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. For instance, we do not need to know whether the object is a loosely cohesive rubble pile or a monolithic rock. For many high-energy deflection approaches (e.g. Kinetic Impactors) this information is important for predicting the effect of the deflection attempt on the NEO. Conversely, the gentle Gravity Tractor approach only requires knowledge of the mass of the asteroid.

A spacecraft flying close to an asteroid acts as a gravity tractor. (Image credit: Airbus Defence and Space)

NEOShield looks at the effectiveness of a number of gravity tractor solutions, as well as at the technical challenges of a gravity tractor mission.

Some of the examined concepts are as follows:

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 gravity tractor, even a large gravity tractor, is that there is only a very small force between the spacecraft and the asteroid. This means that it can take many years to change the asteroid’s course enough so the rock misses the Earth.

The Gravity Tractor poses some quite difficult technical challenges such as spacecraft lifetime, launch mass and spacecraft control.

Multiple Gravity Tractors

An interesting alternative idea which the NEOShield project studies 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 or very little is known about the properties of the target NEO.

The Gravity tractor may also be used in combination with other deflection methods such as a Kinetic Impactor. For instance, the Reconnaissance spacecraft (Explorer) in a typical Kinetic Impactor mission scenario can be additionally utilized as a Gravity Tractor before or after arrival of the Impactor.