[Editor’s Note: When I posted the Enhanced Gravity Tractor article earlier in the week, I didn’t have time to really dig into the concept the way I had originally intended. Josh Hopkins suggested on Twitter that I could cheat and revisit the topic in a follow-on blog post since I’m trying to do this blog-a-day for this last month leading up to the 10th anniversary of Selenian Boondocks. This is probably the first of two or three follow-on posts]
The previously linked-to EGT paper had a great introduction to the concept of using a gravity tractor for deflecting potentially hazardous asteroids. In all gravity tractor concepts you’re using the mutual gravitational attraction between the spacecraft and the asteroid itself as a way of transferring thrust from the spacecraft’s thrusters into the asteroid itself. You can do this using an in-line tractor orientation, where you cant the spacecraft engines outwards at an angle sufficient to avoid plume impingement on the asteroid, and you eat the cosine losses on the thrusters.
Or you can place the spacecraft into a halo orbit around the asteroid, and fire the thrusters due backwards.This shifts the orbit from orbiting around the equator of the asteroid to an offset halo (that looks like a spiral from a sun-centered perspective).
I prefer the halo approach, both because you can probably make it passively safe (where a thruster failure doesn’t involve the spacecraft colliding with the asteroid), because you can probably have your spacecraft a lot closer to the asteroid thus increasing the gravitational acceleration (and thus peak thrust you can impart), because you avoid the cosine losses from canted thrusters, and because it’s a lot easier to add multiple gravity tractors flying in formation with the halo/spiral approach.The equation governing the thrust you can impart into an asteroid in such a halo orbit is:Where rho is the radius of the halo orbit, and z is the axial offset distance of the halo orbit, G is the universal gravitational constant, Mast is the mass of the asteroid, and Msc is the mass of the spacecraft. As you can see, the closer you are in, and the heavier your spacecraft, the more force you can transmit into the asteroid, hence the idea of augmenting the spacecraft mass with locally harvested regolith, rock, and boulder materials. It is pretty easy to increase the effective towing mass by >10x using locally harvested materials. While traditional gravity tractor methods required more than a decade of advanced notice, enhanced gravity tractoring might only take a year or two of advanced notice if you already have the infrastructure in place to deal with a threat.
As an interesting aside, I noticed the other night that it looks like my coblogger John Hare might have actually beat the NASA guys to coming to this conclusion by at least a few months, based on this Selenian Boondocks blog post from February 2013, which describes a concept almost identical to the one shown in Figure 20 from the paper.