One thing you quickly learn as you write a suite of articles about space and the dark beyond is that there are a lot of things in the Universe that can kill you. Gamma rays, black holes, solar flares; there is no shortage of topics to make the hair on the back of your neck stand up. There is one threat, however, that is closer to home than all the rest both in terms of distance and presence in the human mind: Impact.
There is something powerfully unromantic about it. Of all the ways the human race could be brought to its knees, a collision by a comet or asteroid is the most blunt and unimaginative. If the Universe were a movie villain black holes and gamma bursts would be the James Bond disaster scenario. Impacts are the celestial equivalent of getting hit over the head with a pipe.
Even still, we stand in awe at the possibility of being hit with such a massive pipe. A plethora of movies have painted varyingly plausible doomsday situations featuring rocks from the sky, and these movies are where most people get their information from. Unfortunately, Bruce Willis and a team of oil drillers aren’t always the best teachers.
There are three places from which falling death can originate. The first, and closest to Earth is the home of asteroids and it lies in the vast track of space between Mars and Jupiter that marks the boundary between the inner and outer solar system. When most people think of the asteroids belt they envision a tightly packed maze of rocks that spaceships must deftly maneuver through with the greatest of care. In truth, it is a pretty roomy place. A flight through the belt is a bit of a letdown in terms of site seeing. Asteroids of any significant size are an average of 5 million kilometers apart. The collisions between asteroids that send them careening onto a path that threatens the Earth are exceedingly rare with an average rock getting bumped once or twice in its several billion year long life. Nevertheless, it happens.
The other two breeding grounds for swift cosmic death are further out in space and are the hideouts of short period and long period comets. They are known as the Kuiper Belt and the Oort Cloud respectively. The Kuiper Belt lies just beyond the orbit of the most distant planet (Neptune) and is home to both comets and planetoids (AKA dwarf planets) like Pluto and Eris. When comets get knocked out of the Kuiper Belt they take on long elliptical (egg-shaped) orbits that pass through the inner solar system once or twice in a human lifetime. These comets are subsequently called “short period comets.” Objects from the Oort Cloud take on much longer orbits and become long period comet. Their orbits can take 1,000, 10,000, or even a million years to orbit the Sun once.
It is all well and good to know where these things come from but the real question is when can we expect one for dinner? It must be said that comets and asteroids are notoriously bad houseguests and you don’t want to be surprised by their arrival. The dinosaurs can vouch for that. It turns out that a sizeable impact happens once every 300,000 years or so with smaller objects burning up in the atmosphere daily or even hourly.
The major impacts are not something you want to be around for. Even a relatively small object (~1 km across) would release a million times more energy than the bomb dropped on Hiroshima and create global chaos. Evidence of past collisions can be seen at meteor crater in Arizona (3.8 km/2.4 miles in diameter), the Chicxulub Crater in Yucatan, Mexico that is believed to have been left by the object that marked the end of the age of dinosaurs (180 km/112 miles in diameter), and in the Sudbury Basin in Ontario, Canada (250 km/160 miles in diameter).