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The Conversation

Local astronomer on the probability of an asteroid collision — and the interplanetary defense plan

Pan-STARRS Observatory haleakala nasa uh
R. Ratkowski
The Pan-STARRS Observatory is a 1.8-meter telescope located at the summit of Haleakalā.

The University of Hawaiʻi Institute for Astronomy has another $15 million for its coffers, thanks to a new NASA grant. What’s the money for? Monitoring asteroids, of course. The Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS, on top of Haleakalā identifies the majority of asteroids and comets near Earth, some of which could be catastrophic if on a collision course. But just how worried should we be when we look up into the night sky? The Conversation spoke with astronomer Ken Chambers to find out.


KEN CHAMBERS: Everyone knows that there was an enormous asteroid 63 million years ago that wiped out 95% of the species on Earth, wiped out the dinosaurs. The potential of an asteroid impact can be from something that large to something much more modest, right, and then a smaller one is going to be much more frequent. The bigger ones are rare, the smaller ones are much more frequent. But you could still have one that had a devastating impact on an inhabited area. So our mission is to scan the skies repeatedly to try to find these objects.

SAVANNAH HARRIMAN-POTE: This idea of a giant celestial object hurtling itself towards Earth is something that has captivated people's imaginations for decades. I'm thinking of one movie, 1998's "Armageddon," in which we essentially looked up at the sky and saw that there was an asteroid the size of Texas that was going to hit us in less than a month. What's the likelihood that something that big could sneak up on us?

Well, it's very rare. I mean, one that's so large that will change the biosphere of the planet, like the one 63 million years ago, that's one in 60 million years. But you could still have very, very devastating consequences from a much smaller one, right? Even a 140-meter one, if it hit near a large inhabited area would be terribly devastating — it would be the equivalent of many, many thermonuclear warheads. So you want to find as many of these as we possibly can. I look at Pan-STARRS as an insurance policy, your house is unlikely to get hit by a hurricane, but you still buy hurricane insurance because if it does hit, the consequences are so devastating — you would lose everything, right? So it's better to pay a little bit of insurance every month, just on the chance that that happens.

How big does an object have to be in order to do the type of damage you described?

One line is drawn. This is by congressional mandate — they picked the number of 140 meters. Congress has mandated that we should try to find 95% of all asteroids greater than 140 meters. And that's because 140 meters can do... that's a city killer. That's not a planet killer, that's a city killer. And we find many, many, many of those every year — many more even smaller ones, and very few ones that are bigger than a kilometer in size, but we're still finding ones that are bigger than a kilometer and that would be very devastating.

What's the one you're most worried about?

At the moment, there aren't any that are on the west where we see an impact. There are ones that way in the distant future have a very small probability of impacting the Earth. But you do come across things — we had one not so long ago, weeks ago, where we found this asteroid, it was a fairly large one, two-tenths of a kilometer, so 200 meters, and the initial orbit was projected to potentially hit the Earth in about 10 years, which is about, you know, the timescale on which you would really want to do something about it. But as we took more and more observations, we refined its orbit more and more — it used to take us about a month or so to get an orbit that was good enough where you could really say whether it was gonna hit or not — in this case, after about 10 days of observations, we were able to find it in our archival data. So we've been surveying the sky for 10 years now with Pan-STARRS1, and we have all that archival data. If you go back with the prior information that there might be something in some particular point, you can then go look at it. "Oh my gosh, there was something there that we didn't pick up the first time." And so we did that. And we found observations of that object going back to 2010, 2013, 2018. And then with all that archival data, we were able to get a very good orbit. The all-clear signal went out "no, this one's not potential, it's not going to intersect the Earth at any time." If we can find it in the archive, we can have a much more accurate orbit much quicker than we couldn't before.

In a pandemic, people are making decisions about risk for themselves and their family every time they leave the house. And we're also starting to face down some of the truly serious impacts of climate change. Where should the average person put near-Earth objects in their list of daily worries?

They should not worry about it because the risk is so low compared to those other things. But what these other things have taught us like the pandemic is rare things do happen. So you want to be prepared. So you want to have a system like Pan-STARRS. In terms of, do I lose any sleep at night worrying about an asteroid impact? No, not at all. It's very unlikely. But boy, did we get excited when we found the one like this one where it looks like it potentially might be, and again, it could happen. One will hit eventually, we want to find them because there's one out there, right? 65 million years is nothing in the 4.6 billion life of the planet. There are objects out there. We want to find them all. It could hit tomorrow, might not hit for a million years.

What would we need, for instance, to stop a 140-meter near-Earth object from colliding with Earth at all? Do we have the ability to do that? Do we have any type of defense mechanisms already in place?

Yeah, so the international planetary defense community gets together every couple of years and does a simulation to work out all these particulars: how to actually deal with all the different aspects of it, right? So we're deeply engaged in day one when it's the astronomy part. Then over the course of the week, when you get into these political discussions, and you get into disaster preparedness, and all these other aspects of it that you want to exercise. There are international agencies that are prepared to deal with disasters and stuff. And so, you know, for a very short period of time they participate with us to prepare to try to scope out what kinds of things we would do in that case. So usually the simulation is for something bigger than 140 meters, it might be for a kilometer or a couple of kilometer object, right? And there are various different ideas out there — Kinetic impactors, where you just send a big slug to hit it so that it changes direction, there are things called gravity tracking... a massive spacecraft next to it and try and pull it out of its orbit with the gravitational attraction.

But the one that's most likely going to be used and in fact, in the simulations always ends up being one used, is you do have to use a nuclear warhead. And in that case, you don't want to blow it up, the destruction is not the goal. The goal is to just give it a little push, and the earlier you can give it a little push, the more leverage you have as time goes on, and it's gonna miss the Earth by a wider margin. So what you want to do is detonate your thermonuclear warhead just off to the side of the thing, and you give it a little push, you don't try to blow it up, you just try to give it a little shove on the side to change its orbit. Now there is a political wrinkle in this. We have this thing called The Outer Space Treaty, which was written back in the 1960s and back in the peak of the Cold War, which forbids the detonation of nuclear weapons in space, or even putting nuclear weapons in space. And that's a very, very important treaty, it's critical, you don't want nuclear weapons in orbit, and you could have World War III in a very, very short period of time. So it's been a very, very important treaty. It's protected us all this time. So the space lawyers look at this and their recommendation is don't open up the treaty for negotiation. Because anytime you open up a treaty for negotiation, it gets weakened. And if there happens to be an asteroid coming, then the President makes a decision: violate the treaty, nuke the asteroid, and you'll litigate it later.

This interview aired on The Conversation on Nov. 2, 2021.

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