Tuesday night, asteroid YU55 buzzed between the earth and the Moon, as reported Monday on this website by Miles Clark. What would have happened if its path changed slightly, and it veered Earthward and tumbled down through our atmosphere and impacted the surface of our planet?

The history of the Earth is littered with impact events. In its early formative (first few billion) years, the volume of debris flying around our solar system was much, much higher than what’s out there right now. The young Earth was bombarded by asteroids, comets, rocks, and other cosmic debris. Some theories even hold that the moon was formed when a comet or asteroid smashed into the Earth and broke off a large enough chunk to form the moon (I wrote about this last month.)

Our moon is pockmarked by craters. Every single one of these features was caused by an impact, going back all the way to the formation of the moon around 2 billion years ago. Craters this old are still visible on the moon because unlike the Earth, the moon lacks plate tectonics, weathering, or erosion to change the features of it’s surface over time.

Another difference between the Earth and the moon that contributes to the millions of craters that pockmark its dusty surface is the fact that the moon has no atmosphere. Here on Earth, the atmosphere that holds in the gases we breath acts as a small barrier to objects falling from space. Intense heat from friction causes smaller objects to vaporize, and larger objects to burn as they tumble towards Earth. Satellites burn up on re-entry, but larger pieces of them can and do impact the earth and are capable of causing substantial damage if they impact anything manmade.

The amounts of space junk flying around above our heads are staggering. NORAD, deep in a mountainside in Colorado Springs, is charged with tracking these objects. NASA’s Orbital Debris Program website shows impressive maps of the clouds of debris flying around over our heads. According to NASA, over 95% of the objects seen here are pieces of junk and debris (i.e not satellites). Click here to open a website that offers real-time satellite tracking.

Two months ago, a 6.5 ton NASA satellite called UARS fell to earth completely out of control. NASA literally had no idea where it was going to splashdown, although they were able to say it would be “somewhere between latitude 57 north and 57 south, and over a 500-mile long swath.” That range covers Australia to Northern Canada. Some warning! People in Florida reported seeing pieces of it burn up in the atmosphere, yet it ended up coming down in the Pacific Ocean, causing no harm to anyone or anything. Check out this animated computer model of the re-entry. Only one person has ever been hit by a falling piece of space junk – Lottie Williams, in 1997 was hit by a Pepsi-can sized piece of a Delta 2 booster rocket, which fluttered to earth and tapped her gently on the shoulder.

Satellites and pieces of junk hitting earth are one thing – asteroids and comets are a whole different story. Basically, the larger an object, the more potential it has to do serious damage to the earth. Not just destroying some forests, or creating an impressive crater – I’m talking biblical, mass extinction, end of the world levels of damage.

The most commonly known significant (i.e. massive) impact event is probably the mass extinction at the end of the Tertiary period, called the K-T event (Cretaceous / Tertiary, thought to have been partially caused by a giant asteroid or comet impacting the earth in the vicinity of what is now the Yucatan peninsula of Mexico. This explanation was first offered in 1980 by a team of geologists led by father-and-son Luis and Walter Alvarez. The Alvarez Hypothesis, as it is now known, is one of the classic tales of scientific discovery.

The Alvarezes and their colleagues noticed a distinct layer of sediment that could be dated to 65.5 million years in age was found in rocks everywhere around the earth where rocks of that age were preserved. Not coincidentally, there were abundant fossils below this layer, and barely any above it (or, any large, complex organisms) – which led them to conclude that they were looking at the record of a mass extinction. Upon further study, they noticed that the unusual sediment layer also contained 30 to 130 times the amounts of a trace element iridium. This element is very rare on earth, but it happens to be quite abundant in asteroids. All of these pieces could add up to only one conclusion – a giant asteroid smashed into the earth and was a major part of the cause for the extinction of >80% of the species living on earth in a period of a year to a few thousand years (an instant in the geologic record).

The only thing their theory was missing was a crater. Ten years after they put forth their theory, the location of impactor was discovered to be the Chicxulub crater in the Gulf of Mexico.

This crater is 111 miles across and oval shaped, which matches that theorized as necessary by the Alvarez team. It’s not readily visible from the surface like the meteor crater in Arizona, because of the proximity to the ocean, the older age, and the thick vegetation, but it is easily seen in the seafloor via radar imagery.

How big of an asteroid would have been required to cause this big of a crater, and to wipe out more than 80% of the species living on the entire planet, including the dinosaurs? The answer is somewhat surprising – only 6-9 miles across. Given that the earth has a diameter of ~7,900 miles, that’s pretty small for the havoc it wreaked.

Just what was that havoc? Bad, bad things. Global firestorms, intensified by much higher amounts of oxygen in the atmosphere. Earthquakes ripping apart the earth for days as it vibrated like a rung bell. Volcanoes erupting everywhere, acid rain burning flesh and vegetation, vaporized seabed rocks and sulfuric gases blocking out the sunlight for years, and even megatsunamis, up to 2 miles (yes, you read that right) TWO MILES high. Just how much energy – well around 1 billion times the amount released by the nuclear bombs dropped on Japan.

Scientists at Princeton University recently published a paper in the journal Geophysical Journal International that may just shake up some commonly held views about this event. I mentioned large volcanic eruptions, and the fact that this K-T boundary impact asteroid is only part of the cause of the mass extinction. One of the other leading theories is a massive volcanic eruption of basaltic lava in India, known as the Deccan Traps. The traps are the cooled remains of lava flows about 6,500 feet thick that cover an area of 194,000 square miles. And they happen to have been erupted between 60 and 68 million years ago – the same period as the asteroid impact discussed above. That much lava would have released an enormous amount of climate-altering gases, which almost certainly contributed to the extinction preserved in the fossil record (yes there is ample evidence of global warming and cooling cycles causing extinctions repeatedly in the ancient history of our planet).

Some people have speculated that the massive impact over at the Gulf of Mexico caused the eruption of the enormous amounts of lava in India, as it was at the approximately opposite location on the earth from the impact location (i.e. straight down through it, a.k.a. the “antipode”). The thought was that the enormous amounts of seismic energy released at the time of impact would converge on the opposite side of the planet at the antipode and cause the ground to basically explode, releasing the lavas of the Deccan Traps.

The aforementioned recent Princeton research, led by German professor Matthias Meschede looked at paleogeography, and also applied a new computer model for the dispersion of seismic energy through the earth that took into account the actual makeup of the crust. Turns out all the old models had been based on the Earth being a perfect, smooth sphere. In reality, it has a wide variation in crustal composition, and it’s oblong. Based on this work, the Cicxulub meteor couldn’t have caused the Deccan Traps eruption. But, the two events are still thought to be the major players in the Cretaceous / Tertiary extinction.

So what would have happened if the solar wind blew strong enough to direct YU-2005 down into the Earth? It does pass by the vicinity of the earth again in 2041, and each time it flies by it’s orbit changes slightly, so this is not beyond the realm of possibility over a long enough timeframe.

As Miles reported a few days ago, this asteroid is relatively tiny at only~1,000 feet across. This is about the same size as an aircraft carrier is long. Nevertheless, it would have struck the earth at a velocity of 11 miles per second, and gauged out a crater about 4 miles wide and 1,700 feet deep, accompanied by a Mw 7.0 earthquake, and tsunamis up to 70 feet tall if it struck the ocean. Yikes. Although extremely unlikely, its not hard to imagine what would happen if it hit a city.

As a comparison, Meteor Crater in Arizona is one of the most famous impact craters in the US. The crater is 4,000 feet across, and 570 feet deep. It’s rim rises 150 feet above the flat desert floor. This crater was thought to have been caused by a rock about 165 feet across. Much smaller than YU-2005. Objects the size of YU-2005 strike the earth about once every 100,000 years, which means that in a thousand years, there is a 1% chance of a collision.

The simple fact is that meteors and comets are out there, and they will strike our planet. Not if, but when. These events are extremely rare, but they have high consequences for all of us. If you want to destroy the earth from the safety of your own computer, have a play with Impact: Earth!, the simulation program prepared by Dr. Jay Melosh at Purdue University:

Impact: Earth!