Can Asteroid Mining Technology Serve as a Global Defense System?

[HopDavid]

The initial goal isn't landing platinum on Earth's surface.

Rather, parking a propellant source high on the slopes of Earth's gravity well.

The forum's not letting me post a link. Google: kiss caltech asteroid_final_report

 

[HopDavid]

From conservation of energy,

v_arrival = sqrt(v_escape² + v_{interplanetary}²)

So even if the asteroid is moving in nearly the same orbit as the Earth's, it will crash down at great speed -- v_escape ~ 11.2 km/s.

One could get it into Earth orbit and gradually lower it, and try for a soft landing that way. But even then, it'll likely crash into the Earth's surface at close to low-Earth-orbit velocity, about 7.9 km/s.

Using 3 body mechanics, there are some bodies that can be captured at EML1 or 2 with relatively small delta V.

From EML1 a .7 km/s acceleration suffices to drop the perigee into Earth's upper atmosphere. Once this is accomplished, each perigee pass through the upper atmosphere sheds a little velocity. Thus circular low Earth orbit can be accomplished with relatively little reaction mass.

Low Earth orbit is about 7.9 km/s as you say. But this doesn't mean the object would hit the Earth's surface at 7.9 km/s. You have to take into account ballistic coefficient which includes, among other things, ratio of object's mass to cross sectional surface area.

 

[lpetrich]

There is a certain problem with aerobraking: the square-cube law.

(acceleration) ~ (force)/(mass) ~ (area)/(volume) ~ 1/(size)

This explains why large objects reach the Earth's surface while small objects burn up in the upper atmosphere.

 

[HopDavid]

There is a certain problem with aerobraking: the square-cube law.

(acceleration) ~ (force)/(mass) ~ (area)/(volume) ~ 1/(size)

This explains why large objects reach the Earth's surface while small objects burn up in the upper atmosphere.

If you're arguing against soft landing kilometer sized asteroids, this argument's valid. But that's not what Space Resources is suggesting.

Again, nobody seems to have done much research. The KISS paper proposes returning ~7 meter rocks and parking in high lunar orbit.

For two reasons:

1) Parking rocks this size at EML1 is doable.
2) It's much safer. 300,000 kilometers is fairly distant from Earth's surface so an accidental impact is unlikely. In the unlikely event of such an impact, a 7 meter rock would burn up in the upper atmosphere.

 

[alensmith4]

Asteroid mining could result in costs so high that it would be better to find those elements the other way.

 

[negativzero]

Regardless of whether the industry paid off initially, the technology required for mining would provide the requisite super-structure for a world-wide asteroid defense. Protection needs to be in situ. Trajectories of known asteroids are difficult to predict with precision, and "new" asteroids can pop up.
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It would be foresighted to practice various means of moving asteroids around too.
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On the subject of both, i suggest a version of Clarke's elevator could be used. Most asteroids spin. That spin could be used with an elevator, perhaps a cable or cable on a tower, to launch ore projectiles beyond escape velocity.
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i'm dubious how much such a technique could deflect the orbit though. Because it seems like the launches would use up the spin rather than the velocity. i guess the average distribution of ore on the cable might shift the center of gravity a little. i can't figure out how much the orbit would change as a result of using an elevator as a mass thrower. The overall effect would look kind of like a bolo lasso. The more massive and longer the cable is, the more complicated the whole thing would be. It boggles. Yet if all that changes is the rate of rotation and the mass, the orbit should be substantially the same, assuming the center of gravity doesn't move around.
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i assume every asteroid will be unique, in orbit, size, composition, spin, and that a panoply of techniques will be needed to deflect any dangerous examples.
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A world-wide subsidization would be necessary to seriously fund asteroid defense. The world should be happy to survive if mining even brings back a fraction of the initial investment because the alternative is unacceptable.