承前*1
Jonathan Amos “China's space lab set for fiery re-entry” http://www.bbc.com/news/science-environment-43557446
2011年に打ち上げられ、2016年９月頃から制御不能になっている中国の宇宙ステーション「天宮１号」は今週末、つまり明日or明後日に地球に突入・落下する可能性が高くなってきた。勿論、詳細は「突入」の寸前にしかわからないわけだけど。
ところで、今引用した部分で、revolutionという言葉がその語源に近い「回転」という意味で使われていて、一瞬あれっ！ と思った。でも、考えてみれば、そう珍しい使い方ではないのだろう。地球は１日１回「革命」し続けているわけだ。
Thirteen space agencies, under the leadership of the European Space Agency, are now following Tiangong's path around the globe, modelling its behaviour as it descends deeper into atmosphere.The collective, known as the Inter-Agency Space Debris Coordination Committee (IADC), are trying to forecast the most likely time and place for the laboratory's re-entry.
The many uncertainties involved mean definitive statements can only be made close to the end of Tiangong's flight.
"A confidence of one hour is only reached about four hours beforehand. And one hour still means almost one revolution around the Earth," said Holger Krag, the head of Esa's space debris office. "But that's still good enough to exclude many countries and even some continents."
What can be said with certainty is that nothing will fall outside of 43 degrees from the equator, north or south.
This encompasses a region up to the Mediterranean and down to Tasmania, for example. It is governed by the inclination on which Tiangong was launched.
China has limited national tracking facilities around the globe and so had no choice but to keep the vessel on a reasonably tight equatorial path.
The International Space Station by contrast reaches 52 degrees north and south.
Although about 5.2 billion people live within the re-entry zone, most of it is ocean, which explains the high probability that any debris that survives to the surface will hit water.
