Author Topic: Could parabolic mirrors in low earth orbit be used to melt incoming comets?  (Read 4998 times)

Offline DOCOrion

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Could parabolic mirrors in low earth orbit be used to melt incoming comets?  Power plants focus sunlight to make steam power. 

Online edkyle99

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Could parabolic mirrors in low earth orbit be used to melt incoming comets?  Power plants focus sunlight to make steam power. 

Using light to modify asteroid orbits has been studied, but the methods were more direct.  They involved visiting the asteroid years before a potential Earth impact to either set up a solar sail/collector/mirror to gradually use solar pressure to change the orbit or to use solar power to burn away ice if present, creating thrust, etc.  One idea was to "paint" part of an asteroid with a light reflecting or absorbing substance.

Since it would probably take years to modify an asteroid orbit using solar pressure, it seems unlikely that an asteroid close enough to be affected by a LEO mirror could have its orbit changed in time.  As for "melting" an asteroid, if it is largely made of rock, the "melted" version will probably end up weighing pretty much the same as the "unmelted" version. 

 - Ed Kyle
« Last Edit: 01/28/2009 05:27 pm by edkyle99 »

Offline CriX

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Solar-pressure, and thrust produced from the comet's own material ablating are two different ideas, if I understand what OP is suggesting.  If this were a physics word problem I would look at a comet's orbit, try to calculate the force being imparted on it from the sun, and then subtract out the light pressure to calculate the pressure from the gases escaping.  Then you could look at heat energy received at a comet compared to this force and and figure out what size mirror, or (a more fun idea) what size solar cells or power source you would need for a large laser to ablate the comet and induce a desired force.
« Last Edit: 01/28/2009 06:34 pm by CriX »

Offline alexterrell

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In short, no. To be more powerful than sunlight, the mirror has to occupy a greater area of sky than the sun. To melt an asteroid you might need 25 times the power of sunlight so you'd need a mirror that looks five times the diameter of the sun.

As comets come in fast and don't spend much time in LEO, not possible.

You could steer a parabolic solar sail to a comet and then burn off part of it to change its course.

Offline jee_c2

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alextrerrell: "... To be more powerful than sunlight, the mirror has to occupy a greater area of sky than the sun. To melt an asteroid you might need 25 times the power of sunlight so you'd need a mirror that looks five times the diameter of the sun"

The term 25 Sun power refers to that the concentrated ray of light is 25 times stronger ("denser"), than the natural Sun light. To achioeve this, you don't have to have 25x area than the Sun's cross section. The question is, ho wmany energy do you wantto concentrate on how big surface (what should be the cross section area of the light ray - at the impact point). The area (ortogonal projection for the dierction of the Sun) of the collector mirror should be so many times greater than the planned impact area (where it reflects the light) as many Sun power you want to achieve. I.e. the comet's is cross section is 1000 m2. It's calculated, that 100x powerfull ray of light (100 Sun)  would be enough to melt it (on the given time). So we need 100 000 m2 of concentrator mirrors (actually even more, because  they will not all be just facing the Sun). With these and a mirror system, that directs the concentrated ray, you can "shoot" the comet to melt.

To come back to the original question: it depends on how big is the comet, and what is it made off, how dense it. Even: what is the albedo of it's surface, how many gasous material is surronding it's core...

Offline khallow

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jee, the problem is that mirrors cannot concentrate a diffuse source more than it's peak radiance per unit solid angle (not sure I have the right physical quantity here, but should be close). The Sun appears to be a 6000K black body. So any mirror is going to appear to be at best a 6000K body. The only thing that differs is the apparent size of the mirror to the asteroid. Now, in theory you could take a particularly enormous mirror and concentrate the light coming from a solar flare, which is higher intensity than the photosphere. But that would be much less viable than the original concept.
Karl Hallowell

Offline Art LeBrun

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What about the direction of the sun relative to the target?
1958 launch vehicle highlights: Vanguard TV-4 and Atlas 12B

Offline alexterrell

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jee, the problem is that mirrors cannot concentrate a diffuse source more than it's peak radiance per unit solid angle (not sure I have the right physical quantity here, but should be close). The Sun appears to be a 6000K black body. So any mirror is going to appear to be at best a 6000K body. The only thing that differs is the apparent size of the mirror to the asteroid.
That's what I thought, just wasn't sure how to explain it...

It means that if you need 100xsolar, the mirror has to appear 10 times the diameter of the sun. Now a comet is going to come in at 10km/s at the very least. If you're trying to burn with 100 x solar at 10,000km range, I calculate you need a mirror or 750km diameter.

What might work is the idea of a Mars halo mirror. This is a halo around Mars balanced by light, gravity and centripetal forces, which keeps Mars warm. But this has 30 million square kilometres of mirrors and weighs over a billion tons, and is out of scope for next 100 years at least.

Offline jamesbdunn

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Could parabolic mirrors in low earth orbit be used to melt incoming comets?  Power plants focus sunlight to make steam power. 

A solar sail of 1 square kilometer setup as a mirror, at the distance from the Sun as the Earth, can direct 1 gigawatt of energy, per mirror.

Correspondingly, a 170 kilogram solar mirror will accelerate 1500 kilometers per hour per day (rough estimate) due to photon pressure. 

Both of these parameters diminish with distance from the Sun.


At some point the solar sail would have to reverse direction; perhaps slingshot around a large celestial body.

At twice the distance from the Sun the solar power available would be about 250 Megawatts per mirrored solar sail.  Though there would have to be some system setup to counter the decelerating force of photon pressure while focusing the related solar energy (perhaps an ion drive).

If the ice were to off-gas in floating ice crystals that formed over many thousands of miles, I'm not sure what that effect would be compared to a central impact.  Same mass, perhaps a different trajectory for much of the ice crystals due to photon pressures.  But I don't know.

Would the gravity of the ice crystals counter the effect of the steam cones in the comet?

More questions than answers on my part.


Offline nomadd22

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 How much of that light do you think is going to penetrate the vapor cloud surrounding the comet? That's the reason the sun doesn't destroy comets in the first place. You melt a tiny bit of the surface and you have a fifty thousand mile (or more) thick, highly reflective shield around it.

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