Is the Hubble Space Telescope, and every other space telescope for that matter, completely outside the Earths atmosphere?
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3$\begingroup$ What do you mean by "every other telescope"? What about this? Or this? The Hubble telescope is roughly in the exosphere which is not a complete lack of atmosphere but is nevertheless exceedingly thin. $\endgroup$– called2voyage ♦Commented Jun 26, 2015 at 19:42
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1$\begingroup$ "completely outside if the Earths atmosphere" is not well defined, at least for any telescope in low Earth orbit. What do you consider the boundary of Earth's atmosphere? We know the gas density near all the space telescopes and it is not zero. $\endgroup$– Ross MillikanCommented Jun 27, 2015 at 2:47
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$\begingroup$ @called2voyage I didn't mean it like that I meant space telescopes, sorry for poor wording. $\endgroup$– DashboarrdCommented Jun 27, 2015 at 23:14
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2 Answers
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A handful of space telescopes are located in Langrange point L2, 1.5 million km from Earth. This is much farther away than the Moon, and far outside Earth's atmosphere.
WMAP and Planck, which measure the cosmic microwave background (CMB), are located here because Earth is a hundred times brighter than the CMB in this wavelength region. Herschel observes in the infrared, which is heat radiation, so it was also put in L2 to avoid Earth's heat. Others can be found in the list provided in Keith's answer.
Another advantage of putting a telescope in L2 is that the Earth and Sun are always in the same direction, making it easier to observe most of the sky. The cost of putting it here is probably more than an order of magnitude higher, though.
Whether or not low Earth orbit telescopes like Hubble at 550 km are outside Earth's atmosphere is a bit subjective. The atmosphere doesn't stop at some particular point, instead gradually thinning. These space telescope are definitely far enough away that they are not heated up. Neither does the atmosphere affect their observations (by seeing). They are, however, slowly decelerated and thus need a boost once in a while not to fall down.
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1$\begingroup$ Okay, I don't actually know if it's an order of magnitude more pricey. That was just a number I pulled out of my hat. Feel free to correct me. $\endgroup$– pelaCommented Jun 26, 2015 at 21:49
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1$\begingroup$ And Spitzer and Kepler (now K2) are in earth-trailing orbits, now a good fraction of an AU away from the Earth and receding. $\endgroup$– eshayaCommented Jul 4, 2015 at 16:59
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This article contains a list of space telescopes. It's likely to be nearly complete.
The extent of the Earth's atmosphere is not very well defined. The altitude at which Hubble orbits (about 550 kilometers above the surface) is above almost all of the atmosphere, but there's still enough residual air to cause some slight drag. It's not higher because it was deployed from the Space Shuttle, which couldn't carry it much higher that its current orbit. It was also useful to be able to fly servicing missions.
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2$\begingroup$ "It was also useful" - in fact as it turned out, it was close to mission-critical for Hubble's optical capabilities. $\endgroup$ Commented Jun 27, 2015 at 1:20