The Perseid meteor shower occurs every year when the Earth passes into a cloud of debris trailing behind the comet Swift-Tuttle. The shower has already passed its peak for the year, but several quick-fingered photographers have shared their photos of the meteors streaking the sky:

Turn Basin at NASA's Kennedy Space Center: This may actually be an incidental meteor rather than a Perseid, since its path doesn't point back to the shower's radiant. [APOD]
Perseid shooting across star trails by Izzyman1.
Perseid over Mudanya, Turkey by Ugur Ikizler.
Shooting Star Inn & Observatory just outside of Flagstaff, Arizona by Tom Taylor.
Perseid over Malta by stefni082.
Meteor over Halifax, Nova Scotia by Michael Boschat.
Perseid over Laleh-zar, Kerman, Iran by Ehsan Rostamizadeh.
Rooftop view of Perseid in Washington Heights, NYC by meg21210.
Perseid over Glade Park, CO by Thad V'Soske.

Researchers reported this week in Proceedings of the National Academy of Sciences that they found have found shock-synthesized diamonds — known to result from impact events — in the Arlington Canyon on Santa Rosa Island in California, and have previously found similar diamonds in the Greenland Ice Sheet. The placement of the Arlington Canyon diamonds coincide with North America's oldest known human remains — from the Clovis people, who went extinct nearly 13,000 years ago — and the disappearance of the pygmy mammoth from Santa Rosa. This fits with the team's earlier speculation that a comet strike led to a mass mammalian extinction across North America:

In 2007 researchers theorized that a comet set off continental fires that led to the mysterious disappearance of the Clovis people and the extermination of 35 mammal genera, including mammoths, mastodons, ground sloths and camels. The team documented a "black mat" of charcoal throughout North America that contains high levels of iridium, magnetic spheres, and nano-diamonds, which are consistent with such an airburst.

However, the hypothesis remains controversial, and other geologists and archeologists are reluctant to buy the diamonds as evidence of a comet-induced die-off, especially given the absence of an impact crater.

Did a Comet Cause a North American Die-Off around 13,000 Years Ago? [Scientific American]

Anthony Wesley reports that he noticed the spot (seen at the top of the photo above) while observing Jupiter yesterday from his home observatory near Murrumbateman in New South Wales, Australia. Wesley, who had observed the planet two days earlier but had not seen the black spot, speculates that the spot might be an impact spot, possible the result of a comet or asteroid strike.

And, while Phil Plait, Bad Astronomy blogger and author of Death from the Skies!, initially cautioned excited armchair astronomers that the spot might be a mere weather event, he agrees that the emerging data seems to point to an actual impact.

Impact Mark on Jupiter

This gorgeous photo of Lulin, discovered a little over a year ago, was taken last week at New Mexico's Caldera Rim Observatory by James Maxwell. Stars around the comet look a bit streaky because of the long exposure required to make the greenish comet pop like that - you can even see its tail and anti-tail (in front). According to Sky and Telescope, this photograph is a composite of "24 x 90 second exposures, totaling 36 minutes, tracking the comet."

If you want to find out more about the best times to view Lulin check out Sky and Telescope's article about it.

The traditional theory has been that comets and other debris get caught in Jupiter’s gravity well and neutralized by the immense planet. But that theory has come under fire in the past year, and new simulations indicate that Jupiter’s gravitational field could be doing more harm than good:

Using a model of some 40,000 planetesimals, Kevin Grazier of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues found that debris in the outer solar system initially had circular orbits and posed no threat to Earth or the other inner planets early in the history of the solar system. But the researchers showed that, through a series of close gravitational encounters with the outer planets, especially Jupiter, the objects assumed more elongated orbits and were handed down to the inner solar system.

In the simulation, Jupiter was responsible for most of the debris that entered in the inner solar system. And, dinosaur-killing comets aside, such debris may have had an impact on the development of our planet:

Not all of the bullets were destructive, Grazier emphasizes. Some of the material that had been delivered to Earth from the outer solar system contained water and other compounds that could have helped life to gain a foothold.

Sniping at Jupiter [via Science Not Fiction]

Located in the Australian outback, this 14-mile long crater was created about 140 million years ago. Today it's called Gosses Bluff and this hilly area pictured is the 2-mile-wide center of the impact.

Check out more planet-smashing goodness in the full photo essay.

Target Earth [via National Geographic] (Thanks, Marilyn Terrell!)

USA Today, New Scientist, the awesome astronomy blog Bad Astronomy and the BBC and Nature and just about every sciency news outlet all have items devoted to the centennial. But they disagree on what the Tunguska Event was. USA Today calls it an "impact,' but Bad Astronomy says "air blast" and says there's no evidence anything hit the ground. New Scientist has posted a video in which their reporter circles Lake Cheko nearby the blast site in a helicopter and speculates whether it's the smoking gun of an impact.

What's going on here? Tunguska is probably the most heavily studied impact/air blast/space rock encounter on Earth and we know almost nothing about how it happened. It's also hard to say how likely it is that it will happen again, though one scientist's guess isn't comforting:

In terms of risk to Earth, astronomer David Morrison of NASA's Ames Research Center says a Tunguska-magnitude strike could happen once every two centuries and a bigger impact, a "civilization-threatening" million-megaton strike, could happen once every 2 million years. Even though astronomers have spotted more of these nearby asteroids in the last two decades, the estimated odds of an impact have actually declined, as Morrison notes in a May issue of NEO News, his asteroid newsletter.

If Morrison's right, we've got at best another century to learn as much as we can from Tunguska before another similar event hits home — maybe less. And in the mean time, we'll have plenty of close calls reminding us that we are basically sitting ducks unless we start doing something about one of the greatest threats to our survival as a civilization.

NASA's list of potentially hazardous asteroids (PHAs) currently numbers 959. That's 1,000 asteroids that astronomers pretty much know are going to get closer than 7.5 million kilomters to Earth, about 20 times the distance from here to the Moon. Five of those are expected to come between Earth and the Moon over the next century.

So we'll have a few close shaves but nothing to worry about, right? Not so fast. The total number of PHAs and comets astronomers think are out there is probably more like 20,000. That means we've mapped about 5% of the objects that stand a good chance of hitting us. So take the future part of this chart as a best-case scenario. The past five close encounters, however, show just how vulnerable we are:

1) The Comet of 1491. This one must've scared the hell out of some folks. At a little less than four times the distance to the moon, this was the closest pass ever recorded at the time, and no one knows for sure how big it was. Little did our ancestors know how much more interesting things would get.

2) Tunguska, 1908. One of the most famous Earth lcose calls of all time, it was also a pipsqueak. For a long time scientists believed a comet perhaps 60 meters in diameter exploded over Siberia with a force of as much at 30 megatons, or about 2,000 times more powerful than the atomic bomb dropped on Hiroshima, though nothing solid ever hit the planet. All those pictures of flattened forest certianly look impressive, but last year, scientists re-crunched the numbers and found that the comet oculd've been as small as 30 meters, and the blast just 5 megatons. In other words, much smaller objects can do way more damage than we ever thought before. Gulp.

3) The Great Daylight Fireball of 1972. The name says it all — it doesn't get much closer than this. Size estimates range from 3 to 14 meters in diameter, depending on whether it was ice or rock. Whatever it was, the object called US19720810 burned through the atmosphere from Utah to Canda for about a minute and a half. Luckily, the space rock struck a glancing blow — had it hit Earth directly, it could've blasted us with 1/2 a Hiroshima worth of energy.

4) 2004 FH and 2004 FU162. At 30 meters in diameter and made of solid rock, 2004 FH would be a thumper of Tunguska proportions if it ever hit home. In the right (or wrong) place, it could detroy a city. As it was, it passed 43,000 kilometers above Earth on on March 18, 2004.

Three weeks later, FU 162 came whizzing along. Astronomers basically discovered it at bascially the same time as the 6-meter in diameter rock soared just 6,400 kilometers above Earth's surface.

5) Comet Hyakutake. Now we're getting into civilization-threatening territory. At 2 kilometers in diameters, this comet only got within about 40 lunar distances to Earth in 1996. Compared ot our other close calls, that's pretty comfortable, but considr this: it was discovered less than two months before its closest pass. Had it been on a collision course with Earth there's almost nothing we could've done other than brace for the millions dead, massive climate disruption, crop failure, 500-foot high tsunami...you get the idea.

FUTURE:

6) 1999 AN10. In a little less than 20 years, our usually quiet Earth-Moon system is going to have a lot of visitors. In August 2027, AN 10 is going to get about one lunar distance from Earth, and we'll get a chance to see just how big this bad boy is. Estimates range from 1/2 to 2 kilometers in diameter, plenty large to leave a dent in humanity if it ever gets closer.

7) 2001 WN5. Just six months after AN10 comes a callin' WN5 will get even closer, just about splitting the difference between Earth and the Moon. At 700 meters in diamters, this asteroid has a got potential for major dmaage, but current odds of impact are rated a negligible.

8) 99942 Apophis. By far the most famous of the end-bringing objects we know about in our solar system, astronomers thought for a while that this 270 meter-wide rock had an almost 3% chance of hitting us. Since then, odds have been lowered to 1 in 43,000 that it could slam into Earth in 2029. But if it passes through a gravitaitonal keyhole — a tiny region in space that could tweak its orbit ever so slightly — an impact could still happen on April 13, 2036.

9) 2005 WY55. Just 200 meters wide, astronomers think this asteroid could still pack a wallop. Right now it's scheduled to get within about 75,000 km of Earth, but impact odds are big enough to kep in mind — currently they're rated at around 1 in 70,000. If our number comes up on that faeful dayin May 2065, look out — blast yield estimates from this rock range to 1100 megatons.

10) 2000 WO107. Depending on how well humanity holds up under climate change, bird flu, and all the other things that could potentially kill us off, we might be able to look up and see WO107 zoom by in December 2140. The 400 meter-wide rock isn't scheduled to hit us — it should get about half way between Earth and the Moon — but if calculations are off by even a little bit (and all of the future examples here have some uncertainty) we could care a lot.

Sources: NASA's Near Earth Object Program, Harvard List of PHAs

Additional research by Nivair H. Gabriel. Image by Stephanie Fox.

The data fits with the extinction of the dinosaurs 65 million years ago and another extinction at the end of the Paleocene period, 35 million years ago. It also means our planet's probably in for another life-ending black eye in the not too-distant future.

But what's Galactic density got to do with comet impacts? As the solar system moves through denser parts of the galaxy the extra gravitational pull upsets comets orbiting the Sun, sending them hurtling towards Earth. The theory's been tossed around for a while (and there are other good ones, like supervolcanoes, that haven't been discounted yet) but this new evidence makes it seems a little more believable.

It sucks to think that gravity — a pretty immutable force of nature — will be the source of our demise rather than something we can avoid, like global warming or nuclear war. But the study suggests there may be a silver lining to our extinction: comet impacts could be the driving force behind panspermia:

While the "bounce" effect may have been bad news for dinosaurs, it may also have helped life to spread. The scientists suggest the impact may have thrown debris containing micro-organisms out into space and across the universe.

Centre director Professor Chandra Wickramasinghe said: "This is a seminal paper which places the comet-life interaction on a firm basis, and shows a mechanism by which life can be dispersed on a galactic scale."

Source: Cardiff University via Science Blog

Image: The Alien Next Door