Wikipedia:Reference desk/Archives/Science/2015 August 23

= August 23 =

Storing USB flash drives
I know that USB flash drives only have a limited number of writes before they die. However, if a flash drive was just stored and never used, would it still work in say 30 years time? I am not asking about preserving data on it, just if it would work as blank media after that amount of time. Or does it degrade without any use like CD-RWs do due to chemical break down. Thanks — Preceding unsigned comment added by 62.37.237.15 (talk) 14:47, 23 August 2015 (UTC)


 * An unused flash drive should still work unless it has been physically damaged. Data retention is a more complicated issue (i.e., will you still be able to read data stored today in 30 years time). The real question is whether there will be any devices with USB ports 30 years from now. My guess is "no", but my forecasting record is imperfect to say the least. Short Brigade Harvester Boris (talk) 15:12, 23 August 2015 (UTC)
 * For that reason (future data retention), companies who are obliged to keep records for decades for legal reasons often to store whole computer systems along with their backups. About flash retention, this article here (http://www.forbes.com/sites/tomcoughlin/2014/06/29/keeping-data-for-a-long-time/) mentions a retention time of only ~100 weeks (cold storage at 25°C) according to "JEDEC JESD218A endurance specification". Another spec sheet here (http://www.ti.com/lit/an/slaa334a/slaa334a.pdf) mentions "MSP430" where the retention time is specified as 100+ years. Does that mean that most/all new flash drives comply to the 100 years, or what's the chance of getting a drive which only holds data for 2 years? Rh73 (talk) 13:31, 24 August 2015 (UTC)

Gravity Movie Reentry
In the movie Gravity, Ryan uses a Shenzhou spacecraft to come back to earth. When it detaches from the space station it is tumbling. It jettisons the service module and the airlock and it continues to tumble for a bit, but then it straightens out and flies right. Presumably the guys who designed this thing knew what they were doing and chose that shape so that it would do just that. Or did they? Somehow it just doesn't look stable. Is there some secret involved in designing the shape of a reentry capsule so that it will self-stabilize? They aren't counting on active control, are they? (YouTube clip https://www.youtube.com/watch?v=SN9GuV8siEg) 50.43.33.62 (talk) 15:50, 23 August 2015 (UTC)


 * Hypersonic aerodynamics can be counter-intuitive at times. While I havn't read up much on the Soyuz / Shenzhou spacecraft, I know that when they were designing the Mercury space craft it was found to be stable in two positions; heat shield forward, or nose cone forward. To ensure a safe reentry, they added a reentry spoiler to the nose, making that position unstable. You can read more on the theory of blunt bodies during reentry at our article on atmospheric entry. WegianWarrior (talk) 16:30, 23 August 2015 (UTC)
 * All capsules are/were designed to be passively stable, to re-enter heat shield first even if all control is lost. In nominal reentries they usually fly slightly lifting trajectories to reduce g-loads. Fgf10 (talk) 18:52, 23 August 2015 (UTC)


 * Yes, reentry modules (from MIRVs to spy satellites) are designed to passively self orient so that the heatshield is facing the direction of travel. Early Soviet photographic-return satellites (Zenit (satellite)) used the simple Vostok spacecraft reentry module (which is very similar to Soyuz's) which oriented itself automatically. For a complex spacecraft (like Apollo or Soyuz) where only part of the orbital system is supposed to reentry, there is the danger that the separation is imperfect, and you end up trying to reenter with the orbital stuff still attached. Soyuz 5 and Soyuz TMA-11 (at least) experienced failures where the bolts holding the orbital element to the reentry element failed to separate, and the two started reentry together (at a funny angle). In both cases, due to aerodynamic pressures in the upper atmosphere, the whole reoriented so the heavy reentry module was facing forward, before the air resistance tore off the unwanted orbital stuff and left the reentry capsule to descent as designed (albeit heavily). This story discusses TMA-11's unhappy descent. There is the concern that, while previous failures like this have resolved themselves with the spacecraft self-orienting with its heatshield down (once the other stuff had burned off), it's not impossible that a "hatch-first" reentry could occur, which would destroy the spacecraft. -- Finlay McWalterᚠTalk 22:45, 23 August 2015 (UTC)

April 2015 Nepal earthquake
There was a report by some North American University that the earthquake caused significant portion of southern plain to slip underneath the hills. Does this mean the area of Nepal decreased? Of the Asian landmass? Of the world's land? Percentage of earth surface covered by land? Will books be updated? Will some countries vanish from earth in the future just by virtue of plate tectonics? How far in the future is it? Need Nepal worry? Usedtobecool (talk) 20:43, 23 August 2015 (UTC)


 * The Indian Plate continues to move northwards towards the Eurasian Plate at a rate of 45 mm per year. Because of this, the active thrust front of the Himalayas, which takes up a proportion of that convergence, continues to move relatively southwards, overriding the Indo-Gangetic Plain. The displacement involved with the earthquake is estimated at a few metres (maximum ~4), so to that extent Nepal has got smaller. In the very long term (millions of years) countries may disappear, but not something to worry about in the short term. Mikenorton (talk) 21:31, 23 August 2015 (UTC)


 * In the nearer term, the places that need to worry about their land area disappearing are the ones at very low elevations. For example, see Maldives. --65.94.50.17 (talk) 16:42, 24 August 2015 (UTC)


 * Yes, at subduction zones, the Earth's crust is recycled back into the mantle. Meanwhile, at divergent boundaries, new crust is created. That's how plate tectonics works. But all of this occurs on the order of millions of years. The usual comparison I see given is that the average speed at which tectonic plates move is about the same as the speed at which your nails grow. What Nepal and other countries need to worry about are the immediate effects of earthquakes that we're all familiar with. Many of those killed in Nepal would have lived if the government had been able to enforce adequate building and safety codes. For comparison, the 2011 Japan earthquake had a death toll not even twice as high, despite being much more violent and having an associated tsunami; this is because Japan has fairly strict seismic construction standards, better infrastructure, etc. An interesting illustration of earthquakes' power is that they change the length of the day. The change in the mass distribution of the Earth changes Earth's moment of inertia, changing the speed of its rotation. Of course, because the Earth is so large, these changes are on the order of fractions of a second. --71.119.131.184 (talk) 20:27, 24 August 2015 (UTC)