Gravity Tractor

Slowly but surely pulling the NEO to a safer course

The gravity tractor is an idea that was first proposed by two astronauts, Ed Lu and Stan Love. The idea is extremely simple – a spacecraft hovers above an asteroid and relies on the small gravitational attraction between the two to ever-so-slightly change the asteroid’s course. The beauty of this method is that it does not require a lot of information about the asteroid to allow the outcome to be accurately predicted – only the mass of the asteroid is needed.

NEOShield will look at the effectiveness of a number of gravity tractor solutions, as well as look at the technical challenges of a gravity tractor mission. Some of the concepts that will be looked at are as follows.

Gravity Tractor
(Image courtesy Astrium)

The Large Gravity Tractor:

Large gravity tractors can be used as the main method of deflecting asteroids. The larger and more massive the spacecraft, the more effective the method. The gravity tractor is also more effective if it hovers close to the asteroid. The major drawback of the large gravity tractor is that there is only a very small force between the spacecraft and the asteroid which means it can take many years to change the asteroid’s course enough so the rock misses the Earth. These result in some quite difficult technical challenges such as spacecraft lifetime, launch mass and spacecraft control.

Multiple Gravity Tractors:

An interesting alternative idea which will also be studied in detail during the NEOShield project is the concept of using multiple, smaller spacecraft flying in formation near an asteroid to perform the gravity tractor manoeuvre. This could help reduce problems with spacecraft reliability by using multiple, redundant, replaceable spacecraft to do the job.

Combined Gravity Tractor Solutions:

The gravity tractor has a great deal of potential, particularly when fine control of an asteroid’s course is needed. Often, the bulk of a deflection can be done using another method, such as a kinetic impact, with a gravity tractor being used to tweak the position of an asteroid into its final desired orbit.