Light is fast, but not instant
Light travels at about 300,000 kilometers per second, which is the speed of light in a vacuum. It feels instantaneous in daily life, but across space it is slow enough to matter. Light from the Moon takes a little over one second to reach us. Light from the Sun takes a little more than eight minutes. If the Sun vanished, we would not know for about eight minutes.
This means that whenever you look at something, you see it as it was when the light left. For nearby objects, the delay is negligible. For astronomical objects, it becomes significant. The distance light covers in a year is called a light year, roughly 9.46 trillion kilometers, and it is used as a unit of distance, not time.
Looking farther means looking earlier
The Andromeda galaxy, the large galaxy nearest to ours, is roughly 2.5 million light-years away. The light reaching us now left it about 2.5 million years ago. We see the galaxy as it looked then, not as it looks today. A galaxy ten billion light-years away is seen as it was ten billion years ago, when the universe was much younger.
So a telescope is, in effect, a time machine for the past. It does not let us see Earth’s own past from far away, or let us travel. It simply collects light that has been en route for a very long time. By observing objects at different distances, astronomers can assemble a sequence of snapshots of how galaxies formed and changed. The farthest observations come close to the earliest times in the universe’s history. These ideas fit with the Big Bang model, in which the universe has a finite age of roughly 13.8 billion years. The same fact is why the observable universe has a limit: light from beyond that distance has not had time to reach us.
Why put telescopes in space?
Ground-based telescopes have excellent advantages, including size and easier repair. But Earth’s atmosphere gets in the way in several ways.
- Blurring. Moving pockets of air distort starlight, which is the cause of twinkling stars. Telescopes on mountains use special techniques to partly correct for it, but a telescope above the air sees steady images.
- Blocked wavelengths. The atmosphere absorbs much of the ultraviolet, X-ray and infrared light from space. Some of these can only be studied from orbit or above.
- Background glow. The air and artificial lights add brightness that hides faint objects. Space has a much darker background.
- Weather and daylight. Ground telescopes can only work on clear nights, while space telescopes can observe more continuously.
The tradeoff is cost and difficulty: building, launching and operating a space telescope is very expensive, and repairs are usually impossible or limited.
Hubble
The Hubble Space Telescope was launched in 1990 and orbits Earth at roughly 540 kilometers up. It observes in visible light, along with some ultraviolet and near-infrared. Its mirror is 2.4 meters wide, which is modest compared with the biggest ground telescopes, but being above the atmosphere gives it exceptionally sharp images. Early on, a flaw in the mirror’s shape was discovered, and astronauts installed corrective optics in 1993 during a Space Shuttle mission. Further servicing missions upgraded its instruments. Hubble’s observations have helped measure the age and expansion rate of the universe, revealed distant galaxies and captured detailed views of planets in our own solar system.
The James Webb Space Telescope
The James Webb Space Telescope launched in December 2021 and works mainly in infrared light, with a primary mirror about 6.5 meters wide made of gold-coated segments. It does not orbit Earth. Instead, it sits near a point about 1.5 million kilometers from Earth, on the side away from the Sun, where it can keep Earth, Moon and Sun in roughly the same direction. A large sunshield keeps the instruments extremely cold, which is necessary because warm objects glow in infrared and would swamp faint signals.
Why infrared? The universe is expanding, so light from very distant galaxies is stretched to longer wavelengths on its way to us. This is called redshift. Light that started out as visible or ultraviolet may arrive as infrared. Infrared also passes through some dust clouds better than visible light, which helps in observing stellar nurseries and the early stages of the stellar life cycle. Webb is designed to study some of the earliest galaxies, as well as the atmospheres of planets around other stars.
Other telescopes in space
Different kinds of light reveal different things, and several space telescopes specialize in them.
- X-ray telescopes, such as Chandra, study very hot gas, black holes and exploded stars.
- Telescopes that survey in gamma rays observe the most energetic events.
- Missions like Kepler and TESS monitor the brightness of many stars to find exoplanets.
- Microwave observatories have mapped the faint glow left over from the early universe, called the cosmic microwave background. It comes from a time of roughly 380,000 years after the Big Bang, when the universe first became transparent.
What these images are, and are not
The pictures released by space telescopes are real data, but they are often processed. The detectors record brightness, not color in the way a camera does, and infrared and other invisible wavelengths must be assigned visible colors. Scientists choose colors to bring out structure and to show different wavelengths or elements. This is called false color or representative color. It is a normal practice, not a deception, and observatories usually explain how an image was made.
A telescope can also only record what its light-collecting area and detectors allow. Faint, far objects may look like a few pixels, and astronomers combine many long exposures to bring them out. Researchers interpret the data with models, and conclusions about the earliest galaxies are refined as more observations come in.
The wider picture
Because light from distant objects is old, astronomers can compare galaxies near and far in order to see how they grow. The same approach connects to how astronomers measure cosmic distances, since knowing how far an object is tells us how long its light has traveled. Space telescopes therefore do not just look outward; they extend our view backward through time.