Palermo Technical Impact Hazard Scale
Encyclopedia
The Palermo Technical Impact Hazard Scale is a logarithmic scale
used by astronomer
s to rate the potential hazard of impact of a near-earth object
(NEO). It combines two types of data
—probability
of impact, and estimated kinetic
yield—into a single "hazard" value. A rating of 0 means the hazard is as likely as the background hazard (defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact). A rating of +2 would indicate the hazard is 100 times more likely than a random background event. Scale values less than −2 reflect events for which there are no likely consequences, while Palermo Scale values between −2 and 0 indicate situations that merit careful monitoring. A similar but less complex scale is the Torino Scale
, which is used for simpler descriptions in the non-scientific media.
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. The Palermo Scale value, P, is defined by the equation:
where
The annual background impact frequency is defined for this purpose as:
where the energy threshold E is measured in megatons
.
The near-Earth object was the first near-Earth object detected by NASA
's latest NEO program to be given a positive rating on the scale of 0.06, indicating a higher than background threat. The value was subsequently lowered after more measurements were taken and 2002 NT7 is no longer considered to pose any risk, and was removed from the Sentry Risk Table
on 1 August 2002.
For a brief period in late December 2004, asteroid 99942 Apophis
(then known only by its provisional designation ) held the record for Palermo scale values, with a value of 1.10 for a possible collision in the year 2029. The 1.10 value indicated that a collision with this object was considered to be almost 12.6 times more likely than a random background event: 1 in 37 instead of 1 in 472. With further observations, the possibility of a 2029 impact was eliminated, but as of August 2011 a maximum Palermo rating of −3.08 applies, due to a possible event in 2036.
Since September 2002, the highest Palermo rating maintained has been that of asteroid (29075) 1950 DA
, with a value of 0.17 for a possible collision in the year 2880. 1950 DA is the only known asteroid whose hazard could be above the background level.
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...
used by astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
s to rate the potential hazard of impact of a near-earth object
Near-Earth object
A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to...
(NEO). It combines two types of data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
—probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
of impact, and estimated kinetic
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
yield—into a single "hazard" value. A rating of 0 means the hazard is as likely as the background hazard (defined as the average risk posed by objects of the same size or larger over the years until the date of the potential impact). A rating of +2 would indicate the hazard is 100 times more likely than a random background event. Scale values less than −2 reflect events for which there are no likely consequences, while Palermo Scale values between −2 and 0 indicate situations that merit careful monitoring. A similar but less complex scale is the Torino Scale
Torino Scale
The Torino Scale is a method for categorizing the impact hazard associated with near-Earth objects such as asteroids and comets.It is intended as a communication tool for astronomers and the public to assess the seriousness of collision predictions, by combining probability statistics and known...
, which is used for simpler descriptions in the non-scientific media.
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. The Palermo Scale value, P, is defined by the equation:
where
- pi is the impact probability
- T is the time interval over which pi is considered (measured in years)
- fB is the annual background impact frequency
The annual background impact frequency is defined for this purpose as:
where the energy threshold E is measured in megatons
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...
.
The near-Earth object was the first near-Earth object detected by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's latest NEO program to be given a positive rating on the scale of 0.06, indicating a higher than background threat. The value was subsequently lowered after more measurements were taken and 2002 NT7 is no longer considered to pose any risk, and was removed from the Sentry Risk Table
Sentry (monitoring system)
Sentry is a highly automated collision monitoring system that continually scans the most current asteroid catalog for possibilities of future impact with Earth over the next 100 years...
on 1 August 2002.
For a brief period in late December 2004, asteroid 99942 Apophis
99942 Apophis
99942 Apophis is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a small probability that it would strike the Earth in 2029. Additional observations provided improved predictions that eliminated the possibility of an impact on...
(then known only by its provisional designation ) held the record for Palermo scale values, with a value of 1.10 for a possible collision in the year 2029. The 1.10 value indicated that a collision with this object was considered to be almost 12.6 times more likely than a random background event: 1 in 37 instead of 1 in 472. With further observations, the possibility of a 2029 impact was eliminated, but as of August 2011 a maximum Palermo rating of −3.08 applies, due to a possible event in 2036.
Since September 2002, the highest Palermo rating maintained has been that of asteroid (29075) 1950 DA
(29075) 1950 DA
-External links:* from JPL /...
, with a value of 0.17 for a possible collision in the year 2880. 1950 DA is the only known asteroid whose hazard could be above the background level.