Due to the lasting nature of communicating on these various internet sites, I am on record in several places as saying that gravity tractors for moving asteroids are stupid. I could see no way that gravitational attraction between an asteroid of a few hundred thousand tons and a spacecraft of a few thousand pounds could make sense as a means of linking them for thrust, however gentle. So after my last snark over at transterrestrialmusings.com, Doug Jones destroyed my little conceit with numbers. John Schilling weighed in with a bit more conceit destruction.
Doug brought in something that I hadn’t realized with his numbers. He worked them with a spacecraft mass a couple of hundred tons. I had made the assumption that the craft would be yet another of the fragile constructs similar to most probes flying out there now. With a couple of hundred tons of spacecraft though, it becomes possible to have enough coupling to actually move the asteroid, albeit very slowly. The other two things Doug and John Shilling brought in were the difficulty of finding good attachment and the fragile nature of rubble pile asteroids. The lasso and drag that I had assumed could be more difficult than I had thought and might just tear the rubble pile apart.
Without the numbers from a known rocket
scientist, plumber, I might have quibbled a bit more even on the losing end. I am not, after all a graceful looser, nor do I like being shown up as a fool. So as revenge, I modify the concept to fool people into thinking that maybe it was my idea all along. If two hundred tons is good, two thousand tons would be better, especially with ISRU.
Sending a two hundred ton vehicle to a dangerous asteroid would be a bit difficult today. IMLEO would probably be on the order of a thousand tons or so. Even under the dinokiller or city buster threat, the empire builders might just delay things by scarfing funding for heavy lift, nuclear thermal, or other program that could easily extend beyond the launch window for the threat.
I suggest it might be possible to get similar results with less spacecraft with a bit of low tech ISRU. The spacecraft uses the previous flyby surveys to locate a boulder that masses enough to generate the gravity attraction desired. It lassos the boulder from the rubble pile and pulls it free of the rest of the asteroid. The spacecraft then pulls the boulder into the appropriate location to attract the rest of the asteroid along the desired thrust vector. The craft lets out enough tether to hold the boulder in the right place while being well clear of the asteroid itself.
The advantages would be less cosine losses from the thrusters as the spacecraft itself would be well clear of the asteroid at the end of the lasso/tether. Acceleration could be higher with more mass of ISRU boulder than deliverable spacecraft. And less spacecraft to get the job done, which could be the difference between on time success and too little too late with the original concept if too much time is wasted on ‘perfection’.
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