explain xkcd - User contributions [en]

2952: Routine Maintenance

2024-06-29T21:17:35Z

172.68.50.120: /* Explanation */

{{comic
| number = 2952
| date = June 28, 2024
| title = Routine Maintenance
| image = routine_maintenance_2x.png
| imagesize = 299x413px
| noexpand = true
| titletext = The worst was the time they accidentally held the can upside down and froze all the Earth's magma chambers solid.
}}

==Explanation==
{{incomplete|Created by an ONCOMING LETHAL DUST CLOUD - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

A recommended routine maintenance step for many electronics, such as desktop computer towers, is to remove the buildup of dust on a regular basis. This bit of routine maintenance can help prevent the electrical components from overheating, and lengthen the lifetime of these electronics. There exists {{w|Gas duster|cans}} of high-pressure gas, as depicted, to blow dust out without a person blowing themselves, thus allowing them to keep their distance and not get a faceful of dust, or adding unintended moisture to the electronics.

This is suggesting that this is a maintenance step performed on the Earth itself, blowing gas into the Earth to force out the dust. However, filling the atmosphere with dust would be unhealthy and fatal to living beings, so as a safety measure everyone would have to take shelter. This is a reference to one theory about the extinction of many creatures, especially the (non-avian) dinosaurs, that a crashing meteor sent so much dust into the air that it killed off all non-avian dinosaurs in a much wider area than were directly affected by the initial impact. Those lineages that chanced to survive the global effects (including our own mammalian ancestors, and the avian dinosaurs that led to todays birds) must have been able to escape the worst of the disrupted ecosphere, perhaps some of them by already being more inclined/suited to living in burrows while the worst of the atmospheric effects subsided and let them exploit various newly vacant (and/or changed) environmental niches.

The image suggests that the "routine maintenance" for Earth would involve using the {{w|Hawaii hotspot}} (possibly <em>via</em> its most active volcano, {{w|Kilauea}}), as the point to insert the high-pressure gas, causing volcanoes to erupt in Iceland, the {{w|Aleutian Islands}} or the {{w|Kamchatka Peninsula}}, the {{w|Andes}}, and elsewhere; the two geographically-indeterminate plumes may represent Italy and Indonesia.

The title text mentions using the can upside-down, and this freezing solid the magma chambers. Pressurised canisters of air, as with similar aerosol sprayers, make use of a propellant gas that condenses into a liquid when compressed. As the can is sprayed, the release of pressure allows some of the liquid to evaporate and take the place of the released gases, become some of the gas subsequently released (or all of it, if there are no other contents required to fulfil its purpose as a spray). The propellant liquid/gas acts as a refrigerant, as its transition from dense liquid to space-filling gas requires it to 'boil off', this process needing to pick up {{w|Enthalpy of vaporization|heat (or 'enthalpy') energy}}. Under typical operation, this happens within the can itself, the heat being taken initially from the can itself as it tries to attain thermal equilibrium. As a result, the can itself (and the expelled gases) will be cooled a little as the heat 'loss' is taken from the general mass of the can, including its contents, and then (ultimately) anything touching or surrounding the slightly cooled can and its spray. This is precisely how a purposeful refrigerant acts, either as a one-time process or as a reversible cycle where repressuring a suitable gas can 'release' heat (the heat/enthalpy of condensation) at the 'hot side' of a refrigerator, leaving the system with more liquid it that it can later let boil and cool the 'cold side' of the system.

It is not normally useful for such a can to allow the liquid propellant-in-waiting to exit the container, as it would waste its usefulness as a source of pressure once it does. But by holding the container the wrong way up (which way that is being dependant upon its design, and intended use...) the pressurised contents push the liquid out via the nozzle's stream. The now exposed propellant is now free to evaporate into the air at atmospheric pressure, typically much lower than the constraints it had within the can, after landing directly upon whatever the can was sprayed at. The resulting demand for heat energy (much more rapid than normal, and likely concentrated upon a much smaller target than the can itself represents) produces a greater localised drop in temperature and can lead to freezing nearby liquids (which may or may not be intended/useful). Of course, the total 'cooling effect' of such a can does not change, depending upon how it is (mis)used, it merely changes the extent (and lifetime) of application, and how extreme the temperature change may be within a much more limited 'liberation' of its cooling ability.

Spraying canned "air" in reverse is a party trick used to very quickly cool beverages, being able to bring them down from room temperature to ice cold in seconds, if performed correctly. Some 'spray cans' are ''designed'' to let you freeze things (e.g. to help in plumbing repairs) or safely chill surfaces (e.g. for first-aid purposes), but these are exceptions (that require judicious use) and generally it is a wasteful use of a spray-can, if not actually unwise.

Given the location of the planetary-scale dust-busting 'air canister', it may be considered confusing which 'way up' is the correct orientation, given that Earth-gravity would be pulling the contents sideways (however that changes what the nozzle ends up ejecting from the can itself). But it would also have its own significant internal 'can-centric' gravity that possibly (depending upon how full of still-liquid propellant it is) exceeds that of the Moon, possibly letting all the denser liquid hold itself into the centre of the canister, even against the nearby Earth's gravity. Although, as significantly closer to the Earth, it could also be a far greater influence upon Earth's own tides (not alluded to in the comic), making the dusting of the atmosphere or the freezing of some of its magma secondary issues to the sheltering population.

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}

:[The nozzle of a "Dust-Off" gas duster can is pointing into a hole on the Earth's surface in the Pacific Ocean around where Hawaii is located, and its trigger is pressed as an arrow indicates, resulting in dust clouds being released from five visible spots of the Earth. These eruptions can be seen in the Aleutian Islands or Kamchatka Peninsula, Iceland, the Andes, and two further in the eastern hemisphere on the other side of the Earth.]

:[Caption below the panel:]
:I know routine maintenance is important, but I hate how we all have to take shelter for 48 hours every year while they flush out the Earth's magma system for cleaning.

{{comic discussion}}

[[Category:Geology]]

Talk:2906: Earth

2024-04-11T06:06:24Z

172.68.50.120:

I originally read the caption as "how badly ''we'd'' messed up", which... changes Sagan's tone somewhat. [[Special:Contributions/172.71.155.54|172.71.155.54]] 08:02, 14 March 2024 (UTC)

At first I thought the joke was that the rocket firing had somehow gone so catastrophically badly that the entire Earth had literally been reduced to dust. [[User:Fabian42|Fabian42]] ([[User talk:Fabian42|talk]]) 08:37, 14 March 2024 (UTC)
:I also had this notion at first. That after the failed burn Earth had been destroyed... But I think not so anymore. So thx explain xkcd. ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 12:43, 14 March 2024 (UTC)

So, according to explainxkcd, that’s a ''square'' “spacecraft window”?? Why have we never seen a square spacecraft window in any other context, ever? Did Randall screw up that badly in the original comic, or was it a previous explainxkcd editor who screwed up here? [[Special:Contributions/172.70.214.60|172.70.214.60]] 08:58, 14 March 2024 (UTC)
:I'm not sure what you're on about and why anyone has to have screwed up. Why can't it be a rectangular (we don't know it's square) spacecraft window? [[User:Bischoff|Bischoff]] ([[User talk:Bischoff|talk]]) 09:53, 14 March 2024 (UTC)
::This is very clearly a triangle shaped window in a very elongated spaceship [[User:Whimsical|Whimsical]] ([[User talk:Whimsical|talk]]) 11:24, 14 March 2024 (UTC)
::: Maybe it is part of a huge spider-shaped window? (I home people here will remember that meta-reference to What If) [[Special:Contributions/172.71.94.208|172.71.94.208]] 12:28, 14 March 2024 (UTC)
::::This picture from the [https://en.wikipedia.org/wiki/Cupola_(ISS_module)#/media/File:Tracy_Caldwell_Dyson_in_Cupola_ISS.jpg Cupola] module of the ISS has trapeze like windows. But the one behind the astronaut could easily have been a rectangle from what can be seen in the picture. So to argue that this window could not have been shot the same is just silly. Of course it was important to the joke that you did not realize it was a window until reading the caption. Also if this space craft has held up to go so far form Earth with living inhabitants it is obviously not a space ship in use today! --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 12:43, 14 March 2024 (UTC)
:::::There is precedent with the [https://www.sciencephoto.com/media/990677/view/crew-at-window-of-space-shuttle-discovery-2006 Space Shuttle] (aft flight-deck window, others were round, the 'forward flight-deck' ones were of course the main flight/piloting ones with awkward quadrilateral shapes and pesky instrument panels where none are in the comic). The windows in the Shuttle were actually a weight issue (certainly, at first, they were plain (chunky!) glass, and added a ''lot'' of weight to the design.
:::::Also look at {{w|File:Blue Origin M7.jpg|Blue Origin's capsule}} for more current design that could end up eventually on an orbital/extra-orbital vessel. Although Crew Dragon is more conservative, and {{w|File:MACES in Orion mock-up.jpg|Orion's interior}} looks like it isn't so big (while Starship's eventual window configuration might eventually be vastly more conservative than the Dan Dare/Flash Gordon aesthetic of the concept imagery).
:::::So... Possible, but depends upon the design needs for the craft (fully space-capable whilst ''intended'' to undergo re-entry, is all we really know). [[Special:Contributions/172.70.90.92|172.70.90.92]] 16:13, 14 March 2024 (UTC)

In my mind is the scene in C.S. Lewis's religious novel Out of the Silent Planet, where an English philologist, Ransom, is abducted by criminals into outer space and meets aliens. In chapter fifteen, a wise sorn tries to figure out which planet Ransom is from. Probably Thulcandra, the garbage planet of the Solar System. Ransom doesn't like the sound of that, but the sorn gets out something that isn't a telescope and he shows Thulcandra to Ransom, and yup, that's us. Lewis writes it better. I don't know if Carl Sagan had read this. --Robert Carnegie rja.carnegie@gmail.com [[Special:Contributions/141.101.99.75|141.101.99.75]] 13:12, 14 March 2024 (UTC)
:Hoo boy, yep that book (and its sequel) are beyond even Narnia in their religious symbolism (though the later environmentalist movements could definitely find an allegory in them, too, not sure how intended that was, in CSL's time, some time before a practical Gaia Hypothesis/etc). I can imagine Randall knows of the book (though clearly more influenced by Sagan in a direct lineage). Not entirely sure Sagan will have taken interest in that genre, nor taken the above to heart. Probably no more than his genuine scientific and rhetoric interests, which may be sufficient genesis for his own coined meme. But that's just my gut feeling. i.e. Worthy of note, but not directly (or singly-indirectly) connected. [[Special:Contributions/172.70.85.153|172.70.85.153]] 14:20, 14 March 2024 (UTC)
:: The Gaia Hypothesis is also religious, so it makes sense that it could be written into C. S. Lewis. Protecting the environment is important, but Mother Nature is basically a deity for misanthropes. {{unsigned ip|162.158.155.91|14:02, 15 March 2024}}
:::It ''can'' be believed religiously, and is named after a goddess, but also can be used to describe the (long-term) self-regulating interactions of the various elements of biomes and atmos/geos-/pedospheres that interact, in a more scientific way. (Some people religiously believe in evolution as God's "fire and forget" Creationist act to get us from the very start of Genesis until Revelations, for example, but it doesn't stop a strictly agnostic scientific analysis of natural selection getting us here, with or without writings that describe everything in traditional concepts that may not ''necessarily' be entirely accurate - but sustained "God's chosen people" in ways that mattered... to put just one teleological spin on it.)
:::It can just be considered a more complex (and entirelg natural) global thermostat. Which might have difficulty dealing with people lighting unexpected fires in the middle of their bedrooms (e.g.), and can't stop them burning their own house down if they start a runaway effect that it has no power to stop, but ''normally'' it can counteract variations of temperature by eventually adjusting the heating/cooling elements. Or, I suppose, decides that a hotter/colder house is better, so long as there are still some pot-plants that will thrive under the new conditions (until nudged back by other effects).
:::...that's simplistic, as an analogy, and in key ways 'not really correct', but it starts to reflect the no-god-needed tenet of the general idea. (Not sure I have confidence in "it'll be alright", but I would say it has better chance than "''we'll'' be alright", as we aren't vital to 'keep things going'. In fact, once we're gone then anything that remains will get its own chance to nudge conditions to a new equilibreum.) [[Special:Contributions/141.101.98.232|141.101.98.232]] 15:23, 15 March 2024 (UTC)

The color for the blue dot seems to be around #B6C8EB. --[[User:1234231587678|1234231587678]] ([[User talk:1234231587678|talk]]) 15:18, 14 March 2024 (UTC)
:Zooming in, the central pixel is definitely #ABBBDC (a very easy color to hand type), with some artifacts around it of varying shades of grey. [[Special:Contributions/172.69.59.184|172.69.59.184]] 20:58, 14 March 2024 (UTC)
::I think we're discussing different versions of the two images. The single central pixel of the dot in the "regular-sized" image on the xkcd site, "earth.png" (364 x 472 pixels), is now #B5C6E9, while the ''six'' central pixels in the 2x image, "earth_2x.png" (728 x 945 pixels), are #BDCFF4. The "regular-sized" version here, "363px-earth_2x.png" (363 x 471 pixels) has a single central pixel, #ABBBDC, while the large image on *this* site, "545px-earth_2x.png" (545 x 707 pixels), has ''two'' central pixels, #BDCFF4. [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 22:33, 14 March 2024 (UTC)
I'd say he nailed Sagan's hair... [[User:Inexplicable|Inexplicable]] ([[User talk:Inexplicable|talk]]) 19:11, 14 March 2024 (UTC)

Carl may *claim* to have messed up, but I find it hard to believe that he wouldn't aim the rocket for deep space, given half a chance.
:Sounds about right. It's Carl Sagan... and besides, who ''wouldn't'' want the opportunity to venture into the final frontier? (Also, please remember to sign your post next time.) [[User:OmniDoom|OmniDoom]] ([[User talk:OmniDoom|talk]]) 23:40, 14 March 2024 (UTC)
::Um. "Spend a little while in LEO then go home and live out the rest of your life" vs. "spend a little while much further out, in the company of a few people who ''really resent you'', then die unpleasantly"? I'd go with the former. [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 16:18, 15 March 2024 (UTC)

Does anybody know what is a typical reentry burn, for instance, when a capsule leaves the ISS? I wrote "some hundreds m/s" but it might be less than that. If the original orbit is very low, even a tiny reduction will lower the perigee enough to intersect the atmosphere. [[User:Rps|Rps]] ([[User talk:Rps|talk]]) 18:57, 18 March 2024 (UTC)
:According to ''What If: Orbital Submarine'', "A typical de-orbiting maneuver requires in the neighborhood of 100 m/s of delta-v, which means that the 24 Trident missiles carried by an Ohio-class submarine could be just enough to get it out of orbit." [[User:Take The A Train To Watertown|Take The A Train To Watertown]] ([[User talk:Take The A Train To Watertown|talk]]) 00:20, 20 March 2024 (UTC)
:Most treatments I see of the respective Delta-V Budgets basically deal with having to overcome the atmospheric and gravity deficits upon launch ''to'' LEO, which don't easily apply/discount in reverse. Also the delta-v needed betwixt low-LEO and high-LEO (or roughly encompassing the difference between current space-stations and a Hubble servicing mission) is almost a full km/s (either way), so you might need to add that to the much smaller(?) final bit of atmosphere-hitting adjustment, whereupon you hopefully are now slowing down entirely passively. (Rather than bouncing off...)
:If you can afford to wait, though, being at ISS heights will bring you down with ''zero'' (active) delta-v. There being tenuous atmosphere, already, that actually requires maintenance boosts every now and then to keep it up there. So maybe you need to consider it much as you do with a launch profile (over-powered rocket first-stage gets you over the atmospheric 'hump' quicker, and requires less total delta-v expendes for the same eventual mission). But the ultimate solution (entirely hand-brake your orbit, just fall straight down from space itself) is also not practical or necessary.
:All in all, your "some hundreds m/s" is probably not far wrong.
:Perhaps chase down the Crew Dragon operating manual, as a current example, if you want to put such actual figure(s) as used. Not sure how public that info is, but there are a lot of armchair experts out there that ''particularly'' reverse engineer vague SpaceX release-info into solid-looking figures that Musk then confirms "sound about right" if he responds to their assessments. If the usual suspects haven't crunched this number, then I'd be surprised. [[Special:Contributions/172.69.195.38|172.69.195.38]] 05:24, 19 March 2024 (UTC)
:I played around a bit with my DIY orbit calculator [https://cavac.at/guest/lithobreak LITHOBREAK], to go from a 300x300 km orbit (calculated above the surface) to a 300x100km one, which puts your perigee inside the upper atmosphere, you need a Delta-V of 59m/s. To raise your apogee to about GEO, you need roughly 2400 m/s. I could be wrong (cobbled the tool togehther a long time ago), but it looks just about right. --[[Special:Contributions/172.68.50.120|172.68.50.120]] 06:06, 11 April 2024 (UTC)
My reading: The original plan for Voyager 1 was it, after LEO to complete its mission and to be destroyed by burning in the atmosphere. So, all the spectacular discoveries of Voyager 1, including the Pale Blue Dot, are unintentional results of Cara’s miscalculations. Contrary to popular opinion, the Carl’svcoworkers are deeply disappointed that Voyager didn’t burn. [[Special:Contributions/162.158.60.145|162.158.60.145]] 12:43, 19 March 2024 (UTC)