The sharp nudge from throwing kilotons of rock can easily dwarf any movement by the boiling rock thruster.

;-P

~Jon

Guys, REALLY, and I mean R-E-A-L-L-Y need to read that.

Via anon at spacetransportnews.com

One was that we were going to harvest the asteroid in the process of changing its’ trajectory to a safer orbit. My mental scenerio was that the launch is 15 years before potential impact just as in the gravity tractor article. Then a low delta V window from asteroid to Earth is located in the 3-6 year from mission launch timeframe. That 3-6 years is used to rendezvois, land, set up all the equipment, and mine material to send. During the launch window, when delta V change to intersect Eath orbit is under 100 m/s, a payload is launched every 4 hours as accurately as possible on a Hohman orbit to a Lunar capture system. Under these conditions, the number of possible launches is limited, so payloads should be as large as possible. Large payloads want large tether masses, which possibly leads to in situ tether manufacture. After this major effort, the orbit is checked to see if it now misses the Earth by an acceptable margin. It might be desirable to wait on the next window to send more material to the Lunar orbital factories, or thrust as much as possible to change the trajectory to something safer.

If the only purpose is to change the orbit to something safe, then a much smaller tether becomes optimal with continous ‘thrust being applied in a constantly changing direction. You probably want the higher Isp available with an Earth manufactured tether, and the higher throughput available from the pure clothesline system. This even allows more than one throw per NEO 4 hour day, as pure propulsion could be scattered across 90 or so degrees since propellant is free and efficiency is less important. Perhaps a launch every 5 minutes for an hour and then a 3 hour break and repeat. This would allow an even lower mass tether per unit of thrust per hour.

NEO survey is an absolute requirement. It wouldn’t do any good to send a regolith scooping mission to a solid nickle iron body with no dust. Or to depend on collecting iron from a body that has none.

Unfortunately, it is very difficult to plan such activities without an accurate characterization of the actual mass, composition, distribution, and rotation of the targeted asteroids. What is really needed is a better understanding of these objects so that we can better plan to exploit them once we finally get the means and the motive to do something with them.

To get us started along this path, I would recommend that someone begin thinking about how to design a class of cube-sat that can be mass produced and then sent out on many parallel survey missions. Each probe would have an ion drive so that many bodies could be encountered and assessed. The Dawn mission to Ceres and Vesta should provide us with some useful experience conducting this kind of survey operation.

Bear in mind that the direction of thrust is not arbitrary, it is constrained to be within the plane of the object’s rotation. Somewhere along that plane, though, it is very likely that there is at least one direction that will give good results for moving the asteroid’s closest approach away from the Earth. (Typically the best is to thrust along the velocity vector to change the orbit period, this moves the impact point 3x faster than any crossrange thrust can acheive, and can be applied continuously to allow maximum impulse in the shortest time.)

That the hurled objects might approach Earth is not particularly likely, with a 100 m/s delta-V they’d probably be wildly scattered. A fat tether with large payloads should not be needed, just keep a skinny one really busy.

If you consider ten tons of xenon reaction mass used in an array of NSTAR thrusters with Isp = 3300 at 92 mN thrust each (http://en.wikipedia.org/wiki/Ion_thruster), the total impulse would be about 3E8 N.s, requiring 20 thrusters to run for 5.5 years.

The same impulse in 100 m/s gravel would require 3E6 kg of material mined (3000 tonnes), and to deliver the same impulse over the same 5.5 years would require a throughput of 0.018 kg/s. If you throw one lump on every four hour rotation of the asteroid, that has to be a 265 kg lump (or bag of regolith). If you have ten lumps in transit at any time, they only have to move up the tether at about 230km/40 hr, or a whopping 1.6 m/s.

Forget a standalone tether plus clothesline. all you need is a circular clothesline-like loop, with a wheel at the bottom rotating it and the bags. A loop of tensioned line hangs into the sky, passing a big flimsy wheel around a hundred meters across turning every 200 seconds or so… every four hours a bag comes down on the cable, gets filled, and swings back up. Ten empty bags are coming down at any moment, ten full ones going up, and the wheel grinds out power for the scraper and bag stuffer.

The hardest part for me is the manufacture of the tether in situ. I don’t think that is really necessary, though! Instead of shipping a micro manufacturing plant, just ship up a pre-made cable. The first 24 km or so is just cable. The lander uses a small rocket to deploy the first 24 km of cable. Then the lander crushes some rock, puts it in a bag, and attaches the bag to the cable and reals it out some more.

The basic idea is that the cable’s mass pulls the bags containing rock slowly past 23 km, at which point the bags of rocks help pull as well. Initially, the bags would hold only a few grams. When the end of the cable is reached, multi-ton bags could be used. Once the entire cable is reeled out, you drop it and get a large, instantaneous acceleration.

This lets you land far less mass on the asteroid, and get a far higher Isp. The equipment is a simpler crusher and bagger. The cable would mass maybe 1/1000th the mass moved.