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Watch out for Falling Satellites What Is Happening?

游客2023-07-11  44
问题                         Watch out for Falling Satellites
    What Is Happening?
    An out-of-control US spy satellite will crash to Earth in the coming months, government officials say. The satellite is large enough that debris are likely to survive atmospheric re-entry and strike the Earth, sometime in late February or early March, says Gordon Johndroe, a spokesman for the National Security Council.
    Is That Normal?
    "This is relatively routine in that satellites deorbit all the time," says Johndroe. Pieces of uncontrolled debris heavier than two stones — mostly discarded rocket stages — crash to Earth as often as once every three weeks, says Jonathan McDowell, an astronomer and launch observer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Many discarded pieces retain some power, so that controllers on Earth can guide them to a point far from human habitation, usually using a final dive into an ocean.
    In 2001, Russian space officials broke up the old Mir space station in this way over the South Pacific. That’s not the case for this US one, however.
    "Obviously, we want to take a look at the potential for it to land in a populated area," says Johndroe. "As background, I’d like to say that over the past 30 plus years, there have been many satellites and other man-made objects falling from space — of course, they falling with very little damage and no injuries. What makes this case a little bit different, however, and in particular for the president in his consideration, was the likelihood that the satellite, upon descent to the Earth’s surface, could release much of its thousand-plus pounds of hydrazine fuel as a toxic gas. "
    What Are the Chances of It Crashing Through My Roof?
    Exceedingly slim, says McDowell. Remember that some 70% of the Earth is water, and most lands are void of people. "There is no reason for people to get alarmed about it," he says. According to the NASA Orbital Debris Program Office, there have been no confirmed instances of serious property damage or injury caused by crashing debris in 40 years. The likelihood of the satellite falling in a populated area is small, and the extent and duration of toxic hydrazine in the atmosphere would be quite limited; nevertheless, if the satellite did fall in a populated area, there was a possibility of death or injury to human beings beyond that associated with the fall of satellites and other space objects normally, if we can use that word. Specifically, there was enough of a risk for the president to be quite concerned about human life, and on that basis, he asks us to review our options.
    What Is This Satellite?
    Since it is a spy satellite, this isn’t public information. But it is likely to be USA 193, which, according to news reports, was launched at the end of 2006 for the National Reconnaissance Office (NRO) and which was lost to ground-based controllers upon reaching orbit. At the time, amateur sky watchers noticed that its orbit was slightly off and wasn’t being corrected. Based on the slight losses in altitude, they had predicted an eventual crash this spring, McDowell says.
    If the dying satellite is indeed USA 193, its specifications can be better defined. USA 193 was launched on a Boeing Delta 2 rocket, which limits the size of the satellite to between two and four tonnes and five or six meters in length — about the size of a minivan, McDowell says. This satellite is now at an altitude of 250 kilometers and is falling a kilometer per day. It orbits in the mid-latitudes, between 58 degrees north and south. Where it will fall within that range is impossible to know now, so McDowell says the chance of landing in the United States, for example, is 2%, based on area. Better estimates should be available a day before landing.
    Is the "Minivan-sized" Satellite a Big One?
    Nowhere near as big as a satellite that fell uncontrollably in 1979: the more-than-70-tonne US Skylab I crashed through the atmosphere and scattered debris across the Indian Ocean and Western Australia.
    What Will Happen to This One on Entry?
    The atmosphere will tear up the satellite. Aluminum parts and outer layers will burn. Twisted chunks of heavier pieces, like the fuel tank, could survive. McDowell says that the satellite could contain as much as a tonne of hydrazine, a highly toxic propellant which could be hazardous to be near but not dangerous when dispersed in the atmosphere.
    Have Bits of Satellites Survived Re-entry Intact Before?
    Yes. The NASA Orbital Debris Program Office hosts some pictures of pretty big chunks, including: a 250-kilogram steel fuel tank and a 30-kilogram titanium tank of a launch vehicle that both landed in Texas in 1997, and a 70-kilogram titanium rocket motor casing that landed in Saudi Arabia in 2001. One noteworthy ill-fated satellite actually crash — landed through the roof of a workshop supporting its launch in 2006, having fallen from a botched take-off.
    How Many Things Are There That Could Potentially Fall Out of Orbit?
    Yes. NASA Orbital Debris Program Office says there are some 11,000 objects bigger than 10 centimeters tracked by the US Space Surveillance Network. Of course, the biggest man-made object in the sky, by far, is the International Space Station. Nearly 45 meters long and more than 200 tones heavy, the concern about the ISS is in protecting it from debris, rather than worrying about it becoming debris itself.
    If It Lands in the Middle of Nowhere, does It Matter?
    While the impact is likely to be a nonevent, the falling satellite does serve as a reminder that the NRO — the United States "eyes and ears in space" — is having a bad time of things. In 2005, the NRO cancelled a next-generation reconnaissance technology project, called Future Imagery Architecture, after Boeing had spent more than $ 4 billion on it, according to the New York Times. "The NRO has had a bad decade," says Jeffrey Lewis, a space policy analyst at the New America Foundation in Washington DC. "We knew this satellite has been dead for a long time. But it reminds us of how much trouble they’ve had." [br] McDowell says that most of the heaviest uncontrolled debris crashing to Earth______.
选项 A、are satellites which have deorbited
B、are then discarded in three weeks
C、come from rocket launchings
D、still have power to use
答案 C
解析 题目中most和原文的mostly意义一致。文章中提到的那些重量在两公吨以上的残骸大多是被遗弃的不同级别的火箭，因此残骸是来自于火箭发射，故正确答案是C。
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