explain xkcd - User contributions [en]

2908: Moon Armor Index

2024-03-19T18:52:58Z

141.101.99.75: /* Transcript */ ...missed changing that.

{{comic
| number = 2908
| date = March 18, 2024
| title = Moon Armor Index
| image = moon_armor_index_2x.png
| imagesize = 740x390px
| noexpand = true
| titletext = Astronomers are a little unsure of the applicability of this index, but NASA's Planetary Protection Officer is all in favor.
}}

==Explanation==
{{incomplete|Created by a ROCK-LOVING MOON ARMOR SAMURAI BOT - Please change this comment when editing this page. Do NOT delete this tag too soon.}}

In this comic, [[Randall]] hypothesizes a situation in which each planet's moons were converted into protective armor to coat their respective planets. For example, the {{w|Moon}} would coat {{w|Earth}} in a 43 kilometer layer if it were molded into protective armor for the planet. It is unknown how the moons would be converted into armor, but with current technology, this would be highly impractical, if not impossible.{{citation needed}} It is also unclear how planets such as Jupiter, Saturn, Uranus, and Neptune, which are gas giants and thus have no solid surface, would sustain such a layer of armor, nor is it immediately evident what threat might necessitate such drastic measures. A 43km moon rock layer might in fact endanger life on Earth by blocking out the sun and impacting ecosystems, rather than protecting it.

Mars's moons {{w|Phobos_(moon)|Phobos}} and {{w|Deimos_(moon)|Deimos}}, being very small compared to Mars, would make a very thin layer over Mars. Huge Jupiter would be covered with almost 3km of "moon", which indicates just how much moon mass there is around Jupiter. This is, to a smaller degree, similar for Saturn, Uranus and Neptune.

Some trans-Neptunian {{w|Dwarf_planet|dwarf planets}} and dwarf planet candidates are included, as well: Only Pluto, having a moon ({{w|Charon_(moon)|Charon}}) of a comparable size to its planet, would have a layer thicker than Earth's. {{w|120347 Salacia|Salacia}}, {{w|Haumea}}, {{w|50000 Quaoar|Quaoar}}, {{w|225088 Gonggong|Gonggong}} and {{w|Eris (dwarf planet)|Eris}} are among the {{w|List_of_trans-Neptunian_objects#List|ten largest such objects}}. However, {{w|Makemake}} and {{w|90482 Orcus|Orcus}} are not included in this comic, despite both being known to have an available moon.

The comic uses the ≈ sign to show that the formula is only an approximation: this is because it does not take account of the armor having a larger surface area as it gets thicker. This approximation would be perfect for a shield of thickness zero, but for the thickest shield (Pluto) the error is around 4% (52.5 km by approximation, but 50.4 km by calculation). If P, M and A are the Planet radius, Moon radius and Armour thickness respectively, we can use V(P+A) = V(P) + V(M) to get A^3 + (3P)A^2 + (3P^2)A - C^3 = 0 and then solve the cubic to make an accurate calculation. For multiple moons, this calculation can be used repeatedly to armor each moon with the previous moon, before using the final moon-conglomerate to armor the planet. Compression effects are not taken into account, the combined planet's mass could settle some of the moon's original material (and forcing the planet below) into a slightly tighter volume than it may have occupied within its own gravitational field.

The title text states that astronomers are "unsure" about the applicability of protective armor made from moons, possibly because they are wary of {{w|Giant-impact_hypothesis|moons and planets getting too close}}. It is dubious whether there would be an actual increase in planet safety due to the procedure. Moons as they are already serve a protective purpose by deflecting incoming asteroids, and by covering a small portion of sky.

The title text continues that NASA's [https://what-if.xkcd.com/117/ Planetary Protection Officer], whose job it is indeed to protect planets, is purportedly in favor of the idea. However, they are in fact responsible for keeping other celestial bodies safe from Earth's contamination, not for shielding planets in armor, as the title text suggests. There is no known method of shielding other planets from contamination by physically armoring them. {{citation needed}}

{| class="wikitable"
|-
! Planet/<br>dwarf planet !! Surface area (km²) || Moons || Volume (km³) || Moon shield thickness 
|-
| {{w|Earth}} || 5.1007*10^8 || {{w|Moon|1}} || 2.196*10^10 || 43 km (27 mi)
|-
| {{w|Mars}} || 1.4437*10^8 || {{w|Moons of Mars|2}} || {{w|Phobos (moon)|(5695±32)}}+{{w|Deimos (moon)|(1033±19)}} || 5 cm (2 in)
|-
| {{w|Jupiter}} || 6.1469*10^10 || {{w|Moons of Jupiter|95}} || 1.7646*10^11 || 2.87 km (1.78 mi)
|-
| {{w|Saturn}} || 4.27*10^10 || {{w|Moons of Saturn|146}} || 7.651*10^10 || 1.79 km (1.11 mi)
|-
| {{w|Uranus}} || 8.1156*10^9 || {{w|Moons of Uranus|28}} || ||
|-
| {{w|Neptune}} || 7.6187*10^9 || {{w|Moons of Neptune|16}} || ||
|-
| {{w|Pluto}} || 1.7744*10^7 || {{w|Moons of Pluto|5}} || {{w|Charon (moon)|(9.322×10^8)}}+{{w|Moons of Pluto|(approx 87100+38800+900+200)}} || 52.5 km (32.6 mi) (by XKCD)
50.4 km (31.3 mi) (by full calculation)
|-
| {{w|120347 Salacia|Salacia}} || 2.27*10^6 || {{w|Actaea (moon)|1}} || 1.41*10^7 || 6.21 km (3.85 mi)
|-
| {{w|Haumea}} || 8.14*10^6 || {{w|Moons of Haumea|2}} || {{w|Hiʻiaka (moon)
|(17.2*10^6)}}+{{w|Namaka (moon)|(2.57*10^6)}} || 2.43 km (1.51 mi)
|-
| {{w|50000 Quaoar|Quaoar}} || 3.78*10^6 || {{w|Weywot|1}} || 4.19*10^6 || 1.11 km (0.69 mi)
|-
| {{w|225088 Gonggong|Gonggong}} || || {{w|Xiangliu (moon)|1}} || ||
|-
| {{w|Eris (dwarf planet)|Eris}} || (1.70±0.02)*10^7 || {{w|Dysnomia (moon)|1}} || ||
|}

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[Text above diagram:]
:Moon armor index:
:How thick the shells around various worlds would be if their moon(s) were converted into protective armor
:≈Total moon volume/Planet surface area

:[Above the diagram, there is a depiction of two moons orbiting a planet, an arrow pointing to the right of it, and on the right is the same planet with an additional layer around it without orbiting moons.]

:[The diagram consists of vertical bars showing "moon armor" thicknesses for the Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, Salacia, Haumea, Quaoar, Gonggong and Eris. Earth's bar has a label named "43 km thick" and is compared to the height of Mt Everest. The zoom in to Mars's bar has a thickness label of 2" and is compared to a rover wheel over it. Pluto's bar has a label named "(Mostly Charon)", with arrows pointing away from the label.]

{{comic discussion}}

[[Category:Astronomy]]
[[Category:Bar charts]]

Talk:2906: Earth

2024-03-14T13:12:10Z

141.101.99.75: Another writer's take on it.

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)
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)

2822: *@gmail.com

2023-08-31T02:47:12Z

141.101.99.75: /* Explanation */

{{comic
| number = 2822
| date = August 30, 2023
| title = *@gmail.com
| image = gmail_com_2x.png
| imagesize = 305x269px
| noexpand = true
| titletext = Hi all, just replying to loop in *@outlook.com and *@yahoo.com.
}}

==Explanation==
{{incomplete|Created by BOT@COMPUSERVE.NET - Please change this comment when editing this page. Do NOT delete this tag too soon.}}
A wildcard symbol, such as the asterisk, is not generally usable via email servers, although email ''clients'' may sometimes implement a function, internally, perhaps to support mailing-list functions (though more commonly via address-book 'groups'). That said, the asterisk character is a valid one for a mailbox, including group-boxes to facilitate server-side distribution.

The address *@gmail.com, as illustrated in the comic, is a proposed feature from Randall that would send an email to ''every'' Gmail user, without having each and every valid gmail address at hand (manually typed in or via the email client's address books). For obvious reasons, this is not actually a feature, but Randall sugests that if Google ever wanted to shut Gmail down, they could do either do it this way (possibly causing a service-ending overload of resources) ''or'' allow someone this one last boon (as a farewell gift, knowing that there would be relatively little repurcussions to deal with).

The title text suggests the possibility of a similar iteration over two further (im)famous mail-servers.

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

:To: *@gmail.com (+expand)
:Cc: [Empty field]
:Bcc: [Empty field]
:Subj: New Friends

:Hey all! Go ahead and introduce yourself!

:[Caption:] If Google ever decides to shut down Gmail, they should let one user trigger a global reply-all apocalypse.

{{comic discussion}}

[[Category:Email]]