Why do astronomers use space telescopes for infrared instead of ground telescopes?

In one line

The rule

Infrared astronomy is best done from space because Earth's atmosphere absorbs and blocks much of the infrared radiation coming from space.

There is no single formula, but the key idea is: Transmission = Amount of infrared light that passes through the atmosphere to the ground.

Let's understand it simply

Imagine you are trying to listen to your friend's voice from across a busy market. If there is a lot of noise (like people talking, horns, music), you can't hear your friend clearly. Now, imagine you both go to a quiet park. Suddenly, you can hear every word.

The same thing happens with infrared light (a type of light we can't see, but can feel as heat). The Earth's atmosphere is like the noisy market. It blocks and absorbs most of the infrared signals coming from stars, planets, and galaxies. Only a little bit gets through. But if you put your 'ears' (telescopes) above the atmosphere, in space, it is like being in the quiet park—you can 'hear' (detect) all the signals clearly.

How it works

1. Stars and galaxies emit infrared light, which travels through space.
2. When this light reaches Earth, it hits the atmosphere.
3. The atmosphere (especially water vapor and carbon dioxide) absorbs most infrared light.
4. Only a few specific wavelengths (called 'infrared windows') make it to the ground, and even those are weak.
5. A telescope on the ground misses most of the infrared signals.
6. A space telescope, above the atmosphere, can catch almost all the infrared light, giving a much clearer and complete picture.

The story behind it

Infrared astronomy began in the mid-20th century, but astronomers quickly realized that ground-based telescopes could not see most of the infrared universe. The first major space infrared telescope was IRAS (Infrared Astronomical Satellite), launched in 1983. It showed how much more could be seen from space. Later, telescopes like Spitzer and the James Webb Space Telescope (JWST) were built to study the universe in infrared from above the atmosphere.

One memorable detail: When IRAS was launched, it discovered entire galaxies and clouds of dust that were completely invisible to ground telescopes, opening a new window to the universe.

Where you see it in real life

• James Webb Space Telescope (JWST): Orbits far from Earth, sees deep into space in infrared, revealing stars and galaxies hidden by dust.
• Spitzer Space Telescope: Operated from space, found planets and studied the birth of stars using infrared.
• Ground-based telescopes in high, dry places: Like in Ladakh or Chile's Atacama Desert, where the air is thin and dry, a little more infrared gets through, but still much less than in space.
• Remote sensing satellites: Use infrared to study Earth's weather, forests, and crops from space because the atmosphere blocks less infrared when looking down than when looking up.
• Thermal cameras: On Earth, these use infrared to see heat, but for astronomy, they need to be in space to avoid the atmosphere's blocking effect.
• Monsoon clouds: Just like thick clouds block sunlight, the atmosphere blocks infrared from space, so astronomers go above it.

Why it matters

• See hidden objects: Infrared can see through dust clouds that block visible light, revealing new stars, planets, and galaxies.
• Study the early universe: Distant galaxies are redshifted (their light stretched into infrared), so only infrared telescopes can see them.
• Find exoplanets: Many planets around other stars are easier to spot in infrared.
• Understand star formation: Stars are born in dusty regions; infrared shows what is happening inside.
• Future studies: Many new discoveries in astronomy depend on seeing in infrared, so space telescopes are essential.

Worked example

Let's compare how much infrared light reaches a telescope on the ground vs. in space.

Given: 100 units of infrared light from a distant galaxy
Step 1: Ground telescope (atmosphere absorbs 95%)
         Light reaching ground = 100 × 0.05 = 5 units
Step 2: Space telescope (no atmosphere)
         Light reaching telescope = 100 × 1 = 100 units
Step 3: Comparison
         Space telescope gets 20 times more signal than ground telescope

Common mistakes

• Thinking all light passes through the atmosphere: Only some types do; most infrared is blocked.
• Believing high mountains solve the problem: They help a little, but space is much better.
• Confusing infrared with visible light: Visible light passes through the atmosphere better than infrared.
• Assuming space telescopes are only for sharp images: Their main job in infrared is to see what ground telescopes cannot.

Remember it

Infrared telescopes work best in space because Earth's atmosphere blocks most infrared light—like a thick curtain blocking a view.