Chicxulub dinosaur impact

A gravity anomaly map of the Chicxulub Crater in the Yucatan Peninsula. Credit: NASA Shuttle Radar Topography Mission

This one was the dinosaur killer.

The 65 million-year-old Chicxulub site, buried beneath the Yucatan Peninsula today, appears on gravity anomaly maps — which show how much the planet's gravity field, which is dictated by mass, differs from a hypothetically uniform surface. Today, it appears as about half of a huge crater.

The infamous six-mile-wide asteroid struck in shallow water, blowing prodigious amounts of pulverized earth into the skies which drastically cooled the climate. "The enormous amount of energy generated by this impact, equivalent to 10 thousand times the world's nuclear arsenal, ejected into the atmosphere huge quantities of dust particles and gases," NASA explained.

Ultimately, the asteroid impact killed off around 70 percent of Earth's species. Though some dinosaurs survived.

Meteor Crater

The Meteor Crater in Arizona. Credit: NASA

The Meteor Crater in Arizona is relatively young, proof that Earthlings should track and be aware of potential incoming space rocks.

Some 50,000 years ago, a metal asteroid around 100 to 170 feet across slammed into modern-day Arizona. This is an object considerably smaller than those discussed above. Yet such a rock can still create a tremendous, regionally catastrophic blast.

"A similar-size impact event today could destroy a city the size of Kansas City," David Kring, an impact expert at the Lunar and Planetary Institute, told NASA.

The Clearwater Lakes Craters

The Clearwater Lakes Craters in northern Quebec, Canada, as seen from space. Credit: NASA

A double-whammy.

Two big asteroids slammed into Quebec, Canada, right next to each other. Though researchers argue that these impacts are in fact separated by many millions of years.

Today, the impact basins are lakes. Clearwater West is estimated at some 280 million years old, while Clearwater East formed much earlier, around 450 million years ago. You can spot a ring of islands in the western lake that measures around 6 miles in diameter.

The evidence dramatically carved into Earth's crust makes it clear: Giant objects have slammed into Earth, particularly when the solar system was a chaotic place. But big or catastrophic impacts have become rare. That's why traces of impacts on Earth are relatively few.

"The fact is that through geologic history these impacts are fairly infrequent," said UNLV's Jowitt.

Small rocky particles, however, hit Earth every day, but quickly vaporize in the sky. Here are today's risks from objects both small, and very large.

• Every single day about 100 tons of dust and sand-sized particles fall through Earth's atmosphere and promptly burn up.

• Every year, on average, an "automobile-sized asteroid" plummets through our sky and explodes, explains NASA.

• Impacts by objects around 460 feet in diameter occur every 10,000 to 20,000 years.

• A "dinosaur-killing" impact from a rock perhaps a half-mile across or larger happens on 100-million-year timescales.

Other, developing solar systems are far more treacherous places. The solar system around the bright star Vega, for example, is likely filled with violent collisions. It's a young star surrounded by a rich disk of dust— which is evidence of ongoing impacts.

The young planets or objects out there must be unsettling places.

"They're pretty dangerous, I would guess," Dodson-Robinson said.