AUSTRALIAN SPACEGUARD SURVEY
Additional Information on Asteroids and Comets
[Predicting a NEO's orbit] [Books]
Predicting a NEO's orbit
How difficult is it to predict the orbit of a newly discovered asteroid
to determine if it is a threat to the Earth?
The recent excitement with asteroid 1997XF11 gives an indication of
the difficulty of the task. Consider an asteroid which is 2 kilometres
in diameter and has an orbital period of just over 1 year. The length of
its orbit will be about 1,000 million km and in 30 years (the timescale
of concern with 1997XF11) it will have travelled about 30,000 million km.
this huge distance is equivalent to the circumference of the Earth (about
40,000 km) - our scaling factor is therefore about 800,000:1. On this scale
the Earth is about the size of a house (15m) and the asteroid is the size
of a small ant (a few millimetres), crawling along a straight line at 150
metres per hour (4km/day, 100km/month, 1,300km/year). We watch the ant
crawling along for a while and need to determine whether it will hit our
house by the time it has crawled right around the world (40,000km) some
30 years later. It is very important that the ant's bearing (trajectory)
is measured precisely!
Now, just to make it more challenging, we are only allowed to watch
the ant with a telescope from a distance of 150 km (assuming 120 million
km between the Earth and the asteroid when it is first spotted).
For an idea of the damage that one "ant" can cause if it hits the "house"
see this table. Of course, we need
to find and track more than 1,000 ants (estimated number of asteroids larger
than 1 kilometre in our region of the solar system) and sometimes these
pesky ants deviate from their path (due to the gravitational effects of
the planets and some unpredictable effects such as outgassing). Finally,
there are many more smaller "ants" that we also need to find and track.
Note: this analysis is not intended to trivialise the importance of
the Spaceguard Program. On the contrary, it is intended to give some idea
of the exceptional work that a handful of scientists are undertaking (or
should be undertaking).
See also Tracking Asteroids - Astrometry
* KT stands for Cretaceous Tertiary, Kriede is the German word for Cretaceous.
Asteroids and Doomsday Comets" by Duncan Steel, John Wiley and Sons,
to Comets and Asteroids" by Tom Gehrels, ed.
Sagan's Universe" edited by Yervant Terzian and Elizabeth Bilson, Cambridge
University Press, 1997. A collection of excellent papers by scientists
at the Sagan Symposium - held to celebrate Carl's 60th birthday. Many of
the papers refer to the influence of asteroid/comet impacts on the development
(and future) of life on Earth. In particular, see the papers by Chris Chyba
and David Morrison.
March 1996: "Collisions
with Comets and Asteroids" by Tom
Gehrels "Small rocky or icy bodies, left over from the formation
of the planets, normally follow distant, stable orbits, but rare mischance
can send one hurtling into the inner solar system. A leader of the Spacewatch
team that tracks near-earth comets and asteroids describes their awesome
beauty, the odds of a collision with our world and what could be done to
prevent a cataclysm."
October 1990: "What caused the mass extinction" by Walter Alvarez and Frank
Asaro. At the time this article was written the Chicxulub Crater had not
been discovered. The authors talk about the world-wide signs of a huge
comet/asteroid impact at the KT* boundary and the search for a 150km impact
crater (Chicxulub is the right age and is about 170km in diameter). The
same issue contains an article "A volcanic eruption" in which Vincent Courtillot
proposes that a massive volcanic eruption caused the extinctions. The interesting
thing is that, 65 millions years ago the volcanic eruptions in the Indian
sub-continent appear to be directly opposite Chicxulub and there could
be a relationship.
April 1990: "Impact cratering on Earth" by Richard Grieve.
January 1982: "The mass extinctions of the late Mesozoic" by Dale Russell.
Destined to be a historic piece. The detective story which led Walter Alvarez
and his team to propose that an asteroid or comet wiped out the dinosaurs.
"Rain of iron and ice : the very real threat of comet and asteroid bombardment"
by John Lewis, Reading, Mass.(240 p.)
"Impact!" The threat of Comets and Asteroids by Gerrit Verschuur.
and Asteroid Impact Hazards on a Populated Earth" by John S. Lewis,
Academic Press. It was released late in 1999. See review.
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Geological Boundaries and Eruptions and Shiva Hypothesis
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Last update 30 July 1998
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