Why does the earth's magnetic field not cause a collision with another planet?

Rob Thacker
23 January 2012

The Earth's magnetic field is just like a bar magnet, with a North and South poles. We can measure its strength using a magnetometer. As it turns out, the strength of the Earth's magnetic field is about 100 times less than a typical bar magnet. So it's actually really weak. But that makes sense because paper clips and keys don't go zooming off to the poles!

Did you know? The Earth's magnetic field actually protects us from harmful fast travelling particles, called cosmic rays, that mostly come from the Sun. On a human mission to Mars protecting the astronauts from these rays is incredibly important.

But not only is the field very weak, it also falls off with distance very quickly. If you go twice as far away and measure the magnetic field strength you'll find it is only one eighth what it was before. For gravity it only falls to one quarter – so you feel the effect of gravity much more the further away you are. If you look at the strength of magnetic fields around other planets, Venus and Mars have magnetic fields that are even smaller than the Earth's! Jupiter's magnetic field is ten times that of the Earth's at the surface, but Jupiter is over 629 million km away. So as it turns out, gravity is actually far more important than magnetic fields.

Did you know? The poles of the Earth's magnetic field can flip! The last “reversal” happened about 780,000 years ago.

But even the gravitational attraction between planets is pretty small. What really matters in the Solar System is the Sun's gravitational field. It isn't too hard to calculate that the force between the Earth and the Sun from gravity is at least 26,000 times that between the Earth and Venus. So even gravity the gravity between planets can't overcome the effect of the Sun's pull. Which is a good thing! That's why the planets have orbited smoothly for billions of years.

Did you know? The magnetic field on the surface of a neutron star is at least 2,000,000,000,000 times stronger than Earth's!

There are systems in astrophysics where magnetic fields are incredibly strong. On a neutron star, a dead star left over from the explosion of a giant star known as a supernova, the magnetic fields are thousands of times larger than the strongest man-made magnetic fields. But there's one problem. The gravitational field on a neutron star is 500,000,000,000 times that of the Earth. You'd be squashed to pulp instantly.

Photo Credit: Wikipedia Media Commons

Rob Thacker

I am a Professor of astronomy at Saint Mary's University in Halifax, Nova Scotia. I use supercomputers to answer questions about how galaxies form and evolve. As well as being interested in things in the sky, I am also passionate about our planet and take time to go backpacking or diving with my wife Linda whenever possible. English by birth, I still haven't quite gotten used to Canadian winters!

Comments are closed.


Avatar  GT Stroller

Hi Rob,
If magnetism is weak compared to gravity as you say, why is it that with a fridge magnet weighing 20g I can make a pin jump upwards off a table in defiance of the gravity being generated by the 5.9e24kg mass of the Earth?