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Comment: Re:chinese anti-satellite lasers (Score 1) 248

by hankwang (#49162771) Attached to: 20-Year-Old Military Weather Satellite Explodes In Orbit

But with your point about the coating, I'm only off by a factor of ten...

1000 times smaller beam diameter means 10^6 times higher irradiance. With 1% absorbance it's still a factor 10^4 off.

how much power do we really need on target to make it overheat given that it's designed to cycle between daylight and night

The solar panels are designed to receive 1500 W/m2 irradiance from the sun, of which maybe 500 W/m2 comes out as electricity and the other 1 kW/m2 is heat load. A black-body radiator against 4 K background temperature on one side and earth (279 K) on the other side will reach an equilibrium temperature of 330 K (57 C). To heat the solar panel to a damaging 150 C (just a guess), you'd need about 2.5 kW/m2 extra (on top of the solar irradiance).

It wouldn't surprise me if the back side of the solar panels is used as a radiative heat exchanger to get rid of the heat generated inside the body of the satellite. The gold-coated foil around the satellite body acts as a kind of thermos bottle to insulate the electronics inside from extreme temperature swings, but still the electronics need to get rid of its own dissipation, so you would not only destroy the solar panels but indirectly also the electronics.

I doubt that it's practical to aim specifically for the solar panels for an object that's travelling at 7 km/s, with a mechanism that can do that within 1 microrad. It might be more practical to have a 10 m diameter spot size, which would require 250 kW, which might be doable with a CO2 laser at 10.6 micrometer wavelength (the atmosphere seems to be fairly transparent at that wavelength).

If the solar panels have a mass of 15 kg/m2 (just a guess), then you would need to maintain this 2.5 kW/m2 for about 10 minutes, over which time the satellite will be travelling 4200 km. This doesn't sound easy...

Comment: Re:chinese anti-satellite lasers (Score 4, Informative) 248

by hankwang (#49160223) Attached to: 20-Year-Old Military Weather Satellite Explodes In Orbit

"A 1um laser fired out of a 1m aperture spreads to an 800m circle at an altitude of 800km"

You're off by a factor 1000. The divergence is about 1e-6 rad, which makes 0.8 m diameter at 800 km.

Now another issue is that satellites tend to be wrapped in gold-coated foil, which will reflect 99% of the light at 1 micrometer. It would be difficult to overheat the body of the satellite, although the solar panels might be damaged more easily.

Comment: Re:Yes, it's a conflict of interest. (Score 1) 445

by hankwang (#49104557) Attached to: How One Climate-Change Skeptic Has Profited From Corporate Interests

Having a conflict of interest is understandable; hiding a conflict of interest is problematic.

The strange thing is that his management (Alcock and Kress) is claiming that they were unaware of his funding. The research grants pass through the accounting department of the institute, right? Large sums of money deposited onto or withdrawn from the institute's bank account need to have proper paperwork in place and be signed off by the management? His management must have been actively looking the other way.

Comment: Etching does the same thing (Score 2) 133

by hankwang (#49099639) Attached to: Crystal Pattern Matching Recovers Obliterated Serial Numbers From Metal

It has been possible for decades to recover serial numbers by chemical etching, which is sensitive to changes in the crystal structure. All you need is a polishing implement and a bottle of etching gel. What is the added value of this EBSD technique? I can see a big disadvantage: you need equipment (a scanning electron microscope) worth a few hundred kEUR and the object with the erased serial number needs to fit into the vacuum chamber of said equipment.

The article mentions that etching techniques don't always work, but they don't state that their technique does work on samples for which the etching method doesn't work...

Comment: Re:You're both right, and both wrong. (Score 1) 311

by hankwang (#49068525) Attached to: Nuclear Plant Taken Down In Anticipation of Snowstorm

What makes you both partially correct is that there isn't a delay in fission, but a delay in neutron release.

From reading the linked Wikipedia page, I get that the delayed neutrons are responsible for 0.6% of all neutrons emitted from fission, and most of them with half-life times between 2 and 20 seconds. While this makes for some nice control bandwidth (you can afford to be 0.6% off with the control rods without getting an instantaneous nuclear explosion), I don't see how these delayed neutrons can be responsible for the majority of the 7% decay heat.

Comment: Re:Nuclear plants don't like sudden shutdowns (Score 2) 311

by hankwang (#49064637) Attached to: Nuclear Plant Taken Down In Anticipation of Snowstorm

"Normally there is some time between neutron capture and actual nuclear fission (I have heard a figure of 15 minutes)."

The fact that you can detonate a nuclear bomb by bringing together two subcritical pieces of U-235 shows that this can't be true.

In a nuclear reactor, 7% of the heat output is from the decay of the fission products (alpha and beta decay). This 7% will continue to be generated regardless of control rods or neutron absorbers. It will last hours to weeks, depending on where you put the threshold for "finished". Remember Fukushima: it became a disaster when the water circulation backup pumps failed 12 hours after the reactor shutdown.

Comment: Re:The Greater Danger (Score 1) 180

by hankwang (#49030321) Attached to: US Gov't To Withdraw Food Warnings About Dietary Cholesterol

"until the discovery of the chirality (and chirality affects a lot of things) of saturated fats, they probably got mixed together (they're still saturated fats, after all) and the original problem was because of the hidden trans- version."

I think you are mixing up things. Trans and cis fats are both unsaturated fats, with double bonds. Saturated fatty acids have no chirality. (Well... Triglycerides are technically often chiral (the middle carbon atom of the glycerol backbone), but that's not relevant for the cis/trans/saturated discussion.)

Comment: Re: Be careful not to justify government corruptio (Score 1) 215

by hankwang (#49015265) Attached to: Silk Road Drug Dealer Pleads Guilty After Federal Sting

"I recently heard about a case called "Kids For Cash", where a judge was sending lots of juveniles to a privatized detention center and getting paid for each."

Actually, two judges:

Quote: For example, Ciavarella adjudicated children to extended stays in youth centers for offenses as minimal as mocking a principal on Myspace, trespassing in a vacant building, or shoplifting DVDs from Wal-mart.

Comment: Re:I don't think this [release] matters at all... (Score 1) 193

by hankwang (#49012379) Attached to: Xfce Getting a New Version Soon

"I use lightweight WMs such as XFCE or Openbox. Not a fan of the bloat..."

XFCE isn't that much lightweight compared to MATE (i.e., Gnome 2), only compared to Gnome3/Unity/KDE.

Where did XFCE get this lightweight reputation? It surely doesn't look very polished (based on looking over other people's shoulders only).

Comment: Re:Not Photosensitive (Score 1) 192

by hankwang (#49012123) Attached to: Xenon Flashes Can Make New Raspberry Pi 2 Freeze and Reboot

"A 100mW red laser pointer aimed"

You play around with a HAND-HELD class 3b laser? Still have eyesight in both your eyes? (And your pets/household members)

Legally, those should be operated only by trained staff and I believe they must be equipped with a key lock and interlock connection. That stuff from WickedLasers should really be banned. The laser inside a dvd writer is of similar power, but interlocked and much less dangerous due to the focusing lens.

Comment: Re:Kind of.. (Score 1) 481

by hankwang (#48986971) Attached to: DOT Warns of Dystopian Future For Transportation

"Except you forgot to pay gas, maintenance and insurance on those 40 miles."

And depreciation, say $15k over 150k miles is another $0.10 per mile (I'm not from the US, not sure of typical car prices and lifetime mileages. YMMV)

Car owners typically don't count depreciation "because they have the car anyway". However, once infrastructure (or choice to live and work close to mass transit) is available, you can choose not to own a car and rent one for the few occasions you need one.

Apart from those costs, your own time may also have value. IMO, time spent driving is a waste and costs me EUR 20/h in loss of life quality. Time in the train I use to read the newspaper or slashdot (or post comments, like now). The bike ride to the station is my primary form of exercise (no gym subscription).

Comment: Re:Here's a great idea... (Score 1) 481

by hankwang (#48986931) Attached to: DOT Warns of Dystopian Future For Transportation

"GPS receivers have been in cell phones for years. The cost of the receiver doesn't add $1,000 to the phone."

The ones on phones don't need to be certified, tamper-proof, and linked to a database with financial consequences. Because of privacy issues, you'd probably want the unit to only log distance per day, not full location information. So,.reliability is extremely critical.

See alcohol locks (whatever they're called) for people with a multiple DUI conviction. You can get an uncertified breath analyzer gadget for a few dollars/euros, but the ones that are actually attached to a car are hundrds of dollars.

Nothing is finished until the paperwork is done.