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.
This comic is an infographic representing the launch mass of various spacecraft and artificial satellites, and the low Earth orbit payload capacity of various space launch vehicles. Rather than using standard units of mass such as kilograms or pounds, Randall has assigned values based on the mass of a horse. Based on cross checking researched masses and payloads with the number of horses depicted, it appears that one horse unit is defined as 450 kg (with an average of 432.82 kg), or perhaps 1000 lb. 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 (with an average of 48.05 kg, or perhaps 100 lb.). 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 (3.5 lb.?) mass.
The overall comic may be 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 particular spacecraft, and of the maximum payload launch vehicles can carry).
The top pane of the comic (black background) shows the mass of various spacecraft, while the bottom (white background) shows the payload capacity (to low Earth orbit) of launch vehicles. Along the bottom of the image is a timeline, relating to the launch date of the entries.
There are also several joke insertions:
- T-Rex - A dinosaur, but fairly unlikely to be found orbiting Earth.
- Pegasus - An actual launch vehicle, but also the name of a mythical flying stallion. The payload is given as "one Pegasus", which comes out to be slightly less than "one horse".
- Atlas-Centaur - Again, an actual launch vehicle, but also a reference to the half-human half-horse creatures of Greek mythology. The payload is given in "centaurs", which come out to be slightly more than "horses".
- 1981 Oldsmobile - Not a launch vehicle, but in fact a car. The payload is given as 4 horses, which may relate to the carrying capacity (by weight) of the Oldsmobile, not the ability of an Oldsmobile to launch that payload into low Earth orbit. While there are no known examples of an Oldsmobile reaching low Earth orbit, The Blues Brothers movie shows an Oldsmobile performing a very, very long flight, and this might be the reason why Randall chose this specific car.
The Pegasus, 1981 Oldsmobile, and Stratolaunch spacecraft are depicted horizontally, because these vehicles launch from a horizontal starting position and use forward momentum to facilitate their launch.
An unlabelled launch vehicle is shown below the H-IIA near 2002. From the payload and date it is believed to represent the Delta IV M. Whether its lack of labelling is intended or a mistake is unknown.
The title text refers to a favourite subject of Randall's - The space elevator. A space elevator is a (currently theoretical) mechanism for travelling into space, consisting of a very long (>35,000 km) cable and counterweight connected to the Earth at the equator. The cable rotates at the same rate as the earth, and thus appears stationary when viewed from earth. It is then possible to climb the cable into space, and even use it as a slingshot to launch vehicles.
In the title text, the amount of power required to lift a horse into space has been investigated, with the launch capacity of a backyard solar array and large power station compared. The orbit to launch horses is not precised, though; from the space elevator, the only circular orbit easily achievable is geostationary orbit, and getting into Low Earth Orbit is only slightly easier than without the elevator. Assuming the lowest stable orbit (that is, above the atmosphere), required power output of the solar array is about 315-350 kW and the power station at 3.3-3.7 GW.
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
Where the researched launch date or mass/payload don't seem to match the comic, they should be identified with ?
*Terrastar is believed to be a misspelling of TerreStar, based on its mass and launch date.
†Keyhole 3 and 7 seem to be errors; Keyhole 3 satellites were launched between 1961 and 1962 and Keyhole 7 between 1963 and 1967.
- [This comic is a wide drawing, with a larger drawing that can be reached by clicking the small picture on xkcd. In the smaller picture shown on xkcd only the text that is not red can be read. The transcript below is thus for the large drawing. It is divided into three horizontal sections. The first section is black and shows spacecrafts, the second is white and shows launch vehicles, and the third is black again showing a timeline ranging from 1950 to the future. The vehicles are shown by the proper number of horses, and when that weight is less than one full horse also in the weight of other smaller animals.]
- [The black section:]
- Spacecraft mass
- Measured in horses
- Sputnik -- <1 horse (2 dogs )
- Vanguard 1 -- <1 horse (Squirrel )
- Pioneer 5 -- <1 horse (Large dog)
- Mariner 2 (United States) -- <1 horse (3 dogs )
- Venera 1 (USSR) -- 1 horse
- Apollo -- 67 horses
- Venera 7 -- 3 horses
- Pioneer 10 -- <1 horse (7 dogs)
- Skylab -- 171 horses
- Venera 9 -- 11 horses
- Voyager 2 -- 2 horses
- Shuttle (Total) -- 206 horses
- Shuttle Payload -- 54 horses
- Mir -- 288 horses
- T-Rex -- 15 horses
- Hubble -- 25 horses
- Compton Gamma Ray Observatory -- 38 horses
- Keyhole 3 -- Spy satellite
- International Space Station -- 932 horses
- Cassini -- 11 horses
- Huygens lander -- 1 horse
- Rosetta -- 6 horses
- Opportunity -- <1 horse (5 dogs)
- Dawn -- 3 horses
- Terrastar -- 15 horses
- Dragon -- 17 horses
- Tiangong-1 -- 19 horses
- Curiosity -- 2 horses
- Keyhole 7 -- 40 horses
- Orion (capsule) -- 20 horses
- James Webb Telescope -- 14 horses
- [20 horses:]
- [25 horses:]
- Orion Service Module
- [65 horses:]
- Orion Deep-Space Habitat
- [The white section:]
- Launch vehicle capacity
- (Payloads to low earth orbit)
- Measured in horses
- Sputnik Launcher -- 1 horse
- Thor -- <1 horse (3 dogs)
- Mercury-Atlas -- 3 horses
- Saturn I -- 20 horses
- Proton-K -- 44 horses
- Atlas-Centaur -- 8 centaurs
- Titan IIIA -- 7 horses
- Saturn IB -- 45 horses
- Soyuz -- 14 horses
- Saturn V -- 262 horses
- Black Arrow -- <1 horse (4 dogs)
- N1 -- 211 horse -- Exploded on Launch pad
- Long March 1 -- 2 horses
- N-I (Japan) -- 4 horses
- Delta 0900 -- 3 horses
- Ariane 1 -- 3 horses
- SLV (India) -- <1 horse (1 dog)
- N-II -- 4 horses
- 1981 Oldsmobile -- 4 horses
- ASLV -- <1 horse (4 dogs)
- Long March 4A -- 9 horses
- Ariane 4 -- 16 horses
- Shavit (Israel) -- <1 horse (6 dogs)
- Energia -- 218 horses
- Pegasus -- 1 Pegasus
- Atlas I -- 13 horses
- PSLV -- 8 horses
- J-I -- 2 horses
- Long March 3B -- 27 horses
- H-IIA -- 22 horses
- Delta IV-H -- 64 horses
- Falcon 1 -- 1 horse
- Ariane 5ES -- 47 horses
- H-IIB -- 37 horses
- Unha (North Korea) -- <1 horse (2 dogs)
- Atlas V 541 -- 38 horses
- Falcon 9 -- 29 horses
- Antares -- 14 horses
- Stratolaunch -- 14 horses
- Falcon Heavy -- 118 horses
- SLS Block 1 -- 156 horses
- SLS Block 1B -- 217 horses
- SLS Block 2 -- 289 horses
- [The timeline:]
- 1950s, 1960s, 1970s, 1980s, 1990s, 2000s, Future
A larger transcript with image descriptions can be found here: Full transcript
- Later after the initial release of this comic Randall added a link to this page. It's viewable in the HTML-source or here: https://xkcd.com/1461/info.0.json. The text is: "This comic is an infographic. A very good transcription is available in complete form here: http:\n\nwww.explainxkcd.com\nwiki\nindex.php\n1461". At the time this was added here was no written transcript but a complete explain section.
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