After reading this answer I found out that the space shuttle normally spent its time upside-down. Why is that? I'd guess it has to do with shielding from the sun, but I can't back that up with anything.

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    $\begingroup$ If it flies upside down normally, isn't THAT orientation the real up-side? Just sayin'... $\endgroup$ – corsiKa Oct 14 '15 at 23:41
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    $\begingroup$ I'd think that it would be nice to look up and see the Earth from space, which isn't something people normally get to do. $\endgroup$ – jamesdlin Oct 15 '15 at 2:08
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    $\begingroup$ Define "upside down" in zero gravity... $\endgroup$ – Jon Story Oct 15 '15 at 18:06
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    $\begingroup$ @JonStory: no shuttle was ever in "zero gravity". $\endgroup$ – Martin Argerami Oct 16 '15 at 7:29
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    $\begingroup$ @JonStory You know exactly what he means (in relation to the Earth, it has its dorsal facing the ground). $\endgroup$ – user001 Oct 16 '15 at 9:10

The main reason is heat rejection. NASA was asked this very question, and the answer was identified. Basically, the waste heat from the shuttle is expelled via the cargo bay doors. You don't want to ever point a radiator at the Sun, so the easiest thing is to point it at the Earth. Sometimes, if the heat was too high, they would actually point the shuttle away from the Sun in eclipse, but this was rare.

If space debris was the main reason, then the shuttle would actually fly with the belly towards the direction of motion. Most impacts of space debris will be head on. The shuttle did fly backward, with the tail in the direction of orbital motion. This protected it against debris, as the tail was the less critical portion of the spacecraft for small holes that Debris would cause. See this link.

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    $\begingroup$ I remember reading that one of the Shuttles was hit on a front window by a fleck of paint. It took a chip of glass out but didn't hole the window. $\endgroup$ – Howard Miller May 22 '16 at 22:45
  • $\begingroup$ Now just why did it launch upside-down? I know these were quirks of the construction, but what quirks? $\endgroup$ – SF. May 23 '16 at 2:05
  • $\begingroup$ @SF. Sounds like a different question. The dynamics during launch were almost certainly different than those during in-orbit freefall. $\endgroup$ – a CVn May 23 '16 at 8:38
  • $\begingroup$ @MichaelKjörling: The current question is skim in details enough to fit ;-) $\endgroup$ – SF. May 23 '16 at 8:46
  • $\begingroup$ @SF: see space.stackexchange.com/questions/12006/… $\endgroup$ – Hobbes Jun 28 '16 at 7:35

Yes, one reason is that it is flown upside down (with the top facing towards Earth) to use the heat shield tiles to shield the astronauts from the sun, but I think a bigger reason is to shield from space debris.

One reason is to protect against space debris. Here on Earth, most of the space junk (small rocks and ice, etc.) that falls towards the planet burns up in the atmosphere before it can get to us. The space shuttle is outside the atmosphere, though, so it is much more exposed to debris (which may be traveling at very high speeds). The space shuttle's belly is designed to take intense heat and pressure so that it doesn't fall apart when it re-enters the atmosphere, and is therefore much better suited for taking hits from flying space junk.

From here.

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    $\begingroup$ At the end of the program, when not docked or using a different attitude for some specific reason, shuttle mostly flew in -ZLV -XVV attitude, which means not only did the -Z axis (out of the payload bay) point at the Earth, the -X axis (out of the tail) was pointed in the direction of motion, to protect the nose cap. Ass backwards into the unknown! You can see the attitude callouts in the flight plan here nasa.gov/centers/johnson/pdf/567071main_FLT_PLN_135_F.pdf and the axes are defined here www-lite.larc.nasa.gov/level1doc/bacs.html $\endgroup$ – Organic Marble Oct 14 '15 at 21:14
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    $\begingroup$ Also the view from the windows would be nice blue planet instead of pitch black sky with BLAZING HOT SUN in it. Honestly it was bad enough being above the clouds in a Boeng 787 Dreamliner with the sun in your face, I can't imagine what it would be like in space. 787 Dreamliners have (partially) dimmable windows instead of conventional pull down shutters. Avoid them. $\endgroup$ – Level River St Oct 14 '15 at 23:47
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    $\begingroup$ That debris part is highly dubious. First of all, if something is going to hit you, it's a lot better if it hits the pressurized compartment which would make micrometeorite hits detectable and any damage easier accessible. Those heat shield tiles are also pretty much the most important asset for safe return to Earth, they could easily shatter on high velocity impact, and there was no easy way to fix them even if the damage was detected. NASA actually didn't have an answer to fixing tiles on orbit until after the Columbia disaster (main problem was gluing replacement tiles on in vacuum). $\endgroup$ – TildalWave Oct 15 '15 at 0:24
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    $\begingroup$ First time I've heard a cabin leak referred to as a desirable scenario! Cabin leaks were deemed to be pretty much unfixable unless they occured in a golden, easily accessible location. A kit was flown for this. An unpluggable cabin leak would result in an emergency deorbit. MMOD hits on the radiators could also force a quick deorbit. Late in the program radiator isolation valves were added for that very reason. $\endgroup$ – Organic Marble Oct 15 '15 at 0:34
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    $\begingroup$ @OrganicMarble Emergency deorbit > not surviving reentry ;) $\endgroup$ – TildalWave Oct 15 '15 at 1:06

During launch, it is for aerodynamic reasons, and line-of-sight with the ground:

"The orbiter flies upside down during the ascent phase. This orientation, together with trajectory shaping, establishes a trim angle of attack that is favorable for aerodynamic loads during the region of high dynamic pressure, resulting in a net positive load factor, as well as providing the flight crew with use of the ground as a visual reference. By about 20 seconds after lift-off, the vehicle is at 180 degrees roll and 78 degrees pitch."

Basically, the once-off fuel tank is taking the brunt of the wear and tear during the climb up through the atmosphere, and sheltering the orbiter somewhat. Although I don't have a reference for it, I remember reading somewhere that the communication antenna work best on the 'top' side of the orbiter, so pointing it at the ground is desirable when on orbit.

Source https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_mes.html#mes_1st_stage

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