# Could Earth be knocked out of orbit?

21 April 2015

## Question

I have always been wondering if the earth's orbit is due to a perfectly balanced equation where the sun's gravity is cancelled by the inertia of our velocity, why is it that with all the meteors that have hit the earth why we haven't unbalanced slightly and with compounding, eventually fallen to the sun or broken free from our orbit. Does it have something with gyroscope properties due to our spin, and if so, how much meteoric impact can we take before our orbit will be forever lost?

We put Rod's question to astronomer Carolin Crawford...

Carolin - The first one that you asked is, what would happen if the earth stopped orbiting? First, let's just look at why the Earth orbits the sun. it's because it's just gradually being pulled around by the gravity of the sun. you know, if the sun wasn't there, the Earth would just continue on in a straight line in space. But because the sun's gravity is there, it just keeps being deflected all the time. so, the minute it stops orbiting, there's nothing left for it to do other than to fall straight into the sun. it doesn't take long to do this sort of back of the envelope calculations suggest it's something over 2 months, less than 100 days anyway. It's just going to fall straight towards the sun. it's going to get hotter as it does - Ginny is looking very dismayed at this news. I'm afraid it's not good news.

Chris - She likes the hot bit, but she doesn't like the fact she only got 100 days to enjoy it.

Carolin - She won't like this hot bit. It gets too hot for life pretty rapidly. Very rapidly, we kind of cross the orbit of Venus, that moves us out of this habitable zone, the safe zone for life around the sun. And by the time you're well into the orbit of Mercury, you're going to start getting temperatures in the surface, thousands of degrees, you're going to get rock melting. And when you get really close to the sun of course, you've got tidal forces so that the front end of the Earth feels a different gravity from the back end of the Earth so it's going to get stretched and squeezed. And kind of the whole Earth would disintegrate. So, that's really what would happen if the Earth stops orbiting. But just to stress, this is a thought experiment. So, we come on to the last question about how safe is our planet from losing that orbit. I mean, if you're going to change the Earth's orbit in that way, you have to change its momentum and that's not easy to do. You have to kind of apply huge external force or you've got to lose a big chunk of the Earth's mass and there's no easy way. I mean, even an asteroid collision would not lose enough mass to stop the Earth from its orbit. So, our planet is really safe from losing orbit. There's one little point talked in the middle there. we talked about the Earth stopping orbiting. The fact is not going to happen. The other thing that was mentioned was, what happens if the earth stops rotating. And again, thought experiment is not going to happen but imagine a great big finger comes out of the sky and just breaks the Earth like that.

Chris - National lottery.

Carolin - That's right. So, imagine it's as really sudden movement and it's going to be a big show. Everything is going to carry on moving sideways. We've all got momentum and the surface of the Earth is spinning. Well, it depends where you are in latitude but probably, where we are is pretty about a thousand kilometres an hour. So, everything is going to continue moving sideways at about a thousand kilometres an hour. So, we would all go flying. Anything lose, not tethered to the ground will go flying. Not just that though. All the oceans would slosh around and go sideways a thousand kilometres an hour, and the air. So, all the atmosphere would carry on going and just like scrape everything off. So, it does getting very serious if that happens.

Ginny - We did an experiment recently where you spin a boiled egg or a raw egg. And if you spin the raw egg and stop it and then let go again, it starts spinning again because of the momentum of the fluid inside. If our imaginary finger stops the earth and then let it go again, would the fluid in the atmosphere, the core and the oceans, would that be enough to get it going again?

Carolin - It would start it moving again, but it would probably still peter out in the long run. I mean, an interesting side effect, if you stopped the Earth spinning of course, you're going to stop perhaps that dynamo effect that drives the Earth's magnetosphere or the Earth's magnetic field. So, the earth would stop having magnetic field as well if you managed to stop all those internal motions.

Chris - Doesn't sound like a very attractive prospect?

Carolin - No. And the other thing is your day and night would last a whole year because instead of - when your day is depends on when the Earth rotates into view of the sun, it would just have to be when that part of the Earth came into view of the sun, as you follow the orbit. So, you've got a 6-month day, a 6-month night. I mean, just think what that would do to your climate changes and the wind patterns.

Chris - Roy Orbison said that, "I could drive all night." It could be a long journey then, wouldn't it?

Carolin - It could be a very long journey, yes.

Kat - But I think, given the likelihood of these things happening is very small, I'm going to put them lower down the list of things that keep me awake at night.

Carolin - Let it stress these are not things that we expect to happen at all, but they're still interesting to just sort of contemplate a bit.