An asteroid impact is also the stuff of science fact. There are obvious craters on Earth (and the moon) that show us a long history of large objects hitting the planet. The most famous asteroid ever is the one that hit Earth 65 million years ago. It's thought that this asteroid threw so much moisture and dust in to the atmosphere that it cut off sunlight, lowering temperatures worldwide and causing the extinction of the dinosaurs.
In movies, an incoming asteroid is usually a very last-minute shock: a big, deadly rock hurtling right toward Earth like a bullet out of the darkness, with only weeks or days between its discovery and its projected impact. That is a real threat, according to an April 2019 presentation by NASA's Office of Planetary Defense that Live Science attended. But NASA believes that it's spotted most of the largest, deadliest objects that have even a small chance of striking Earth — the so-called planet killers. (Of course, there are probably plenty of smaller rocks — still large enough to kill whole cities — that remain undiscovered.)
First off, could there be an undiscovered 20-mile-wide asteroid headed our way in the foreseeable future? The answer is a qualified yes. The killer could not come from the asteroid belt. Astronomers have already plotted the orbit of every asteroid that size (and a lot smaller) in great detail. There is no object that size that could plausibly hit Earth any time in the next few thousand years—probably not in the next few million years.
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Because most of the large objects in Earth's neighborhood are already being closely watched, we'll likely have plenty of warning before one strikes Earth. Astronomers watch these space rocks as they get near Earth to see whether they're likely to cross through one of their "keyholes." Every Earth-threatening asteroid gets closer and further from Earth at different points in its orbit around the sun. And along that path, near Earth, it has keyholes. Those keyholes are regions of space that it has to pass through in order to end up on a collision course during its next approach to our planet..
There is one way that an object like that could be on its way without anyone knowing, however. If it were a giant comet or dislodged Kuiper Belt Object coming toward us on an extremely elliptical path (ie. falling almost straight toward the Sun), it would be very hard to detect. We plausibly might not spot it until it was somewhere between the orbits of Uranus and Neptune. An object at that distance would, in fact, take about 20 years to reach us.
Keep in mind that 20-mile-wide objects are extremely rare, relatively speaking. There is no record of Earth being struck by anything that size in the past two billion years. The likelihood of it happening in the next few years is, well, astronomically small. But let’s play out the scenario.
A number of recent movies have been about this subject. It is important to remember that they are just movies and often times do not represent good or true science. Sometimes, however, there is a tiny seed of truth behind a movie, although it may be hard to tell what is true and what is fiction. This is the case for the movies about asteroids hitting the Earth.
Let us say right away that at this moment, we know of nothing currently on a collision course with Earth. Last year, it was announced in the news that an asteroid was on a collision course with Earth and would hit in the year 2028. We now know that the calculations were in error and that asteroid will miss the Earth completely.
"If this asteroid hit the Earth, a crater with a diameter of 2-4 km and a depth of 400 m would be created" - says asteroid researcher Dr. Anna Łosiak from the Institute of Geological Sciences of the Polish Academy of Sciences and the University of Exeter. She adds that it would be even larger than the famous Meteor Crater in Arizona in the US. The aforementioned crater, formed about 50 thousand years ago, is 1.3 km in diameter and 170 m deep. It was formed by an impact of an asteroid that was about 50 m in diameter.
If a one-kilometre meteorite were to hit Montreal, the city would be completely wiped off the map. The size of the resulting crater would be 10 to 20 times the diameter of the meteorite—so that’s a 10- to 20-kilometre hole at the impact site. But that’s not all. The shock of the collision would cause a tremendous earthquake of a Richter-magnitude greater than 10 that would end up affecting all of northeastern North America. If the meteorite were to hit water instead, it would create a large-scale tsunami.
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It's difficult to imagine 1 million megatons, so let's try some smaller sizes. Let's say that an asteroid the size of a house crashed on Earth at 30,000 mph. It would have an amount of energy roughly equal to the bomb that fell on Hiroshima -- perhaps 20 kilotons. An asteroid like this would flatten reinforced concrete buildings up to half a mile from ground zero, and flatten wooden structures perhaps a mile and a half from ground zero. It would, in other words, do extensive damage to any city.
Because most of the large objects in Earth's neighborhood are already being closely watched, we'll likely have plenty of warning before one strikes Earth. Astronomers watch these space rocks as they get near Earth to see whether they're likely to cross through one of their "keyholes." Every Earth-threatening asteroid gets closer and further from Earth at different points in its orbit around the sun. And along that path, near Earth, it has keyholes. Those keyholes are regions of space that it has to pass through in order to end up on a collision course during its next approach to our planet.
There is one way that an object like that could be on its way without anyone knowing, however. If it were a giant comet or dislodged Kuiper Belt Object coming toward us on an extremely elliptical path (ie. falling almost straight toward the Sun), it would be very hard to detect. We plausibly might not spot it until it was somewhere between the orbits of Uranus and Neptune. An object at that distance would, in fact, take about 20 years to reach us.
OK, what would happen if the asteroid struck? We’re talking about an object that is 2–3 times the diameter of the asteroid that hit us at the end of the Cretaceous. Given its steep path toward the Sun, it would be moving at a high velocity as well. It might pack 100 times the energy of the impact that ushered the old dinosaurs off the scene. This would be a full-on extinction-level event. There’s no precedent in the history of complex life on Earth, so we can only extrapolate.
But will it work? That’s really hard to say. We’ve never attempted anything like this. The closest is missions like Hayabusa-2 and OSIRIS-REx, which will rendezvous with asteroids and touch the surface, or New Horizons, which did a precision high-speed flight to Pluto. But we’d be trying to deflect an object while knowing little about its shape and composition. Probably we would launch the nukes as quickly as possible, then adapt the mission parameters as we learned more about the asteroid.
As part of her work, Dr. Łosiak investigates asteroid impact craters and works on the possible effects of future space object impacts. "I want to understand how dangerous these phenomena are. So if we know, for example, that in three days an asteroid with a diameter of 50 m will hit a specific place on Earth, then we should also know whether to evacuate people from the area of 5 km, 100 km or 10 thousand km. It is important so that we can avoid a tragedy" - she emphasizes.
Well, the fact is, we’re struck by approximately 10 to 100 tonnes of extraterrestrial materials every day. The shooting stars you see at night are small particles from space that break up as they enter the atmosphere, which causes them to generate a trail of light. Most of the objects are small, about the size of your thumb, but there are also larger ones. Sky-survey programs have identified close to 20,000 Earth-crossing asteroids in the past 30 years. In early 2019, these ongoing monitoring programs, which are a legacy of Ronald Reagan’s “Star Wars” defence plan, had found around 900 asteroids measuring over one kilometre in diameter.
All of Earth’s surface would be set on fire. There would be tremendous earthquakes and tsunamis, followed by massive volcanism around the impact zone. The ozone layer would be destroyed. The oceans would turn acidic. The Sun would be blotted out, probably for decades. All surface infrastructure would be destroyed. Most complex species would surely perish in the aftermath.
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By the time you get up to a mile-wide asteroid, you are working in the 1 million megaton range. This asteroid has the energy that's 10 million times greater than the bomb that fell on Hiroshima. It's able to flatten everything for 100 to 200 miles out from ground zero. In other words, if a mile-wide asteroid were to directly hit New York City, the force of the impact probably would completely flatten every single thing from Washington D.C. to Boston, and would cause extensive damage perhaps 1,000 miles out -- that's as far away as Chicago. The amount of dust and debris thrown up into the atmosphere would block out the sun and cause most living things on the planet to perish. If an asteroid that big were to land in the ocean, it would cause massive tidal waves hundreds of feet high that would completely scrub the coastlines in the vicinity.
Paek and his co-authors tossed out most of the more exotic asteroid-deflection schemes out of hand, leaving only nuclear detonation and impactors as serious options. Nuclear detonation is problematic as well, they wrote, because it's uncertain exactly how an asteroid will behave after a nuclear explosion and because political concerns about nuclear weapons could cause problems for the mission.
Now that we have the scope of the problem mapped out, we can think about the response. Idea one: Can we deflect the thing? The most likely approach, based on a recent study (Scientists design conceptual asteroid deflector and evaluate it against massive potential threat) would be to explode multiple nuclear warheads right next to the asteroid, vaporizing part of its surface and changing its orbit.
It is believed that it was an asteroid that contributed to the mass extinction of dinosaurs. According to Dr. Łosiak, that object was 10 km in diameter, and the crater remaining in Mexico measures 180 km. "It was a very unfortunate event for dinosaurs. A lot of carbon dioxide and sulphur dioxide was released from the rocks, which the asteroid hit, which caused huge climate changes on the planet. Nuclear winter came" - the researcher says.
It’s not any more likely to be struck than Earth. But because there’s no atmosphere around the moon, the impacts are more violent and remain visible. That said, most of the craters on the moon’s surface were formed about four billion years ago, when there was more debris floating around our young solar system.
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