Stand back, humanoid! Here comes the next installment of the Codex Futurius project, this blog’s never-ending quest to explore the ineffable scientific ideas raised by science fiction. This question on killer asteroids goes to Kevin Marvel, head of the American Astronomical Society. Thanks to Dr. Marvel for the scary info and to Jennifer Ouellette, the director the NAS’ Science and Entertainment Exchange (SEEx) program, for connecting us with him. Question: How big an asteroid would be needed to completely destroy a planet? That’s easy. It would have to be really, really big or moving very, very fast (or both for a real whopper of an impact), but there are some subtleties that are worth explaining. First off, let’s admit that we’re really concerned with how big an asteroid would destroy planet Earth, especially life on Earth. I’m a bit more worried about my home planet than Mars, Jupiter, or even Pluto and even more worried about all the life we see around us (not to mention ourselves!). Earth is far more important from the human perspective, so let’s tackle that question. Frighteningly, many large objects have hit Earth. Real whoppers. That’s a bit scary to think about. The good news is that the Earth is still here, so apparently large impacts of the planet-destruction kind rarely happen. We do know that smaller impacts have happened, such as the meteorite that hit the high Arizona desert just east of Flagstaff, at the site known as Meteor Crater. If we could count the impacts, we could gauge how frequently and when the impacts took place. However, it is hard for us to find evidence of all the impact craters on Earth today. This is mainly due to erosion, which washes away the evidence by slowly filling in the craters, but looking at the Moon, where erosion is for all intents and purposes non-existent, we see that our nearest companion has been pummeled a lot, though mainly in the distant past. It, too, is still here and in one piece. The far side of the Moon, which always points away from the Earth, has a lot more craters than the side facing the Earth, which makes sense because the far side is more likely to be hit—it’s a bit harder for asteroids to sneak by the Earth and hit the shielded side of the Moon (though some have) than to hit the exposed side. In fact, the Moon itself holds the key to what was probably the largest impact that the Earth has experienced (and hopefully will ever experience). Before I explain what we know about this biggest of all collisions, it is important to understand what we currently know about the formation of the solar system. Stars form when dense and cold gas and dust that is prevalent in galaxies like the Milky Way slowly collapses under the influence of gravity. Astronomers see these forming stars just about everywhere we look—from regions practically next door, like the Orion Nebula, to the most distant galaxies we can see with the Hubble Space Telescope. As the star forms, a disk of leftover material takes shape through the combined effects of angular momentum and the force of gravity. These disks become fairly violent places as small particles of material slowly accumulate to form specks of dust, then pebbles, boulders, and ultimately planets. Astronomers have seen such disks in various stages of evolution with powerful telescopes. Current models of planet formation gauge the time to go from a disk of gas and dust to a fully formed planetary system at about a million years, depending on the mass of gas and dust available and some other factors. Astronomers are not entirely sure how the process proceeds, but they have developed telescopes designed to peer through the material surrounding these forming stars to try and pin down the details. A prime example is the Spitzer Space Telescope, which observes in the infrared portion of the spectrum. Radio telescopes like the Very Large Array or the Atacama Large Millimeter Array (now under construction in Northern Chile) can also be used to effectively study the star- and planet-formation process, because the long-radio wavelengths they receive can escape the dense molecular clouds, unlike visible light. It is now generally accepted that the Moon formed when a large, Mars-sized object crashed into the Earth very soon after the Earth itself formed. This collision dug deep into the Earth’s crust and threw off material from as deep as the Earth’s mantle into orbit where it was pulled together by its own gravity to form the Moon. This explains why rocks brought back from the Moon are composed of fairly lightweight minerals and rocks, containing little to no iron or nickel (metals found at the core of the Earth rather than the mantle). It also explains why the orbital plane of the Moon doesn’t line up with the orbital plane of the Earth itself (the impactor came from a different orbital plane). From dating the ages of rocks, geologists know the Earth is 4.65 billion years old, while the Moon is a bit younger, about 4.6 billion years old, evidently created in a subsequent massive collision. So, in some sense, Earth wasn’t “destroyed” by an impact of an object the size of Mars that hit the Earth a somewhat glancing blow, but a more direct impact of an even more massive object could easily have had enough energy to seriously disrupt the Earth. Even so, in this case some kind of residual object would have formed, perhaps even two, and if life had taken hold after the planet and its companion cooled down, we might live in a true double planet system. Imagine looking up each night and seeing a blue companion planet in place of the Moon, with its own continents, weather, and oceans. That would be quite a sight. What about life-ending impacts? By studying the fossil record, geologists have identified sudden mass extinctions of species. They count the type and number of species in different layers of rock and can see when the number of species changes significantly. Two of the most significant extinction events are called the K-Pg boundary (a.k.a. the Cretaceous-Paleogene event) and the Permian-Triassic event. The Permian-Triassic event took place about 251 million years ago. Although it is not entirely clear that major impacts caused this extinction, it is clear that the Earth’s life suffered an extreme setback. This extinction event led to the loss of 96% of marine species and 70% of terrestrial vertebrate species. Ponder this for a minute: This means that nearly all marine life was completely wiped out. More than two-thirds of all terrestrial animal species disappeared. Even many insect species—among the best survivors on the planet—were wiped out as well. This event is commonly referred to as the “Great Dying”—suffice to say it would not have been pleasant time to be alive. Although multiple impacts by large asteroids is a likely explanation for the Permian-Triassic event, there are other possibilities and research continues. The K-Pg event took place 65.5 million years ago and is fairly clearly caused by the impact of a large asteroid. A thin layer of sediment with a high concentration of iridium was laid down around the world in a very short period of time. Iridium is very rare in the Earth’s crust, because it sank along with iron to the Earth’s core, but it's often found in asteroids. There is also evidence of significant geologic activity around the time of this extinction event, which led to the loss of about 75% of all extant species, but most geologists believe it was caused by a giant impact near today's Yucatan peninsula, forming the so-called Chicxulub crater. It is still not clear if the impact and its debris cloud (and tsunamis) were the sole cause of the extinctions or if secondary causes (chemical changes in the atmosphere or oceans) had a role to play. Again, research continues. What can we take away from these extinction events? Life is both pretty tough and pretty disposable. Although life as a whole goes on, your species may not get a golden ticket. Impacts happen that can destroy most life on Earth. The good news is that life managed to survive and ultimately re-conquer the ocean and land, just not in the same forms that existed before. It is one of the amazing things about life on our planet that evolution guides both the long-term survival of life generally and the development (and extinction) of individual species. Life goes on, but any individual species may not. Astronomers have begun multiple projects to scan the solar system and identify potential asteroids that might impact the Earth. Hopefully by identifying possible life-threatening objects, we could come together worldwide to somehow save ourselves (and all the other life on the planet). Right now, destroying or nudging an asteroid on a collision course would be a tremendous challenge, but it seems that impacts are few and far between, so we probably have enough time to develop the technology necessary for planetary protection.