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
Francois-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

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 Francois-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 2,000 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


Leo Metcalfe, ISO project scientist

European Space Agency, Vilspa, Spain

Tel: +34 91 8131372


PR contact:

Clovis De Matos

ESA Science Programme Communication Service

Tel: +31 71 565 3460


For more information about ISO visit the ESA Science website at:

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