Science Questions

How do objects orbit in space?

Mon, 5th Sep 2016

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Leslie Wolf asked:

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...GPS satellite

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 thatís swinging the ball around is the Sun or whatever the orbiting object is orbiting. So say, weíre 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 itís 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 doesnít crash into the sun, doesnít fling off into space. Itís kind of in a perfect happy medium.

Kat - Is it Newtonís Ė 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 thatís 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 thatís 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 wouldíve 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 itís very difficult to define the age of the Earth because which of these parent bodies do we regard as a proto Earth(?) Until youíve got a body thatís 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 thatís everything just accreting together tumbled up.

David - Itís 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 youíve got runaway growth and it all kind of speeds up.


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I can modify it slightly to become a more accurate statement:

Where it went off track:
all orbiting bodies: Most orbiting bodies are not in circular orbits (with the larger body at the center), but an ellipse (a squashed circle), with the larger body at one of the two focal points of the ellipse. Even if you launched a satellite on a circular orbit around the Sun, it would not stay that way for very long, because the tug of other planets in the solar system would distort it into an ellipse.
free-falling faster and faster: If you dropped an object off a high building (in a vacuum), the longer it fell, the faster it would go. In this case, it is free-falling faster and faster. But this is a poor model for an an object in a circular orbit - for one thing, as it falls, it is getting closer to the Earth.
faster and faster: An object in a circular orbit maintains a constant speed as it progresses around its circular orbit.
faster and faster: Perhaps recently you saw an animation of two black holes colliding? In this case, they did go faster and faster as they spiraled inwards, but this only occurs in extremely strong gravitational fields, and does not apply to any detectable amount in the Solar system (or anywhere else I would like to be). But this is not a circular orbit - it is a spiral. evan_au, Tue, 12th Apr 2016

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