Something weighing over 8,500kg fell out of the sky this week. As you can imagine, that’s not an object you want slamming into your head. Or anyone’s head. What kind of damage would it have caused had it fallen on, let’s say, INA Market in New Delhi? You can imagine that too. Luckily, that didn’t happen, nor did it hit any other human habitation. Instead, it sank into the waters of the South Pacific.

Or actually, it mostly burnt up as it tore through our atmosphere. But certainly some parts of China’s Tiangong-1 spacecraft survived re-entry to hit the waves and sank to the bottom of the Pacific.

“Tiangong" is Mandarin for “Heavenly Palace". That sort of describes its purpose for the nearly six-and-a-half years that it was in space, orbiting our Earth. In 2012 and 2013, two three-man crew stayed on 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 bona fide space station in Earth orbit", according to a report on Space.com. (Already in Earth orbit, of course, 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 fine. But there were no real Chinese plans about what would happen beyond those two years. It stayed in orbit—but in March 2016, it stopped communicating with Chinese space authorities. Over time, its orbit naturally began “decaying". Imagine whirling a stone tied to the end of a string, and then you slow down and eventually stop whirling. You’ll see the stone slow too, and it makes a couple of final circles, and then it will fall. In much the same way, it was only a matter of time before the “Heavenly Palace’s" orbit decayed to the point when it would fall to Earth in a fireball.

And that’s just what happened early on Monday morning, 2 April, 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 at all, the great Isaac Newton spelled out a thought experiment that explains this.

Think first of all of trying to throw a tennis ball across the court. If you do it feebly, the ball will plop to the ground in front of you. Throw it harder, and the ball travels some distance, maybe even over the net, but will still eventually fall to the ground. That’s gravity at work, overcoming the force with which you fling the ball. Newton imagined lugging a cannon to the top of a tall mountain, and using it to fire a cannonball horizontally, meaning parallel to the Earth’s surface. Of course, as soon as the ball leaves the mouth of the cannon, gravity goes to work on it just as it does with your tennis ball. If it’s a feeble cannon, the ball will quickly fall to Earth, possibly on the side of the mountain itself. A more powerful one, and the ball will travel further, but still fall. Gravity wins, every time.

And what happens if Newton’s cannon is powerful enough to fire the ball with a force greater than that of gravity? The ball will soar off into space, breaking free of Earth’s gravitational embrace at a speed known as the escape velocity. On the earth’s surface, that speed is just over 40,000kmph. It’s a little less on Newton’s mountain, because gravity is a little weaker there. In fact, the higher you go, the weaker Earth’s gravity gets and the lower the escape velocity your cannonball will need. And what if the cannon’s force is precisely equal to gravity’s pull? The ball won’t escape, but neither will it fall to Earth. It will go into orbit, whirling around the planet just like that stone whirls around your wrist, at a speed just below the escape velocity.

Now no cannons—and certainly no human arms—are capable of force like this. (Don’t believe me? Try flinging a tennis ball at 40,000kmph). But we have rockets that are. This is exactly how we have put Tiangong-1 and the ISS and thousands of other satellites into orbit around our planet. We make them fly fast enough so they don’t fall back down, but not fast enough to escape the Earth for good.

Of course, no object can stay in orbit like this indefinitely. Friction as it barrels through the Earth’s atmosphere—thin though it is at those altitudes—slows it down. This is what happened with Tiangong-1, and what will eventually happen with the ISS too. And as the “Heavenly Palace" slowed, it began slipping from its orbit 300km above us, flying gradually but inexorably ever closer to our Earth.

Now usually we can very precisely track and predict the motion of objects we have sent into space. For example, there are websites which can tell you exactly where the ISS is right now and when it will next pass over your particular head —with an accuracy of 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 touch down —and if that spot was on land, we could alert residents to the danger, even evacuate them if necessary and if we had enough warning.

But in contrast to the ISS, it was very difficult to predict the behaviour of Tiangong-1 as its orbit began decaying. Why so? There’s a nice analogy that helps me understand this. Think of travelling in a car and sticking a small wooden board out of the window. If the car is crawling along, you’ll easily be able to move the board about, turn it this way and that. But suppose the car is really zooming, let’s say at 150kmph. Then the pressure of the air rushing past will make it much harder to turn the board, even to merely hold it steady. It might do unpredictable things.

Something like that must have happened to Tiangong-1. In its orbit, it was screaming along at about 28,000kmph. At speeds that high, the Earth’s atmosphere exerted enormous drag on the “Heavenly Palace". Unpredictable things happened, and at some point Tiangong-1 began spinning and tumbling about as it flew and as it slowed. No longer was it flying stably like the ISS, and this made predictions about its future near-impossible.

Yet even so, various space agencies were tracking Tiangong-1, trying to determine when and where it would fall to Earth. In early March, all the European Space Agency (ESA) could say was that it would touch down somewhere between 27 March and 9 April. That’s a two-week window, which is so wide as to be near-worthless. But as March wore on, the predicted window shrank. On 27 March itself, ESA called it within a two-day window, between 31 March and 2 April. On 1 April, the day before it landed, that was down to two hours. That’s impressive, but it still covers a huge part of the Earth’s surface—after all, at 28,000kmph, Tiangong-1 took just 90 minutes to orbit the Earth.

As it happened, it plunged into the ocean more or less in the middle of that final two-hour interval. We are lucky indeed that it targeted the South Pacific and not INA Market.

Other man-made objects in space—there are some 18,000 satellites up there, besides sundry debris—will also eventually see their orbits decay and fall back to Earth. Many will burn up, but especially when something as massive as the ISS comes back down, there will be chunks that survive re-entry. Thus it’s vital to be able to direct their orbital decay to the extent we can, and then to track and predict their path to the extent we can. Consider: surely it’s better to have a two-hour window than a two-week one, or no window at all; and surely it would be good to have that two-hour window nailed down as early as possible.

All of which may be a quiet reminder, in an age when we expect precision in so many things, of how science really works. Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, put it like this while speaking to Space.com about the demise of Tiangong-1: “Science is not about being able to calculate things precisely. It’s about being able to know how wrong you might be."

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

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