Sometime on Friday, a 6.5-ton satellite the length of a small bus will plunge back to Earth . Deployed from the space shuttle Discovery in September 1991 and decommissioned in December 2005, the Upper Atmosphere Research Satellite  (artist's concept in image) measured chemical compounds found in the ozone layer, wind and temperature in the stratosphere, and energy streaming from the sun. According to NASA analyses, as many as 26 components weighing a total of 532 kilograms could survive reentry and strike Earth at scattered sites from 500 kilometers to nearly 1300 kilometers downrange of where the satellite first enters the atmosphere. Potential crash sites lie anywhere between 57° N (about the latitude of Sitka, Alaska, and Aberdeen, Scotland) and 57° S (a little more than 100 kilometers south of the southern tip of South America). Because Ireland lies beneath the flight path of the satellite, bookmakers there are getting in on the action: the most likely spot for the craft to crash is the Pacific Ocean, but odds are about 66-to-1 that any space debris lands on the Emerald Isle. If any pieces strike land, the most likely continents to be slammed are Africa (9-to-4 odds) and South America (11-to-4 odds), both of which lie fully within the danger zone. Because significant swaths of Asia and North America lie outside the danger zone, odds of debris landing there are lower, at 3-to-1.
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