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Peter Brown of the University of Western Ontario looked at the data of eight years and found 300 in-air explosions of space rocks ranging in size from large television sets to studio apartments. But his analysis also revised downwards the statistical probability of Tunguska-size impact from once every hundred years to once every thousand years. He found one-hundred years to be the likely interval between impacts one-tenth the Tunguska-size, or of one megatonne since - six times the force of Hiroshima.
Considering the last one to happen in 1908, statistically speaking, we may be due for a collision any time now. An asteroid named 'Apophis' caused a brief period of concern in December 2004, because initial observations indicated a small probability of up to 2.7% that it would strike the Earth in 2029. In 2005, scientists calculated that Apophis had a one in 5,500 chance of colliding with Earth. As of October 7 this year, the impact probability has decreased considerably to a 1 in 250,000.

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Risk Management Solutions, a catastrophe-modelling firm, is certainly thinking outside the atmosphere. In a 17-page report, it estimates a repeat of the 1908 Tunguska event -- when a comet or asteroid exploded above Siberia -- could cause 3.2 million deaths and $1.2 trillion in damages if it occurred over New York.

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What Global Hotspots are Most at Risk Of Getting Hit by an Asteroid?
"The threat of the Earth being hit by an asteroid is increasingly being accepted as the single greatest natural disaster hazard faced by humanity," according to Nick Bailey of the University of Southampton's School of Engineering Sciences team, who developed the identifying program.
The team used raw data from multiple impact simulations to rank each country based on the number of times and how severely they would be affected by each impact. The software, called NEOimpactor (from NASA's "NEO" or Near Earth Object program), has been specifically developed for measuring the impact of 'small' asteroids under one kilometre in diameter.

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Comets: Cosmic Life Preservers
Comets have been blamed for a lot of death and destruction over the years. Showers of them are thought to have triggered at least one and maybe even several of the major and minor mass-extinction events that have wiped out millions of species in the 4 billion years since life on earth first began.
But new research from the University of Washington defends comets, those gassy balls of ice and dust that swing by earth every once in a while to show off their tails. Doctoral student Nathan Kaib ran some data that suggests that the number of comets available to rain down on earth from time to time has been hugely overestimated.


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Nasa is charged with spotting most of the asteroids that pose a threat to Earth, but doesn't have enough money to complete the task, according to a new report.
The space agency was charged with finding 90 per cent of the potentially deadly rocks
hurtling through space by 2020.


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Earth could be blindsided by asteroids, panel warns
Existing sky surveys miss many asteroids smaller than 1 kilometre across, leaving the door open to damaging impacts on Earth with little or no warning, a panel of scientists reports. Doing better will require devoting more powerful telescopes to asteroid hunting, but no one has committed the funds needed to do so, it says.

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Asteroid Watch
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NASA's Jet Propulsion Laboratory is introducing a new Web site that will provide a centralised resource for information on near-Earth objects - those asteroids and comets that can approach Earth. The "Asteroid Watch" site also contains links for the interested public to sign up for NASA's new asteroid widget and Twitter account.

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Near-Earth asteroids
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Title: Asymmetric impacts of near-Earth asteroids on the Moon
Authors: Takashi Ito, Renu Malhotra

Recent lunar crater studies have revealed an asymmetric distribution of rayed craters on the lunar surface. The asymmetry is related to the synchronous rotation of the Moon: there is a higher density of rayed craters on the leading hemisphere compared with the trailing hemisphere. Rayed craters represent generally the youngest impacts. The purpose of this paper is to test the hypotheses that (i) the population of Near-Earth asteroids (NEAs) is the source of the impactors that have made the rayed craters, and (ii) that impacts by this projectile population account quantitatively for the observed asymmetry. We carried out numerical simulations of the orbital evolution of a large number of test particles representing NEAs in order to determine directly their impact flux on the Moon. The simulations were done in two stages. In the first stage we obtained encounter statistics of NEAs on the Earth's activity sphere. In the second stage we calculated the direct impact flux of the encountering particles on the surface of the Moon; the latter calculations were confined within the activity sphere of the Earth. To represent NEAs' initial conditions, we considered two populations: one is the currently known NEAs, and the other is a synthetic population created by debiasing the orbital distribution of the known NEAs. We find that the near-Earth asteroids do have an asymmetry in their impact flux on the Moon: apex-to-antapex ratio of 1.3-1.4. However, the observed rayed crater distribution's asymmetry is significantly more pronounced: apex-to-antapex ratio of ~1.67. Our simulations suggest the existence of an undetected population of slower (low impact velocity) projectiles, such as a population of objects coorbiting with Earth.

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Scientists say it is quite unlikely and in fact, you can sleep well as the article reveals that "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."
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A few years ago, a book titled "The Black Swan" came out. No, it's not about swans that get singled out by the Cambridge Police Department for breaking into their own roosts, but about sudden, unpredictable events occurring far more often than we'd like to think. There are flocks of black swans out there.
In 2008, science writer Gregg Easterbrook surveyed preparedness for a "space-object strike" for the Atlantic magazine. He found that even though serious experts believe there's as much as a 1-in-10 chance of a significant Earth-strike within the next century, NASA doesn't much care.

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