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Top Document: [sci.astro] Solar System (Astronomy Frequently Asked Questions) (5/9) Previous Document: E.16 What's the Oort Cloud and Kuiper Belt? Next Document: E.17.1 What would be the effects of an asteroid impact on the Earth? See reader questions & answers on this topic! - Help others by sharing your knowledge Much of the material in this section is drawn from the SpaceGuard Survey report, <URL:http://ccf.arc.nasa.gov/sst/spaceguard.html>. A crucial point about asteroid impacts is that they are random. Below are various estimates of the frequency with which the Earth is struck by objects of various sizes. These estimates are, roughly speaking, averages over the Earth's history. For instance, the average time between the impact of a 100 m diameter object is roughly 100--200 yr. The actual time between the impacts of such objects could be shorter than 10 yr or longer than 1000 yr. For more information about Near-Earth Objects, those asteroids (or minor planets) that have orbits similar to Earth's, see the following. A list of "Potentially Hazardous Asteroids" (PHAs) is at <URL:http://cfa-www.harvard.edu/iau/lists/Dangerous.html>. These have a projected closest distance to Earth of less than 0.05 AU (7.5 million km, about 1000 Earth radii). A list of closest approaches to the Earth by PHAs between 1999 and 2099 is available at <URL:http://cfa-www.harvard.edu/iau/lists/PHACloseApp.html>. A list of moderately close (to within 0.2 AU) approaches to the Earth by asteroids and comets between 1999 and 2032 is available at <URL:http://cfa-www.harvard.edu/iau/lists/CloseApp.html>. It is worth emphasizing that, at the moment, *none* of the known objects presents a serious risk of collision. 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.16 What's the Oort Cloud and Kuiper Belt? Next Document: E.17.1 What would be the effects of an asteroid impact on the Earth? 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