Author Topic: Preventing asteroid disaster (via laser)  (Read 6282 times)


Offline simonbp

Re: Preventing asteroid disaster (via laser)
« Reply #1 on: 02/26/2007 02:16 PM »
Hooray for UAH, defending the world from asteroids with gigantic lasers!

Simon ;)

Offline Stowbridge

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Re: Preventing asteroid disaster (via laser)
« Reply #2 on: 02/26/2007 03:43 PM »
Good article. Although I thought the laser option was always unworkable.
Veteran space reporter.

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #3 on: 02/27/2007 12:22 AM »
It is interesting, but it doesn't explain what the trick would be. Are we applying radiation pressure to deviate the asteroid or we plan to focus the laser so that by overheating material on the asteroid surface we have a propulsive effect?

I recently operated robotic laser welding systems. Focusing the laser to obtain material heating was already challenging enough from 1 meter distance.

Besides, a laser in space is going to be considered by somebody "weaponization of space".
"Paper planes do fly much better than paper spacecrafts."

Offline simonbp

Re: Preventing asteroid disaster (via laser)
« Reply #4 on: 02/27/2007 01:09 PM »
Quote
sandrot - 26/2/2007  7:22 PM

It is interesting, but it doesn't explain what the trick would be. Are we applying radiation pressure to deviate the asteroid or we plan to focus the laser so that by overheating material on the asteroid surface we have a propulsive effect?

I recently operated robotic laser welding systems. Focusing the laser to obtain material heating was already challenging enough from 1 meter distance.

Besides, a laser in space is going to be considered by somebody "weaponization of space".

The ablative effect of a laser pulse is a direct function of the irradiance/intensity at the target spot, which is basically the laser pulse power, divided by the spot area. Most likely the laser welder is a big dirty CO2; the pulse power is pretty low, and so you have to focus it down to increase the intensity. Something with a stronger pulse power, like a diode-pumped ND:YAG, can do a lot of damage, even unfocused; I have several shirts with holes blown in them from accidentally standing in front of an ND:YAG (thankfully no limbs, though)...

At any rate, firing a really big IR laser at a silicate asteroid would blow off chunks of material off the surface, producing a propulsive effect (without any need to physically put anything on the asteroid, unlike other mass-driver concepts). An iron asteroid could allow a laser-sustained plasma plume, effectively turning the asteroid into a gigantic ion engine...

Simon ;)

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #5 on: 02/27/2007 02:00 PM »
You are correct about my big dirty CO2, still I have difficulties to believe we can transfer significant energy to the surface of the asteroid from, say, 50 million km. Different the story if we plan to "land" a laser on the asteroid surface.
"Paper planes do fly much better than paper spacecrafts."

Offline simonbp

Re: Preventing asteroid disaster (via laser)
« Reply #6 on: 02/27/2007 09:19 PM »
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sandrot - 27/2/2007  9:00 AM

You are correct about my big dirty CO2, still I have difficulties to believe we can transfer significant energy to the surface of the asteroid from, say, 50 million km. Different the story if we plan to "land" a laser on the asteroid surface.

You're thinking atmospherically. :)

In vacuum, the only thing that reduces the laser's strength is spreading of the beam by the inverse square law. A laser, though, is inertly collimated (all light pointed in the same direction), so the beam will not significantly spread out over long distances. Coupled with a properly focused laser, there is no reason that the energy coming out of the laser aperture will not be exactly the same as the energy at the target asteroid. The real trick (not surprisingly like missile defense) is in tracking the target, and we're getting increasingly better at that...

Simon ;)

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #7 on: 02/27/2007 09:59 PM »
Properly focused is also part of the trick. Your focusing optics must be built resilient to the laser pulse, or they will be damaged.

I googled around some info regarding the airborne laser project, which might be somewhat relevant to asteroid deflection by laser.

http://www.lockheedmartin.com/wms/findPage.do?dsp=fec&ci=11457&rsbci=5&fti=0&ti=0&sc=400
http://www.space.com/businesstechnology/airborne_laser_techwed_041117.html
http://www.fas.org/spp/starwars/program/abl.htm
"Paper planes do fly much better than paper spacecrafts."

Offline joema

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RE: Preventing asteroid disaster (via laser)
« Reply #8 on: 02/27/2007 10:45 PM »
The Boeing Airborne Laser actually illustrates why current or planned military lasers aren't sufficiently powerful for asteroid deflection.

The ABL is a 1 megawatt Chemical Oxygen Iodine (COIL) laser, emitting a 1.5 meter diameter beam at 1.35 micron wavelength (infrared).

It supposedly will be able to fire about 20 rounds of (maybe) several seconds each. If we take 3 sec/round as typical, that's 60 sec total firing time. Total energy emitted would be about 60 million joules, but you'd lose energy from atmospheric absorption, even if adaptive optics maintained focus.

Due to beam divergence, it would also only be useful at very close ranges. Beam divergence is given by the formula:

divergence in radians  = (wavelength * 4) / (pi * beam diameter)

So divergence would be 1.15 microradians. At distances of more than a few thousand km, the beam would be much wider than the asteroid.

Even if 100% of energy was delivered,  60 megajoules is roughly the energy of 1/2 gallon of gasoline. It's just not enough, when the object weighs 2.1E10 kg.

The article is dealing with hypothetical ground-based lasers of vastly larger power than anything planned. The ABL actually delivers more energy within 1/10th sec than the Lawrence Livermore NOVA laser used for inertial confinement fusion experiments: http://en.wikipedia.org/wiki/Nova_laser.

In general lasers aren't a very effective method of delivering energy in asteroid-deflecting quantities.

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #9 on: 02/28/2007 02:33 AM »
In my opinion the article was referencing more space based applications. I made a connection with ABL because it is a powerful and compact system. How much compact has to be seen, the laser sources are as big as six Suburban SUV's. If the plan is to send that much mass (and more, as the lasers would need a megawatt class power supply) in close vicinity to an asteroid, we need NASA to complete at least the development of Ares V.
"Paper planes do fly much better than paper spacecrafts."

Offline sandrot

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"Paper planes do fly much better than paper spacecrafts."

Offline joema

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Re: Preventing asteroid disaster (via laser)
« Reply #11 on: 02/28/2007 03:06 AM »
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sandrot - 27/2/2007  9:33 PM

In my opinion the article was referencing more space based applications. I made a connection with ABL because it is a powerful and compact system...
You're correct, I was thinking about this article which discussed more powerful ground-based lasers for asteroid deflection (1 MB .pdf): http://www.llnl.gov/planetary/pdfs/Interdiction/04-Phipps.pdf

The problem is powerful laser systems are very heavy. Putting them in space imposes a huge additional burden, beyond the already considerable task of just making the laser. The ABL is a good example. It's already been tested on the ground, the problem is fitting everything in the space/weight constraints of a 747 and making it function reliably in that harsh environment.

The above article says you need multi-gigawatt power to deflect asteroids of meaningful size. That's thousands of times bigger than the Boeing ABL. Putting that in space would be impossible with current technology.

I think the newspaper article in this thread assumed deflection just sufficient to miss the gravitational keyhole, not to miss the entire planet. That makes the task much easier, but there are still likely better techniques.

Offline TyMoore

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Re: Preventing asteroid disaster (via laser)
« Reply #12 on: 02/28/2007 09:14 AM »
I would imagine that laser pumped ablation of asteroid material would create a plume of material that moves significantly slower than the jet from an ion engine--this in itself isn't really a big deal. It has the entire asteroid  to supply reaction mass. But the power system onboard the spacecraft presents a problem. If we go with a CO2 LASER, then the pumping is usually done with either a rf system or electrical discharge. In which case, depending upon the position of the spacecraft and the average power output of the laser, could either be powered by solar power or nuclear power.

How about a Solar Powered Gas Dynamic laser?

If CO2 is heated to pretty high temperature and pressure using a large solar concentrator, and then expanded through a two dimensional de Levaal nozzle with a semi-transparent window on one side and an optical mirror on the other--conditions for a far-infrared laser can be set up.  The CO2 could be compressed and expanded using a closed cycle Brayton gas turbine coupled with an ammonia based heat rejection system to condense the CO2...

Such a system ought to have an optical efficiency of 20% or so and deliver a few of megawatts of continuous beam power. If a beam chopper is used, it might be possible to raise the peak power to several times this--but obviously peak power won't be anything near a modern Q-switched, diode pumped lasers--but the continuous power rating will exceed just about anything else on the planet...

I wonder if a direct nuclear reactor pumped xenon-chloride excimer laser is possible?

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #13 on: 02/28/2007 11:13 AM »
The article points out that we need a better threat detection system. We had a number of close calls in recent years. In one occasion the asteroid was discovered four days before it transited by Earth.
"Paper planes do fly much better than paper spacecrafts."

Offline meiza

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Re: Preventing asteroid disaster (via laser)
« Reply #14 on: 02/28/2007 12:11 PM »
Interesting, TyMoore.

I understand semiconductor lasers are advancing at a quick pace. They have the benefit of not having moving parts. Maybe in ten years there could be thousands of solar powered satellites in space (at some L point perhaps?) with kilowatt class lasers, and they could fire focusedly to an asteroid to get very hot conditions. I'm not very familiar with the optics here though, and how big the beam spot could be from a few meter mirror.

If the laser wavelength was something that wouldn't penetrate the atmosphere, it wouldn't be useful as a weapon against earth surface targets. (High flying airplanes and space resources though...?) And how easy would it be to just change the diodes?

It is interesting to think these questions, it makes it possible to avoid having to send anything to physically meet the asteroid. It is somewhat related to laser or microwave power transfer, which could power scientific probes and enable them to attain huge speeds.
I think solar power is very useful in this regard as we're not so heavily constrained by mass at the beam station.

Offline simonbp

Re: Preventing asteroid disaster (via laser)
« Reply #15 on: 02/28/2007 04:43 PM »
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TyMoore - 28/2/2007  4:14 AM

I would imagine that laser pumped ablation of asteroid material would create a plume of material that moves significantly slower than the jet from an ion engine--this in itself isn't really a big deal. It has the entire asteroid  to supply reaction mass. But the power system onboard the spacecraft presents a problem. If we go with a CO2 LASER, then the pumping is usually done with either a rf system or electrical discharge. In which case, depending upon the position of the spacecraft and the average power output of the laser, could either be powered by solar power or nuclear power.

The thrust is really dependent on the asteroid's material. In laser propulsion, the term "coupling coefficient" is typically used, and defined as the the thrust obtained per laser pulse power. Typically, it is inversely related to the specific impulse. Thus, a strong IR absorbing solid (like ice or plastic) has a high coupling (~ 10-100 dynes/watt) and low Isp (~ 100-10 sec), while a conductor (like copper or iron) will have closer to 0.1-1 dyne/watt and up to 5000 sec Isp.

Therefore, a non-metal asteroid or comet would be best deflected by a series of short, high power bursts, while a metal asteroid would need a long, continuous beam to create a laser-sustained plasma.

If you have a really big IR laser in orbit already, though, it might be better just to launch a 5 tonne block of IR-absorbing plastic (Teflon is real good), and use the laser to accelerate it to several tens of km/s into the offending asteroid...

Simon ;)

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Re: Preventing asteroid disaster (via laser)
« Reply #16 on: 02/28/2007 08:16 PM »
I propose we build this thing on the dark side of the moon if at all possible.  Otherwise some nefarious evil doer will commandeer it and point it at (what's left of) the polar ice cap.  Then James Bond will have to steal (what's left of) the space shuttle to go up and save the world and get the girl.

Just a side note, as someone who once got a masters in optical signal processing, I'm constantly annoyed by the political use of the term "focus like a laser beam".   A laser beam isn't focused, the light is parallel - so, like a politician, it just goes on and on, never coming to a point.

We now return you to the discussion already in progress.

Offline sandrot

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Re: Preventing asteroid disaster (via laser)
« Reply #17 on: 02/28/2007 09:40 PM »
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bhankiii - 28/2/2007  4:16 PM
Just a side note, as someone who once got a masters in optical signal processing, I'm constantly annoyed by the political use of the term "focus like a laser beam".   A laser beam isn't focused, the light is parallel - so, like a politician, it just goes on and on, never coming to a point.

In the ABL system a number of lasers is focused in a single point. In a laser system I recently used three lenses installed on a linear slide and linked by an electronic cam "focus" (whatever this means) the laser in a single point. I don't know if there is a better term.
"Paper planes do fly much better than paper spacecrafts."

Offline TyMoore

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Re: Preventing asteroid disaster (via laser)
« Reply #18 on: 03/01/2007 01:29 AM »
You know, maybe something really 'low' tech might work just as well. If a thin mylar sheet were tensily formed into a circular parabolic reflector, say 1km across with a focal length of say 20km--standing off from an asteroid, such a mirror would intercept:

1350 W/m^2 * pi*(500m)^2 = 1.060 GW of solar power at 1 a.u. from the sun.

If the overall optical efficiency was only 60% and could focus onto a spot say 50m across, the effective irradiance on that spot would be about:

0.60*1060MW/(pi*(25m)^2) = 0.324 MW/m^2  or about 240 suns. Going with a higher 'quality' optical system that allows spot size to shrink to 10m across would put the equivalent of 8.10 MW/m^2 or about 6000 suns on the spot.

Knowing absolutely nothing else about the composition of the 'asteroid' I would have to say that even though this is formidable, I don't think it is enough to cause significant volatilization of the surface materials unless they were composed of ice, or were very rich in volatiles. 6000 suns sounds like enough to melt iron and silicates but you'd probably need closer to 50,000 suns worth of irradiance to get a good vapor jet going.

Just a hunch though...

Offline simonbp

Re: Preventing asteroid disaster (via laser)
« Reply #19 on: 03/01/2007 07:46 PM »
Ah, ha! University PR has put up a snazzy ppt: http://www.uah.edu/News/newsread.php?newsID=664

Explains alot more than the AP piece...

Simon ;)

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