The Infrared Telescope ISO was in operation from November 1995 to May 1998. To avoid extra space debris, the ESA directed the satellite telescope into the Earth's atmosphere at the end of its life and burned it. (Source: ESA)
Read aloud According to a study by the European Space Agency (ESA), the asteroid belt between Mars and Jupiter has about 1.5 million asteroids more than one kilometer in diameter. That's twice as many as previously thought. The results of the analysis of data acquired by the infrared observation satellite ISO are published by Edward Tedesco of TerraSystems and François-Xavier Desert of the Observatory de Grenoble in the April issue of the Astronomical Journal (Vol. 123, p. 2070). The discrepancy with previous estimates is explained by the difficulty of observing asteroids in the visible light range. The brightness of the overall very faint asteroids can change drastically within a few minutes, depending on the angle at which the sunlight is just reflected by the surface of the asteroid.

By contrast, infrared heat radiation does not depend on the actual solar radiation, but on the heat that the asteroid surface has absorbed by the sun within the last hours. It is therefore more uniform. But since the Earth's atmosphere is not permeable to a large part of the wavelengths of infrared radiation, infrared images are possible only with a satellite telescope.

According to the researchers, the new study does not impact recent estimates of Earth's risk of being hit by an asteroid. It is estimated that the Earth is hit every 100, 000 to 300, 000 years by a near-Earth asteroid (NEO) with a diameter of one kilometer or more. The NEOs are located on strongly elliptical orbits that come close to the earth's orbit. It is believed that the NEOs were originally members of the asteroid belt, but catapulted by Jupiter's gravity into their near-Earth orbits.

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