Talk-Polywell.org

Close approaches will be in 2029 and 2036.

http://news.yahoo.com/s/space/20110206/ ... sianreport

Is that enough time to get ready?

Motivations: LEO resources and something really cool to do.

I went to an asteroid deflection conference a couple of years ago and listened to the options. At that time about the best they could offer for Apophis (which was the poster child for potential threats at that point) was that they might be able to deploy a few "gravity tugs" near it and use the mutual attraction to nudge it to one side a little. The gravity tugs would use a few DS1-style ion thrusters to station-keep off to one side. By applying this constant force, they figured they could possibly deflect it something like 30 km if they worked on it for over a decade.

Not much. But it might be enough that the 2029 pass was moved off a "keyhole" that would bring it back for a collision on the next pass. Or, conversely, maybe aim it for a more useful pass.

But to apply sufficient delta-V to it to drop it into a useful orbit is way beyond our present capability. For that you need a nuclear powerplant and a serious thrust system, such as a VASMIR or QED. You also have to de-spin the asteroid before you can use thrusters mounted on it. Nice application for a Polywell.

What we have is 2.7 kW ion motors with about the thrust of a half-hearted fart. What we need is gigawatt ass-kickers.

But yeah, I hate the idea of considering a resource pile coming that close as a threat to be gotten rid of.

What kind of deltaV are we talking about to drop it into orbit on the 2036 pass? No need to put it in a circular orbit all at once. But safest to do most of the slowing as it pulls away so Earth's gravity can't bring the point of closest approach any closer.

If that plan isn't viable, what about attaching thrusters on the 2029 pass? 7 years to alter the orbital eccentricity for a nice slow approach.

Apart from not having a suitable technology available yet to capture an asteroid with a good safety margin, there is also the consideration that a 270 meters rock is pretty small and you can't hope to realistically collect some valuable resources from it.

It would be cool thought.

Tom Ligon wrote:But to apply sufficient delta-V to it to drop it into a useful orbit is way beyond our present capability. For that you need a nuclear powerplant and a serious thrust system, such as a VASMIR or QED. You also have to de-spin the asteroid before you can use thrusters mounted on it. Nice application for a Polywell.

This is a Polywell forum, yes? Even if Polywell doesn't work out as well as our wildest dreams, could it still be useful for applications like asteroid deflection?

hanelyp wrote:What kind of deltaV are we talking about to drop it into orbit on the 2036 pass?

From the relevant wikis:
Apophis average orbital speed: 30.728 km/s
earth average orbital speed: 29.783 km/s
difference: 0.945 km/s

mass ~2.7 x 10e10 kg
approximate energy required = 1.2 x 10e16 J

Giorgio wrote:there is also the consideration that a 270 meters rock is pretty small and you can't hope to realistically collect some valuable resources from it.

mass ~2.7 x 10e10 kg! Perhaps not. But it looks like a good place to learn how.

Approximate mass of the Great Pyramid at Giza = 5.9 million tonnes. So Aphophis is about 4.5 times the mass of the Great Pyramid. At the least, I'd propose Apophis as the orbital headquarters of the Space Guard.

Paul March better hurry with his experiments.

rjaypeters wrote: [approximate energy required = 1.2 x 10e16 J

This would put it in co-orbit with the Earth around the sun. To put it into Earth orbit, you need to answer the question... how high? to put it into any orbit that would be useful, you would probably have to use about 16 times that energy. (delta V ~4km/s, not 1)

Okay, thanks. If we have 1.2 x 10e16 J available, then we could probably reach 19.2 x 10e16 J. Also, I don't think an Orion-style drive is the best idea for this project.

[Edit]Alternatively, we could shoot for one of the LaGrange points and reduce the energy required closer to my original, wrong, estimate.

There are several problems to consider. Using information from this source, the inclination of the passage will be ~ 40 degrees, the speed will be ~ 6.4 km/sec, and it will pass at a distance ~ the same as the geosynchronous satellites or ~ 30,000 km.

http://neo.jpl.nasa.gov/apophis/Apophis ... _PAPER.pdf

Apophis's speed will be ~6.4 km/s =~ 23,000 km/hr.
Geosynchronous satalites travel at ~8,000 km/hr.
That means that the asteroid would need to be slowed by ~ 15,000km/hr. to achieve a similar orbit.
Lets see, a typical Shuttle launch may add ~ 30,000 km/hr (being generous) to a mass of ~ 300,000 pounds (again being generous) or ~ 150 tonnes.
If Apophis weighs ~ 20 million tonnes, then the weight ratio would be ~ 130,000/ 1.
The speed ratios would be ~ 15,000 km/hr required delta V divided by 30,000km/hr per Shuttle flight. or ~ 0.5

So the amount of thrust needed would be ~ 130,000 * 0.5, or ~ 60,000 Shuttle flights. This is a simple analysis, but it illustrates the magnitude of the effort needed.
Other considerations like gravitational assists, multiple orbits, higher efficiency thrusters, etc would modify this somewhat, but still a stupendous effort would be needed.

And you would end up with a large rock (or collection of rocks and dust) in a high orbit inclined at 40degrees. Much harder to access than a LEO or geosynchronous equatorial orbit.

If you are going to expend such stupendous efforts for a relatively small rock, I suspect you would be tremendously ahead if you instead established and maintained lunar access. You might even be ahead if you flew to Mars and utilized Demos and Phobos. There are the locations for your Space Command.

Alternately, boosting your supplies from Earth and building a very large orbital station would probably be cheaper than capturing and mining Apophis for a similar amount of useful material or real estate. Using Shuttles, you could boost ~ 1 millions tonnes of material to LEO with the same effort required to capture Apophis into an inconvenient orbit.

Efforts to change the orbits of Earth crossing asteroids, if started soon enough) only involves changing the velocity or direction of the Asteroid by only a few meters per hr (or less). Over a 10 yr period this tiny change in velocity (1 M/hr) would result in a miss distance of ~ 1M/hr * 24 hr/day *3,650 days =~ 87 Kilometers. That may be enough, along with other orbital dynamics effects. If a NEO is heading for a bulls Eye hit on the Earth and the time is short, much higher delta V's may be needed, and the time to max out your credit cards would be at hand. If the Asteroid is small you might be able to blast it to pieces with a series of hydrogen bombs. This probably wouldn't change its course much, but if many of the pieces are small enough to burn up in the upper atmosphere and the pieces that reach the surface are dispersed, the global consequences would probably be much less. You might not appreciate that if one of the chunks is heading for your city (or State).

Dan Tibbets

I'm thinking the efficient approach: ensure Apophis does not hit the astrodynamic keyhole necessary to hit the earth by emplacing the thruster(s) ASAP. Also, if we are happy with a Sun-Earth LaGrange point, the mission requirements relax further. Earth-Moon LaGrange points increase delta-V and then LEO orbits even higher.

I'm not sanguine about the heroic, high-thrust, short timeline approaches (e.g. Orion), especially with this "LL chondrite." Lower thrust, higher isp is the way to go, IMO. Since the surface is already so dusty, we might use the asteroid itself for reaction mass.

Apophis is merely the worst case we know about. The probabilities look low for a threat to the earth, this time.

Back to my original question: Is there enough time to get ready? I think so, especially if the Polywell concept works as we hope.

Oops, should be "Spaceguard" a la Arthur C. Clarke.

Part of the "attraction" of gravity tugs is they will apply a gentle force that will affect even a rotating body, and will work on a loosely-bound rubble pile. Any attempt to attach a rocket motor to a rubble pile is probably doomed. We don't know much about how solid most asteroids are.

The asteroid Hartley imaged a few months ago by the EPOXI probe is neat ... looks like a peanut. It has jets, probably indicating considerable water or other volatiles. The astroid is probably actually two rocks in contact with each other, with the space between filled with loose debris. This assembly tumbles slowly, in two axes IIRC. To attach a rocket motor to t one must first stop the tumble and spin. Then you would have to strap the two pieces together or it would be like trying to push one marble with another.

http://epoxi.umd.edu/3gallery/20101104_Sunshine3.shtml

We don't know nearly enough about asteroids to try this yet. Each one is unique. We should have sent out a swarm of probes long ago to put something like rovers on them, to return data and probably samples, too. Today nobody is sure if any given piece of rock is worth exploiting, so nobody is pushing to do it. Finding one that contains, for instance, both metal of reasonable utility plus a lake's worth of water ice might change things considerably.

It might be possible to rig some fancy Venus-Earth-Moon slingshot maneuver to get rid of much of the excess energy needed to bring one of these things into orbit. This would take some years, and you would still have to de-spin and add thrusters, or else manage it with a gravity tug. De-spun even a solar sail or mag sail might work.

Tom Ligon wrote:... Any attempt to attach a rocket motor to a rubble pile is probably doomed.

Not probably, certainly.

Which is why I like wrapping our unknown composition asteroid in a not-too flimsy bag, cinching it down and attaching the rocket motor of our choice (or use a gravity attractor). Heck, with some bodies we could solar heat the bag and asteroid and use the ejecta for reaction mass. But, we'd be wasting the volatiles we want...

Disadvantages? Yes, additional mass, something we haven't done, etc. But we haven't done any of these things.

Advantages? I think we can use the technique on all the not-too-solid bodies, etc. Not one size fits all, but one technique fits many.

EPOXI is an interesting case for efficient orbit changes. This is the spacecraft that did the Deep Impact mission, slamming comet Tempel 1. It was then repurposed for the Hartley 2 flyby. I correct my earlier post ... Hartley 2 is a comet nucleus, not an asteroid. But that's OK as some of the Apollo asteroids probably are as well.

Anyway, I wondered how they made such a big orbital change so as to intercept a second target. I expected to find that it has xenon ion thrusters of the same type as DS1, but in fact it has plain old hypergolic fuel thrusters. The total delta V available was tiny, some few hundred meters per second if memory serves. It carried only 84 kg of fuel on a 517 kg dry mass spacecraft. With this they managed an amazing dance. And Cassini is similarly incredible, doing a continuous series of billiards shot from one keyhole to another around Saturn's moons, with only tiny amounts of fuel used.

But returning to Earth and dropping in to orbit would probably take about a 2 mile per second (3219 meters/sec) delta V at a minimum (the difference between LEO orbital velocity and escape velocity from LEO). Parking an asteroid will take something like that (the higher the orbit the less one needs to scrub off, but the harder it is to get to).

I agree, using water from the asteroid to make thrust probably defeats the whole exercise. Obviously the reaction mass expenditure to move an asteroid is going to be stupendous. Insitu resources are needed, but better to use rock dust. This is not the best choice for high Isp, but it is the least valuable material. Charge it up to high voltage and accelerate it away. Use any power source we have ready to go, preferably a boron-burning fusion plant. With enough lead time one could even do this with solar, but in the time it would take to do that we might have fusion running.

My thoughts exactly.

"In situ resources" reminded me about chrismb's P + 15N fusion. On some classes of heavenly bodies, it might be worth mining for fusion fuel for power during the boost.

A stray thought here.

Your typical automobile can do 0-60 in something like 12 seconds. I made a WAG that you might, doing a series of mild accelerations followed by slowing down, get an hour of acceleration out of a tank of gas. Turning the crank, that would give the car something like 8000 meters/sec of total delta V on a tank of gas. Eight kilometers per second ... five miles per second.

Would that we could do this with space probes!

Just something to think about as you press the pedal of your own personal little high-tech George Jetson toy. Just think, SSTO on a tank of gas, and escape velocity with the optional larger tank.