Wikipedia:Reference desk/Archives/Science/2018 March 16

= March 16 =

How can a pedestrian bridge fail so quickly?
Tragic news from Florida. I know it'd be speculation at this point, but surely this is one of the surest things to design for? Imagine Reason (talk) 01:57, 16 March 2018 (UTC)
 * I've been warning people to avoid speculation at the article, but there are several possibilities. One of the supports could have shifted or been pushed out of alignment (one appears to be tilted, but that could be an effect rather than a cause). There could have been an undetected flaw in the reinforcing connections, or a post-tensioning cable could have failed. Post-tensioned structures are designed to withstand cable failures within reason, but a failure is a spectacular event that could have unforeseen results - they release a lot of energy. Failures like this usually are the result of a bad connection of some kind. The design appears to be a kind of cable-stayed structure, but the component that failed was apparently designed to support itself during construction without the stays. This newspaper article contains a great deal of speculation, but it describes some of the common adjustments that are made to this kind of structure, such as camber adjustment. Construction is often the most hazardous time for a structure, as the redundant systems of a fully-completed design are not in place and the structure may be subject to unusual stresses.   Acroterion   (talk)   02:10, 16 March 2018 (UTC)
 * Your link doesn't work, you lost an l at the end . I also came across this from the same paper which IMO is a little better as while it's speculates on possible reasons, it makes it clearer it is just speculation and doesn't really suggest anything is the cause of he collapse . Nil Einne (talk) 05:31, 16 March 2018 (UTC)
 * Oops, sorry. Here's another article that's fairly measured in its coverage . Our article on L'Ambiance Plaza collapse is worth a read, as it also involved the placement of prestressed concrete structure that was fabricated elsewhere - in that case the movement was vertical, not horizontal. At L'Ambiance it was apparently a problem with evenly distributed  support that led to a failure at a connection point. We can expect reports from construction standards organizations in the long run for the FIU accident.   Acroterion   (talk)   12:22, 16 March 2018 (UTC)


 * According to this report, the cables were being tightened after a stress test. Akld guy (talk) 21:40, 16 March 2018 (UTC)


 * Courtesy link to thus unfamiliar: Florida International University pedestrian bridge collapse. Matt Deres (talk) 12:47, 18 March 2018 (UTC)

It sounds like people are surprised it failed so quickly. Bathtub curve suggests an early failure is likely, as is a late failure. RJFJR (talk) 16:26, 20 March 2018 (UTC)

sending probe on Sedna
In 2076, planetoid 90377 Sedna reaches perihelion. Why dont the humans try to land a probe on sedna? I mean, if it lands successfully; it will be a free journey for a very very long distance. All have to be done is to tell the probe to "stay still" and "do nothing" for a few centuries. Once sedna is far long, the probe cab be awaken. Then it can study the outer rim, and edge of solar system. It will take around 12,000 years for sedna to reach the perihelion again. During that time, the probe can go to sleep again. It doesnt need to return by itself. In 12,000 years humans might have gone extinct, or must have built faster space ships. One of such ship can pick-up the probe.

Are there any plans of a mission similar to this? If not, why? — usernamekiran (talk)  12:44, 16 March 2018 (UTC)


 * It's not that much of a "free journey". It takes considerable energy to match the velocity of such a body.  If you can do that, then the probe could have simply put itself into such an orbit anyway.  A rendezvous and sampling mission is interesting in itself, see Philae et al, but it doesn't really change the orbital dynamics of a mission. Andy Dingley (talk) 13:02, 16 March 2018 (UTC)


 * To amplify Andy's point, landing a probe does not provide a "free journey" at all. There is a way to get assistance from an orbiting body such as a planet or asteroid, but it doesn't involve a landing; you have to do a slingshot maneuver. If the probe was able to bounce off a hard object like an idealized rubber ball, then bouncing off the asteroid would give the same benefit that a slingshot maneuver does.  (That's because in the asteroid's frame of reference the slingshotting probe is in a hyperbolic orbit.) --69.159.62.113 (talk) 20:15, 16 March 2018 (UTC)


 * My gut feeling is that for a slow trip to put a permanent memorial on Sedna with minimum fuel expenditure, it would be better for it to be moving away from the Sun. That's because our probe, when it eventually reaches the dwarf planet, will need to slow down and otherwise change velocity to match its orbit, and that is easier if it's moving away than if it is moving sideways or coming toward Earth.  The idea is delta v.  This would not be the case if you wanted as fast a trip as feasible with a lot of fuel to burn, in which case the distance is what matters most. Wnt (talk) 21:48, 16 March 2018 (UTC)
 * Voyager 1 crossed the Heliosphere of our sun on August 25, 2012. Voyager 2 is currently still in the Heliosheath, aiming to measure its thickness. So the "study the outer rim, and edge of solar system" is already under way and done.
 * The United States dont seem as dedicated to the Space Race anymore tho its participation in the joint venture James Webb Space Telescope, in preparation to be launched 2019, will probably deliver more new science than all of the historic "spaceprobes" together. Other states dont seem to have very big plans too. Maybe that changes, maybe someone is secretly planning to become/send the first martian (Mr. Musk?). Btw. planning for the second space telescope (James Webb) started in 1996, more than 20 years(!) ago, so its not like finding a target and starting a probe to it some month later. The process to get politics to decide about the money for it can easily take a decade alone. --Kharon (talk) 23:13, 16 March 2018 (UTC)

Aphelion versus perihelion at those distances actually doesn't have much of an effect on the minimum fuel cost of the mission. The vast bulk of the fuel is simply spent getting off of Earth, and then out of its orbit. It would certainly be faster to go when Sedna is closer to the Sun, but still crazy expensive. Also, from an engineer/scientist/politician's perspective, it would be kind of silly to launch something now, intended to go into hibernation for 60 years (and we can't possibly test in advance if it can survive for that long), when instead would could spend money on something that would yield results now, and not even think about visiting Sedna until its much closer. Someguy1221 (talk) 01:43, 17 March 2018 (UTC)
 * I'm assuming that the OP is thinking about planning/preparing for this now rather than necessarily launching it now although I agree it's way too far advanced. That said, in addition to all the problems highlighted above there seems to be another big challenge. If putting something into hibernation for 60 years is tricky, and I agree it is, imagine trying to put it into hibernation for a few centuries! I'm not convinced our current tech is able to produce electronics that can survive that long, especially in outer space, even given some protection by the planetoid. Even the power source is likely to be tricky. The Radioisotope thermoelectric generator plans using Americium-241 would probably do although with RTGs being in hibernation is irrelevant to the power source (don't know about the other components); hence most plans don't propose that. And as you've indicated there's both producing things that can last that long, and being confident they will last that long. Nil Einne (talk) 04:03, 17 March 2018 (UTC)


 * In theory Sedna could contribute useful resources, such as water for propellant and ores for solar panels and mirrors. There is no way to rule out some kind of geothermal energy, even toasty warm spots, without a better look-see.  But you'd better bring a very big nuclear plant with you if you need power to mine enough material to make enough mirror to concentrate sunlight at 80 AU out (1/1600 the sunlight!).  It might be best to see some bots making fully equipped living space on the Moon and Mercury before we try that one.  But yes, as a base, Sedna can be seen as providing free passage to a tremendous amount of raw materials for future space intrigues. Wnt (talk) 21:19, 17 March 2018 (UTC)
 * 1/6400 the sunlight at 80 AU, ~1/875,000 at aphelion... 93.136.39.109 (talk) 02:46, 18 March 2018 (UTC)


 * To build on the basic point already expressed, but not fully explained: in space, objects do not have the same familiar drags on them that they do on earth. Your scenario suggests a hitch-hiker jumping aboard a passing train to get a free ride out to the Oort cloud. A satellite attempting to land on Sedna would have to match Sedna's speed (i.e. the hitch-hiker would have to run as fast as the train). However, unlike earth-bound hitch-hikers and trains, the satellite and Sedna do not have to worry about friction - they obey Newton's first law of motion in a very different environment than what we experience in everyday life. Once a satellite gets up to the same speed as Sedna, hitch-hiking gives you no benefit as that speed is easily maintained - the satellite will keep going that speed forever (until something else interferes with it). Matt Deres (talk) 13:00, 18 March 2018 (UTC)
 * Slightly related, if humanity ever want to leave the Milky Way then getting a ride with (probably not on) exiled stars leaving the galaxy may be of help in a distant future – but only because there may be useful resources to stay alive for a really long time. Your spaceship still has to catch up with the speed of the star like with Sedna, so there is no "free ride" advantage. PrimeHunter (talk) 02:57, 20 March 2018 (UTC)


 * FWIW, there are Jupiter gravity assist windows for 2033 and 2046 launches to go to Sedna (mentioned at 90377 Sedna, actually). But no space agency is currently considering launching a probe to go there. Double sharp (talk) 14:36, 20 March 2018 (UTC)

Would babies and toddlers need less nap time in the day if they sleep more hours at night?
If pre-school children all sleep enough hours during the night time, do they really need "nap time" during the day? SSS (talk) 23:48, 16 March 2018 (UTC)


 * There seems to be a link: " For example, one toddler may sleep 13 hours at night with only some daytime catnapping, while another gets 9 hours at night but takes a solid 2-hour nap each afternoon.". Count Iblis (talk) 02:36, 17 March 2018 (UTC)
 * Sleep Duration From Infancy to Adolescence: Reference Values and Generational Trends. Alansplodge (talk) 12:33, 17 March 2018 (UTC)


 * Since retiring, I've drifted back to being polyphasic. --Aspro (talk) 19:38, 17 March 2018 (UTC)


 * Be aware that the amount and duration of naps and sleep anyone should get is a contentious issue and - at the very least - varies with age. Quick example here. Parents of newborns are inundated with conflicting information, but it's usually formulated the opposite way of the OP. For example, the received wisdom when my child was a newborn (15 years ago) was that having naps improved night-time sleep habits; babies that missed their nap(s) would be too cranky to properly get to sleep at night. That was the theory, anyway. Matt Deres (talk) 16:10, 18 March 2018 (UTC)