Wikipedia:Reference desk/Archives/Science/2019 March 4

= March 4 =

Zircon missile
Something about the Zircon missile doesn't add up: if in flight "the missile is completely covered by a plasma cloud" which "absorbs any radio waves and makes the missile invisible to radar", then by what conceivable means do the missiles "exchange information in flight and can be controlled by commands if necessary"??? Wouldn't the very same plasma cloud absorb all the radio waves carrying the guidance commands??? And also, how can such a missile have any guidance system capable of finding moving targets such as ships??? Because it can't use radar guidance (either active or semi-active) or passive anti-radiation homing (the selfsame plasma cloud will absorb any radar signals reaching it), and infrared homing won't work either (its own IR radiation from the plasma -- and from aerodynamic heating! -- will mask the target!) It's the same thing as with The Invisible Man -- if he's completely invisible (including his eyes), he's also blind! 2601:646:8A00:A0B3:FD4C:F594:146:3B5C (talk) 03:03, 4 March 2019 (UTC)


 * Plasma does not universally absorb electromagnetic radiation, but rather has a complicated relationship with it. Put simply, a given plasma may strongly absorb some frequencies, while transmitting others. You can read more at Plasma stealth and the links and sources therein. Obviously, this means the stealth cannot be perfect. Someguy1221 (talk) 05:38, 4 March 2019 (UTC)
 * But the plasma cloud is of course visible to a radar. Ruslik_ Zero 20:10, 4 March 2019 (UTC)
 * Just not well enough for missile guidance, right? 2601:646:8A00:A0B3:C119:5F8:1314:9E22 (talk) 21:06, 4 March 2019 (UTC)


 * The truth is too complicated to distill into such a broad statement. For a real example, like this one, experiments (which were done in a lab rather than on a rocket) showed the first plasma they describe strongly absorbed radio waves between 4 and 7 GHz, did nothing for higher frequencies, and actually increased visibility at lower frequencies. The behavior of the plasma also had a strong dependence on the angle of incidence of the radio waves, plasma thickness, and plasma density. The total effect was not a simple linear combination of individual variable effects. This paper didn't address the plasma's own emissions. So, again, you can't look at this and say "these missiles are invisible" or "radar is blocked but communications still work".


 * It's not like that. Just try and look at the math in that paper. Look at the graphs. Plasma physics is not a simple subject, and plasma-EM interactions are not simple either. A given plasma stealth system will have radio frequencies that are strongly absorbed and thus useless for radar. The absorption is not 100%, but 99 or 99.9% is good enough. The same plasma will have other frequencies that transmit freely through it. "Radar" is also not one thing, but dozens of variations on the sample principle, each with their own advantages and disadvantages. You could certainly imagine a system like this, especially if it is tunable mid flight, being extremely useful for avoiding detection or an inbound missile. Simply detect type of radar being used to track or home in on you, and then generate a plasma that makes you completely invisible to that system. One could also imagine tracking and homing systems for anti-missile missiles developing their own adaptive systems to counter this, as well as passively just always using multiple systems that would be difficult to defeat simultaneously with a single countermeasure. In other words, the sort of stuff that is already done today, just now there's plasma to think about. Someguy1221 (talk) 23:47, 4 March 2019 (UTC)
 * So in other words, it's like an advanced broadband frequency-agile jammer? 2601:646:8A00:A0B3:C119:5F8:1314:9E22 (talk) 00:29, 5 March 2019 (UTC)
 * "Jammer" would imply overwhelming the enemy receiver with signals. This is really the reverse. It's like painting the plane black so you can't see it at night. Obviously we think of black as the color that absorbs all wavelengths in the visible spectrum. A plasma-stealth rocket would be black for some region of the radio spectrum, and potentially that region could be changed at will. Radar emissions aimed at the sky do not normally come back to you, so empty sky also looks black to radar. There are indeed radiation-absorbent materials used in aircraft to the same effect, but these tend to be much more limited than (claimed) plasma-stealth in terms of both absorption and frequency-range, and of course cannot be adjusted without time on the ground. Someguy1221 (talk) 04:50, 5 March 2019 (UTC)
 * So like adjustable radar-absorbing material then? 2601:646:8A00:A0B3:C119:5F8:1314:9E22 (talk) 12:59, 5 March 2019 (UTC)


 * Military technology is in many parts an impressive history of fairy tales! Since its secondary function is always to impress, its potential is often exaggerated. The facts are naturally top secret and no one will get a sample to do an Radar cross-section just as no one will get an enemy radar module to check how well it detects objects - or someone in an secret agency is up for promotion to General. --Kharon (talk) 23:16, 8 March 2019 (UTC)

Kepler's second law of vectors
Am I right in thinking that Kepler's second law can be paraphrased thus? —Tamfang (talk) 04:29, 4 March 2019 (UTC)
 * The cross product of the radius vector and the velocity vector is constant.
 * Yes, you can put it like that. You can also say that the angular momentum of the orbiting body is conserved. PiusImpavidus (talk) 09:35, 4 March 2019 (UTC)
 * I beg to differ: The angular momentum of the system about its barycenter is conserved. DroneB (talk) 00:18, 6 March 2019 (UTC)
 * I agree with PiusImpavidus. However, I question the use of the words “of vectors” in the title. Kepler’s second law is universally true for planetary motion, but it isn’t universally true for vectors. Vectors are applied to much more than displacements and velocities, including to things that aren’t the subject of conservation laws. Dolphin ( t ) 10:39, 4 March 2019 (UTC)


 * I'd also use caution in attempts to reformulate or rephrase Kepler's laws, purely from the perspective of historical accuracy. Clearly, Kepler was on to something, and later scientists used new math and new theory to generalize his statements, but if we want to be historically accurate, those generalizations were not done by Kepler.  Since he did not use vector math to derive or to explain his work, any generalization you make - even if it's mathematically and physically valid - sort of ceases to be "Kepler's" law.  Nimur (talk) 16:18, 4 March 2019 (UTC)


 * My wording was a bit facetious, as it usually is in such headings. —Tamfang (talk) 22:15, 4 March 2019 (UTC)
 * Indeed, but while you've made a (valid and correct) mathematical deduction, it is one that was probably was never made by Kepler nor by anyone else in his lifetime. It was almost certainly found by many subsequent mathematical physicists, and can be summarized, for example, in the works we usually attribute to Emmy Noether or Lagrange.
 * In the study of the history of math, we sometimes observe facts that are obvious today, thanks to modern formalisms; but these same facts could not have been obvious to very smart people who worked the same problems across many centuries. This is, if anything, a testament to the incredible analytical power of modern mathematical formalism, from which many great minds of the early scientific era could not yet benefit.  Imagine if Isaac Newton had the benefit of vector-notation ... or even symbolic-algebra notation ... how much faster might he have arrived at the conclusions that in his era took an entire working lifetime to derive?   Nimur (talk) 23:39, 4 March 2019 (UTC)

How was Chernobyl operated for 14 years after the disaster?
I was reading the article on Chernobyl Nuclear Power Plant which famously had an explosion and meltdown in 1986, and was shocked to discover that the other 3 reactors continued operating for various periods such that the power plant continued producing electricity until 2000. How, logistically, did that work? I thought the surrounding area was too radioactive to safely live and work, hence Pripyat being abandoned etc. And even if the risk isn't at the level of certain death or short exposure is safe (hence the modern day tourism), surely going to work inside the building itself day after day wasn't feasible. Or was it? 143.159.150.105 (talk) 21:18, 4 March 2019 (UTC)
 * Most of the radiation was in Building 4, and that was contained when they built the "sarcophagus". 2601:646:8A00:A0B3:C119:5F8:1314:9E22 (talk) 22:39, 4 March 2019 (UTC)
 * I'm probably making a mistake shooting from the hip here, but I would presume that buildings (somewhat) robustly designed to keep radiation in would also be effective at keeping radiation out. Wnt (talk) 01:27, 5 March 2019 (UTC)
 * You might assume them to be equally robust in either direction... the finest in Soviet technology. ←Baseball Bugs What's up, Doc? carrots→ 01:41, 5 March 2019 (UTC)
 * Global technology, for that matter. Wnt (talk) 11:59, 5 March 2019 (UTC)
 * The technology thatbrought us Windscale and Three Mile Island, eh... ——  SerialNumber  54129  12:25, 5 March 2019 (UTC)
 * I've never been that impressed with Business Insider but this story [//www.businessinsider.com/chernobyl-reactors-14-years-disaster-2016-4] may do an okay job of discussing the political and other issues surrounding keeping the plant running, then shutting it down. This is a contemporaneous account around the time of the final plant being shut down [//news.bbc.co.uk/2/hi/europe/1071344.stm] It's worth noting that the decommissioning one or more of the other reactors is AFAIK still going on. [//www.vice.com/en_nz/article/jpa853/what-life-looks-like-for-people-living-in-chernobyls-nuclear-exclusion-zone] [//www.sortirdunucleaire.org/IMG/pdf/maltinif-2016-07-08-chernobyl_decommissioning.pdf] [//www.theguardian.com/cities/2016/mar/07/chernobyl-30-years-residents-life-ghost-city-pripyat] [//www.powermag.com/ignalina-decommissioning-chernobyls-big-sister/] [//www.dailymail.co.uk/news/article-3019536/Living-working-Chernobyl-Fascinating-insight-lives-work-live-exclusion-zone-nuclear-plant-nearly-30-years-disaster-shook-world.html] (warning last one is Daily Mail). Per those sources, many of them live at Slavutych and need to pay attention to exposure management, although the Daily Mail article says some live within the exclusion area for up to 14 days. (Besides the earlier things, the sources also discuss other stuff like the various social and economic issues leading to people living nearby, and working at the plant.) Also things like building the Chernobyl New Safe Confinement clearly required a number of people for a reasonable length of time, and probably closer to the disaster area than the other reactors [//www.rferl.org/a/inside-chernobyl-nuclear-power-plant-conferment-shelter-photo/29583945.html] Nil Einne (talk) 12:22, 5 March 2019 (UTC)
 * Very interesting, many thanks for all the sources especially. 143.159.150.105 (talk) 13:48, 5 March 2019 (UTC)