Ok, so once the asteroid collector has delivered the asteroid to high lunar orbit, what does the spacecraft do then?
Well, if its got even a tiny fraction of its propellant left over (remember it just towed something maybe 100x its size clear across the inner solar system) , it slowly spirals down to low earth orbit and... REFUELS.
Now here's where things get interesting. Once it's refueled (remember its main consumable is up to 12,000 lbs. of Xenon, it gets its energy from solar power), it can do any number of things. Of course it could be sent out again to get another asteroid (including, as I mentioned in a previous post, one with precious WATER) but that might be boring. How about having it PAY FOR ITSELF by moving satellites from LEO to geosynchronous orbit. (This is very expensive as it typically requires an additional booster, I think the cost per pound is at least double that to low orbit). I think this market is on the order of $5B per year.
The reason why this would work is because the asteroid tug would clearly be capable of moving very(!) large payloads. It wouldn't even have to be very slow, if it can accelerate a 500 ton asteroid at 1/10,000th of a g, it could accelerate a 5 ton satellite at say 1/200th of a gee (taking into account the tug's own weight). So it could deliver the satellites in weeks if not days. Of course there would need to be a few minor design modifications to the tug. The collapsible "bag" would have to be removable and some sort of industry standard docking ports added. There would need to be some provision for refueling ports and critical components (gyroscopes, reaction wheels, electronics) would need to be replaceable/upgradeable like the Hubble space telescope. Of course servicing this "space tug" in this way is probably beyond the near term capabilities of robotics. However, rather than this being a problem, it could be an opportunity -
- for the International Space Station to actually be USEFUL. Here it could serve as a fuel depot, servicing "garage" and interchange point for these "space tugs". The kind of problem that robotics can't handle yet are ideally suited for an astronaut with a wrench (and maybe some elbow grease). The fact that the main propellant for these tugs is Xenon, an inert noble element, makes handling the fuel much less problematic (no problems with corrosion or toxicity) and safer (no fear of explosive combustion). Even the fact that these tugs use ion thrusters would be an advantage meaning that everything would be happening very slowly, if one went out of control they could probably move the entire station out of the way (like they do when avoiding space junk). The station could also keep spare, interchangeable parts for these tugs such as additional "bags" or robot arms or other modules. In short, the ISS would have a PURPOSE.
With even a little thought, these space tugs have lots of additional uses. The same high power ion engines that can move a 500 ton asteroid could also send 500 tons of cargo cheaply (if slowly) to Mars. The same collapsible bag that can capture a tumbling asteroid can easily capture a much lighter piece of space junk. All it takes is for a government with foresight to make the initial investment that may (as I've suggested) quickly repay itself perhaps many times over. And isn't that the purpose of government (if not NASA)?
(By the way, putting the mini-asteroid in high lunar orbit may be useful as a last resort because, if we detect a threatening object heading our way, it might be in a good position that we could put the mini-asteroid on a new trajectory to hit the object and thus deflect it out of the way. With luck the 500 ton mass will strike the incoming object at a high incidental angle and at a significant velocity since it'll be coming from a completely different orbit. Of course it would be much preferred to nudge the incoming object years before in deep space off of an intercept trajectory but if we're caught with our pants down it would be nice to have a big rock whirling in the sling of its lunar orbit. In that case, we coud call it "David's Rock" or "The Goliath Killer".)