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"  UPDATE, Friday morning: NASA says today that the satellite&amp;#039;s descent has slowed. As a result, it may not re-enter until Friday night or early Saturday, and the delay means that there is now a slim chance debris could land in the U.S. That possibility was discounted earlier when UARS was descending faster. UPDATE, Thurday: As we get closer to UARS&amp;#039; return, the predictions are getting more detailed. NASA now says it expects the satellite to enter the atmosphere Friday afternoon Eastern time, and that it won&amp;#039;t fall over the continental U.S. Debris from space falling into the atmosphere is nothing new. It happens every day. There are thousands of pieces of space junk in low Earth orbit; most of them are too small to track, and they continually burn up in the high atmosphere. In the entire history of space exploration, no one has ever been injured by a piece falling back to Earth\u2014not even from Skylab&amp;#039;s return more than three decades ago or from the Russian Mir space station that fell out of orbit in 2001.  But NASA&amp;#039;s Upper Atmosphere Research Satellite (UARS), which should come back to the planet&amp;#039;s surface this weekend, is an unusual case that brings up some intriguing questions.  1. Why Is This Particular Satellite Falling Out of Orbit? NASA launched UARS back in 1991. When its work was finally done in 2005, there wasn&amp;#039;t enough propellant left on board to lift it into a stable orbit. Perhaps in the future, we&amp;#039;ll have space &amp;quot;tugs&amp;quot; that can grapple objects like this and put them wherever we want. But for now, the prudent thing to do was to take UARS out of orbit as soon as possible so it doesn&amp;#039;t endanger other satellites, particularly the International Space Station. So, NASA used the last bit of UARS&amp;#039;s propellant to lower the orbit and make it fall to Earth more quickly. UARS is a lot like one of the other major orbiters to fall from the sky: Skylab, the 1970s space station that returned to Earth in 1979. Skylab is a good example of the difficulties of predicting what will happen to objects in orbit. NASA planned to reboost the station to a higher altitude to save it. But increased activity in the sun boiled the atmosphere higher than NASA expected, which increased the drag on Skylab; plus, the space shuttle, which was to do the boosting job, wasn&amp;#039;t ready until 1981. So NASA tumbled Skylab end-over-end to make it enter into the atmosphere faster and hopefully fall into the Indian Ocean. But the operators didn&amp;#039;t have sufficient control, and Skylab ended up dropping debris on Australia. (The Russian Mir station, by contrast, was still an active station when it entered, so its operators used a deorbit burn to direct its path over the South Pacific). And like both Skylab and Mir, the UARS satellite falling this week has pieces that are bulky enough to survive all the way to the ground. In fact, it is the largest U.S. satellite to reenter since Skylab. 2. Why Don&amp;#039;t We Know Where Those Pieces Will Land? NASA now has no control over when the satellite enters the atmosphere. All it can do right now is predict which pieces will fall in what order, based on their size and mass. Heavier objects for a given area will move faster and thus fall farther downrange than lighter ones. NASA expects the grapple-fixture abutment plate to hit the ground first, a little more than 300 miles downrange from the point that the satellite enters the atmosphere, followed by a bracket, the propellant tanks, the grapple extension, a stainless steel reaction-wheel rim, and finally the forward bulkhead fitting. The total debris trail will stretch about 600 miles. But where will this 600-mile path fall? Even now, a day or two before entry, no one can precisely predict it. UARS is in an orbit inclined at 57 degrees, which means that once every hour and a half or so, its ground trace (the point on the ground directly below it) will change in latitude from 57 degrees north to 57 degrees south. It could hit anywhere in that latitude band, and there&amp;#039;s no way to predict where until you know the precise time of entry, when it falls deeply enough into the atmosphere that it can no longer maintain orbital velocity.  Even then, it&amp;#039;s still difficult to predict the path because it depends upon chaotic, unpredictable fluctuations in the atmosphere. For example, the speed with which the orbit decays depends on the density of the upper atmosphere, which can vary even from hour to hour as the earth turns and is heated by a variable sun. Finally, the satellite is tumbling end over end, so even if the atmosphere were predictable, it would be an enormous task for NASA to model the satellite&amp;#039;s path. Even a couple hours before the event, the Joint Space Operations Center (JSpOC, formerly known as NORAD) will be able to predict the location only to within a few minutes and a few thousand miles\u2014about a quarter of the way around the planet. (It&amp;#039;s worth noting, however, that NASA can constrain the possible paths somewhat to rule out areas. New York and Washington, D.C., for example, won&amp;#039;t be hit.) 3. What Are Your Chances of Being Hit? Well, if you are above 57 degrees north or below 57 degrees south latitude, your chance is zero. But even inside that range, where the vast majority of people around the world live, the chance of being struck by debris is astronomically low. It is estimated that the chance of anyone at all being hit is on the order of one in 3000 (that low risk, combined with the rarity of falling satellites, is why no one has ever been hit). Remember, one in 3000 is the chance that debris will hit someone. The risk of you as an individual being hit is more like one in a trillion\u2014or lower. You&amp;#039;re more likely to win the lottery, and much more likely to be struck by lightning.  4. Will You Get a Souvenir if a Piece of UARS Lands in Your Backyard? Not legally. All the pieces of UARS are U.S. government property, and NASA wants to get all of the pieces back, both to gather data and because some of the parts may be hazardous (for example, the propellant tanks could have toxic hydrazine in them). So, you&amp;#039;ll be expected to report any falling debris so the agency can send a hazmat team. 5. Okay, but What if Pieces Fall in Foreign Countries? If the debris does any damage, the U.S. government would be responsible as the launching state under the Outer Space Treaty of 1967 and the 1972 Liability Convention. For example, in 1978 a Soviet satellite containing a nuclear reactor fell on Canada, making a radioactive mess in a remote wilderness. (Had reentry happened an hour later, the satellite would have hit near New York City during rush hour.) The Canadian government billed the USSR for 6 million Canadian dollars in cleanup charges. The Soviets eventually paid about half of that. 6. Do Other Countries Have to Return Our Space Junk? It turns out that there&amp;#039;s a treaty for that, too, with a mouthful of a name: &amp;quot;Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Objects Launched into Outer Space.&amp;quot; About 90 countries are signatories to it. Article 5, section 3 states that &amp;quot;Upon request of the launching authority, objects launched into outer space or their component parts found beyond the territorial limits of the launching authority shall be returned to or held at the disposal of representatives of the launching authority, which shall, upon request, furnish identifying data prior to their return.&amp;quot; Yes, pieces of UARS could fall in a country that&amp;#039;s not a party to that agreement. But it&amp;#039;s likely that a non-signatory would return it to NASA (unless debris falls in a country that just doesn&amp;#039;t like the U.S.). The agreement almost never comes into play, because it&amp;#039;s so rare that a piece actually falls to earth, is recognized as a piece of space debris, and the nation of origin can be identified. The last time it happened was with Skylab (many of the pieces were returned to the U.S.), and before that, the Russian reactor (the USSR didn&amp;#039;t get anything back). "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/satellites/a7276/6-questions-about-nasas-falling-uars-satellite/>