New study reveals twice as many asteroids as believed
EUROPEAN SPACE AGENCY NEWS RELEASE
Posted: April 7, 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.
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?
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'
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'
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
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.