Space Telescopes: Why We Put Observatories in Orbit

Building a telescope on a mountaintop costs a fraction of what it takes to launch one into space.

Space telescopes cannot easily be repaired, they are limited in size by the rockets that carry them,

and a single launch failure can erase years of work. So why do astronomers keep sending observatories into orbit?

The short answer is Earth's atmosphere.

It protects life on the ground, but it also blurs, dims and completely blocks much of the light coming from the universe.

Here is what the air takes away, what space telescopes get back, and how they are designed to do it.

The Atmosphere: A Blanket With a Downside

If you have ever watched stars twinkle, you have seen the problem.

Pockets of air at different temperatures bend incoming starlight in constantly shifting directions, like looking at a coin at the bottom of a swimming pool.

To your eye that is a pretty sparkle. To a large telescope, it smears fine detail into a blur.

The atmosphere also glows faintly on its own, scatters the light of cities and the Moon, and hides the sky entirely behind clouds.

Observatories are built on high, dry mountains to rise above as much of this as possible,

but even the best sites still look up through a thick layer of moving air.

The Light We Cannot See From the Ground

Visible light is only a thin slice of the electromagnetic spectrum.

Stars, galaxies, black holes and gas clouds also give off gamma rays, X-rays, ultraviolet, infrared, microwaves and radio waves,

and each type of light reveals different physics.

The atmosphere lets visible light and most radio waves through, but it absorbs much of the rest.

For some of these wavelengths, space is not just better, it is the only option.

X-ray astronomy, for example, barely existed before instruments could be lifted above the atmosphere,

and it has since revealed black holes, superheated gas in galaxy clusters and the remains of exploded stars.

A Very Old Idea

The concept of a telescope in space is older than spaceflight itself.

Rocket pioneer Hermann Oberth mentioned it in the 1920s,

and in 1946 the American astrophysicist Lyman Spitzer wrote a detailed paper describing the advantages of an observatory above the atmosphere.

He spent much of his career campaigning for the idea that eventually became the Hubble Space Telescope.

The first successful orbiting observatories flew in the late 1960s and 1970s, studying ultraviolet and X-ray light.

NASA later launched four large "Great Observatories," each covering a different part of the spectrum: Hubble in 1990,

the Compton Gamma Ray Observatory in 1991, the Chandra X-ray Observatory in 1999 and the Spitzer Space Telescope in 2003.

The COBE satellite, launched in 1989, measured the afterglow of the Big Bang so precisely that the work behind it earned a Nobel Prize.

Where Space Telescopes Live

Not every space telescope sits in the same kind of orbit. The location is chosen to suit the science:

• Low Earth orbit: Hubble circles a few hundred kilometers up, close enough for Space Shuttle crews to visit and service it five times. The downside is that Earth blocks part of the sky during each orbit.
• High, stretched orbits: Chandra follows a long, egg-shaped orbit that carries it more than a third of the way to the Moon, giving it long stretches of uninterrupted viewing.
• Trailing Earth around the Sun: Spitzer and the planet-hunting Kepler telescope orbited the Sun rather than Earth, slowly drifting behind our planet, away from its heat.
• The second Lagrange point: About 1.5 million kilometers (roughly a million miles) from Earth, directly away from the Sun, a spacecraft can keep the Sun, Earth and Moon behind it at all times. The James Webb Space Telescope, Planck, Herschel and Gaia all went there.

Some observatories study the Sun itself from a vantage point on the sunward side of Earth.

If you ever want to look at the Sun yourself, never do it without certified solar filters,

even for a moment, because it can cause permanent eye damage.

Keeping Cool and Pointing Straight

Infrared telescopes face a special challenge: warm objects glow in infrared, including the telescope itself.

To avoid being blinded by their own heat, instruments must be chilled to extremely low temperatures.

Earlier infrared telescopes carried tanks of liquid helium, and their coldest observations ended when it boiled away.

Later designs rely on large sunshields and mechanical coolers instead.

Space telescopes must also point with astonishing steadiness while orbiting at high speed. They use gyroscopes,

star trackers and spinning reaction wheels to lock onto targets.

Hubble's pointing precision has been compared to holding a laser beam on a coin about 320 kilometers (200 miles) away.

All the data then travels to Earth by radio, captured by large antennas on the ground.

The Trade-Offs

Space telescopes are powerful, but they are not a replacement for observatories on the ground.

Ground-based telescopes can be far larger, since they do not have to fit inside a rocket,

and they can be upgraded with new instruments for decades.

Many now use adaptive optics, flexible mirrors that change shape hundreds of times per second to cancel out atmospheric blurring.

The strongest science often comes from combining both. A space telescope might spot an unusual object in infrared or X-rays,

and then giant ground telescopes follow up with detailed spectra, or the other way around.

Each type covers the other's blind spots.

Space telescopes also have limited lifetimes.

They can run out of the propellant or coolant they depend on, and parts such as gyroscopes eventually wear out.

Since almost none can be visited by astronauts, engineers design them with backups and careful plans for keeping them working as long as possible.

Why It Matters for You

The discoveries reach far beyond pretty pictures.

Space observatories helped pin down the age of the universe, showed that its expansion is speeding up,

charted the positions and motions of more than a billion stars in our galaxy and revealed thousands of planets orbiting other stars.

Each of those results depended on light that either never reaches the ground or arrives too blurred to measure precisely.

Space telescopes have also changed how people everywhere picture the universe.

Images of glowing nebulae and crowded fields of galaxies have become part of textbooks, museum walls and phone screens.

Much of the data is placed in public archives, where researchers around the world,

and even volunteers in citizen science projects, can sift through it for new discoveries.

Keep Looking Up

Space telescopes exist because the atmosphere that shelters us also hides much of the cosmos.

By rising above it, they catch light no ground-based instrument can see and deliver views sharper than any mountaintop can offer.

The next time the stars twinkle above your backyard,

remember that the same shimmer is exactly why some of astronomy's best eyes are floating far overhead.