“Primitive and dangerous” aren’t highly desirable features in a neighbor. Add in “more common than previously thought,” and you’ve found a great reason to move.
The trouble is we’re already moving — at 67,000 miles per hour, and unfortunately, the Earth’s average orbital speed isn’t always enough to get us away from these particular neighbors: large, ancient asteroids living in what, until recently, was believed to be a relatively quiet corner of our solar system.
It stands to reason that an asteroid twice as wide as Manhattan, like the asteroid that killed the dinosaurs, shouldn’t be that hard to find. Reason didn’t stick the landing here.
Where Did the Asteroid That Ended the Mesozoic Era Come From?
More than a decade ago, NASA’s Wide-field Infrared Survey Explorer (WISE) mission went hunting for the birthplace of the asteroid that killed the dinosaurs and ended the Mesozoic Era. WISE discovered incredible things, but after more than a year of searching the solar system for the origin of that particular space rock, WISE was empty-handed.
In their 2011 study, NASA’s Jet Propulsion Laboratory (JPL) astronomers looked at reflected light from around 120,000 asteroids in the asteroid belt. In the end, they still couldn’t pinpoint the origin of the 6.2-mile-diameter asteroid that fell to Earth in what is now the Gulf of Mexico, leaving an immense crater and a layer of meteor-like material in the middle of the fossil record: the Mesozoic Era.
This telltale material suggests that the object that struck our planet probably wasn’t from a nearby part of the asteroid belt — a belt that, for the record, isn’t cinched very tightly around the sun. The asteroid belt extends from just beyond Earth and exists between Jupiter and Mars. Millions of asteroids scattered throughout the belt are made up of combinations of rock, ice, metal and minerals left over from the formation of the solar system.
Around 60% of the closest asteroids to us are made primarily from silica. Known as S-type asteroids, they are the gray hue of sand mixed with dirt. Darker C-types made from carbon tend to live farther out in the belt — three times farther away than the S-types. Like the recently visited asteroid Bennu, these carbonaceous chondrite-like asteroids can be the color of coal, reflecting less than 4% of the light that shines on them.
This dark origin helps explain how a 6.2-mile-diameter asteroid can sail around between Earth and Jupiter undetected. Sorting objects out of an infinity of blackness depends on light. Unless that object is blocking out starlight, bending it like a black hole, radiating it like an oven or reflecting it back at our telescopes, seeing an object of that size is not an easy feat, even with the best telescopes we possess. Fortunately, even if we can’t see where something is, we can try to model where it should be.
That’s where the Southwest Research Institute (SwRI) group comes in. Dr. Simone Marchi and colleagues re-examined the data gathered by WISE. They ran their own calculations regarding where certain sizes of asteroids are in the belt and how often those asteroids travel inward toward the sun to strike the Earth. They came to a number of conclusions; two of which, if correct, would significantly impact our views of how that Mesozoic Era came to an end.
What We Can Conclude From the Evidence Gathered
The first conclusion is that asteroids from the outer and middle belt impact Earth far more often than previously believed.
Previous models indicated that less than 10% of Earth impactors came from the outer or middle belt. Marchi, Bottke and Nesvorný concluded that asteroids from that region actually supply closer to 35% to 40% of our large impactors. They also calculated that these darker asteroids constitute up to 50% of Near Earth Asteroids (NEAs) and that about 70% of dark NEAs come from this farther-out region.
The second Earth-shattering conclusion was about how often a C-type asteroid (the same size of asteroid that killed the dinosaurs) can be expected to visit our planet. Their conclusion was about once every 250 million years, on average. Given that the Chicxulub event that marked the end of the Mesozoic Era occurred close to 65 million years ago, if these calculations are correct, we should have some time before another 6-mile-wide asteroid from the outer belt pays us a visit.
It’s important to remember that kilograms of space dust, chunks of ancient ice and rocks the size of fists — not to mention our own satellites — hit our exosphere head-on every single day. Very few of these objects pose a threat. Most are the size of a sand grain. Asteroids larger than 100 meters can cause earthquakes on land and tidal waves in the ocean, but according to our observations, asteroids of that size only come by every 1,000 years or so.
What Does This Mean for the Future of the Planet?
Millions of years pass between era-ending events, where an asteroid 1,000 meters or larger manages to make direct contact with the surface of the Earth. To better understand where these killer asteroids are now, where they come from in space and where they’ll be going in time, we need detailed observation of dark asteroids in all parts of the belt. Planetary defense organizations are working to solve that problem now, as well as the issue of how to deal with an asteroid of any type and significant size that comes our way.
The SwRI team also concluded that since asteroids have been escaping from the belt for more than a billion years, the number of asteroids remaining to maraud us has declined significantly and will continue to do so over time. According to the team’s model, these days, only one asteroid with a diameter greater than 3 miles escapes from the main asteroid belt every 93,000 years or so.
Though these asteroids are, beyond a shadow of a doubt, potentially dangerous — and more common visitors to Earth than we thought — the SwRI group’s simulations demonstrate, perhaps for the first time, how the orbits of large asteroids from this part of space can bring them, or not bring them, near Earth. The models suggest that when they do go wandering, the vast majority of these city-size, oddly shaped boulders from the dark outer belt avoid our planet altogether.
At this moment in real space, this phenomenon is playing out before our very eyes. Asteroid Bennu, sampled by NASA’s OSIRIS-REx spacecraft in 2020, is drifting slowly in our general direction. It will continue to do so for more than 100 years. Even so, it only has a 1-in-2,700 chance of falling to Earth sometime between 2175 and 2199, which means we have some time to decide how to reason with it.
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