Halley's comet is easily recognizable because of its tail. This does not apply to all comets. Photo: NASA / Kuiper Airborne Observatory
In the outer reaches of the solar system, flocks of dark comets could swirl around, which could also be dangerous to the earth. Astronomers Bill Napier from the University of Cardiff and David Asher from the Armagh Observatory in Northern Ireland estimate the number of these celestial bodies up to 3, 000. So far, only 25 active comets are known. The oscillation of the solar system around the galactic plane repeatedly distracts comets from distant regions towards the sun. These comets have higher speeds than asteroids and are therefore more dangerous. Astronomers should therefore keep an eye out for these comets in their observations, the scientists suggest. The researchers analyzed the known impact craters of celestial bodies on Earth in the past 250 million years. Their distribution is best explained by periodic cometary swarms that penetrate the solar system. The driving force here is the sun itself: its gravity disturbs remote cometary reservoirs and distracts them. Many of these comets could still travel on long orbits through the solar system. Around 25 active comets that develop their characteristic tail near the sun are known to astronomers. These include, for example, the Halley Comet. However, astronomers Napier and Asher estimate about 3, 000 comets of this type in total, based on their studies.

Presumably, the reflective layers of ice on this dark comet have long evaporated. The remaining surface crust hardly reflects the light and is therefore difficult to detect for telescopes. Some observations support the thesis of the dark comets: At the near-Earth-moving comet IRAS-Araki-Alcock in 1983 only about one percent of the surface was active. The 2001 observed comet Borrelly had big black spots. However, black does not mean invisible, say other astronomers. Nevertheless, such comets could absorb the sunlight and be detected by the emitted heat radiation.

Astronomers emphasize that dark comets are twice as fast as asteroids in near-Earth orbits. The early warning time would be shorter and the dangers to life on earth would be greater. A small specimen of these comets could have triggered the so-called Tunguska event, in which in 1908 an impact in Siberia destroyed 2, 000 square kilometers of forest.

New Scientist (No. 2695, p. 11) ddp / science.de? Martin Schäfer advertisement

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