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An improved estimate of the number
of nearby asteroids still capable of causing local destruction
suggests these pesky rocks are likely to hit Earth about once
every 1,000 years. Astronomers had thought such minor catastrophes
occurred about once per century.
The new calculations, from Alan Harris
of the Space Science Institute in Boulder, CO, show there
are about 500,000 relatively small asteroids that inhabit
roughly the same region of space through which Earth orbits.
The asteroids are in the 50-75 meter (165-245 foot) size range.
Rocks this size can flatten a forest
and would cause tremendous damage and even death if it targeted
a city.
About 1,100 large Near Earth Objects
(NEOs) are thought to exist. More than 600 have been found
by programs stemming from a NASA effort to locate 90 percent
of them by 2008. None are known to be on a collision course
with Earth.
Smaller asteroids are harder to detect,
however. Now and then, one passes relatively close to the
planet and some are spotted only after such flybys. A few
vocal scientists have long called for stepped-up funding to
find these smaller NEOs, because they are more likely to strike
and are a more immediate threat.
NASA's line has been to find the big
ones first, then consider going after the smaller ones. Only
recently, however, have discussions on how to do this become
serious. New telescopes would be needed, and telescopes generally
take a few years to design and build.
Suppose a giant asteroid or comet is
heading toward Earth right now. Impact is certain. The consequences
are expected to be globally devastating, with the human race
among the casualties. The chances of doing anything about
it are zero, the government decides.
Would you want to know?
(Article modified from Space.com -
Feb. 2003)
I
WANT TO READ MORE ABOUT THE METEORITE IN THE TOMB
(Screen opens as a Flash presentation)
MINOR
PLANET 4179 TOUTATIS: A POTENTIAL MENACE
According to the scientific
article, "Motion of the Minor Planet 4179 Toutatis: Can
We Predict Its Collision with the Earth?, written by Sitarski,
G. (1998), Minor planet 4179 Toutatis is an Apollo type object
with a very small orbit inclination (i=0.47 arcdeg), hence
it has a possibility to approach closely the Earth (an encounter
to within 0.01 a.u. is expected in 2004) and might be a good
candidate for a future collision with the Earth.
The author collected 640
astrometric observations of Toutatis from the period 1934-1997
to improve the orbit. The data had to include a nongravitational
term into equations of motion expressed by a secular change
dot{a} of the semi-major axis a of the Toutatis orbit to obtain
a fully satisfactory solution of the orbit determination.
A value dot{a}=-0.16*10^{-10} is two orders smaller than that
determined in the case of short-period comets with known nongravitational
effects. To investigate the long-term motion of Toutatis the
researcher numerically integrated the equations of motion
by recurrent power series taking into account perturbations
caused by the eight planets from Mercury to Neptun, treating
the Earth and Moon as separate bodies, and also by the four
biggest asteroids.
Data was randomly varied
the orbital elements to examine the Toutatis' motion for a
number of different orbits. The author presents a new method
of the random orbit selection which allows the simulation
to find a set of different orbits but representing well all
the observations used for the orbit correction. Their results
confirm a conclusion found by other authors that Toutatis
orbit is exceptionally chaotic. Therefore, he was not able
to predict the motion of Toutatis further than for 300 years.
However, their integrations
spanning 1500 years showed that the evolution of position
of the descending node of Toutatis' orbit might go also in
such a direction that the orbits of Toutatis and of the Earth
would intersect in the future. Hence a possibility of the
Toutatis-Earth collision is not excluded but it is completely
unpredictable.
To investigate conditions
of a hypothetical collision of a minor planet with the Earth
the author made the following numerical simulation. Based
on the Toutatis' orbit he deduced such orbital elements for
a fictitious minor planet "Fatum" that a shape of the orbit
was very similar to that of Toutatis, but he knew in advance
that "Fatum" would certainly collide with the Earth in September
2004 and he calculated values of the impact parameters. The
author created a set of 638 artificial observations of "Fatum"
in 1988-1997 for the same dates and with the same random observational
errors like those of Toutatis. Then he corrected the "Fatum's"
orbit for different observational intervals to examine the
exactness of the impact prediction in 2004.
The numerical simulation
found that in 1993 we would be sure that the collision is
inevitable, and in 1997 we could determine an impact area
on the Earth's surface in range of a square of 100*100 km.
The author shows that if we knew the impact date so early
we could undertake an action to avoid the collision by trying
to change the "Fatum's" heliocentric velocity only by one
cm/sec.
Title: Motion of the Minor
Planet 4179 Toutatis: Can We Predict Its Collision with the
Earth? Authors: Sitarski, G. Journal: Acta Astronomica, v.48,
pp.547-561, (1998). Publication Date: 09/1998.
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