The problem with looking through air
Earth’s atmosphere is a very good shield for life, but a poor window for astronomy. It affects starlight in two main ways.
It blocks some light entirely. Air absorbs most ultraviolet, X-ray and gamma-ray radiation, and much of the infrared. That protects living things, but it also means whole ranges of the spectrum cannot be studied from the ground. Some infrared and radio wavelengths do get through, and ground telescopes use those “windows”.
It blurs what does get through. Pockets of air at different temperatures bend light unpredictably, which is why stars twinkle. For a telescope, this turbulence smears fine detail, a limit astronomers call poor “seeing”. The air itself also glows faintly, adding background light, and cloud, weather and light pollution can interrupt observations.
A telescope in orbit sits above nearly all of this. Its images are not affected by turbulence, the sky behind them can be much darker, and it can observe wavelengths that the atmosphere blocks.
What orbit makes possible
- Sharper images in visible light, limited mainly by the telescope’s own optics rather than the air. Ground telescopes can partly compensate with adaptive optics, which adjusts a flexible mirror many times per second to correct for turbulence, but this works better in some conditions and wavelengths than others.
- Access to ultraviolet, X-ray and gamma-ray light, which reveal hot, energetic phenomena such as gas around black holes, and the remains of stellar explosions.
- Infrared observations, useful for seeing through dust clouds, studying cool objects, and observing very distant galaxies whose light has been stretched to longer wavelengths by the expansion of the universe.
- Long, steady observations, since there is no weather. A telescope can keep pointing at a target for many hours, though many orbits do pass through Earth’s shadow, and the Sun, Moon and Earth limit which directions are usable at a given time.
Choosing where to put it
Different missions pick different locations.
Low Earth orbit is the easiest to reach, and repair or servicing is at least conceivable there. The Earth blocks part of the sky during each lap of roughly 90 minutes, and nearby heat and scattered light can interfere.
Higher or more distant orbits offer a better view and more stable conditions. A popular choice for some observatories is a region near the second Lagrange point (L2), about 1.5 million kilometers from Earth on the side away from the Sun. Gravity from the Sun and Earth together makes it possible to stay there with modest fuel use, and the Sun, Earth and Moon all lie in the same general direction, so one sunshade can keep the instruments cool and shaded. The price is that the telescope is much too far to service with current crewed vehicles.
Infrared telescopes need cold. Warm objects glow in infrared, so an infrared instrument must be kept very cold or it will drown out the faint signals it is looking for. Spacecraft use sunshields, distant orbits and sometimes coolant to manage this.
Drawbacks of working in space
Space telescopes have real costs.
- Expense. Building something that survives launch and operates in vacuum, extreme temperatures and radiation is costly, as is launching it. Cheaper launches from reusable rockets may eventually help, though large observatories remain major investments.
- Size limits. A rocket’s payload space restricts mirror size. Designers use folding mirrors and segmented designs to get around this, which adds complexity. Ground telescopes can have much larger mirrors, so they can collect more light from faint objects in wavelengths where the atmosphere is transparent.
- Difficult repair. A fault that would be a quick fix on a mountaintop may end a mission, though a few have been serviced by astronauts.
- Limited lifetime. Fuel, pointing hardware and electronics eventually wear out.
- Data constraints. Everything must be sent to Earth by radio, so observations are planned in advance and data rates are limited.
Ground and space work together
It would be a mistake to think space telescopes have replaced ground-based ones. The two complement each other. Large ground observatories collect huge amounts of light and can be upgraded, while space telescopes provide access to blocked wavelengths and extremely sharp, stable images. Many discoveries combine both: for example, a survey from the ground may flag an interesting object, and a space instrument may then study it in infrared or X-rays.
From telescope to knowledge
Space telescopes produce large volumes of data that are archived and shared with researchers worldwide. Analysis relies on software, including machine learning for sorting and classifying objects, and on computing resources in data centers. The same engineering that supports observatories, such as precise pointing, thermal control and reliable communications, also benefits other spacecraft, including those used for Earth observation.
In short, observatories go to orbit because the atmosphere blocks and blurs the light astronomers want. The decision balances better data against higher cost, limited size and the difficulty of repair.