NEO News (07/25/02) Confusion on 2002 NT7
Dear Friends and Students of NEOs:
As you probably all know, we are in the midst of an orgy of
misinformation and confusion in the press, concerning asteroid 2002
NT7.
There have been “false alarm” stories in the past about threatening
asteroids, some originating in poorly informed or misguided
statements made by astronomers. The general pattern has been that a
warning is issued and a day or two later retracted, reflecting either
improved calculations or new data or both. It is entirely normal that
a very low probability prediction of impact will go to zero as more
information is processed. This is not a “failure” of the system, but
rather the normal working of the Spaceguard Survey and supporting
dynamical calculations. However, the press sometimes portrays this as
a “mistake” by astronomers. Consequently, most of us prefer to see no
press coverage of such low-probability predictions. It was in this
spirit that no formal announcement was made concerning 2002 NT7,
since new observations are accumulating and the whole situation is
likely to resolve itself within a few days.
Unfortunately, the media themselves seem to have created the current
flood of publicity surrounding NT7. Initial statements from the
British press stated that 2002 NT7 was on a collision course with
Earth with the impact predicted for February 1, 2019. The only
qualification was that the prediction of an impact was still somewhat
uncertain. There was no hint of the true situation, in which the
probability of impact was at the 1-in-a-million level. As noted by
Don Yeomans and others, the position of 2002 NT7 on February 1, 2019,
is actually uncertain by many millions of kilometers!
The situation has been made more complex by press references to the
Palermo technical scale for classifying an asteroid risk. Astronomers
use this Palermo scale in communicating among themselves, but several
years ago they agreed to use the simpler Torino risk scale in talking
to the press and public. The Torino scale was invented to facilitate
such communication. As recently noted by Rick Binzel: The largest
rationale for the Torino Scale is that we all have a common lexicon
for communicating with the public. If each and every one of us would
say: “That object is only zero (or one) on the Torino scale, meaning
we are carefully monitoring it, with no cause for public concern.”
eventually the responsible press and responsible people would learn
to categorize these “events” as being the same as the last ones that
“just went away.”
Below are two items: (1) a statement that I am sending to people who
e-mail me expressing their concern that this asteroid will hit in
2019, and (2) a selection of the distorted initial press coverage in
the UK taken from Benny Peiser’s CCNet.
David Morrison
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(1) NOTE TO CONCERNED CITIZENS
This newly discovered asteroid (2002 NT7) is very unlikely to hit the
Earth. The current odds are a million to one against hitting. In any
case, astronomers are continuing to measure its position, and within
a few days we should know for sure what the circumstances are for
2019. Meanwhile, the story has been blown out of proportion.
Statements from the press that the asteroid is on a collision course
with Earth are simply false.
The probability of impact is so small that this asteroid remains at a
risk level on the Torino scale of 0 or 1 – meaning that the chances
of impact from another unknown NEA of the same size or larger is
similar to the chances of being hit by 2002 NT7. However, the impact
probability is not yet zero, and additional observations are needed
to ensure that this object will not hit the Earth in 17 years.
Although 2002 NT7 is unlikely to pose any danger, the long-term risk
of asteroid collision is real.
The Earth orbits the Sun in a sort of cosmic shooting gallery,
subject to impacts from comets and asteroids. It is only fairly
recently that we have come to appreciate that these impacts by
asteroids and comets (often called Near Earth Objects, or NEOs) pose
a significant hazard to life and property. Although the annual
probability of the Earth being struck by a large asteroid or comet is
extremely small, the consequences of such a collision are so
catastrophic that it is prudent to assess the nature of the threat
and prepare to deal with it.
Studies have shown that the risk from cosmic impact increases with
the size of the projectile. The greatest risk is associated with
objects large enough to perturb the Earth’s climate on a global scale
by injecting large quantities of dust into the stratosphere. Such an
event could depress temperatures around the globe, leading to massive
loss of food crops and possible breakdown of society. Such global
catastrophes are qualitatively different from other more common
hazards that we face (excepting nuclear war), because of their
potential effect on the entire planet and its population. Various
studies have suggested that the minimum mass impacting body to
produce such global consequences is several tens of billions of tons,
resulting in a groundburst explosion with energy in the vicinity of a
million megatons of TNT. The corresponding threshold diameter for
NEOs is between 1 and 2 km. Smaller objects (down to tens of meters
diameter) can cause severe local damage but pose no global threat.
We don’t know when the next NEO impact will take place, but we can
calculate the odds. Statistically, the greatest danger is from an NEO
with about 1 million megatons energy (roughly 2 km in diameter). On
average, one of these collides with the Earth once or twice per
million years, producing a global catastrophe that would kill a
substantial (but unknown) fraction of the Earth’s human population.
Reduced to personal terms, this means that you have about one chance
in 20,000 of dying as a result of a collision. Such statistics are
interesting, but they don’t tell you, of course, when the next
catastrophic impact will take place-next year or a million years from
now.
How much warning will we have? With nearly half of even the larger
NEOs remaining undiscovered, the most likely warning today would be
zero — the first indication of a collision would be the flash of
light and the shaking of the ground as it hit. In contrast, if the
current surveys actually discover a NEO on a collision course, we
would expect many decades of warning. Any NEO that is going to hit
the Earth will swing near our planet many times before it hits, and
it should be discovered by comprehensive sky searches. This is the
purpose of the Spaceguard Survey. In almost all cases, we will either
have a long lead time or none at all.
Meanwhile, the Spaceguard Survey has already discovered more than
half of the near Earth asteroids 1 km or larger, and we are on track
to find 90% of them before the end of this decade.
For more information see the NASA websites: David Morrison NASA Ames Research Center
