50,000 years ago, a dense chunk of nickel and iron, over 100 feet across, entered the Earth’s atmosphere. It sped towards the surface at 45,000 miles per hour, and slammed into the lush woodlands of North America. The impact left a giant hole—590 feet deep and three-quarters of a mile wide—in the landscape of what is now northern Arizona.
Today, we know the site as Meteor Crater. It has fascinated generations of people, but we are still only just beginning to understand asteroids, the phenomenon that excavated this remarkable location. To really get the scope of this natural wonder, Popular Science sent extreme science correspondent Jake Roper to take us on a video tour.
In order to understand what occurred at Meteor Crater, we have to go back to the start of the solar system. As the sun was first forming, dust and gases started orbiting the new star. These tiny objects crashed into each other, building up into larger and larger bodies as they went through constant collisions. Many of these became planets and their moons, but there some were left over, especially between Mars and Jupiter. There are tens of thousands of these bodies in the space between the two planets.
To reiterate a quick vocabulary lesson from the video: According to NASA, asteroids are small, rocky bodies orbiting the sun; comets are similar to asteroids, but they tend to be icier, and can leave trails of vapor in space as they get close to the Sun; meteors are what we call the fiery streaks across the sky when an asteroid enters the earth’s atmosphere; and meteorites are the remnants of asteroids that make it to the Earth’s surface.
Occasionally an asteroid will hit another asteroid, or cross through a planet’s gravitational pull and get shot into the inner part of the solar system. When that happens, the asteroid might move into a new orbit, fall into the sun, or hit another planet—like Earth.
“It’s almost like a pinball machine,” says Amy Mainzer, the principal investigator of the NEOWISE program, which maps out near-earth objects (NEO) such as asteroids with the Wide-field Infrared Survey Explorer (WISE) spacecraft.
During its initial run from 2009 to 2011, the WISE spacecraft identified and charted over 34,000 asteroids in our solar system. Since it started back up in December, 2013, scientists have identified 178 new asteroids in our solar system, including 58 that are close enough to be considered Near Earth Objects.
The Turning Point
Until 20 years ago, nobody was really focused on whether another event like the one that created Meteor Crater was likely to happen again. When many astronomers considered asteroids, they thought of them as the “vermin of the sky,” says Mainzer—not because they posed a threat, but because they obstructed observations of other astronomical objects.
But in 1994, the world watched in awe as fragments of the Shoemaker-Levy comet slammed into Jupiter—the very first time that anyone had observed an asteroid-like object colliding with a planet.
The crash left huge, visibly discolored scars in the gassy giant’s atmosphere the size of several Earths, and they lingered there for months. Before the Shoemaker-Levy collision there was only a relatively modest effort to track down objects capable of doing serious damage to Earth. But after Jupiter got hit, interest in asteroids (and asteroid impacts) soared. Hollywood made films. And the U.S. government threw money at NASA programs like NEOWISE and its predecessors, requiring them to find out how many earth-destroying asteroids were out there, and what would actually happen if an asteroid struck.
Anatomy of a Strike
Seeing Meteor Crater for the first time can shift anyone’s perspective—including someone who studies asteroids professionally. Amy Mainzer has catalogued many asteroids much larger than the one that created Meteor Crater, but she called the experience of seeing the site for the first time a visceral reaction. “It just goes to show you that it doesn’t take a large asteroid to make a very large hole in the ground,” she says.
But how, exactly, did that hole get made?
“In the 1870s, people had a very incorrect idea of what a large meteor impact was like,” explains Jay Melosh, a geophysicist at Purdue. “They thought it was like firing a bullet into the ground.” When you fire a gun at the ground, the bullet creates a small crater, and the bullet disappears into the ground, buried in the soil.
“At the speeds that [the asteroid is] moving, the atmosphere is almost like a brick wall,” Melosh says. Smaller asteroids, or asteroids with less metal, would break apart and burn up entirely. But even the stronger, denser asteroid that hit Arizona was strained by its trip through the atmosphere. When it did finally make impact, it created a huge crater, but instead of burrowing into the ground like a bullet, it exploded, strewing the landscape with debris.
Those physics were a huge disappointment to Daniel Barringer, a geologist who was the first to theorize the crater was made by an impact. He hoped to strike it rich by mining the ground for raw metal, and dug deep into the heart of the crater in search of treasure, not realizing that all that was left of the asteroid were chunks of metal strewn across the landscape.
Even after Barringer died in 1929, scientists continued to visit the site. They mapped out those bits of metal around the crater and were able to discover what really happened. Others, including Melosh, added to the general knowledge about the site, and calculated the angle of the strike and the speed at which the asteroid was traveling when it eventually hit the earth.
Melosh created an interactive website that shows what could happen if different types of meteors hit. Dense, metal-rich asteroids might leave a substantial mark even if they’re on the small side, while larger icy or rocky bodies might only leave a visual impact, similar to what happened at Chelyabinsk in Russia in 2013.
Cameras mounted on Russian dashboards captured a sight that was unusual even by Russian dash cam standards: a giant fireball streaking across the sky.
The meteor exploded before it ever hit the ground, blowing out windows in the city of Chelyabinsk, but not leaving a crater.
Events like Chelyabinsk are far more common than events like Meteor Crater, but in the future we have to be prepared for anything.
Simulation: Will a Meteor Wipe Out Humans Anytime Soon?
Types of Meteors
The simulator sends meteors hurling toward Earth over a period of 100 million years, with diameters that vary according to research by U.S. and U.K. planetary scientists. For this simulation, the meteorites are rocky, are traveling at 38,000 mph when they encounter the atmosphere, and are striking the planet at an angle of 45 degrees—all most-likely values, but simplified from the true range of impactor properties. We’ve divided the meteors by size into five approximate classes:
- Non-damaging meteors likely burn up in the atmosphere.
- Moderately damaging impactors usually strike the ground as meteorites, but bestow very little—if any—harm. Some explode in the air as fireballs, like meteor that burst above Chelyabinsk, Russia in 2013 with enough energy to break windows.
- Seriously damaging meteorites leave craters that are 10 times the size of the meteorite itself. Meteor Crater formed from an impact in the middle of this range.
- Catastrophic events generate Nepal-level earthquakes and ignite trees and grass fifty miles away with their impact energy.
- Extinction events inflict so much damage that the planet descends into an “impact winter” from all the dust, soot, and ash they send into the atmosphere. An event of this scale—responsible for the Chicxulub crater in Mexico’s Yucatán—is likely what led to the die-off of the dinosaurs 65 million years ago.
The implications of a truly massive asteroid strike (like the one that killed the dinosaurs) are so serious (extinction, climate change, disruption of life as we know it), that scientists are keeping an eye on the sky.
Mainzer estimates that over 90 percent of the truly huge, extinction-causing asteroids currently in space have been found. But what about the smaller asteroids, like the one that created Meteor Crater? Only about one percent of those are known to researchers. Scientists have an estimate of how many are left to be found based on previous surveys of the sky and computer models, but because asteroids are tiny dark rocks set against a dark background they are notoriously difficult to detect.
Knowing about only one percent might seem scary, but don’t panic.
“If [asteroid impacts] were more common, humans wouldn’t be here.” Mainzer says. She hopes that within the next year NASA will approve a dedicated long-term asteroid detection spacecraft called the Near Earth Object Camera or NEOCam. WISE, though it has done a stellar job, wasn’t intended to be used for asteroid detection (it was originally supposed map the entire sky.) If approved, NEOCam would watch for small asteroids, keeping track of even small, dark objects difficult to see with traditional telescopes. The tracker would be able to run for years (instead of the months-long lifetime of WISE), enabling researchers to actually track asteroids over long periods. Then there are missions to the asteroids themselves. NASA’s Dawn mission is currently visiting huge protoplanets in the asteroid belt, gathering information about the composition of asteroids and their environment. (It’s also sending back some great pictures.)
By 2025, NASA plans to capture an asteroid and bring it back into earth’s orbit. NASA is identifying candidates that are close enough to the earth that they could send a spacecraft there quickly. So far, they’ve identified four potential asteroids that could be towed from their current location to somewhere near the moon.
This Asteroid Redirect Mission, or ARM, is mostly intended to be a training ground for astronauts, practicing skills that might prove useful if a mission to Mars ever materializes. They would use tools to take rock samples and explore the surface of the asteroid, potentially even bringing samples back to Earth.
But there’s another side to the mission as well. If NASA can successfully direct an asteroid into Earth’s orbit, than it might be able to usher another one out, potentially preventing a future Meteor Crater-level impact.
If an impact did happen, the damage would depend on the where the asteroid hits. If a rock went into the ocean, it would likely do very little damage. If it hit on land, it could form another huge crater, destroying anything standing or growing in the landing zone. Though city centers are expanding in all directions, the chances of a large asteroid on a collision course with a densely populated spot like Times Square is very, very low.
But one of the more feasible ideas is simply doing a better job of mapping asteroids. The non-profit B612 foundation plans to launch the Sentinel Space Telescope in 2018 with the goal of identifying and tracking objects that enter the Earth’s space.
With programs like NEOCam and Sentinel on their way, future generations that marvel at Meteor Crater should at least be able to rest somewhat easier.