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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.17 Asteroid Impacts Next Document: E.17.2 What can we do about avoiding impacts? See reader questions & answers on this topic! - Help others by sharing your knowledge Author: Joseph Lazio <jlazio@patriot.net> The Earth is constantly pelted by bits of cosmic debris. Most of this simply burns up in the atmosphere (as one can attest by simply watching meteors on a dark night). However, if an object is big enough it can survive passage through the atmosphere. The damage done by a meteorite (an object that strikes the Earth) depends upon its initial size. 10--100 m: Objects in this size range can produce devastation similar to that of an atomic blast (leading to them occasionally being called "city-busters"). Effects include severe damage to or collapse of standing buildings and the ignition of flammable materials leading to widespread fires. The radius over which such effects occur would vary depending upon the size and composition of the object, but could easily exceed 10 km. The Tunguska event, in Siberia, of 1908 is thought to have been from an object about 60 m in size; it led to trees being flattened out to 20 km and trees 40 km away being damaged. At the small end of this size range, objects about 10 m strike the Earth about once a decade. Fortunately, only the densest objects, those containing iron, survive to the surface; most of the objects of this size explode sufficiently high in the atmosphere that there are no effects (other than maybe a loud noise) on the ground. At the larger end of this size range, it is estimated that the Earth is struck several times a millennium or about 1 impact every 100--200 yr. 100 m--1 km: Objects in this size range are likely to cause severe damage over a regional area, possibly as large as a continent (hence the name "continent-busters"). If they strike land, they will almost certainly produce a crater, while an ocean impact will generate large tidal waves. A 150 m object might produce a crater 3 km in diameter, an ejecta blanket 10 km in diameter, and a zone of destruction extending much farther out. For a 1 km impactor the zone of destruction might reasonably extend to cover countries. The death toll could be in the tens to hundreds of millions. A 1 km impactor could begin to have minor global consequences, including global cooling caused by vast amounts of dust in the atmosphere. Estimates from the geologic record suggest that craters are formed on the Earth roughly once every 5000 yr. 1--10 km: Objects in this size range are likely to cause severe global effects ("species-busters"). An impact 65 million years ago by an object of 5--10 km in diameter is thought to have been partially or fully responsible for the extinction of half the living species of animals and plants at the time, including the dinosaurs. The crater alone from such an impact will be 10--15 times larger than the object itself. World-wide crop failures from dust injected into the atmosphere could imperil civilization, and the largest-sized objects could make the human species extinct. The frequency with which the Earth is struck by such objects has to be estimated from the geological and paleontological record. At the low end of this size range, estimates are that such impacts occur roughly every 300 000 yr; at the upper end of the size range, impacts occur about every 10 million years. User Contributions:Comment about this article, ask questions, or add new information about this topic:Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.17 Asteroid Impacts Next Document: E.17.2 What can we do about avoiding impacts? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP