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The three largest impact craters on Earth are Vredefort (250 kilometers across), Sudbury (200 kilometers), and Chicxulub (170 kilometers). Most astronomers assume that these were created by asteroids roughly 10 kilometers across. But what of the possibility that comets may have been responsible for one or more of these impacts?

Today David Minton and Renu Malhotra from the Lunar and Planetary Laboratory at the University of Arizona present some interesting calculations about the rate at which planet-smashing asteroids are sent our way.

Contrary to the conventional view, the asteroid belt is a vibrant, dynamic region of the solar system with a complex structure resulting from the gravitational interaction of the major planets. Over billions of years, these orbital resonances have scoured the belt clean in various places. The famous Kirkwood gaps are one result of this process.

But the major planets continue to cause some asteroids to orbit chaotically, causing collisions that inevitably send debris our way.

The new work is a computer model of the rate at which this process produces large asteroids (10 kilometers across and larger) that threaten Earth.

Minton and Malhotra say their model indicates that the rate of Earth-threatening asteroids has fallen logarithmically over the last 3 billion years or so (since the end of the Late Heavy Bombardment).

But interestingly, they say that the numbers suggest that Earth would have been hit perhaps once in that period by a large asteroid, a rate that’s somewhat less than observed.

They also calculate impact rates for Mars and Venus and find the number of large impact craters to be greater than the number of larger asteroids around to create them.

What might account for the difference? Minton and Malhotra discuss several options, such as the possibility that planet-impacting asteroids may have a larger relative velocity than thought, and so make larger craters.

But the most intriguing is this possibility that comets may be responsible for most large impacts. “The fraction of terrestrial planet impacts that are due to comets vs. asteroids has long been controversial,” write Minton and Malhotra.

The conventional thinking is that comets are responsible for less than 50 percent of impacts, but these results suggest otherwise–that comets are responsible for an order of magnitude more impacts than asteroids are.

In their discussion, Minton and Malhotra dismiss this as unlikely, but it bears some what-if contemplation about the mechanism that might generate Earth-threatening comets in the Kuiper Belt and Oort Cloud.

Our understanding of the dynamics of the Kuiper Belt and Oort Cloud is at a very early stage, but what’s the betting that these turn out to be vibrant dynamic structures dominated by resonances, gaps, and chaos, just like the asteroid belt?

And if that’s the case, it may well be possible that the rate at which comets enter the inner solar system changes dramatically from time to time, as collisions in these clouds send showers of icy visitors our way.

Perhaps a computer model of the Kuiper Belt and Oort Cloud would help. Anybody got one they could adapt?

Ref: arxiv.org/abs/0909.3875: Dynamical Erosion of the Asteroid Belt and Implications for Large Impacts in the Inner Solar System

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