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The final fall of a heavenly palace

  A radar image of the Tiangong-1 space station taken at a height of about 270 km above the Earth. The defunct Chinese space laboratory burned during its re-entry to Earth and its remaining debris sank into the South Pacific Ocean. Photo: AFP "title =" A radar image of the Tiangong-1 space station taken at a height of about 270 km above the Earth. The defunct Chinese space laboratory burned during its re-entry to Earth and its remaining debris sank into the South Pacific Ocean. Photo: AFP "class =" img-responsive "/><p> A radar image of the Tiangong-1 space station taken at a height of about 270 km above the Earth The defunct Chinese space laboratory burned during its reentry to the Earth and its remaining debris sank in the South Pacific Ocean Photo: AFP</p></div><p xmlns:fn= Some of the 8,500 kg weight fell from the sky this week, as you can imagine, that is not an object that wants to hit your head. What kind of damage would it have caused if it had fallen into, say, INA Market in New Delhi? You can also imagine that, fortunately, that did not happen, or hit any other human room, it sank in the waters of the South Pacific. [19659003] Or, in fact, it burned for the most part as it passed through our atmosphere, however, parts of the Chinese spacecraft Tiangong-1 survived re-entry to hit the waves and sink to the bottom of the Pacific.

Tiangong "is Mandarin for" Palace celestial. "That kind of description is his purpose during the almost six and a half years he was in space, orbiting our Earth. In 2012 and 2013, two crew members of three men stayed in Tiangong-1 for a couple of weeks each time. China's purpose with this spacecraft was to "master the technologies required to assemble and operate a good-faith space station in Earth orbit," according to a report on Space.com. (Already in Earth orbit, of course, there is the much larger International Space Station, or ISS).

Tiangong-1 was meant to be operational only for a couple of years. It served that purpose well. But there were no real Chinese plans about what would happen beyond those two years. It remained in orbit, but in March 2016 it stopped communicating with the Chinese space authorities. Over time, its orbit naturally began to "decay". Imagine turning a stone tied to the end of a rope, and then slow down and finally stop spinning. You will see that the stone is also slow and makes a couple of final circles, and then it will fall. In the same way, it was only a matter of time before the orbit of the "celestial Palace" decayed to the point where it would fall to Earth in a fireball.

And that's exactly what happened on Monday morning early, April 2, India time.

But how do objects like Tiangong-1 stay in orbit in the first place? Centuries before humans managed to send objects into space, the great Isaac Newton spelled out a thought experiment that explains this.

Think first of all about trying to throw a tennis ball on the other side of the court. If you do it weakly, the ball will fall to the ground in front of you. Throw it harder, and the ball moves some distance, maybe even over the net, but eventually it will fall to the ground. That's gravity at work, overcoming the force with which you throw the ball. Newton imagined dragging a cannon to the top of a high mountain, and use it to fire a cannonball horizontally, that is, parallel to the surface of the Earth. Of course, as soon as the ball comes out of the muzzle, gravity works the same as with the tennis ball. If it is a weak canyon, the ball will fall quickly to Earth, possibly on the side of the mountain. A more powerful one, and the ball will travel farther, but it will still fall. Gravity wins, all the time.

And what if Newton's cannon is powerful enough to shoot the ball with a force greater than that of gravity? The ball will rise into space and be released from Earth's gravitational embrace at a speed known as the escape velocity. On the surface of the earth, that speed is just over 40,000kmph. It's a bit less in Newton's mountain, because gravity is a little weaker there. In fact, the higher you leave, the weaker the Earth's gravity will be and the lower the escape velocity your cannonball will need. What if the force of the barrel is precisely equal to the pull of gravity? The ball will not escape, but neither will it fall to Earth. It will go into orbit, spinning around the planet as that stone rotates around your wrist, at a speed just below the escape velocity.

Now no cannon – and certainly no human weapon – is able to force like that. (Do not believe me, try throwing a tennis ball at 40,000 kmph). But we have rockets that are. This is exactly the way we have put Tiangong-1 and the ISS and thousands of other satellites in orbit around our planet. We make them fly fast enough so they do not fall again, but not fast enough to escape from Earth forever.

Of course, no object can remain in orbit indefinitely. The friction as it moves through the Earth's atmosphere, although it is thin at those altitudes, slows it down. This is what happened with Tiangong-1, and what will eventually happen with the ISS as well. And as the "Heavenly Palace" diminished, it began to glide from its orbit 300km above us, flying gradually but inexorably closer to our Earth.

Now we can follow and predict very precisely the movement of the objects that we have sent into space. For example, there are websites that can tell you exactly where the ISS is now and when it will then go over your particular head, with fractions of a second. If we could have done the same with a defunct Tiangong-1, we could have predicted exactly where on the planet it would land, and if that place was on land, we could alert residents to the danger, even evacuate them if necessary and if we had enough warning.

But unlike the ISS, it was very difficult to predict the behavior of Tiangong-1 when its orbit began to decay. Why? There is a good analogy that helps me understand this. Think of traveling in a car and placing a small wooden board outside the window. If the car moves slowly, you can easily move it, turn it from one side to the other. But suppose the car is really focusing, say at 150kmph. Then, the rapidly passing air pressure will make it much harder to turn the board, even to keep it stable. I could do unpredictable things.

Something like this must have happened to Tiangong-1. In its orbit, it shouted at approximately 28,000kmph. At such high speeds, the Earth's atmosphere exerted enormous resistance over the "Heavenly Palace." Unpredictable things happened, and at some point Tiangong-1 began to spin around and around as it flew and slowed down. He was no longer flying steadily like the ISS, and this made predictions about his future almost impossible.

Even so, several space agencies were following Tiangong-1, trying to determine when and where it would fall to Earth. At the beginning of March, the entire European Space Agency (ESA) could say that it would fall between March 27 and April 9. That is a window of two weeks, which is so wide that it is almost useless. But as March progressed, the predicted window shrank. On March 27, ESA called him within a period of two days, between March 31 and April 2. On April 1, the day before his landing, that was up to two hours. That is impressive, but it still covers a large part of the surface of the Earth; after all, at 28,000kmph, Tiangong-1 took only 90 minutes to orbit the Earth.

It so happened that it sank in the ocean roughly in the middle of that last two-hour interval. We are lucky that he went to the South Pacific and not to the INA Market.

Other man-made objects in space-there are about 18,000 satellites up there, as well as various debris-will also see their orbits decay and fall back to Earth. Many will catch fire, but especially when something as massive as the ISS goes down again, there will be pieces that will survive the re-entry. Therefore, it is vital to be able to direct your orbital decay as much as possible, and then track and predict your path as far as possible. Consider: it is probably better to have a window of two hours than a window of two weeks, or no window; and it would surely be nice to have that two-hour window as soon as possible.

All of which can be a silent reminder, at a time when we expect precision in many things, of how science really works. Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, put it this way when talking with Space.com about the demise of Tiangong-1: "Science is not about being able to calculate things accurately. know how wrong you can be. "

Once a computer scientist, Dilip D & # 39; Souza now lives in Mumbai and writes for his dinners. His latest book is Jukebox Mathemagic: Always One More Number. Your Twitter handler is @DeathEndsFun.

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