Don’t search up your umbrella or start looking for the nearest underground shelter. The number of impacts striking the Earth and Moon has increased … but only relative to the rate 500 million years ago. According to a study by U.S., Canadian, and U.K. astronomers and published in Science, the Earth is receiving more than two times as many impacts as it did in the deep past.
That’s something of an unexpected result. For a couple of billion years, beginning around the time the Earth first appeared, there was a period known as the “heavy bombardment.” The Solar System wasn’t born with eight (or nine, or 10) planets neatly laid out in their current orbits. Instead, the early Sun was surrounded by a swarm consisting of thousands of much smaller objects (plus Jupiter, which likely formed as it’s own little swirl … but that’s another story). Where the inner Solar System now consists of four planets running from Earth to Mars, it was once home to a staggering number of objects, some of them quite large. Even after the Earth had picked up enough mass to be a sizable planet, there were still some very big objects trying to occupy the same space. It was a collision between the Earth and a Mars-sized planet in a similar orbit that created the Moon and reshaped Earth into the planet we know around 4.5 billion years ago.
For hundreds of millions of years after that, large objects continued to strike with enough frequency that early life had to survive several hellish episodes. Most of the marks of that early bombardment are now impossible to find on Earth, as weathering and continental movement has erased most signs. However the scarred face of the Moon gives a sense of just how heavy that heavy bombardment really was—craters on top of craters on top of craters. And not everything is gone from the Earth. Chesapeake Bay? It’s a 90 km wide impact crater. There are at least 27 known craters in the United States with a diameter greater than a kilometer—so you’re probably closer to a structure caused by an impact from space than you ever realized.
Eventually, Earth’s orbit, and that of the other planets, was cleared out by these collisions. All those other objects either became part of larger planets, or were flung in or out until the lanes were nicely empty. There are still plenty of small objects out there, but most of them are very far away, where their interactions with the named planets are extremely rare. A comet may come cutting through on some 100,000-year orbit, and there may be a dark something out there in the distance that occasionally causes some object to veer our way. But for the most part things are generally—thankfully—very quiet here in the inner system.
But not, apparently, as quiet as many would have thought.
The new study looked at those impact craters across the face of the Moon and tried to assign them dates. Other studies have demonstrated that the actual distribution of objects hitting the Moon and the Earth look to be about the same size, so checking out lunar craters should also be a good measure of just how often Earth should expect impacts. NASA's Lunar Reconnaissance Orbiter has spent 10 years collecting exactly the kind of information, including thermal data, that the researchers needed for this study. And looking at what remains of craters on Earth, the results seem to match.
The results of the study show that impacts have increased during the last 500 million years—since around the time of the Cambrian Period on Earth. While it’s hard to find craters older than 300 million years on Earth because of all that weather, drift, etc. that’s not true of the Moon. So the combined Earth-Moon study should give a pretty clear picture of space rock collisions over the long term.
And here’s something a bit odd: Around 290 million years ago, the rate of impacts really picked up—as in picked up by 260 percent. It’s stayed relatively high ever since. The rate of impacts is now about three times that of the period in the first part of the study.
In fact, the rate of impact now is higher than it was when the dinosaurs had their fatal encounter with a space rock. It seems our nice neighborhood has a few more unruly neighbors than we thought.
However … remember that umbrella advice. The rate of impacts is still quite low, and humanity shouldn’t expect a dinosaur killer any time soon, even if three near-earth asteroids did just go winging by our planet in the last week.
Even so, the idea of keeping an eye on the heavens—and having a plan in case we do see an incoming object the size of a boulder, or a bus, or a blue whale (the size of the three that went by in the last week)—is not a bad idea.