Recent Nearcollisions Of Asteroids With Earth

Collisions between asteroids can alter their orbits and cause them to head into an Earth orbit-crossing path. At this point, the asteroid becomes hazardous to life on Earth and is known as an Apollo object. NASA and the United States Air Force estimate that approximately 20,000 objects in space could be on Earth orbit-crossing trajectories. Presently, about 150 Apollo asteroids with diameters of greater than half a mile (1 km) are known, and a couple of thousand objects this size are known in the entire near-Earth object group. Objects larger than 460 feet (140 m) hit the Earth on average about once every 5,000 years. In 1996 an asteroid about one quarter mile (half km) across nearly missed hitting the Earth, speeding past at a distance about equal to the distance to the Moon. The sobering reality of this near collision is that the asteroid was not even spotted until a few days before it sped past Earth. If the object had been bigger or slightly closer, it might not have been stoppable, and its collision might have had major consequences for life on Earth. A similar near miss event was recorded again in 2001; Asteroid 2001 YB5 passed Earth at a distance of twice that to the Moon, and it too was not recognized until two weeks before its near miss. If YB5 hit Earth, it would have released energy equivalent to 350,000 times the energy released during the nuclear bomb blast in Hiroshima.

The objects that are in Earth orbit-crossing paths could not have been in this path for very long, because gravitational influences of the Earth, Mars, and Venus would cause them to hit one of the planets or be ejected from the solar system within about 100 million years. The abundance of asteroids in an Earth orbit-crossing path demonstrates that ongoing collisions in the asteroid belt are replenishing the source of potential impacts on Earth. A few rare meteorites found on Earth have chemical signatures that suggest they originated on Mars and on the Moon, probably being ejected toward the Earth from giant impacts on those bodies.

Other objects from space may collide with Earth. Comets are masses of ice and carbonaceous material mixed with silicate minerals that are thought to originate in the outer parts of the solar system, in a region called the Oort Cloud. Other comets have a closer origin, in the Kuiper Belt just beyond the orbit of Neptune, including the dwarf planet Pluto. Comets may be less common near Earth than mete-oroids, but they still may hit the Earth with severe consequences. Astronomers estimate that there are more than a trillion comets in the solar system. Since they are lighter than asteroids and have water-rich and carbon-rich compositions, many scientists have speculated that cometary impact may have brought water, major components of the atmosphere, and even life to Earth. The relative risks of impact for different objects are described in the table "Torino Hazard Scale for Near-Earth Objects."

The importance of monitoring near-Earth objects is highlighted by a number of recent events where asteroids or comets nearly collided with Earth, or exploded in the planet's atmosphere. Several "Tun-guska style" atmospheric explosions, where meteorites exploded in the atmosphere and formed air blasts have been noted, including the events in 1930

TORINO HAZARD SCALE FOR NEAR-EARTH OBJECTS

Scale

Description of Hazard

No Hazard

0

The likelihood of a collision is zero, or is so low as to be effectively zero. Also applies to small objects such as meteors and bodies that burn up in the atmosphere as well as infrequent meteorite falls that rarely cause damage.

Normal

1

A routine discovery in which a pass near the Earth is predicted but poses no unusual level of danger. Current calculations show the chance of collision is extremely unlikely with no cause for public attention or public concern. New telescopic observations very likely will lead to reassignment to level 0.

Meriting Attention by Astronomers

2

A discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to reassignment to level 0.

3

A close encounter, meriting attention by astronomers. Current calculations give a 1 percent or greater chance of collision capable of localized destruction. Most likely, new telescopic observations will lead to reassignment to level 0. Attention by public and by public of cials is merited if the encounter is less than a decade away.

4

A close encounter, meriting attention by astronomers. Current calculations give a 1 percent or greater chance of collision capable of regional devastation. Most likely, new telescopic observations will lead to reassignment to Level 0. Attention by public and by public of cials is merited if the encounter is less than a decade away.

Threatening

5

A close encounter posing a serious but still uncertain threat of regional devastation. Critical attention by astronomers is needed to determine conclusively whether a collision will occur. If the encounter is less than a decade away, governmental contingency planning may be warranted.

6

A close encounter by a large object posing a serious but still uncertain threat of a global catastrophe. Critical attention by astronomers is needed to determine conclusively whether a collision will occur. If the encounter is less than three decades away, governmental contingency planning may be warranted.

7

A very close encounter by a large object, which if occurring this century, poses an unprecedented but still uncertain threat of a global catastrophe. For such a threat in this century, international contingency planning is warranted, especially to determine urgently and conclusively whether a collision will occur.

Certain Collisions

8

A collision is certain, capable of causing localized destruction for an impact over land or possibly a tsunami if close offshore. Such events occur on average between once per 50 years and once per several 1,000 years.

9

A collision is certain, capable of causing unprecedented regional devastation for a land impact or the threat of a major tsunami for an ocean impact. Such events occur on average between once per 10,000 years and once per 100,000 years.

10

A collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often.

over the Amazon River, in 1965 over southeastern Canada, in 1965 over Lake Huron, in 1967 in Alberta, Canada, in Russia in 1992, in Italy in 1993, in Spain in 1994, in Russia and the Mediterranean in 2002, and in Washington in 2004. Other asteroid encounters luckily were "near-misses," where larger asteroids narrowly escaped collision with Earth. In 1972, an asteroid estimated to be 6-30 feet (2-10 m) in diameter entered the Earth's atmosphere above Salt Lake City, Utah, formed a huge fireball that raced across the daytime sky, and exited the atmosphere near Calgary in Alberta, Canada. The geometry of the orbit was such that the meteorite just grazed the outer parts of the atmosphere, getting as close to the surface as 36 miles (58 km). In 1989 the 1,000-foot (300-m) diameter Apollo asteroid 4581 Asclepius crossed the exact place the Earth had just passed through six hours earlier, missing the planet by a mere 400,000 miles (700,000 km). If an object that size had collided with Earth, the results would have been catastrophic. On June 14, 2002, a 165-400 foot (50-120 m) diameter asteroid named 2002 MN passed unnoticed at a distance of 75,000 miles (120,700 km), one-third the distance to the moon. Remarkably, this asteroid was not recognized until three days after it passed that closely to the Earth. On July 3, 2006, another asteroid, named 2004 XP14, passed at about 248,000 miles (400,000 km), moving at a velocity of 10.5 miles per second (17 km/sec).

Several asteroids are known to be on near-collision courses with Earth. These include 99942 Apo-phis, which will pass within 20,000 miles (32,000 km) of Earth but will miss the planet. However, it may come closer in 2036, with a possible impact on that orbit. The chances of impact are estimated to be one in 43,000, making 99942 Apophis a Level O danger on the Torino impact hazard scale. On March 16, 2880, asteroid 29075, with a diameter of 0.7-0.9 miles (1.1-1.4 km) will pass close to Earth. Some models suggest that this asteroid has a one in 300 chance of hitting the planet, posing a significant threat for a catastrophic collision, with major changes to climate and possibly triggering mass extinctions. This asteroid has the highest probability of any known large objects of hitting Earth.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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