There is evidence that about 35 million years ago, at least five comets or asteroids collided with Earth. If the effects of a single large meteorite impact seem overwhelming, imagine how life on Earth would reel from a barrage of rocks from space.

One way such impact clustering happens is to have a single bolide break up as it approaches a planet. The comet Shoemaker-Levy 9 provides a recent example. Before striking the planet Jupiter in 1994, the comet was torn into 21 different pieces by Jupiter's immense gravity.

These fragments struck Jupiter over 5.6 days, some creating large fireballs as they entered Jupiter's vast gaseous atmosphere.

Earth's gravity is no where near as powerful as Jupiter's, so the same scenario would not happen to Earth. Yet many asteroids are thought to be rubble piles, loosely bound by gravity, and such an asteroid could rip apart as it approached our planet.

But if an asteroid "rubble pile" broke up before it entered Earth's atmosphere, the pieces would only result in one crater, or at most two, because most of the pieces would fall into the same hole.

"Once such a rubble pile enters Earth's gravity it's too late," says Christian Koeberl, a geochemist at the University of Vienna in Austria. "It would only get into the attraction field of Earth's gravity a few hours before it hits at best, and this is not time enough to spread it out appreciably."

However, having an asteroid break up as it approaches Earth is not the only way to end up with multiple craters.

Within the asteroid belt that orbits the sun between Mars and Jupiter, collisions between asteroids sometimes occur. The resulting fragments can then rain down on Earth.

Simon Kelley, a geologist at the Open University in England, says that such a collision occurred 470 million years ago, and many of those fragments traveled to Earth. In fact, some of the fragments still are impacting Earth today.

A similar shower of fragments can come from a collision within the Oort cloud, a comet-filled region in the outer-most portion of the solar system.

Kelley says such a cometary shower may be responsible for the cluster of impact craters dated to be 35 million years old, including two of the largest impact craters on Earth: the 100 kilometer Popigai crater in Siberia, and the 90 kilometer Chesapeake Bay crater off the shore of Maryland. This cometary shower is thought to have lasted for 2 to 3 million years.

Looking over the Planetary and Space Science Centre's Earth Impact Database, several of the crater dates overlap. Much of this overlap reflects the limitations of current dating techniques, where ages can't be narrowed down further than hundreds of thousands of years.

But it is possible that some of the craters point to a multiple impact scenario.

The Boltysh crater in the Ukraine may be proof that multiple impacts occurred during the Cretaceous-Tertiary (K-T) extinction. Kelley has dated the crater to be about 65 million years old.

The Boltysh crater had previously been assigned ages ranging from 88 million to 105 million years old, a variation that arose due to the different dating methods used. Kelley used Argon-Argon dating to determine the correct age for the crater.

The crater rocks melted in the heat of impact, and when they cooled they trapped argon out of the air. Different isotopes of argon decay at different rates, so by measuring the ratio of argon isotopes, Kelley was able to estimate when the rocks melted, within a margin of error of plus or minus 600,000 years.

The margin of error prevents scientists from saying Boltysh was definitely part of the same meteorite strike as Chicxulub. And Kevin Pope of Geo Eco Arc Research points out that while Chicxulub has been linked directly to the K-T boundary and extinctions by the stratigraphy of its ejecta, the same is not true for Boltysh.

"Craters the size of Boltysh form every few million years, so the fact there is a crater this size close to the boundary is no surprise," says Pope.

The Boltysh crater measures only 24 kilometers in diameter - compared to Chicxulub's nearly 200-kilometer-wide monster - so even if the smaller meteorite that made this crater hit around the same time as Chicxulub, its effects wouldn't have been as catastrophic.

But if Boltysh did occur around the same time as Chicxulub, it could aid in our understanding of the K-T extinction event. Craters can act as time capsules, preserving information about the environment at the moment of impact.

Like Chicxulub, the Boltysh crater is buried underground. But while Chicxulub filled with seawater, the Boltysh crater became a fresh water lake. Kelley is writing a paper with Dave Jolley at the University of Sheffield on the microflora and fauna they have found in the Boltysh crater fill.

They hope to determine how rapidly life recovered in the vicinity of the impact, and how that recolonization occurred.

As for other K-T impact craters, Kelley notes that the record is very poorly dated. He plans to study other craters, currently dated to be from the Devonian (around 380 million years ago) to the Eocene (34 million years ago), to see if their ages are accurate.

By determining the correct dates for impact craters, scientists will be able to better understand how often multiple impact events have occurred in the past.

So do multiple impacts play a role in mass extinctions? Kelley says that as far as we know, they don't. For instance, Kelley says there is no evidence that the barrage of comets 35 million years ago led to a mass extinction event.

"The effects would have been truly devastating locally, but they didn't amount to global catastrophes," says Kelley. "You can argue that minor extinctions are associated, but not a K-T-like event."

A single large meteorite impact like Chicxulub may be more harmful to life than a cluster of several smaller meteorites or comets spread out over a million years or less. Yet determining why certain species die out can often be difficult.

Extinctions are a natural part of the cycle of life, and may occur due to a whole host of interrelated factors, including competition for food, climate change, and even sea level change. Perhaps tossing a few meteorites into the mix also can upset the scales, tipping some species too far off balance to recover.

Part 1, Debating the Dinosaur Extinction. Part 3 of this series will discuss a controversial impact crater off the coast of India. Part 4 will cover the debate over whether the K-T extinction was brought on by global warming.

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