the Category Descriptions for the Torino Scale
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The Torino Scale is a "Richter Scale"
for categorizing the Earth impact hazard associated with newly
discovered asteroids and comets. It is intended to serve as
a communication tool for astronomers and the public to assess
the seriousness of predictions of close encounters by asteroids
and comets during the 21st century. [ top
When a new asteroid or comet is discovered,
predictions for where the object will be months or decades
in the future are naturally uncertain. These uncertainties
arise because the discovery observations typically involve
measurements over only a short orbital track and because all
measurements have some limit in their precision.
Fortunately, for the majority of objects,
even the initial calculations are sufficient to show that
they will not make any close passes by the Earth within the
next century. However, for some objects, 21st century close
approaches and possible collisions with the Earth cannot be
completely ruled out. [ top
The Torino Scale utilizes numbers that range
from 0 to 10, where 0 indicates an object has a zero or negligibly
small chance of collision with the Earth. (Zero is also used
to categorize any object that is too small to penetrate the
Earth's atmosphere intact, in the event that a collision does
occur.) A 10 indicates that a collision is certain, and the
impacting object is so large that it is capable of precipitating
a global climatic disaster.
The Torino Scale is color coded from white to
yellow to orange to red. Each color code has an overall meaning:
- White - "No Hazard" meaning they are virtually certain
to miss Earth or are so small that any impact would almost
certainly dissipate in the atmosphere. White corresponds
to category 0.
- Green - "Normal" refers to objects that have predictable close
approaches with some very small, but not seriously concerning,
chance of a collision. Nonetheless, prudence dictates their
orbits should be tracked closely so that the collision chance
becomes refined, and probably in all cases, will ultimately
be reclassified within Torino Scale category zero. Green
corresponds to category 1.
- Yellow - "Meriting attention by astronomers" are close approaches by objects that have higher
collision chances than the Earth typically experiences over
a few decades. These are object for which refinement of
the orbit is of high priority. Yellow corresponds to categories
2, 3, 4.
- Orange - "Threatening"
refers to close encounters with objects that are large enough
to cause regional or global devastation, where the chance
of collision greatly exceeds the level that typically occurs
within a given century. These are objects for which refinement
of the orbits are an extreme priority. Orange corresponds
to categories 5, 6,7.
- Red - "Certain collisions"
refers to objects that are certain to collide with Earth
having sufficient size to likely penetrate the atmosphere
with the capability to cause either local damage, regional
devastation, or a global climatic catastrophe. Red corresponds
to categories 8, 9, 10.
An object is assigned a 0 to 10 value
on the Torino Scale based on its collision probability and
its kinetic energy (proportional to its mass times the square
of its encounter velocity). Categorization on the Torino Scale
is based on the placement of a close approach event within
representation of kinetic energy and
collision probability . An object that is capable of making
multiple close approaches to the Earth will have a separate
Torino Scale value associated with each approach. (An object
may be summarized by the single highest value that it attains
on the Torino Scale.) There are no fractional values or decimal
values used in the Torino Scale. [ top
Yes! It is important to note that the
Torino Scale value for any object initially categorized as
1 or greater _will_ change with time. The change will result
from improved measurements of the object's orbit showing,
most likely in all cases, that the object will indeed miss
the Earth. Thus, the most likely outcome for a newly discovered
object is that it will ultimately be re-assigned to category
0. Any object initially placed in category 0 is unlikely to
have its Torino Scale value change with time. [ top
The Torino Scale was created by Professor Richard
P. Binzel in the Department of Earth, Atmospheric, and Planetary
Sciences, at the Massachusetts Institute of Technology (MIT).
The first version, called "A Near-Earth Object Hazard Index",
was presented at a United Nations conference in 1995 and was
published by Binzel in the subsequent conference proceedings
(Annals of the New York Academy of Sciences, volume 822, 1997.)
A revised version of the "Hazard Index"
was presented at a June 1999 international conference on near-Earth
objects held in Torino (Turin) Italy. The conference participants
voted to adopt the revised version, where the bestowed name
"Torino Scale" recognizes the spirit of international cooperation
displayed at that conference toward research efforts to understand
the hazards posed by near-Earth objects. ("Torino Scale" is
the proper usage, not "Turin Scale.) [ top
For an object making a close approach to Earth,
its categorization on the Torino Scale is dependent upon its
placement within this plot showing kinetic energy versus collision
probability. (One MT = 4.3 x 10^15 J.) The left-hand scale
also indicates approximate sizes for asteroidal objects having
typical encounter velocities. For an object that makes multiple
close approaches over a set of dates, a Torino Scale value
should be determined for each approach. It may be convenient
to summarize such an object by the greatest Torino Scale value
within the set.
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Recommended parameters to be quoted in any public
announcement of a future close approach.
Responsible communication about a potential
close approach should include the following information:
Name of the object:
Size estimate for the object:
Date(s) of close encounter(s):
Collision probability for each close encounter date:
Torino Scale value for each encounter date:
More thorough guidelines and procedures for
public announcement of future close approaches by near-Earth
objects are being developed under the auspices of the International
Astronomical Union (IAU) Working Group on Near-Earth Objects.
Copyright (c) 1999, 2004 Richard P. Binzel, Massachusetts Institute of Technology. Permission is hereby granted to reproduce Torino Scale figures and text for educational and news reporting purposes. [ top
"And Now, the Asteroid Forecast."
vol. 285, p. 655 (1999).
"Scaling the Degree of Danger from an
vol. 400, p. 392 (1999).
"The Torino Scale: Gauging the Impact
& Telescope vol. 98, pp.32-33 (1999).
"Assessing the Hazard: The Development of the
The Planetary Report vol. XIX, pp. 6-10 (1999).
"The Torino Impact Hazard Scale"
Planetary and Space Science vol. 48, pp. 297-303 (2000).