Tonight, December 16, an asteroid known as 3200 Phaethon is going to miss Earth by 6.2 million miles. Given the relative emptiness of space, that's just about grazing us.
The 3-mile wide asteroid has been classified as "potentially hazardous" by NASA, which designates that it has a probability of one day colliding with Earth. That won't be today: 6.2 million miles is not close enough to be of concern. For comparison's sake, the moon orbits us at an average distance of 240,000 miles, or 25 times closer.
But 6.2 million miles is close enough to Earth that amateur astronomers will be able to observe the space rock with a relatively small telescope. Astronomy Now has a map and details on how to see it.
Even though it's just passing by, 3200 Phaethon has made its mark on Earth in other ways. According to NASA, the recent Geminid meteor shower, visible on Earth throughout the past week, may be related to 3200 Phaethon; NASA says that 3200 Phaethon is "widely thought to be the parent body for the meteor stream, due to similarities between [Phaethon's] orbit and that of the meteors." Many meteor showers are caused by comets rather than asteroids, as comets rain particles in a way that can produce meteor showers when they intercept Earth. This would make Phaethon unusual among asteroids: "Most meteor streams are associated with comets, so this raises the question of whether Phaethon could be an inactive comet nucleus," NASA writes.
There are a lot of asteroids of varying size in the solar system, but few which pose a threat. Because 3200 Phaethon orbits the sun once every 523 days in a highly elliptical path, the asteroid occasionally dips closely to Earth's orbit, which may one day put us at risk for collision. (You can look at an interactive model of its orbit around the sun here).
It is not known definitively how devastating a 3-mile wide asteroid impact would be on Earth, and may actually depend on the place that it struck (water vs. land). The asteroid that led to the extinction of the dinosaurs was 6.2 miles wide in diameter, and struck a large gypsum deposit that sent sulphur into the atmosphere, blotting the sun for perhaps years and killing much plant life. If the asteroid had hit a slightly different spot on Earth, the outcome might have been very different.
Major extinction events on Earth, generally caused by asteroid impacts, are believed to happen at least once every hundred million years, and perhaps more frequently. The last major extinction event, known as the Cretaceous-Tertiary extinction event, was caused by the aforementioned asteroid and killed off much of the large land-dwelling life on Earth, including dinosaurs, about 66 million years ago. That impact paved the way for the rise of small mammals who were the ancestors to all humans.
While no potentially civilization-threatening asteroids have been discovered yet, efforts to track and catalogue potentially hazardous asteroids may pay off in the future. If an asteroid on course to strike Earth is detected with enough of a lead time, even a tiny application of force — say, a mirror or a laser pointed at it to slowly push it to one direction — would be enough to push it off course and prevent an impact with Earth. The trick is finding said asteroid far enough in advance, as a last-minute diversion or tactical nuke, as depicted in films like "Armageddon," is unlikely to be practical nor physically possible.