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Business News/ Opinion / The charms of gravity

The charms of gravity

A few days ago on a celestial object far, far away, a fridge-sized object fell to the ground. But it did not shatter. Instead, it bounced

The image released by the European Space Agency shows lander Philae separating from Rosetta mother spaceship and descending to the surface of comet 67P/Churyumov-Gerasimenko. Photo: AP Premium
The image released by the European Space Agency shows lander Philae separating from Rosetta mother spaceship and descending to the surface of comet 67P/Churyumov-Gerasimenko. Photo: AP

Warning: Do not try this experiment at home. Suppose you lug a fridge to the 10th floor of a building, push it off a balcony and watch as it falls. What happens when it reaches the ground? Will it shatter into a thousand mangled pieces? Or will it bounce gently, touch down somewhere else, bounce again and come to rest on its side?

Before you rush off to try this at home, let me offer my considered opinion which I suspect is yours too: the pieces.

A few days ago on a celestial object far, far away, something very much like this actually happened. Not a fridge, but a fridge-sized object fell to the ground—in fact, from a few kilometres up. But it did not shatter. Instead, it bounced, then a second time, and then landed on its side.

Boggles the mind a bit, right? It’s not the only mind-boggling thing about the space mission that was in the news last week, though perhaps it’s the most dramatic. After all, imagine what it must involve to send this spaceship Rosetta hurtling through the vast emptiness of space for a full 10 years, have it rendezvous with a rock that is itself hurtling through space at a frightening speed, and then drop a fridge-sized object called Philae onto the rock in the hope of learning what it’s made of.

It’s as if you went searching for a needle in a gigantic haystack that, for good measure, is whizzing through the air at many kilometres a second. And once you find the needle, it’s as if you dropped a tiny ball precisely onto its point. If you ever try that, you’ll get a taste of the precision this mission to meet a comet demanded.

Still, why didn’t Philae explode when it fell onto Comet 67P?

In a word: gravity. The force you feel every day, that makes things fall. By virtue of its mass, any given object exerts a gravitational force on other objects around it. (It’s a little more complicated, but we’ll leave it there). But your average everyday objects—tables, cricket bats, your neighbour—are too small for their gravities to be noticeable. Our Earth, on the other hand, is massive enough for you to notice the force, which of course you do every time you jump, or stumble, or an apple falls on your head.

In the same way, Comet 67P is massive enough too. But there’s a difference: compared to Earth, it is tiny. It measures about 4km by 3km, about the size of Delhi’s airport. A big rock, for sure, but vanishingly small as far as celestial objects go. The force of gravity it exerts is correspondingly weak. By some estimates, it’s about 10,000 times weaker than Earth’s gravity.

Think of it like this: one second after you drop that fridge from the 10th floor, it will be falling at a speed of 10m/sec. One second after Rosetta dropped Philae above 67P, it was falling at a speed of 1mm/sec. Like travelling through a thick paste, really. No wonder Philae did not shatter.

But it bounced. And there’s another problem. Bouncing off 67P meant the very real risk that Philae would fly off into space. On Earth, you can only escape the clutches of gravity if you’re able to reach a speed of about 11km/sec (the so-called escape velocity). Which is hard.

Which is why you need powerful rockets to fling Apollos and Mangalyaans into space. But puny 67P’s escape velocity is correspondingly low. One scientist recently calculated it as 46cm/sec, which is considerably slower than walking speed. So if you were on 67P and decided to take a stroll, you’d do well to tie yourself down.

That’s just what Philae’s designers had planned to do, but unfortunately its anchors failed to deploy. So Philae bounced.

The attractions of gravity, as you see.

There’s one more. When two objects of radically different sizes come close enough, the gravitational force between them pulls the smaller one into orbit around the larger one. Strictly, they are in orbit about each other, and this is clearer if they are comparable in mass.

(Picture whirling around you a pebble tied to a string, as opposed to an Olympic hammer-thrower who rotates his whole body to get the hammer moving). But if one is much heavier, the universe at large sees the little fellow orbiting the big guy. Think of the Earth and the Sun, or our moon and the Earth, or Mangalyaan and Mars. In the same way, Rosetta actually settled into an orbit around 67P—a delicate task, given 67P’s weak gravity—before it set Philae free.

Something about planning all this years in advance—down to how to place Rosetta into a precise orbit around a tiny speeding rock hundreds of millions of kilometres away—is a remarkable tribute to the practitioners of science. And to some certainties of science. Like gravity.

Now where’s that fridge?

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers will explore the joy of mathematics, with occasional forays into other sciences.

Comments are welcome at To read Dilip D’Souza’s previous columns, go to

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Published: 20 Nov 2014, 05:25 PM IST
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