The Earth’s atmosphere provides a life-sustaining shield, protecting the surface from harmful solar and cosmic radiation. For astronomers, however, this dense gaseous envelope presents a conflict: while it offers protection, it severely degrades the quality and quantity of light reaching ground-based telescopes. The atmosphere is mostly detrimental to high-resolution observation and the comprehensive study of the cosmos.
Why the Atmosphere is Primarily Detrimental
The atmosphere functions like an opaque filter across the majority of the electromagnetic spectrum, limiting what can be observed from the ground. Only a few specific frequency ranges, known as atmospheric windows, are transparent enough for light to pass through efficiently. The primary windows are the visible light spectrum and a large portion of the radio wave spectrum, which are the traditional domains of ground-based astronomy.
Beyond these narrow ranges, the atmosphere blocks a vast amount of cosmic information. A telescope on Earth can only access a minute fraction of the total light and energy emitted by celestial objects. This limited transparency forces astronomers to rely on small windows, restricting the types of phenomena they can study without space-based instruments. The atmosphere also introduces constant, unpredictable changes to the light that passes through, further complicating observation.
The Phenomenon of Light Distortion
The most immediate negative effect of the atmosphere is the blurring and distortion of images, known as astronomical seeing. This phenomenon is caused by atmospheric turbulence, which consists of moving air masses with varying temperatures and pressures. Since the refractive index of air depends on its density and temperature, these moving “cells” of air act like tiny, irregular lenses.
As light from a distant star travels through these turbulent layers, its wavefront is randomly bent and scattered before reaching the telescope. This rapid, irregular refraction causes the light’s apparent position to shift thousands of times per second, which is perceived as the characteristic “twinkling” of stars. Planets, which present an extended disk rather than a point source, appear to shimmer or boil instead of twinkling, but they are still blurred.
The quality of seeing is measured by the diameter of the “seeing disk,” the smallest size a star can be focused to under given atmospheric conditions. Even at the best observatory sites, the seeing disk diameter is limited to about one arcsecond or more, regardless of the telescope’s size. This atmospheric limit prevents ground-based telescopes from achieving the theoretical resolution their massive mirrors are capable of.
Blocking the Electromagnetic Spectrum
In addition to distorting light, the atmosphere absorbs and scatters entire bands of the electromagnetic spectrum, rendering them inaccessible from the ground. High-energy radiation, such as Gamma rays, X-rays, and most of the ultraviolet (UV) spectrum, is absorbed high in the atmosphere by molecules like ozone and molecular oxygen. While this absorption is beneficial for life on Earth, it blinds astronomers to the most energetic processes in the universe, such as those involving black holes, supernova remnants, and active galactic nuclei.
The atmosphere also presents a barrier to infrared (IR) astronomy, which is essential for studying cool objects and star formation within dusty stellar nurseries. Water vapor, an effective absorber of IR radiation, blocks large sections of the infrared spectrum, particularly the mid- and far-infrared wavelengths. Even at very high altitudes, where water vapor is reduced, only narrow IR windows remain transparent.
This opacity means that astronomers studying phenomena like the cosmic microwave background or the formation of the earliest galaxies must rely on space-based instruments. The inability to observe the full spectrum from Earth limits the ability to assemble a complete picture of a cosmic object, which often emits light across all wavelengths.
Strategies for Mitigation
Astronomers employ several strategies to overcome the atmosphere’s negative effects, beginning with the careful selection of observatory locations. Ground-based telescopes are sited on high mountain peaks in dry climates, such as the Atacama Desert in Chile or Mauna Kea in Hawaii. This placement puts the telescope above a significant portion of the atmosphere, particularly the water vapor that absorbs infrared light.
For correcting distortion, modern observatories use Adaptive Optics (AO). An AO system measures the incoming distorted light wavefront using a sensor and sends a signal to a deformable mirror. This mirror rapidly changes its shape, sometimes thousands of times per second, to precisely counteract the atmospheric distortion in real-time. By correcting the turbulence, AO can sharpen images to near-theoretical limits, allowing large ground-based telescopes to compete with the clarity of space-based instruments.
The only way to observe the cosmos across the entire electromagnetic spectrum is to place telescopes above the atmosphere altogether. Space telescopes, such as the Hubble and James Webb Space Telescopes, are positioned in orbit to bypass the issues of seeing and atmospheric absorption. This allows them to capture the full range of wavelengths, from high-energy X-rays to long-wavelength infrared, providing the clearest and most complete view of the universe.