guest blogger john hare
I briefly got into another discussion about moving asteroids recently. It involved parking an eleven ton spacecraft next to the asteroid and letting the gravitational attraction between the two shift the asteroids orbit. Then when the spacecraft gets too close, use the thrusters to open the gap again. When I said that a piece of thread would have just as much pull, and that just as much propellant would be used in either case, it was suggested that I learn something about conservation of momentum. Without further explanation from the other guy, I just assume that it is another of those concepts over the intellectual head of this redneck. If the rock is dangerous though, we need to do something about it.
Thinking about the subject, with the eleven ton vehicle and fifteen year time frame in the article, I think much more efficient use of mass, time and money can be done. They suggest in the article that a half kilogram or so of force would be applied by this gravitational tractor concept. I’m going to try newtons this time to see if I can get them right. Fortunately there are several people here to straighten me out if I screw it up. 5 newtons force for an hour is 18,000 newton seconds applied. In a day that is 432,000, and a year is 157,680,000, and 15 years is 2,365,200,000 newton seconds. That’s a real big number, maybe. The F1 Saturn engine averaged about 7,000,000 newtons, so in about 338 seconds, one of them could apply as much total force. The problem is propellant of course.
During the discussion the ideas of painting part of the NEO and vaporizing the surface for reaction came up. In other discussions nuclear bombs, electric engines and fancy flyby trajectories are mentioned. I think most people miss the point that most asteroids have enough internal energy to move themselves if properly persuaded. The rotational energy alone is more than enough to shift an orbit to safety if we are clever enough to tap into it for propulsion purposes. The ideas here are not new, just my interpretation.
An asteroid is a natural for a beanstalk. A tiny rock with 10 m/s escape velocity and a four hour day would have a NEOsync orbit at about 23 km. If the composition contains enough steel, iron, or other tensile useful materials in attainable form, then an in situ material beanstalk is feasible. Here I am assuming that something can be extruded with material properties half as good as terrestrial rebar. A 100 m/s stalk tip velocity would be at 230 km which would be the total length of the system. With a taper ratio of two, a mass ratio of one results for tether to tip payload.
If a thousand tons of NEO material can be extracted for beanstalk use, then thousand ton payloads become feasible from the tether tip at 100m/s. It would seem that throwing a thousand tons at 100m/s would deliver a 100,000,000 newton impulse to the NEO. 24 such payloads would exceed the impulse delivered by the gravity tractor in the other article. One a beanstalk is set up though, sending dozens or thousands of payloads is just a matter of extracting and bagging NEO material.
The neat thing about a beanstalk much longer than NEOsync is that the energy needed to lift and throw the payloads is supplied by the rotational energy of the NEO itself. Once past NEOsync, the payloads fall out to the end point, and they are quite capable of lifting the next payload off the ground with a light spectra tether that masses a fraction of a percent of the material lifted. This spectra tether is separate from the beanstalk and just used for propulsion purposes, returning to the ground after each lift. There is no need for energy delivery to the vehicles, which are basically bags with tether brakes.
One the beanstalk is built and reaction mass collected, the operation waits until the proper orbit to throw the payloads/propulsion sacks to Earth or Lunar orbit. It seems likely that this window will be during perihelion and last a month or so. A thousand ton payload every four hours for a month puts 180,000 tons of material on trans Earth/Luna trajectory. At the same time, it delivers an 18,000,000,000 newton impulse to the NEO, which should move it out of the danger zone during the flyby that happens in a dozen years or so. This impulse, delivered earlier and more concentrated than the gravity tractor idea, should be proportionately more effective, even discounting the nearly eight times more power.
If harvesting is not wanted for some reason, and mission mass is the critical restriction, then one ton payloads could be done with a hundred kg spectra tether every four hours for the fifteen years suggested in the article. The thousand ton units seem more worth chasing and catching to me for a space faring economy.