Could a Gravity Tractor be used for mining asteroids?

Question

Recently, I came across the concept of the Gravity Tractor for the first time. And I was intrigued.

https://en.wikipedia.org/wiki/Gravity_tractor

A Gravity Tractor, as I understand it, is a concept for a small unmaned spacecraft which is launched into space, and sent to an asteroid. It rotates around it to change the direction of the asteroid. The major benefits is it's easy- no fancy rockets, no need for theoretical solar sails, no need to have any real understanding of the asteroids make up. The major downside is it's slow.

Now, every time I've come across this idea, people talk about using it to redirect an asteroid from impacting the Earth. This runs into the problem 'Can it redirect the asteroid in time?' since a Gravity Tractor is so slow.

But what about non-time-critical applications? Like mining asteroids?

Mining asteroids is hard because they're so far away- we'd need a very large ship to go all the way out to the asteroid belt and mine it and send resources back- and it would probably need to be manned.

But if those asteroids could be towed back to Earth orbit, suddenly mining them becomes a lot easier.

So- could a Gravity Tractor realistically be used to relocate mine-able asteroids closer to Earth? What size asteroids? What size ship would be needed? And what sort of time frame would it take?

Answer 1

Down the bottom of the linked page is an example case, of an asteroid that needs 1cm/s to not hit earth earth, applied over 10 years.

The gravity tractor is not magic, it still needs the tractoring vehicle to expend fuel to change orbit of the combined total mass, and in fact has to do so inefficiently due to the need to not have exhaust hit the towed object.

So if you have an asteroid that only needs 1cm a second change to get into a stable earth orbit than yes it can work. For reference getting something from outside the earth moon system into stable orbit is the reverse of getting a probe out, so things like this can be used to identify that we would expect to need more than 3km/s to bring that asteroid into stable LEO, so we need a lot more zeros than the described 1cm/s. Time would also be a problem, since the very low thrusts you would end up spending a lot of time crossing and recrossing the moon's orbit needing to avoid being either ejected or turned into an impactor.

So this only works if you have a massively powerful craft. And if you have a massively powerful craft adding the hardware to strap the payload down and push it home starts to look better since you can use that engine to it's full capability and avoid melting the payload with the spill from the exhaust.

There should be some cheap(ish) orbits that reach Lagrange points but they are not particularly easy to reach from earth.

This process starts to make more sense if you are working amongst gas giant moons or from one of the larger asteroids. There you are more likely to find a 'cheap' candidate object that only need minimal changes to reach you.