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The Palermo Technical Impact Hazard Scale is described in the
following article:
Quantifying the risk posed by potential Earth impacts
Steven R. Chesley (JPL), Paul W. Chodas (JPL), Andrea Milani
(Univ. Pisa), Giovanni B. Valsecchi (IASF-CNR) and Donald K. Yeomans
(JPL)
Icarus 159, 423-432 (2002)
ABSTRACT
Predictions of future potential Earth impacts by Near-Earth Objects
(NEOs) have become commonplace in recent years, and the rate of these
detections is likely to accelerate as asteroid survey efforts continue
to mature. In order to conveniently compare and categorize the
numerous potential impact solutions being discovered we propose a new
hazard scale that will describe the risk posed by a particular
potential impact in both absolute and relative terms. To this end we
measure each event in two ways, first without any consideration of the
event's time proximity or its significance relative to the so-called
background threat, and then in the context of the expected risk from
other objects over the intervening years until the impact. This
approach is designed principally to facilitate communication among
astronomers, and it is not intended for public communication of impact
risks. The scale characterizes impacts across all impact energies,
probabilities and dates, and it is useful, in particular, when dealing
with those cases which fall below the threshold of public interest.
The scale also reflects the urgency of the situation in a natural way,
and thus can guide specialists in assessing the computational and
observational effort appropriate for a given situation. In this paper
we describe the metrics introduced, and we give numerous examples of
their application. This enables us to establish in rough terms the
levels at which events become interesting to various parties.
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