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Comment: Re:Occam's razor. (Score 3, Interesting) 85

No, Astronomers have asked the WIMP vs MACHO question for many decades now, and WIMPs are winning.

Occam's Razor has always been applied here, and that is why it is still an open question, because the simple and obvious answer (MACHO) is not working and extraordinary evidence is being found, eg the Physic's Nobel Prize 2011.

This article is not about MACHO vs WIMP. It says they found a nearby MACHO with water vapor, and that is very interesting for life questions, not dark matter questions.

Comment: Re:Simulations are limited by imagination (Score 1) 173

by idji (#47736037) Attached to: Google Wants To Test Driverless Cars In a Simulation
You are right that real life is far more creative, but that is not the point being argued here. The point here is that simulation is a better testing environment than a test track. The test track will have much less creative scenarios than simulation because they are so much harder to stage. A test track will not test scenarios that people didn't think of. The simulator is a much better environment to test dog+stop sign+rain - try doing that on a track. Put some creative people in the simulator and they'll also test elephant+storm+headlight failure.

Comment: Wrong (Score 2, Insightful) 145

by idji (#47661217) Attached to: The Quiet Before the Next IT Revolution
No, you IT people are no longer the great revolutionists - your time is gone. You are now just plumbers, who need to fix the infrastructure when it are broken. Other than that, we don't want to hear from you, and we certainly don't want your veto on our business decisions - that is why a lot of us business people use the cloud, because the cloud doesn't say "can't work, takes X months, and I need X M$ to set it up", but is running tomorrow out of operational budget.

Comment: Re:Another Silver Bullet? I don't think so... (Score 1) 294

by idji (#47654095) Attached to: The Technologies Changing What It Means To Be a Programmer
The most useful programming skills I learnt were in 1982 on the Sinclair ZX Spectrum. Nothing I learnt then has become irrelevant, just the languages changed. I give programming courses and am amazed how 21st century programmers are missing the basics and cannot write algorithms.

Comment: Re:In Orbit? (Score 5, Informative) 54

by idji (#47617491) Attached to: Rosetta Achieves Orbit Around Comet
The comet 67P has a mass of 3.14E12 kg
Today the comet is 186,444,271 km from the Sun Where is Rosetta?
Using F=GMm/R^2, the Sun's gravity on Rosetta is equal to 67P's gravity on Rosetta at 700m from the center of Rosetta on 6 August 2014, which means that Rosetta will never really be completely within 67P's field. (At Perihelion on 13 Aug 2015, 67P's gravity field will be as strong as the Sun's only 250m from the centre) However, now that Rosetta is in the same orbit as 67P we can mostly disregard the Sun's gravity and the elliptical path that Rosetta and 67P now share as of today. (Earth's pull on Rosetta is at least a million times weaker than the Sun's pull - so forget any influence from the Earth's mass.)

The "orbits" at 100km are called hyperbolic because Rosetta is not trapped in 67P's gravity well since the gravity is so weak and because Rosetta is still moving FAST at 1 m/s. But this hyperbola is so weak it is effectively a straight line.
Rosetta will turn 60 degrees after every 100 km of a hyperbolic path to make a triangular "orbit". This triangular path cannot be called an orbit because it is not a conic section, nor is the comet at a focal point of the conic section Kepler's First Law.

These "straight"/"hyperbolic" paths of 100km and 50km are deliberately done for two reasons:
-to calculate exactly the gravity field of the comet, because it is clearly not a uniform sphere. They will likely use radar&cameras to continuously measure the precise distance to the comet
-to keep in front of the comet to avoid its coma and tail.
After these maneuvers, Rosetta will go into a 30 km "orbit", so that the task of mapping 80% of the surface all happens from the same distance. This orbit is not natural and will be powered because a natural 30km orbit of 67P takes 26 days.

Here's how to calculate the natural circular orbits for 67P (it won't be circular, because of the crazy shape, but close enough). Kepler's 3 Law gives us
T^2=4pi^2/GM*r^3. 4pi^2/GM=0.19 for this comet. G=6.67×1011 N(m/kg)2
if r=30km=3e4m, the natural orbit would have a period of T=2.3e6 seconds=26.11 days
If r=2.5km, the natural orbit would have a period of T=15 hours
If r= 5km, the natural orbit would have a period of T=1.77 days
If r= 100km, the natural orbit would have a period of 159 days So I could imagine that when Rosetta gets within 5km it is mostly using the natural orbit and hence saving fuel.

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