Every month about 40 known or recently discovered asteroids come within 0.05 astronomical units, or about 19 lunar radii, from our planet. In some cases, such as in the month of December 2016, four or even five objects reach their closest approach distance on the same day. Clusters of asteroid flybys often attracts the interest of the media and of the public, but are these occurrences unusual?
Upon applying statistical considerations to the present discovery rate and assuming a uniform probability of discovery every day, it turns out that they are not. A cluster of at least four objects approaching on the same day is expected about three months every four; a cluster of five about three times per year.
In reality, the groupings are likely to happen even more often. It is well known that astronomical observations peak around new Moon, when the sky is darker. Therefore asteroids flying-by near full Moon, while equally common, are more likely to go undiscovered and therefore not be included in the counts. (ESA NEO Newsletter February 2017)
Also new technologies and studies are playing a role. Although it might seem there are a lot more asteroids than before, this isn’t the case, we’re just getting better at finding them.
Some facts presented by NASA:
Every day, Earth is bombarded with more than 100 tons of dust and sand-sized particles.
About once a year, an automobile-sized asteroid hits Earth’s atmosphere, creates an impressive fireball, and burns up before reaching the surface.
Every 2,000 years or so, a meteoroid the size of a football field hits Earth and causes significant damage to the area.
Only once every few million years, an object large enough to threaten Earth’s civilization comes along. Impact craters on Earth, the moon and other planetary bodies are evidence of these occurrences.
Space rocks smaller than about 25 meters (about 82 feet) will most likely burn up as they enter the Earth’s atmosphere and cause little or no damage.