I think he used horses as a reference to the unit of work, horse power, but in this case instead of being the 550 foot pounds per second, it is the force required to put a horse at that altitude 199.27.128.195 08:10, 17 December 2014 (UTC)

The top one clearly says "Spacecraft mass" and the bottom says "Capacity" (which is normally either the mass or volume something can hold), so I don't think either refers to force. 173.245.54.175 08:32, 17 December 2014 (UTC)

Horses might also be used as a length unit... I am wondering if this is somehow related to the size of SRBs [1] ;-) -- Ld75 (talk) (please sign your comments with ~~~~)

There is the famous tail (no pun intended) of how the width of the Space Shuttles SRBs are related to the width of a horses, er, um, butt -- which apparently is not true. However, similarly to the "Upgoer Five", Randall may just be trying to relate a very difficult to grasp concept (weight of a huge object) to something that with which a large number of people may be familiar. Jarod997 (talk) 14:34, 17 December 2014 (UTC)

A few mass calculations: (All masses from wikipedia)

• ISS - 450,000 kg / 932 Horses = 483 kg/Horse
• Skylab - 77,088 kg / 171 Horses = 450 kg/Horse
• Mir - 129,700 kg / 286 Horses = 453 kg/Horse
• Shuttle Payload - 24,400 kg / 54 Horses = 452 kg/Horse
• Compton GRO - 17,000 kg / 38 Horses = 447 kg/Horse

It looks like Randall probably used 450kg as a standard horse, which seems like a fairly average weight for a horse. --Pudder (talk) 09:06, 17 December 2014 (UTC)

Curious: the Atlas-Centaur rocket is listed as lifting Centaurs, not horses. 141.101.98.244 09:11, 17 December 2014 (UTC)

Plus the Pegasus rocket is labelled as lifting one (mythical, horse-sized?) Pegasus. I took this as synchronicity, but currently someone (who missed the Centaur reference in the part of the main text about "joke additions/deviations") who edited the the main text seems to think that it's purely a mythical reference. 141.101.98.245 13:27, 17 December 2014 (UTC)

The Keyhole captions are wrong; assuming 450 kg/horse, 40 horses for the Keyhole 7 would be around 18,000 kg. Documents declassified a couple of years ago give the mass as around 2,000 kg. The Keyhole 3 was even smaller. The mass and dates are about right for Keyhole 11 satellites but I don't know where the 3 and 7 have come from (the dates are wrong for the third and seventh Keyhole 11s) 141.101.99.85 09:43, 17 December 2014 (UTC)

Initially I thought that the position of 'T Rex' along the X axis (approx 1985) may be a link to the band T.Rex, but according to wiki that was 1967-1977. I guess it could be a random date, but thats not usually Randall's style... --Pudder (talk) 10:51, 17 December 2014 (UTC)

The T Rex "Sue" was discovered in August 1990. Not quite right, either. 108.162.250.214 11:10, 17 December 2014 (UTC)
Perhaps a reference to the date of 'Jurassic Park'? Not sure exactly when that came out but I think it's in the right range. 108.162.238.189 14:55, 17 December 2014 (UTC)
Hadn't thought of that.. Though after checking, Jurassic Park was released 1993, and I believe set in 1990. Still doesn't seem to tie up.. --Pudder (talk) 15:22, 17 December 2014 (UTC)
I never knew there was ever a space craft called "T-Rex". Learn something new everyday! ;) -- Dangerkeith3000 (talk) (please sign your comments with ~~~~)

The Oldsmobile is probably a reference to the movie Mom and Dad Save the World. The title characters' station wagon was from around that era -- and it did go into space in the movie. --Aaron of Mpls (talk) 11:47, 17 December 2014 (UTC)

Terrastar may be referring to TerreStar-1, with a launch mass of 6,910 kg, divided by 15 horses is about 460 kg/Horse. This is consistent with the above calculations. Jarod997 (talk) 14:23, 17 December 2014 (UTC)

Based on that and its accurate position on the X axis, I would agree with you. I've added it to the table, with a note below. --Pudder (talk) 14:38, 17 December 2014 (UTC)

Thor may be referring to the Thor-Able launch vehicle. First flight of 1958 (good position on the graph), and payload of 120 kg is about 40 kg / dog -- that's a bit high for an average, but depending on the breed it could be accurate. Jarod997 (talk) 14:58, 17 December 2014 (UTC)

Yes, I believe that is correct. Added to tables. --Pudder (talk) 15:22, 17 December 2014 (UTC)

Based on horse payload, location on X-axis (time of first launch?), and some general wikipedia browsing, I believe the unlabeled launch vehicle on the bottom is a Delta IV Medium. According to Wikipedia, it was first launched on 11 March 2003, which fits the location on the timeline. It had an LEO payload capacity of 9,420 kg, which equates to 20.9333333 horses at the estimation of 450kg/horse. For reference, all my information was pulled from http://en.wikipedia.org/wiki/Delta_IV#Delta_IV_Medium. Screamsquad (talk) 16:20, 17 December 2014 (UTC)

Certainly seems a likely candidate..--Pudder (talk) 17:08, 17 December 2014 (UTC)
Title Text Calcs

I did some calculations on the title text, though I'm not sure how correct they are. We can use our standard horse at 450kg, and assuming 9.81m/s2 gravity (neglecting the slight drop in gravity as it moves up the space elevator), the formula I've used is:

```1 Watt = 1 Newton raised 1 Metre in 1 Sec
Power[Watts] = (450[kg] * 9.81[m/s<sup>2</sup>]) * LEO[m] / Time[s]```

A large power station is a pretty vague measure, but I would guess we are looking at around 1GW. I found a 30 acre solar farm (in the not-so-sunny UK) with an output of 5MW, scaled linearly (which may well not be valid) a 0.6 acre (2500m2 back yard could yield 93.5kW. Using these power guesstimates, we can calculate the altitude at which the 500 horses/year and 10 horses/minute converge. It works out at roughly 1350km. Depending on where you look, LEO is classified as anything under 2000km.

Now I know there are a lot of assumptions in there, and really the whole formula is backwards, but hey... its a starting point! --Pudder (talk) 17:05, 17 December 2014 (UTC)

I think Randall may have been talking to Andrew Hussie --141.101.106.89 21:50, 17 December 2014 (UTC)

https://what-if.xkcd.com/58/ Getting to space is easy, staying in space is hard.

The energy required to get to 100km, briefly, is about 1 MJ/kg The energy required to stay in low earth orbit is a bit over 30 MJ/kg

It's tricky to get to low earth orbit with a space elevator, you can go up 200km, but as soon as you let go of the elevator you come down. If you put enough mass at 200km the whole elevator comes down. Not sure which number to use, but I'd go with the 30 MJ/kg, or 15 GJ/horse. So the solar array is about 240kW (averaged over the whole year) and the power plant is 2.5GW. This looks a bit high so maybe the horses don't stay in orbit.

--108.162.219.248 15:49, 18 December 2014 (UTC)

It depends on what he means by launch I suppose. I had simply looked at getting to LEO altitude, but staying there once you let go of the cable is a whole different ball game. I believe the only way to get into orbit from a space elevator (lacking additional power) is to go right to geostationary height? Let go anywhere under that altitude and you go into orbital decay.
Out of interest, where did you get the 1MJ/kg and 30MJ/kg values? As far as power stations, the new generation twin reactor Hinkley Point C nuclear power station (only know that one because its local to me) is designed to output 3.2GW, so 2.5GW isn't over the top. I would say that 240kW for aback yard solar farm is pretty large, depending on your location and size of back yard. --Pudder (talk) 17:11, 18 December 2014 (UTC)
1MJ/kg comes from mgh for 1kg at 100km we get 9.81 * 100,000
30MJ/kg comes from 1/2 mv^2 I happen to know that orbital velocity is about 8km/s so 1/2*8000^2 = 32,000,000
Normally I'd say close enough for goverment work, but in this case NASA probably uses a bit more precision. If you want to be more precision, visit wikipedia "specific orbital energy" and "circular orbit".
If we release a horse from the space elevator at any altitude it will be in orbit, at least until it contacts the earth's atmosphere. It turns out that if we release it at 30,000km (from the earths center = 23,600km altitude) then at perigee the altitude is 100km. High enough that a horse would stay in orbit for a while. Releasing 750 km higher raises perigee to 1000km altitude. So we don't have to go all the way to geostationary orbit, just high enough. But still 23,000km is not LEO.
--108.162.219.248 04:32, 19 December 2014 (UTC)
Ok, I'm just about following you I think! So assuming we are launching to orbit, we have to get to 23,600km, at which point gravity is considerably reduced. We definately have to take into account change in gravity as our horses move up the cable.
I did some maths, and I believe it takes 19.9GJ to get a 450kg horse to 23,600km. That is essentially mgh, but taking into account the change in gravity. At 1 Watt, thats gonna take 631 years. Our 500 horses per year solar plant needs to output 315kW (300kW solar array on a roof for reference). Our 10 horses per minute power station would need to output 3.32GW, so a decent nuclear facility. Once the Three Gorges Dam in China is running at full capacity (22.5GW), it could be launching 68 horses per minute. Running constantly at full capacity, we could rid the world of horses (est 58 million) in under two years. --Pudder (talk) 11:09, 19 December 2014 (UTC)
I confirmed the height of 23 600 km with my own calculations; my result is about 189 km altitude at perigeum, and 996 km for 24 350 km (750 km higher, as you wrote). My code seems consistent as entering geostationary orbit altitude results in geostationary orbit. Anyway, I think that 19.9 GJ is slightly wrong - I entered it into Wolfram, and it showet a little more. This results in 350 kW and 3.7 GW for backyard solar panels and power plant, respectively. 162.158.92.100 22:20, 28 January 2017 (UTC)

I filled out the remaining fields. I thought about getting the interior capacities of all 1981 Oldsmobile models but doesn't seem worth it. It's clearly a joke so that kind of research doesn't seem necessary. 173.245.56.152 (talk) (please sign your comments with ~~~~)

Arrow of Time Did anyone else notice the special advert for Xmas gifts? It was up at the top of the main xkcd page yesterday 2014-December-17, but gone today (the 18th). In black, blue and red colored printing it said:

"The XKCD store Christmas shipping deadline is Friday!
Once it passes, there is no going back. The arrow of time drags us all
inexorably forward toward decay and disorder toward the end of all things.
Toward the empty silence of the grave. ORDER NOW!"

I thought it was amusing and saved a bitmap. Was kind of surprised that there was no other mention of this here, or on the fora either. 108.162.216.171 (talk) (please sign your comments with ~~~~)

I did notice it too, though I've always been a bit disappointed by the limited range of T-shirt comics. None of my favourites are on there. As far as this page mentioning it, it isn't relevant to the comic, so doesn't belong on this page. I don't know of a page on explainxkcd where it would really fit in. --Pudder (talk) 11:18, 19 December 2014 (UTC)
Here: xkcd#Header_text on the xkcd page on explain. Just finished the transcript for this comic. Note the 1461: Payloads#Trivia above. --Kynde (talk) 00:58, 30 January 2017 (UTC)
Why not make your own? Here is a fridge magnet, it wasn't too hard to figure out the design tools, Duty Calls magnet Actual size is a little over 2" x 3" (50 x 75mm)--one problem with the design tools is figuring out the final size for some items. Based on my understanding of the CC license, I have not sold these, but I have given a few to friends. Edit--when something I want is in the XKCD store, of course I buy it there so Randall gets the cash. For example, the large poster sized version of Money on the wall in front of me now came from the XKCD store. It would be really cool if Randall made this fridge magnet (or something similar) available on his site. ‎108.162.216.171 (talk) (please sign your comments with ~~~~)
I guess part of the reason is that I like the idea of Randall getting the cash from my purchase. --Pudder (talk) 14:04, 19 December 2014 (UTC)

Actually, I find it strange that no explicit mention of Laika or Sputnik 2 is made. --141.101.75.89 23:50, 18 December 2014 (UTC)

I'm pretty sure this has nothing to do with dimensional analysis. Who wrote that part? have you ever actually done dimensional analysis? He's just using a nonstandard unit of measure. Unit conversion is almost -but not entirely- unlike dimensional analysis. 108.162.241.5 (talk) (please sign your comments with ~~~~)

What's with the mass of squirrel? Apart from marmots, any squirrel weighs way less than a kilogram! 108.162.253.150 (talk) (please sign your comments with ~~~~)

The T-Rex entry may refer to the Wankel Rex or Devil Rex discovered in 1988. See https://en.wikipedia.org/wiki/Specimens_of_Tyrannosaurus#.22Wankel_Rex_or_Devil_Rex.22:_MOR_555 PaperDragon-CA 108.162.216.89 17:57, 30 September 2016 (UTC)