Comet Crash

Last week, astronomers watched in awe as several anonymous comets plunged into the Sun. Many people are surprised to learn that similar events happen almost every day, but the icy fragments are typically too small to be seen. A string of comet fragments struck Jupiter in 1994, causing quite a splash. Should we worry that the Earth might be next?

Comets are generally believed to be leftover debris from the formation of our solar system more than 4 billion years ago. A distant icy halo surrounding the Sun, known as the "Oort cloud", is the source of nearly all of the comets that have been observed throughout history. Some of these comets return regularly (like Halley's comet in its 75.3 year orbit), while others pass through once and head back to the cloud, never to be seen again. Astronomers believe that comets are like "dirty snowballs", containing a loosely packed mixture of ice and dust. As they approach the Sun they warm up and begin to evaporate, spawning a long bright tail that generally makes them much more visible.

There is a population of comets known as the "Sun grazers", which appear to be the leftover fragments of a much larger comet that probably broke up more than 2000 years ago. The orbits of these comets are extremely elongated, and in some cases shoot them directly into the Sun, a fiery demise that is barely noticed by the enormous boiling mass. Most of the fragments are so small that they are nearly invisible until they begin their plunge, where satellites that are always watching the Sun for signs of hazardous space weather can finally see them brighten and abruptly disappear.

In the summer of 1994, the planet Jupiter had a similar encounter with a train of comet fragments known as "Shoemaker-Levy 9". About two years earlier a large comet passed so close to Jupiter that it was broken into many pieces by tidal forces and thrust into an orbit that would strike the giant planet like a string of pearls on the next pass. I was an undergraduate student at the time, working with a group known as SpaceWatch at the University of Arizona. Over the months leading up to the collision, we carefully measured the positions of each fragment to calculate the individual orbits and make predictions of the precise impact times. For the nine largest pieces, we calculated times that were correct to within a few minutes.

As for something similar happening to the Earth -- don't worry! Jupiter is a huge target with significant gravitational pull, and the Sun is over 1000 times more massive. Nothing as large as a comet has struck the Earth since the disappearance of the dinosaurs, 65 million years ago.