Could Earth Be Hit, Like Jupiter Just Was?

From: http://news.yahoo.com/s/space/20090728/sc_space/couldearthbehitlikejupiterjustwas

AP – This image provided by NASA's Hubble Space Telescope taken with it's Wide Field Camera 3 on Thursday …

Could Earth Be Hit, Like Jupiter Just Was?

Charles Q. Choi

Special to SPACE.com

SPACE.com

Tue Jul 28, 10:45 am ET

The recent bruising Jupiter received from a cosmic impact is a violent reminder that our solar system is a shooting gallery that sometimes blasts Earth.

Still, what are the odds of a cosmic impact threatening our planet?

So far 784 near-Earth objects (NEOs) more than a half-mile wide (1 km) have been found.

"If an object of about the same size that just hit Jupiter also hit Earth — it was probably a typical cometary object of a kilometer or so in size (0.6 miles) — it would have been fairly catastrophic," explained astronomer Donald Yeomans, manager of NASA's Near-Earth Object program office at the Jet Propulsion Laboratory in Pasadena, Calif.

Scientists have ruled out the chances of an Earth impact for all of these 784 large NEOs. Still, lesser objects also pose a risk, and researchers estimate more than 100 large NEOS remain to be found.

Small risk

Billions of years ago, impacts were far more common. Our moon retains a record of the pummeling it and Earth took: the moon's craters remain, while on Earth, most scars of ancient impacts have been folded back into the planet or weathered away.

Today's solar system is far less crowded, and in fact Jupiter, having more mass and gravity, scoops up a lot of the dangerous objects, as does the sun.

Currently just one NEO of all the objects scientists are tracking poses any significant chance of hitting the Earth — 2007 VK184. If this roughly 425-foot-wide (130 meters) asteroid hit our planet, it would strike with an energy of roughly 150 million tons of TNT, or more than 10,000 times that of the atom bomb dropped on Hiroshima.

Roughly 100 telescopic observations made so far suggest that 2007 VK184 has a 1-in-2,940 chance of hitting Earth 40 to 50 years from now. However, if the past is any guide, further observations to refine computations of its orbit very likely will downgrade its probability of hitting Earth to virtually nothing, Yeomans said.

Of remaining concern are the NEOs that we do not see. Researchers suspect about 156 large NEOs 1 kilometer in diameter or larger remain to be found, and when it comes to dangerous NEOs in general, "when we get down to 140 meters (460 feet) or larger diameter objects, we think we've discovered about 15 percent of them, and with 50 meters (164 feet) or larger diameter, we've discovered less than 5 percent of them," Yeomans explained.

On average, an NEO roughly a half-mile wide or larger hits the Earth roughly every 500,000 years, "so we're not expecting one anytime soon," Yeomans explained.

"For 500 meters (1,640 feet), we're talking a mean interval of about 100,000 years," he added. "When you get down to 50 meters, the mean interval is about 700 years, and for 30 meters (98 feet), about 140 years or so, but by then you're getting down to a size where you won't expect any ground damage, as they burn up in the atmosphere at about 25 meters (82 feet) in diameter and smaller, probably for an impressive fireball event."

When it comes to truly monstrous NEOs some 10 kilometers (6.2 miles) or larger, of the size thought to have helped kill off the dinosaurs, "that's a 100 million year event, and in fact, I don't think there is anything like that we see right now," Yeomans said. "The largest near-Earth object that can actually cross the Earth's path, Sisyphus, has a diameter of 8 kilometers (5 miles), and the largest that is termed a potential hazard is Toutatis, which has a diameter of approximately 5.4 km (3.35 miles)."

Keeping watch

There are currently four teams worldwide actively looking for both large and small NEOs, Yeomans said. "We're concentrating on the large ones for now, but hopefully with the next generation of search, we'll be more efficient in finding the smaller objects, to find 90 percent of the total population of potential hazards larger than 140 meters," he added.

Keeping an eye on NEOs might not just be healthy for humanity, but also help lead us out into space.

"They're easy objectives to get to, and asteroids have significant metal resources that can be mined, while comets have significant water resources for space habitats or travel," Yeomans said. "If you want to build a habitat in space, you're not going to build it all on the ground and launch it up, since that's too expensive — you want to go up and look for resources instead."

Furthermore, asteroids and comets are among the objects that have changed the least since the birth of the solar system roughly 4.6 billion years ago, and might reveal vital clues behind the mysterious process.

"They may well have delivered the water and carbon-based molecules to Earth that allowed life to form, so they're extremely important for study in that direction," Yeomans added.

Note from Blogger: The IAA is expected to release its White Paper on Planetary Defense from the 1st International Planetary Defense Conference any day now. The conference was held in April.

Jupiter deserves a medal for doing its job of sucking up dangerous rocks

From:http://www.space.com/scienceastronomy/090721-jupiter-impact.html

The dark bruise that appeared suddenly near the south pole of Jupiter several days ago, likely as the result of an impact by a comet or asteroid, is as big as the Pacific Ocean, astronomers report.

The dark spot was first noticed by chance by amateur astronomer Anthony Wesley in Australia on Sunday, July 19.

The blemish is thought to be the result of an impact similar to that ofComet Shoemaker-Levy 9, which pummeled the gas giant 15 years ago.

After he was convinced the spot was not just another storm or the shadow of one of Jupiter's moons, Wesley alerted other astronomers around the world to the scar's appearance.

University of California, Berkeley, astronomer Paul Kalas took advantage of previously scheduled observing time on the Keck II telescope in Hawaii to image the blemish in the early morning hours of Monday, July 20. The near-infrared image showed a bright spot in the clouds of Jupiter's southern hemisphere, where the impact had propelled reflective particles high into the relatively clear stratosphere.

In visible light, the bruise appears dark against the bright surface of Jupiter.

These observations mark only the second time that astronomers have been able to see the results of an impact on the planet, the first being Comet Shoemaker-Levy 9's collision. Many theories were formed after that collision.

"Now we have a chance to test these ideas on a brand new impact event," Kalas, said.

Kalas and his colleagues hope their observations will shed light on the nature of the impact.

"The analysis of the shape and brightness of the feature will help in determining the energy and the origin of the impactor," said Marchis. "We don't see other bright features along the same latitude, so this was most likely the result of a single asteroid, not a chain of fragments like for SL9 [Comet Shoemaker-Levy 9]."

http://pajamasmedia.com/richardfernandez/

An Australian amateur astronomer detected the first indications that a comet had slammed into Jupiter only fifteen years after the Shoemaker-Levy impacts. It left an impact signature on the giant planet the size of earth. William Harwood writes:

“We’re not sure how large this fragment could have been,” Leigh Fletcher, a researcher at the Jet Propulsion Laboratory in Pasadena, Calif., told CNET. …”We’ve compared it in size to the little red spot on Jupiter, you can see the two are roughly a similar order of magnitude in size,” he said. “It’s smaller than Earth-sized, but it’s certainly gargantuan by earthly proportions.”

Over the next few days and weeks, the Hubble Space Telescope, other space-based telescopes, and observatories around the world will focus on the impact site to learn as much as possible about what might have hit Jupiter and how the impact affected the planet’s atmosphere.

This is the second large hit on Jupiter we’ve observed in fifteen years. Does it tell us anything about the likelihood of an impact on earth? The answer is we don’t really know because we don’t have the detection systems in place to gather the data. Glenn Reynolds, writing in TCS notes that politicians have devoted a relative pittance to watching the skies. And even if an incoming object were detected, there are no established protocols to respond to the event, nor any realistic defenses against a massive incoming. This is probably a function of politics more than anything else, I think. Its deemed essential to spend trillions to prevent “climate change” in the name of the precautionary principle, but the ranging shots of the artillerist of the Solar System are left to amateur astronomers from Australia to observe.

But the most interesting issue that Glenn Reynolds raises in his TCS article is that, without a planetary defense and with an inadequate detection system, then humanity will at most have a short time between the detection of a life-ending incoming space projectile and impact. In that case, he asks, would we want to know? Should governments keep it secret from us?

Quite some time ago, I wrote about scientists’ questions on whether to deliver bad news. The news in question had to do with a potential life-ending asteroid strike. Perhaps, I suggested, it might be best not to deliver that news, if things were bad enough that nothing could be done. I also noted that this was an active question within the astronomical community.

Personally, I would want to know if the end were imminent. In the case Reynolds cites, the panic spawned by a sighting was premature. Further observations by astronomers showed the feared object was going to miss. Somebody should do a disaster movie like that one day, when everybody closes his eyes at the anticipated moment, only to look up to see, blazoned in flaming letters across the sky, “Nyah, nyah! Fooled you! Fooled you!”

Holst looked up at Jupiter and heard laughter. Here’s’s his Jupiter, Bringer of Jollity, from The Planets. On winter nights, after the last bus down Trapelo road had gone, I used to run the distance from Harvard Square to Belmont with a Walkman in my pocket. And the everything after 2:38 was my favorite on the track.

Another Wake-up call that will go unheeded?

From: http://www.jpl.nasa.gov/news/news.cfm?release=2009-112

July 20, 2009

Scientists have found evidence that another object has bombarded Jupiter, exactly 15 years after the first impacts by the comet Shoemaker-Levy 9.

Following up on a tip by an amateur astronomer, Anthony Wesley of Australia, that a new dark "scar" had suddenly appeared on Jupiter, this morning between 3 and 9 a.m. PDT (6 a.m. and noon EDT) scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., using NASA's Infrared Telescope Facility at the summit of Mauna Kea, Hawaii, gathered evidence indicating an impact.

New infrared images show the likely impact point was near the south polar region, with a visibly dark "scar" and bright upwelling particles in the upper atmosphere detected in near-infrared wavelengths, and a warming of the upper troposphere with possible extra emission from ammonia gas detected at mid-infrared wavelengths.

"We were extremely lucky to be seeing Jupiter at exactly the right time, the right hour, the right side of Jupiter to witness the event. We couldn't have planned it better," said Glenn Orton, a scientist at JPL.

Orton and his team of astronomers kicked into gear early in the morning and haven't stopped tracking the planet. They are downloading data now and are working to get additional observing time on this and other telescopes.

This image was taken at 1.65 microns, a wavelength sensitive to sunlight reflected from high in Jupiter's atmosphere, and it shows both the bright center of the scar (bottom left) and the debris to its northwest (upper left).

"It could be the impact of a comet, but we don't know for sure yet," said Orton. "It's been a whirlwind of a day, and this on the anniversary of the Shoemaker-Levy 9 and Apollo anniversaries is amazing."

Shoemaker-Levy 9 was a comet that had been seen to break into many pieces before the pieces hit Jupiter in 1994.

Leigh Fletcher, a NASA postdoctoral fellow at JPL who worked with Orton during these latest observations said, "Given the rarity of these events, it's extremely exciting to be involved in these observations. These are the most exciting observations I've seen in my five years of observing the outer planets!"

The observations were made possible in large measure by the extraordinary efforts of the Infrared Telescope Facility staff, including telescope operator William Golisch, who adroitly moved three instruments in and out of the field during the short time the scar was visible on the planet, providing the wide wavelength coverage.

JPL is managed for NASA by the California Institute of Technology in Pasadena.