Death from the Skies
Published on Jun 23, 2006 at 4:45 pm.
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Filed under Earth, asteroids.
Hey, that would be a good title for a scifi movie about alien invaders. Well, that isn’t what I was going to talk about. Seeking Solace asked me last week about doomsday asteroids. I haven’t had a chance to respond, yet. It has been a hellish week for me. Then, possibly something bad being found in my physical (I should know more next week) has had me pretty tied up all week.Â
Anyway, back to impacts. They are real. Earth is bombarded with several hunred tons worth of stuff each day. Now, most of this burns up in the atmosphere. But, some doesn’t, Larger things hit every now and then, too. I read a report a while back where the Earth is supposedly hit by atomic bomb sized impacts every decade. That sounds like a lot, but remember that most of Earth’s surface is ocean, so such an impact doesn’t do much, and most of the land mass is rather sparsely inhabited. Well, in recent years, I have seen reports of nearly four atomic bomb, or nearly atomic bomb, sized blasts in the last 16 years, the latest one being a possible impact in Norway. Some reports indicate that perhaps the Norway impact has been exaggerated, but was apparently still pretty big. Iceland has had a major explosion overhead, Central America, and an impact in the Atlantic Ocean.   This suggests that perhaps we get hit every four or five years by such large impacts. So far, we’ve just been lucky. Even larger impacts happen. In 1908, near Tunguska, in Siberia, a major explosion resulted from an impact event. Not all of these impacts make it to the ground. In fact, many are air bursts. The shock of entering the atmosphere causes the meteor to break up and burn up suddenly before hitting the ground. Interestingly, very small meteors burn up, medium sized ones make it to the ground, and large ones explode before reaching the ground. That doesn’t mean that they do no damage. In fact, and air burst can be worse. When nuclear weapons are used (only two have ever really been used, but I am speaking in theory), they are designed to detonate before reaching the ground because that does more damage due to the overpressure from the blast. Large ones detonate in the atmosphere. Really large ones, even bigger than the ones that detonate in the atmosphere, will make it to the ground. There are hundreds of craters around Earth that show these impacts. People don’t think of Earth as having craters like the Moon, but we do. What happens, though, is that Earth is dynamic, and so erosion and tectonic activity cover up the craters after a while. The Moon does not have these factors at play, so craters there stay until something else comes along and blasts them to bits, or tosses ejecta into them.
How many large things are flying around the Solar System? Millions. Estimates are that perhaps 1000 (more or less) objects big enough to do major damage to Earth’s environment have orbits that cross near Earth’s orbit. We’ve found only 2/3 of the ones that we suspect are there. Of greater worry, though, might be comets. They can come in at random, and they can also strike us. Ice on Mercury, and possible ice on the Moon, is believed to come from comet strikes on these bodies. Only a decade ago, Comet Hyakutake made a near pass to Earth. It was a small comet, but it was big enough to be devastating if it were to hit us, and it was only discovered a few weeks before it passed us.Â
A really major impact would, in fact, be devastating. The crater might be miles across, or even dozens of miles across. The blast zone would extend for hundreds or thouusands of miles. But what would be really bad would be the environmental damage. Tsunamis would wash around Earth, inundating land far inland. Ejecta would be tossed out and create secondary meteorites that might start fires around Earth. If the impact is big enough, the heat of impact could even ignite the nitrogen in the atmosphere, creating nitric oxides and nitric acid. All of this would be bad. In fact, a couple decades ago Walter and Luis Alvarez proposed that just such an event did in the dinosaurs 65 million years ago. In fact, there is a large impact crater near Chicxulub, Mexico, that dates to that time. There are other impact craters that date to about that time, too. Perhaps it was more than one impact? The impact itself would be bad, but such a large impact would crack through the crust of the Earth, permitting massive volcanic eruptions. These volcanic eruptions may continue for hundreds or thousands of years. This would create a long term event. So, not everything would die at once. Rather many would die at once, but the rest would succumb to the extended climate change. I should point out that the Alvarez hypothesis remains controversal. A great many scientists have embraced it, but not all.Â
The K-T extinction of 65 million years ago was not the biggest mass extinction event. An even bigger event that came between the Permian and Triassic periods 251 million years ago killed off about 95% of everything on Earth. Recently, a massive crater was found in Antarctica that may date to about this time. Again, anything surviving the impact would have to deal with a totally wrecked global climate.Â
There are a lot of big things flying around out there that can hit us. If you really worry about this, though, you can also worry about asteroids that are safely out in the asteroid belt. It turns out that their orbits can shift around a bit, and that they can suddenly be kicked into orbits that intersect our orbit. Still, comets may be a big threat, too. It doesn’t really matter if it is a giant rock or a giant chunk of ice that is hitting you. Both do the same thing. In fact, comets might come it at higher velocities, thus carrying more energy. Most astronomers don’t talk about “if” something will hit us, but “when” it will hit. We are pretty sure that another impact will happen. Oh, and if you really are faint of heart, consider this: There seems to be a mass extinction event every 60 to 70 million years. The last one was 65 million years ago.
Of course, this could be a good thing, as long as it comes before I have to grade all those term papers and final exams. Hmm.
Oh, and impacts aren’t the only thing that can kill us. So could a nearby supernova, a passing black hole, a rouge brown dwarf or red dwarf (one is going to pass within the Oort cloud in a few tens of millions of years, and it will likely toss a lot of comets towards us), or even a super solar flare. Yeah. I was in a bad mood this past semester, I think. My students called it the “and then we all die” semester. I kept talking about how things from the sky could kill us all off.
Well, as long as it comes before I have to grade those all those papers …
-Astroprof
