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Explain xkcd: It's 'cause you're dumb.
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We have collaboratively explained Expression error: Unrecognised punctuation character ",". xkcd comics, and only Expression error: Unrecognised punctuation character ",". (Expression error: Unrecognised punctuation character ",".%) remain. Add yours while there's a chance!

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With a space elevator, a backyard full of solar panels could launch about 500 horses per year, and a large power plant could launch 10 horses per minute.
Title text: With a space elevator, a backyard full of solar panels could launch about 500 horses per year, and a large power plant could launch 10 horses per minute.

A larger version of the image can be found here.


Ambox notice.png This explanation may be incomplete or incorrect: Discuss space elevator aspect from title text, touch on horsepower vs rocket equation, thrust, specific impulse, payload to orbit. E.g., compare with question of a horse getting itself into orbit?

This comic lists the payload capacity of several launch vehicles, and the mass of several spacecraft, in number of horses. This could be related to Randall's hobby of abusing dimensional analysis, as horses/ship is technically a perfectly acceptable derived unit, provided the size of a horse is standard (in this comic it appears that 'one horse' is defined as 450 kg). In cases where the mass is less than one horse, an alternative measure of dogs has been used, where one dog appears to be roughly 40 kg. In the case of Vanguard 1, even a dog is too large a measure, so instead the unit squirrel is used to represent its 1.47 kg mass.

The overall comic is an allusion to horsepower, a similar-sounding but completely different concept. Horsepower is a measurement of power (work per unit time). Another commonly referenced unit for power is the watt. 1 horsepower is meant to be approximately the amount of power a horse can deliver. In contrast, Randall uses the horse to measure mass (of spacecraft themselves, and of the payload they carry).

The top pane of the comic shows the mass of various spacecraft, while the bottom shows the amount of mass which various rockets can deliver to low earth orbit. There are also several joke insertions. In the top, one is T-Rex. In the bottom, another is Pegasus (the payload capacity given as one Pegasus); this is a reference to both Pegasus the rocket and Pegasus the mythical flying stallion. Atlas-Centaur is also measured in centaurs, a reference to the half-human half-horse creatures of Greek mythology. The bottom also gives the 1981 Oldsmobile as 4 horses; this references the carrying capacity (by weight) of the Oldsmobile, not the ability of an Oldsmobile to launch that payload into low earth orbit.

The Pegasus, 1981 Oldsmobile, and Stratolaunch spacecraft are depicted horizontally, presumably because these vehicles launch from a horizontal starting position and use forward momentum to facilitate their launch.

There is an unlabeled launch vehicle below the H-11A near 2002. The unlabeled vehicle has a payload mass of 21 horses (9450 kg).


The tables below contain data relating to each entry on the comic image.

  • Name - Should be as shown in the comic
  • Launch Date - Date of first flight
  • Mass/Payload (Horses) - Value as given in comic
  • Mass/Payload (kg) - Independently researched value
Spacecraft Mass
Spacecraft Launch Date ISO 8601 Mass (Horses) Mass (kg)
Sputnik 1957-10-04 2 Dogs 83.6
Vanguard 1 1958-03-17 Squirrel 1.47
Pioneer 5 1960-03-11 Large Dog 43
Venera 1 1961-02-12 1 643.5
Mariner 2 1962-08-27 3 Dogs 202.8
Apollo 67
Venera 7 1970-08-17 3 1,180
Pioneer 10 1972-03-03 7 Dogs 258.8
Skylab 1973-05-14 171 77,088
Venera 9 1975-06-08 11 4,936
Voyager 2 1977-08-20 2 721.9
Shuttle (Total) 206
Shuttle (Payload) 54
Mir 1986-02-20 288 129,700
T-Rex 15 6,800
Hubble 1990-04-24 25 11,110
Compton Gamma Ray Observatory 1991-04-05 38 17,000
Keyhole 3 40
International Space Station 1998 932 450,000
Cassini 1997-10-15 11 5,300
Huygens Lander 1997-10-15 1 319
Rosetta 2004-03-02 6 2,900
Opportunity 2003-07-07 5 Dogs 185
Dawn 2007-09-27 3 1,240
Terrastar* 2009-07-01 15 6,910
Dragon 2010-12-08 17
Tiangong-1 2011-09-29 19 8,506
Curiosity 2011-11-26 2 900
Keyhole 7 40
Orion (Capsule) 2014-12-05 20 8,913
James Webb Telescope 14 6,200
Orion 2014-12-05 20 8,913
Orion Service Module 25 12,000
Orion Deep Space Habitat 65

*Terrastar is believed to be a mis-spelling of TerreStar, based on its mass and launch date.

Lauch Vehicle Capacity
Spacecraft Launch Date ISO 8601 Payload (Horses) Payload (kg)
Sputnik Launcher 1957-10-04 1 500
Thor 1958-04-24 3 Dogs 120
Mercury-Atlas 1960-07-29 3 1,360
Saturn I 1961-10-27 20 9,070
Proton-K 1967-03-10 44
Atlas-Centaur 1962-05-08 8 Centaurs
Titan IIIA 1964-09-01 7 3,100
Saturn IB 1966-02-26 45 21,000
Soyuz 1966-11-28 14 6,450
Saturn V 1967-11-09 262 118,000
Black Arrow 1969-06-27 4 Dogs 135
N1 1969-02-21 211 90,000*
Long March 1 1969-11-16 2 300*
N-I 1975-09-09 4 1,200*
Delta 0900 1972-07-23 3 1,300
Ariane 1 1979-12-24 3 1,400
SLV 1979-08-10 1 Dog 40
N-II 1981-02-11 4 2,000
1981 Oldsmobile 1981 4
ASLV 1987-03-24 4 Dogs 150
Long March 4A 1988-09-06 9 4,000
Ariane 4 1988-06-15 16 5,000-7,600*
Shavit 1988-09-19 6 Dogs 350-800*
Energia 1987-05-15 218 100,000*
Pegasus 1990-04-05 1 Pegasus 443
Atlas I 13
J-I 2
Long March 3B 27
H-IIA 22
(unlabelled) Delta IV M? 21
Delta IV-H 64
Falcon 1 1
Ariane 5ES 47
H-IIB 37
UNHA 2 Dogs
Atlas V 541 38
Falcon 9 29
Antares 14
Stratolaunch 14
Falcon Heavy 118
SLS Block 1 156
SLS Block 1B 217
SLS Block 2 289


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