Meanwhile, the impact [recent impact of an object into Jupiter] has led many to wonder about the chances that something similar might hit Earth instead. Fortunately, Jupiter is so much larger than our own planet that it acts as a gravitational attractor for cosmic debris. That makes Jupiter "our friendly big brother," Orton said.
It so happens that research newly published by the journal Science provides more data on the likelihood of killer comets - specifically, the chance that a shower of long-period comets might be pushed toward Earth.
The bad news is that computer simulations indicate such a comet shower is indeed possible. The good news is that the same simulations suggest Earth should experience a comet shower only once every 500 million years.
Long-period comets are among the wild cards in a thick deck of cosmic threats. In contrast with short-period comets, such as Comet Halley and Comet Tempel-Tuttle, long-period comets trace insanely eccentric orbits that range out beyond Neptune, Pluto and the Kuiper Belt to a little-understood region on the solar system's edge known as the Oort Cloud. The best-known example is Comet Hale-Bopp (which pays us a visit every 4,200 years).
University of Washington researchers Nathan Kaib and Thomas Quinn ran computer simulations of solar system interactions to see how long-period comets could be knocked loose from the inner Oort Cloud, a region that spans the zone between 1,000 and 20,000 AU away from the sun. (One AU, or Astronomical Unit, is equivalent to the distance between Earth and the sun - that is, 93 million miles or 150 million kilometers).
The outer Oort Cloud goes from 20,000 AU to as much as 100,000 AU, or nearly halfway out to the next star. Astronomers have long believed that comets could be jarred loose from the outer Oort Cloud by a passing star. But some of them thought the solar system was structured such that comets came only rarely from the inner Oort Cloud, in deadly bursts.
In the Science research, published online today, Kaib and Quinn report that comets from the inner Oort Cloud can indeed "penetrate Jupiter's orbit via a largely unexplored pathway" and are a "significant, if not the dominant, source" of long-period comets.
That might sound like bad news. The UW researchers see it differently, however: They say the simulations actually suggest there are fewer comets in the entire Oort Cloud, inner plus outer, than astronomers previously thought. Demystifying the inner Oort Cloud has the effect of making the whole region seem somewhat less dangerous.
"For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth," Quinn said in a UW news release. "We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir."
The simulations indicate that Jupiter and Saturn should be able to catch most of the long-period comets coming our way, like goalies catching soccer balls. Even in the worst-case scenario, only about two or three big comets would slip through and hit Earth, the researchers said.
Kaib and Quinn go so far as to suggest that the only time this happened in the past half-billion years or so was during a minor extinction event in the late Eocene geologic period, 33 million to 40 million years ago. It's thought that the late Eocene was marked by cometary impacts in present-day Chesapeake Bay and Siberia.
"If the late Eocene episode was caused by a comet shower, it was likely the most powerful shower since the Cambrian Explosion, implying that comet showers are unlikely to account for other observed extinction events," the researchers wrote.
The calculations published in Science make the specter of killer comet storms look a little less threatening. It's important to remember, however, that Kaib and Quinn are talking purely in terms of statistical analysis. The case of Jupiter's Great Black Spot illustrates that statistics can take you only so far.
Fifteen years ago, astronomers said Comet Shoemaker-Levy 9's collision with Jupiter was an exceedingly rare occurrence. Now we know that's not necessarily so. "The 1-in-a million chance of seeing one of these per century is clearly off," JPL's Orton said.
For years, JPL has been keeping track of potential cosmic threats as part of its Near-Earth Object Observation Program. Now the subject has spawned a brand-new Web site titled Asteroid Watch, which offers blog entries and a Twitter link as well as an asteroid widget. I suspect the Great Black Spot had something to do with all this.
Link: MSNBC Cosmic Log, Alan Boyle, article
"Reassessing the Source of Long-Period Comets"
Nathan A. Kaib 1* and Thomas Quinn 1
1 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195–1580, USA.
Published Online July 30, 2009
We present numerical simulations to model the production of observable long-period comets (LPCs) from the Oort Cloud, a vast reservoir of icy bodies surrounding the Sun. We show that inner Oort Cloud objects can penetrate Jupiter's orbit via a largely unexplored dynamical pathway, and they are a significant, if not the dominant, source of known LPCs. We use this LPC production to place observationally motivated constraints on the population and mass of the inner Oort Cloud, which are consistent with giant planet formation theory. These constraints indicate that only one comet shower producing late Eocene bombardment levels has likely occurred since the Cambrian Explosion, making these phenomena an improbable cause of additional extinction events.
Link: Science Magazine article