Tractor Method of Asteroid Defense

     Inspired by the movie Armageddon, I researched the realistic techniques that Nasa has come up with to defend our planet against asteroids. The method that stood out to me as the most interesting was the Gravity Tractor method. The Gravity Tractor method is based on the most fundamental force in nature, Gravity. 

Image result for moon motion around earth
Orbits of the four inner planets and Jupiter (taken from flickr).

     If you have ever looked at the orbits of the planets around our Sun, or the orbit of the Moon around the Earth, you are looking at gravitation at its finest. Following Newtons 3rd law and basic gravitation equations, two celestial objects exert an identical force of gravity on one another over space that is equal to the product of their masses divided by the square of their distance, all multiplied by the gravitational constant G. The five inner planets all exert the force of gravity on the Sun and the Sun exerts the same force on each planet. The reason why each planet doesn't fall towards the sun is because they are moving at such high speeds that it causes them to fall around the sun. The sun doesn't move towards the planets because of its collosal mass.

     This gravitational force of objects in space is what the Gravity Tractor method is structured around. This is what it looks like in space.

Image result for nasa gravity tractor
     How it works is that there is a space craft sent to an incoming asteroid which then either orbits or remains stationary over the asteroid. The asteroid and the spacecraft will interact gravitationally and will naturally want to move towards one another. While the planets around the sun move too fast to move towards the sun, the spacecraft does not orbit the asteroid at a speed sufficient enough to not fall towards it. To counteract this, the spacecraft would need to undergo a constant thrust perpendicular to the surface of the asteroid. 

In this video it shows a spacecraft interacting with an asteroid. Notice how the spacecraft is constantly using thrust in order to not fall into the asteroid.

The asteroid is always chasing the spacecraft which in time will change the trajectory of the asteroid and miss the Earth. This method of asteroid defense is mesmerizing because the physics behind it is so simple and beautiful.


  1. Cool. One thing to point out, though, would be that methods like this take a lot of time to deflect the orbit of the asteroid by enough to matter, so it is critically important to identify threatening asteroids plenty early.


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