The James Webb Space Telescope

The James Webb Space Telescope (JWST) is the most powerful space telescope ever built. Launched on Christmas Day 2021, Webb is the successor to the famous Hubble Space Telescope. It is much bigger, much more sensitive, and looks at the universe in infrared light rather than visible. Webb can see galaxies whose light has been travelling towards Earth for over 13 billion years, almost as old as the universe itself. Already, in just a few years of operation, Webb has rewritten parts of astronomy.

  • Launched25 December 2021On a European Ariane 5 rocket
  • Mirror size6.5 metresAbout 7x the light-collecting area of Hubble
  • Mirror segments18Gold-coated hexagons that unfold in space
  • Distance from Earthapprox. 1.5 million kmAt the L2 Lagrange point
  • Operating temperature~-223 °CCold enough to detect faint infrared light
  • Cost~$10 billionIncluding 25 years of design and build

Hubble's bigger younger cousin

Webb is not really a replacement for the Hubble Space Telescope; it is a different kind of telescope designed to do different things. Where Hubble looks mostly at visible light (the same kinds of light your eyes see), Webb looks at infrared light: heat radiation just past the red end of the spectrum, invisible to humans but perfect for studying cool objects and very distant galaxies.

The big differences between Hubble and Webb:

  • Mirror size: Webb's 6.5 m mirror is more than 7 times the light-collecting area of Hubble's 2.4 m mirror.
  • Wavelengths: Hubble does mostly visible and a little infrared and ultraviolet. Webb does almost entirely infrared.
  • Location: Hubble orbits 540 km above Earth. Webb sits 1.5 million km from Earth at a special point called L2.
  • Temperature: Webb must be ice-cold to detect faint infrared signals; Hubble does not need to be cold.

Why infrared?

There are two big reasons why infrared light is so useful in astronomy. First, the universe has been expanding for 13.8 billion years. Light from the most distant galaxies has been stretched as it travels (a process called redshift), so by the time it reaches us, light that started out as visible has been pulled out into the infrared. To see the oldest galaxies, you need an infrared telescope.

Second, infrared light can travel through clouds of dust that block visible light. Newborn stars and forming planets are usually hidden inside thick dust clouds; Webb can see them directly.

The folded telescope

The hardest engineering problem in building Webb was its sheer size. The mirror is 6.5 m across, much too big to fit inside any rocket. The solution was to fold the telescope up to fit, then unfold it in space.

Webb's main mirror is made of 18 hexagonal segments, each coated in a tiny layer of gold to reflect infrared light. The segments were folded into three sections inside the rocket, plus an enormous five-layer sunshield about the size of a tennis court that protects the telescope from the heat of the Sun. Over the first few weeks after launch, Webb had to perform a complicated series of nearly 350 separate "single points of failure" steps to unfold itself successfully. Any one of them could have ended the entire mission. Every single one worked.

Where Webb lives: the L2 point

Webb does not orbit the Earth like Hubble does. It sits about 1.5 million km from Earth (about 4 times the Earth-Moon distance) at a special location called the Sun-Earth L2 Lagrange point. At L2, the gravity of the Sun, Earth and Moon all balance out so that any spacecraft put there can sit reasonably still relative to Earth, slowly orbiting around it.

Sitting at L2 has two huge advantages. First, the Sun, Earth and Moon all appear in roughly the same direction in Webb's sky, so Webb can easily keep its sunshield pointed at all three at once and protect its mirrors from heat. Second, the side of the telescope facing away from the Sun gets to a stable -223 °C, cold enough to detect even very faint infrared signals.

Fact Because Webb is 1.5 million km from Earth, it is far too far away for astronauts to visit if something goes wrong. Unlike Hubble (which was repaired and upgraded by Shuttle astronauts five times), Webb has to work perfectly forever. NASA engineers added many redundant systems and tested every part on Earth thousands of times to reduce the risk. The whole mission was a high-wire act and (so far) it has succeeded beyond anyone's hopes.

What Webb has shown us

Webb has been making discoveries since its first scientific images were released in July 2022. Highlights so far:

  • The deepest image of the universe ever made: the first scientific image, released by US President Joe Biden, showed thousands of galaxies in a tiny patch of sky, some over 13 billion years old.
  • Galaxies from less than 400 million years after the Big Bang, much more numerous and more mature than astronomers expected.
  • Stunning new images of the Pillars of Creation, Carina Nebula and many other star-forming regions, with much more detail than Hubble could show.
  • Carbon dioxide detected in the atmosphere of an exoplanet for the first time.
  • The clearest images of the rings of Uranus and Neptune ever taken.
  • The first ever direct images of an exoplanet in mid-infrared light.
Did you know? Webb is named after James E. Webb, who was the head of NASA during the 1960s, including most of the Apollo Moon programme. Webb was a controversial choice because he was not a scientist; the name has been questioned by some astronomers in recent years. The previous space telescope was named after Edwin Hubble, an actual astronomer.
Deeper dive: how Webb looks back in time

One of the most exciting things about Webb is its ability to see the first galaxies ever to form in the universe. Because light takes time to travel, looking at faraway objects is also looking into the past. A galaxy 13 billion light years away appears to us as it was 13 billion years ago, when the universe was less than 800 million years old. Before Webb, even Hubble could only just see the brightest galaxies from a billion or so years after the Big Bang.

Webb's combination of a huge mirror and infrared sensitivity has pushed that limit dramatically. The James Webb has already imaged galaxies from less than 400 million years after the Big Bang: a time when the universe was just 3% of its current age. Some of those early galaxies turned out to be much bigger and more mature than astronomers had predicted, suggesting that galaxy formation in the very early universe was faster than our theories said.

Looking even further back into the cosmic past is one of Webb's biggest goals. Eventually it should be able to see the very first stars switching on, ending the cosmic "dark ages" a few hundred million years after the Big Bang. Webb is expected to operate until at least the mid-2030s, so it should have many years left to surprise us.

For Webb's elder sibling see the Hubble Space Telescope. For other recent missions see Mars rovers and Voyager 1 and 2.