How do objects orbit in space?

06 September 2016



Dear Chris,
My question is about orbiting bodies and Earth orbits in particular.
Is this statement correct? All orbiting bodies in the vacuum of space are perpetually free-falling faster and faster while encircling the larger body, but maintaining a constant distance from the larger body, because of their sideways speed, which, together with gravity, creates the circular orbit.Many thanks for your answer,
Leslie Wolf.


We asked Caroline how the Earth stays in orbit...

Caroline - I like to think of this sort of like a ball on a string. So, the ball is say, a planet, the string is the force of gravity and your hand that's swinging the ball around is the Sun or whatever the orbiting object is orbiting. So say, we're talking about a planet orbiting our Sun. So the gravity attracts the planet at a perpendicular direction to its motion. So it keeps traveling forwards but getting pulled into the sun, but not quite fast enough to spiral in and crash into the sun. But it's also not traveling fast enough to get hurtled into space. If it were to slow down, it would crash into the sun but because there's no particles, there's no air resistance so the planet keeps going and doesn't crash into the sun, doesn't fling off into space. It's kind of in a perfect happy medium.

Kat - Is it Newton's - I can't remember whose example originally of like firing a cannonball to explain that and if you had fired a cannonball fast enough, you could get it into orbit and that's the principle of space flight. Basically, you have to fire something up fast enough and the Earth falls away faster than the thing falls back into Earth.

Caroline - Yeah, exactly so the cannonball is falling towards the Earth, but the curvature of the Earth is such that it can never actually land. It just sort of keeps going and that's exactly the same principle as with planets orbiting the sun and moons orbiting planet.

Andrew - So I was wondering, with these orbits, how do we get so many planets having exactly the right speed to orbit? Because I would've thought loads of them will just fall in the sun and loads of them would shoot off.

David - If you fall sunwards, you start traveling faster and therefore, you can find yourself traveling fast enough to stay in orbit.

Andrew - So it self-corrects it?

David - I think so, yes.

Kat - When was the Solar System formed?

David - When was it formed? 4.6 billion years ago. There were lots of planet size bodies originally. We call them planetary embryos and they would crash into each other. So it's very difficult to define the age of the Earth because which of these parent bodies do we regard as a proto Earth(?) Until you've got a body that's almost an Earth mass, you haven't got the Earth. But it all happened within 10 or no more than a hundred million years at 4.6 billion years ago.

Kat - And that's everything just accreting together tumbled up.

David - It's starting off as gas and dust. The particles bump into each other. They're fluffy if they stick together. They get bigger and bigger then they get enough gravity to start attracting each other and then you've got runaway growth and it all kind of speeds up.


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