[meteorite-list] New Study Reveals Twice As Many Asteroids As Previously Believed

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:23 2004
Message-ID: <200204051601.IAA25568_at_zagami.jpl.nasa.gov>

http://sci.esa.int/content/news/index.cfm?aid=18&cid=41&oid=29762

New study reveals twice as many asteroids as previously believed
European Space Agency
April 5, 2002

Asteroids in our Solar System may be more numerous
than previously thought, according to the first systematic search
for these objects performed in the infrared, with ESA's Infrared
Space Observatory, ISO. The ISO Deep Asteroid Search
indicates that there are between 1.1 million and 1.9 million 'space
rocks' larger than 1 kilometre in diameter in the so-called 'main
asteroid belt', about twice as many as previously believed.
However, astronomers think it is premature to revise current
assessments of the risk of the Earth being hit by an asteroid.

Despite being in our own Solar System, asteroids can be more
difficult to study than very distant galaxies. With sizes of up
to one thousand kilometres in diameter, the brightness of these
rocky objects may vary considerably in just a few minutes. They
move very quickly with respect to the stars - they have been
dubbed 'vermin of the sky' because they often appear as trails
on long exposure images. This elusiveness explains why their
actual number and size distribution remains uncertain. Most of
the almost 40 000 asteroids catalogued so far (1) orbit the Sun
forming the 'main asteroid belt', between Mars and Jupiter, too
far to pose any threat to Earth. However, space-watchers do
keep a closer eye on another category of asteroids, the 'Near
Earth Asteroids' or 'NEAs', which are those whose orbits cross,
or are likely to cross, that of our planet.

The ISO Deep Asteroid Search (IDAS), the first systematic
search for these objects performed in infrared light, focused on
main belt asteroids. Because it is impossible to simply point the
telescope at the whole main belt and count, astronomers choose
selected regions of the belt and then use a theoretical model to
extrapolate the data to the whole belt.

Edward Tedesco (TerraSystems, Inc., New Hampshire, United
States) and François-Xavier Desert (Observatoire de Grenoble,
France) observed their main belt selected areas in 1996 and
1997 with ESA's ISO. They found that in the middle region of
the belt the density of asteroids was 160 asteroids larger than 1
kilometre per square degree - an area of the sky corresponding
to that covered by four full moons as seen from Earth. Then, a
model developed by Tedesco and the astronomers Alberto
Cellino and Vincenzo Zappala (Osservatorio Astronomico di
Torino, Italy), allowed them to estimate the whole asteroid
population in the main belt: between 1.1 million and 1.9 million
asteroids with a diameter larger than 1 kilometre.

"If you consider the average value of 1.5 million asteroids, the
ISO result is about twice as high as estimated by two other recent
studies in visible light," Tedesco says.

The study by Durda et al., published in 1998, gave an estimate of
about 860 000 asteroids larger than 1 kilometre in the main belt.
In 2001, Ivezic et al. obtained an even lower figure of 740 000
asteroids based on preliminary data from the Sloan Digital Sky
Survey.

Why the discrepancy?

The fact that visually dark objects - such as asteroids - are
better detected in the infrared might explain the discrepancy
between visible and infrared results. For an optical telescope, the
brightness of an asteroid depends on the visible light it reflects
from the Sun. Observations with infrared telescopes, on the
other hand, detect the 'heat' of the asteroid, which does not
depend that much on the reflected sunlight, but on the absorbed
sunlight.

As an example, let's consider two spheres of the same size, and
located close to each other in the asteroid belt, one of which
reflects ten times as much of the visible light striking it as the
other. As seen by an optical telescope, the sphere which reflects
more appears ten times brighter than the other sphere which
might be even invisible. However, for ISO both spheres would
be visible. Actually, the 'dark' sphere would appear brighter in the
infrared because it would have a higher temperature (as it has
absorbed more sunlight).

Expert's 'best estimate'

Tedesco assumes that both visible and infrared searches might
have their own biases, which is the reason why the given results
have an error margin. Considering both the visible and infrared
results, the 'best estimate' would be "1.2 million asteroids larger
than 1 kilometre in the main belt, give or take
500 000," Tedesco says.

The best strategy for finding the asteroid size distribution,
according to this expert, is to combine near-simultaneous
observations at infrared and visible light. "They provide different
kinds of information and therefore play a complementary role in
the search for the asteroid population's size distribution," he says.

The 'impact hazard'

A better knowledge of the number and size distribution of
asteroids in the main belt is essential to understand the
population of Near Earth Asteroids (NEAs), since most NEA are
believed to be former main belt asteroids. In the main belt there
are four 'special' regions where Jupiter's gravitational influence is
especially disruptive; originally, most asteroids currently known as
NEA suffered collisions which resulted in them ending up in one
of those four key regions, and because of Jupiter's gravitational
influence their orbits quickly evolved into Earth-crossing orbits.
Therefore, by studying the asteroids near these so-called 'source
regions' in the main belt astronomers can learn about NEA.
About 500 NEAs have been found so far, and none of them
pose any threat to Earth in this century.

The generally accepted impact rate by objects larger than 1
kilometre in diameter is one every
100 000 to 300 000 years. The new 'best estimate' of about 1.2
million asteroids of 1 kilometre or larger in the main belt will not
change the current estimates of impact hazard, the IDAS
astronomers say; at least not yet.

"IDAS has contributed to our knowledge of main belt asteroids.
And, although we did not observe any NEAs, the ISO data will be
used to improve our knowledge regarding asteroids currently near
the NEA source regions. This, in turn, will allow us to better
understand the population characteristics of the NEAs and so
ultimately enable us to refine our estimates of the NEA impact
frequency and the magnitude of the impact hazard," Tedesco says.

Note to editors

The European Space Agency's infrared space telescope, ISO,
operated from November 1995 until May 1998. As an
unprecedented observatory for infrared astronomy ISO made
nearly 30 000 scientific observations.

This note is based on the paper "The Infrared Space Observatory
Deep Asteroid Search" by Edward F. Tedesco and
François-Xavier Desert, published in the April 2002 issue of The
Astronomical Journal.

(1) 39 462 main belt asteroids were catalogued as of 28 March
2002. This number increases by about 2000 per month at
present.

For more information please contact:

Science contacts:

Edward Tedesco
TerraSystems, Inc. (Lee, New Hampshire, United States)
Tel: +1 603 659 5620
Email: etedesco_at_terrasys.com; eft@mediaone.net

Leo Metcalfe
ISO project scientist
European Space Agency
Vilspa, Spain
Tel: +34 91 8131372
Email: lmetcalf_at_iso.vilspa.esa.es

PR contact:

Clovis De Matos
ESA Science Programme Communication Service
Tel: +31 71 565 3460
Email: Clovis.De.Matos_at_esa.int
Received on Fri 05 Apr 2002 11:01:29 AM PST