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If a Near Earth Object was found to be on a collision course with the Earth our response will depend on exactly how much of a hazard it represents. To evaluate the risk posed by NEOs two impact scales, the Torino and the Palermo Scales, have been developed so that potential impact risks can be categorised and prioritised.
The Torino scale is designed to communicate to non-specialists the risk associated with a future Earth approach by an asteroid or a comet. The scale rates the risk using an integer number from 0 to 10 where 10 is the most hazardous and it takes into account both the probability that the collision will occur and the amount of damage the impact would cause. This is shown in the graph opposite as impact probability versus impact energy. The amount of energy released by the impact increases with the size of the object but will also vary with its speed.
Each rating on the Torino scale is described below.
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Events Having No Likely Consequences
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0
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The likelihood of a collision is zero, or well below
the chance that a random object of the same size
will strike the Earth within the next few decades.
This designation also applies to any small object
that, in the event of a collision, is unlikely
to reach the Earth's surface intact.
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Events Meriting Careful Monitoring
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1
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The chance of collision is extremely unlikely, about
the same as a random object of the same size
striking the Earth within the next few decades.
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Events Meriting Concern
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2
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A somewhat close, but not unusual encounter.
Collision is very unlikely.
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3
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A close encounter, with 1% or greater chance of a
collision capable of causing localized destruction.
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4
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A close encounter, with 1% or greater chance of a
collision capable of causing regional devastation.
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Threatening Events
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5
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A close encounter, with a significant threat of a
collision capable of causing regional devastation.
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6
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A close encounter, with a significant threat of a
collision capable of causing a global catastrophe.
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7
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A close encounter, with an extremely significant threat
of a collision capable of causing a global catastrophe.
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Certain Collisions
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8
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A collision capable of causing localized destruction.
Such events occur somewhere on Earth between
once per 50 years and once per 1000 years.
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9
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A collision capable of causing regional devastation.
Such events occur between once per 1000 years
and once per 100,000 years.
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10
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A collision capable of causing a global climatic
catastrophe. Such events occur once per
100,000 years, or less often.
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The Palermo Technical Impact Hazard Scale was developed to enable NEO scientists to evaluate the hazards represented by potential impacts. Unlike the Torino scale, in which the hazard is expressed as an integer number, the Palermo scale has a continuous series of numbers from -2 upwards.
The scale compares the likelihood of the detected potential impact with the average risk posed by objects of the same size or larger over the years until the date of the potential impact. This average risk from random impacts is known as the background risk. For convenience the scale is logarithmic, so, for examples, a Palermo Scale value of -2 indicates that the detected potential impact event is only 1% as likely as a random background event occurring in the intervening years, a value of zero indicates that the single event is just as threatening as the background hazard, and a value of +2 indicates an event that is 100 times more likely than a background impact by an object at least as large before the date of the potential impact in question.
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