will an asteroid hit earth? -- 3/9/22

Today's selection -- from Frequently Asked Questions about the Universe by Jorge Cham & Daniel Whiteson. Just the facts, so you will be prepared:
"To get a sense of how likely it is for a large asteroid to hit us, and whether we would see it coming, we spoke to the good people working for NASA's Center for Near-Earth Object Studies (CNEOS), headquartered at the Jet Propulsion Laboratory in Pasadena, Califor­nia. Really, their name should be 'Asteroid Defense Force' because they are the group tasked with preventing the complete annihilation of the human race by a giant rock crashing into us. (And you thought your job was important.)

"The main approach of CNEOS (together with their international collaborators) is to look for and keep track of all the rocks in the solar system, so that we might have a heads-up if any of them are on a path to hit us. Using telescopes, and after decades of hard work, the CNEOS team has created a pretty good database of all the biggest rocks around us, where they are, and where they will be in the near and far future.

"What they've found is that there is an inverse correlation between the size of the rock and how many of them there are in the solar system. Small rocks are plentiful in our neighborhood, but the re­ally large rocks are hard to find. In other words, the bigger the rock, the rarer it is. This is good news because the rarer a type of rock is, the less likely it is to crash into us.

"For example, CNEOS estimates there are hundreds of millions of rocks out there that are around one meter in size. That's a lot of rocks, and in fact, rocks of this size hit Earth all the time, about five hundred times per year. That means that on any given day, there is probably one of these rocks crashing somewhere on Earth.

"Fortunately, they cause very little damage. As the rocks get bigger, they get rarer. For example, rocks that are five meters wide are in the tens of millions in the solar system, and strike Earth only about once every five years. Twenty ­meter-size rocks (like the one that exploded over Chelyabinsk, Russia) are in the single millions, and on average only hit Earth every fifty years or so.

"But what about the really large ones? Even if those are rarer (there are only a thousand rocks that are one kilometer wide, and just a few dozen greater than ten ki­lometers), you only need one of them to hit us to potentially end the hu­man race.

"Luckily, large rocks like that are not only rare but also relatively visible. If a large rock is on a regular orbit, it's likely that we'll have seen it reflect light from the Sun. That means the CNEOS team is fairly confident they know where most of them are. They've counted them and mapped their trajectories, and so far, none of them seem to be on a collision course with us.

"At least, we don't think so. The good news is that we know where 90 percent of the big rocks in the solar system are. The bad news is that we don't know where 10 percent of the big rocks in the solar system are.

"There could still be large rocks out there that we haven't seen.They might be hidden, or they could be on an orbit that hasn't brought them close enough for us to observe them. Remember that asteroids don't glow on their own, and a few kilometers in size isn't that big compared to the size of our solar system. That means there's still the possibility that a large asteroid can sneak up on us out of the darkness of space.

"Much more concerning to the scientists at CNEOS are the other type of space rock that can hit us: giant snowballs (aka comets). While NASA has a good handle on most of the asteroids in the solar system that can kill us, it turns out that comets are much harder to spot.

"Most comets we see are huge balls of rock and ice that fall in from the Oort cloud toward the Sun on very long orbits. Some­times these orbits can take hundreds or thousands of years to go around the Sun. That means that when a comet visits the inner solar system (our cosmic neighborhood), it might be the first time that we see it.

"Even worse, after their long trip from the cold cosmic suburbs, they are going to be moving much more quickly than an asteroid, which means (a) we won't have time to react (a year at best), and (b) they would have a more devastating impact if they hit us.

"Scientists think that the chance of a comet crashing into us is probably rare, but it's hard to estimate. Very recently, it happened to one of our neighbors: in 1994, the comet Shoemaker-Levy 9 broke into twenty-one pieces on its way toward the sun, and those frag­ments crashed into Jupiter. One of those pieces created a gigantic explosion approximately the size of Earth. In fact, it was this comet collision that sparked NASA to create the Near-Earth Object program to catalog and track all near-Earth objects. After all, if it happened once, it can happen again, and maybe to us.

Jupiter in infrared, Shoemaker–Levy 9 collision (left), Io (right)

"Let's say a comet suddenly comes out of the blue and is on a path to hit us. Or let's say that we find a new big asteroid we haven't seen before and we learn that its orbit intersects with ours in the future. Or let's say that some kind of solar system event knocks a big rock straight toward us. Is there anything we could do about it?

"In the movies, all it takes is a musical montage of scientists in lab coats, a pot of coffee, and a whiteboard full of scribbles to figure out a solution (plus, it helps if you have Bruce Willis). But is that realistic?

"The first option is to try to deflect the asteroid or comet -- that is, to nudge its trajectory so it's not on a collision course with us. Sci­entists have a few good ideas for how to do this:

Rockets: This plan involves shooting a rocket at the incoming rock to either crash into it or blow up enough of it to change its trajectory. It might also be possible (though less likely) to land on the rock and use the booster engine to push the rock into a new trajectory.
Digger: Another idea is to send a giant crane or robot to land on the rock and start digging, pushing the debris out into space. The momentum from all the debris would essentially cause the rock to change course.
Lasers: Another fun idea would be to build a huge laser here on Earth and then shoot it at the asteroid or comet. The goal would be to heat up one side of the rock so that the melting ice or vaporized rock would push the rock out of Earth's path.
Mirrors: If you wanted to get really fancy, you could send a set of lenses and mirrors to gather sunlight and focus it onto the rock. This would boil off some of its material, pushing the rock out of its collision course.

"The second option is, of course, to try to destroy the big rock be­fore it gets to us. In other words, nuke it. One idea is to launch a nuclear missile that intercepts the rock and blows it up, hopefully shattering it into smaller pieces that would then burn up in our atmosphere. Some of them may still hit the ground, but that's a better outcome than if the whole rock struck Earth.

"On the other hand, it might be that the incoming asteroid is mostly just a pile of rubble loosely bound together by gravity. In that case, a single nuclear explosion wouldn't be very effective at dispersing the rock, and we would be better off sending a series of smaller nukes. Perhaps we would optimize the distance of the nu­clear blasts for maximum dispersal, perhaps setting them off a bit above the surface so that they do more deflecting than destroying.

"Of course, the most crucial factor that determines whether any of these strategies will work is how much time we have. According to CNEOS, 'the three most important things you need in order to survive an asteroid or comet impact are: (1) early detection, and (2) the other two don't really matter much.' 

"If we have a lot of warning (years, hopefully), then we might have time to build and deploy one of these strategies. Not only that, but more time also gives us greater opportunity to affect the outcome. For example, if we learned that a particular asteroid would strike Earth in one hundred years, any small nudge we give it today would have a huge impact in its future trajectory. It's like firing a sniper rifle at a target one kilometer away. The slightest deflection of the rifle to the side will give you a large sideways displacement of the bullet over the full kilometer it travels. The same is true for aster­oids: if you see one coming far enough in advance, you only need to give it a little push to knock it off course.

"This is why it's so important to keep track of all the asteroids and comets flying around us, and why the idea of one of them coming out of the blue is so scary."

 | www.delanceyplace.com


Jorge Cham & Daniel Whiteson


Frequently Asked Questions about the Universe


Riverhead Books


Copyright 2021 by Jorge Cham and Daniel Whiteson


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