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When comets attack

Mike Solontoi / Univ. of Washington
A long-period comet called 2001 RX14 (Linear) streaks across the sky in an image
captured in 2002 by the Sloan Digital Sky Survey's telescope in New Mexico.

The black eye that Jupiter suffered this month has sparked a host of questions for astronomers as well as for the rest of us: What exactly hit the giant planet, and why didn't we see it coming? Why is Jupiter's bruise expanding? How often do these things happen, and how vulnerable are we to a similar cosmic pummeling? Astronomers are closing in on the answers - and helping the public get a better sense of perspective.

The first question is a toughie: What was it that caused Jupiter's "Great Black Spot," which was first noticed by an amateur astronomer in Australia back on July 19? "I'm not sure we'll ever know precisely," said Glenn Orton, a planetary scientist at NASA's Jet Propulsion Laboratory who is a member of the team studying the impact and its aftermath.

Orton addressed the "whatdunit" mystery on JPL's Weblog and expanded upon the subject in a phone interview. The best guess is that the impactor was a comet that measured perhaps a quarter of a mile (half a kilometer) wide. Why a comet and not an asteroid? "Almost everything in that part of the solar system is icy," Orton noted.

A comet that small might not have been noticed from Earth, particularly if it came directly at Jupiter from the outer reaches of the solar system, essentially hitting the giant planet from behind. That would explain why any observers who were watching Jupiter at the time missed seeing the impact.

"The impact almost certainly appeared on the far side in the preceding 10 hours," Orton said.

By the time the impact area came into view, all observers could see was a darkish cloud in Jupiter's dense atmosphere. You could call it a scar, or a bruise, or a black eye. On the Unmanned Spaceflight Web forum, some folks have used the term "astrobleme" - coming from the Greek words for "star wound."

H. Hammel (STScI) / NASA / ESA / Jupiter Impact Team
The black-and-white picture at left provides a wide-angle view of Jupiter. The white box outline shows the area taken in by the color picture at right.

Over the past week, the wound has widened, and Orton said that's due to wind shear at different cloud levels. "The upper part of the scar is moving westward, and the lower part is moving eastward," he told me.

As time goes on, astronomers can use those darkish particles to trace the flow in Jupiter's atmosphere. That's something they couldn't do nearly as well 15 years ago when Comet Shoemaker-Levy 9 broke up into scads of pieces and smashed into Jupiter. The multiplicity of impacts made it difficult to figure out which material was going where. In the current case, there appears to have been just one piece, which makes the tracing job much easier.

In weeks to come, astronomers will be analyzing the chemical composition of the material left behind by the cosmic smackdown, the composition of the material dredged up from the depths of Jupiter's atmosphere, and the way all that material diffuses into the clouds.

That means Orton and his colleagues are in for a hectic few weeks. "My days and nights are filled with asking for telescope time, and pulling data down from telescopes," he said.

Virtually every day, astronomers have been posting pictures of Jupiter and its Black Spot on forums such as Unmanned Spaceflight, the Planetary Virtual Observatory and Laboratory, ALPO and ALPO-Japan.

Could it happen here?
Meanwhile, the impact 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."

Back to the Black Spot
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.

For even more perspective, check out our "Close Encounters of the Asteroid Kind" interactive, and conduct your own search for asteroids and comets on msnbc.com.

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