Asteroid impacts on the Earth and moon have increased since the dinosaurs lived.

Some 290 million years ago, as the last trilobites scuttled across the seafloor, the skies above grew just a little more ominous. At that point, large asteroids including the impactor that would later kill off the dinosaurs began to rain down on our planet between two and three times more frequently than they did before.That impact was singularly catastrophic. But, according to a new study published today in the journal Science, that smashup was also just one episode in an ongoing spike of gargantuan asteroid impacts bombarding our neck of the solar system.

After studying 1 billion years of asteroid craters on the Earth and moon, the study's authors found that the rate of huge asteroid impacts on Earth has nearly tripled in the past 290 million years and nobody's sure why.

Before this study, we believed that we couldn't find the oldest impact craters on Earth because erosion or other geological processes wiped them from the surface. And compared to other planets in the solar system, Earth has fewer older impact craters than expected.
So we decided to study the moon instead, a perfect analog for Earth, crater-wise, because both were hit similarly over time. Craters are also more well-preserved on the moon because it doesn't undergo the same disruptive processes that Earth does and by using the Lunar Reconnaissance Orbiter (LRO) data, we were able to assemble a list of ages of all lunar craters younger than about a billion years. We did this by measuring the heat radiating from the Moon's surface, to monitor the rate of degradation of young craters.
Because it takes so long for big rocks to break down into dust, we concluded that craters surrounded by larger, hotter boulders probably resulted from more recent asteroid impacts than did craters carpeted with pulverized dust and this allows us to distinguish rocks from fine particles in thermal images.
By applying this idea we were able to calculate ages for previously undated lunar craters.When compared to a similar timeline of Earth's craters, we have found the two bodies had recorded the same history of asteroid bombardment. and It became clear that the reason why Earth has fewer older craters on its most stable regions is because the impact rate was lower up until about 290 million years ago.

The change in the rate of asteroids colliding with the Earth and moon may be due to the large collisions that were happening in the main asteroid belt. The belt lies between Mars and Jupiter, and this area was highly active 300 million years ago. That would have created a stream of debris that would enter the inner solar system.

Confirming how impact rates vary in time could have repercussions that echo across the solar system. We are using craters (their number, their sizes, how more-recent ones overlap older ones) to estimate the ages of surfaces on planets and moons. This history of impacts throughout the solar system is pretty central to planetary science.

Phedias Hadjicharalambous.
Cyprus Astronomy Organisation