James Webb Space Telescope
What is James Webb Space Telescope?
Probes & telescopes· Launched 2021-12-25 – 5th year observing at L2. An infrared space telescope about 1.5 million km from Earth. It studies the earliest galaxies and the atmospheres of planets around other stars.
Mission timeline
Spacecraft specs
| Launch | 2021-12-25 |
|---|---|
| Location | Sun–Earth L2, 1.5M km |
| Mirror | 6.5 m gold-coated beryllium |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Why Webb's mirror is coated in gold
Webb's main mirror is 6.5 m across and made of 18 hexagonal segments fitted together. The segments are made of a light metal called beryllium and coated with a very thin layer of gold. The coating is 100 nanometres thick, hundreds of times thinner than a human hair, and the whole mirror uses only about 48 grams of gold, roughly the weight of a golf ball.
Gold was chosen because it reflects infrared light so well. Silver and aluminium are good at reflecting visible light, but gold reflects more than 98 percent of infrared, and nearly all the light Webb collects is infrared. The yellow colour of the mirror in photographs is not decoration; it is part of how the telescope works.
Beryllium was chosen for good reasons too. It is light and stiff, and it barely shrinks in the cold at −230 °C, so the mirror keeps its shape. Behind each segment sit seven small actuators that adjust its shape and position in steps of nanometres. Lining up all 18 segments to act as one mirror took months, and the first images were released in July 2022.
The mission in depth
The James Webb Space Telescope was launched on 25 December 2021 on an Ariane 5 rocket from Kourou in South America. It now circles a point called L2, 1.5 million km from Earth in the direction away from the Sun, about four times the distance to the Moon. From there the Sun and Earth always lie on the same side, so shading one side keeps the telescope cold.
Webb sees infrared light. Infrared is invisible to our eyes and is closely tied to the heat given off by warm objects. A warm telescope would be blinded by its own heat, so a five-layer sunshield the size of a tennis court blocks the Sun and cools the telescope to −233 °C.
Infrared was chosen because the universe is expanding. Light that left very distant galaxies has been stretched during its journey of more than 13 billion years until it became long-wavelength infrared. Webb has found galaxies from about 280 million years after the Big Bang. Many early galaxies are bigger and brighter than expected, which has raised a new puzzle about how they grew so fast.
The telescope was too big to launch open, so it flew folded. Over about two weeks after launch it unfolded its sunshield and mirror in stages, and 344 of those steps could each have ended the mission if they failed. Every one succeeded, and the launch was so accurate that plenty of fuel was left over. Webb is therefore expected to operate well beyond its ten-year design life.
Frequently asked
- When did James Webb Space Telescope launch?
- James Webb Space Telescope was launched on 2021-12-25.
- In one line, what is James Webb Space Telescope?
- Probes & telescopes — An infrared space telescope about 1.5 million km from Earth.
- What can I see of James Webb Space Telescope here?
- You can see its live position in the orbit view.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.