International Year of Astronomy - Space Observatories
Earth's atmosphere limits the observation of astronomical
objects by reflecting and distorting light coming from space. The apparent
twinkling of stars is caused by thermal turbulence in the air. This same
turbulence results in blurry images in light collecting devices like
telescopes. Ground-based observatories are usually placed on top of remote
mountains to rise over hot air and minimize blurring, and also to get away from
the light of populated areas that reflects back off the atmosphere. Until
recently, this was one of the reasons why space telescopes were built, since
they are subject to neither atmospheric distortion nor light pollution. Some
new ground-based telescopes, however, have the capability of "canceling"
atmospheric distortions through the use of adaptive optics, and achieve images
almost as sharp as the ones produced by orbiting telescopes.
The most limiting factor of ground-based observatories is
that Earth's atmosphere blocks much of the light coming from space. The
atmosphere is transparent only to optical light (light visible to the human
eye) and some radio wavelengths, which are only a fraction of the full
electromagnetic spectrum. Harmful-to-life gamma-ray, X-ray, and ultraviolet
wavelengths are absorbed by the atmosphere, but they also carry most of the
information from high-energy phenomena in the Universe. A large fraction of
infrared light also gets blocked-precisely the wavelengths in which small stars
and newly-formed planets glow. Space observatories are necessary if these
wavelengths are to be studied, but developing an orbiting telescope and then
launching it into space is very costly. While state-of-the-art technology
allows for ground-based telescopes to be tens to even one hundred meters in
diameter to collect more light, the size of a space telescope is limited by the
size and power of the launching spacecraft.
Space observatories provide the sharpest images of the cosmos that
current technology allows and uncover aspects of the Universe that would
otherwise be undetectable by humans.
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