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FREQUENTLY ASKED QUESTIONS FOR IMPACT RISK ASSESSMENT
How is an orbit calculated?
An asteroid's orbit is computed by finding the
elliptical path about the sun that best fits the available
observations of the object. That is, the object's computed path about
the sun is adjusted until the predictions of where the asteroid should
have appeared in the sky at several observed times match the positions
where the object was actually observed to be at those same times. As
more and more observations are used to further improve an object's
orbit, we become more and more confident in our knowledge of where the
object will be in the future.
How far into the future does Sentry search for impacts?
100 years.
Why do your first calculations of an orbit often look more
threatening than later ones?
Because orbits stemming from very limited observation sets
are more uncertain it is more likely that such orbits will "permit"
future impacts. However, such early predictions can often be ruled out
as we incorporate more observations and reduce the uncertainties in
the object's orbit. Most often, the threat associated with a specific
object will decrease as additional observations become available, and
so objects will be posted to, and later removed from, our Impact
Risk Page. The Palermo Scale values will typically start out at less
negative values when the object's orbit is most uncertain and evolve
to more negative values (and eventually off the list) as more and more
observations allow the object's orbit to be continually
improved.
On the other hand, in the unlikely case where a particular
potential impact event persists until the orbit is relatively well
constrained, the impact probability and associated risk will tend to
increase as observations are added. This is not too paradoxical: If an
asteroid is indeed going to come very near the Earth then a collision
cannot be ruled out early on. The impact probability will tend to grow
as the orbit is refined and alternative and safer trajectories are
eliminated. Eventually, the impact probability will drop (usually
quite abruptly) to zero or, if the asteroid is really on a collision
trajectory, it will continue to grow until it reaches
100%.
How soon after the discovery is a search for potential
collisions initiated?
When the discovery of a new NEA is announced by the
Minor Planet Center (MPC), Sentry automatically (usually within an
hour or two) prioritizes the object for an impact risk analysis. If
the prioritization analysis indicates that the asteroid cannot pass
near the Earth or that its orbit is very well determined then the
computationally intensive nonlinear search for potential impacts is
not pursued. If, on the other hand, a search is deemed necessary then
the object is added to a queue of objects awaiting analysis. Its
position in the queue is determined by the estimated likelihood that
potential impacts may be found.
How often do these results change?
NEA orbits and close approach tables are continuously
and automatically updated whenever new observations are made
available, generally within a couple of hours of the release of the
information. Whenever an NEA orbit is updated the object is re-prioritized
and, if appropriate, it is re-queued for a new potential impact
search. This process is ongoing - taking place anytime, day and night,
seven days a week.
Why isn't 1950 DA listed on the Risk Page?
1950 DA is an asteroid for which there is some
possibility of impact in March of the year 2880. The case is
extraordinary because the current orbit of 1950 DA is very precisely
known, which allows us to explore centuries into the future, much
farther than is usually possible. The Sentry automatic monitoring
system is tailored for objects with poorly determined orbits and it
searches for potential impacts only over the next 100 years. More information on 1950
DA.
Why are the results published by NEODyS not the same as those published by
Sentry?
The differences between the two systems are generally
not substantial, and in some sense they are reassuring. Independent
systems using different software and theoretical approaches are not
expected to produce the same results from statistical
searches. Experience has shown that there is excellent agreement
between the two systems for the more serious potential collision
detections.
One of the differences between the two systems stems from different
approaches to computing the impact probability. This computation is
rough by its very nature, and different techniques may be used; impact
probabilities different by a factor of ten or so are not
extraordinary.
Another important variation is that Sentry uses a different
sampling strategy, one that should detect nearly all potential impacts
with probability greater than 10-8 (1 in 100 million), and
does not expend much effort pursuing less likely cases, although it
may find some anyway. In any case, nothing with impact probability
below 10-10 (1 in 10 billion) is published by Sentry. In
contrast, NEODyS may not detect as many potential impacts at
probabilities below 10-6 (1 in 1 million), but in certain
cases it can detect very low probability events that Sentry does
not.
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