Light takes time to travel, which means any telescope powerful enough to see far enough away is also looking backward in time. One observatory now orbiting far from Earth has pushed that principle further than any instrument before it, routinely capturing light that began its journey when the universe was a small fraction of its current age and the galaxies it came from looked nothing like the ones nearby today.
An observatory built to see in infrared, not visible light
The James Webb Space Telescope, run by NASA in partnership with the European and Canadian space agencies, is designed around infrared rather than the visible-light wavelengths most telescopes and cameras use. That choice is deliberate: light from the most distant galaxies has been stretched by the universe’s expansion into longer, redder wavelengths by the time it reaches Earth, a phenomenon astronomers call redshift, and infrared instruments are what is needed to detect it. Infrared light also passes through the dense clouds of gas and dust that block visible light, letting the telescope see newly forming stars and planets hidden from observatories tuned to ordinary light.
A mirror built in 18 pieces
The telescope’s primary mirror spans 6.5 meters across, assembled from 18 hexagonal segments made of gold-coated beryllium, each about 1.32 meters from flat side to flat side. Beryllium was chosen for its light weight and its stability across the extreme temperature swings the telescope experiences in space, and the segmented design let engineers fold the mirror to fit inside a rocket fairing, then unfold and fine-tune each segment’s position once in orbit. A smaller secondary mirror roughly 0.8 meters across, along with a tertiary mirror and a fine steering mirror, route the collected light to the telescope’s four science instruments.
A sunshield the size of a tennis court
Because infrared detectors are exquisitely sensitive to heat, including the telescope’s own, the observatory carries a five-layer sunshield made of a material called Kapton, unfolded to roughly 14 by 22 meters, about the footprint of a tennis court. The layers work together to block the roughly 200 kilowatts of solar radiation striking the sun-facing side, cutting the temperature difference between the hot and cold sides of the spacecraft by close to 570 degrees Fahrenheit. That temperature gap lets the telescope’s mirrors and instruments run cold enough to detect faint infrared light without the observatory’s own warmth drowning it out. One instrument, the mid-infrared MIRI, needs to run even colder than passive shielding alone can achieve and uses a dedicated helium-based cryocooler to reach about 7 kelvin, while the telescope’s other three instruments cool passively to around 39 kelvin in the sunshield’s shadow.
Parked far past the moon, not orbiting Earth
Unlike the Hubble Space Telescope, which circles Earth at a few hundred kilometers altitude, Webb does not orbit the planet at all. It launched on an Ariane 5 rocket from the Guiana Space Centre in Kourou, French Guiana, on December 25, 2021, and travels around the sun in a position called the second Lagrange point, roughly 1.5 million kilometers from Earth, on the side facing away from the sun. That distance keeps Earth, the moon and the sun consistently on one side of the sunshield, simplifying the job of keeping the telescope’s instruments in permanent shadow.
What the extra reach into the past has already found
Webb‘s combination of a large mirror and infrared sensitivity was built specifically to capture light from the era when the universe’s first stars and galaxies were forming, more than 13.5 billion years ago. That design goal has paid off directly: among its confirmed discoveries, the telescope identified the earliest supernova detected to date, a stellar explosion that occurred when the universe was only about 730 million years old, a small fraction of its present age of roughly 13.8 billion years. Observations like that one are only possible because the telescope is not just measuring distance — it is reading light old enough to describe conditions in the universe long before Earth, the sun or the Milky Way as it exists today had formed.
Four instruments splitting the work of seeing that far back
Reaching those distances requires more than a large mirror; it requires instruments tuned to different slices of the infrared spectrum, since no single detector performs well across the whole range. The Near-Infrared Camera, or NIRCam, and the Near-Infrared Spectrograph, NIRSpec, both operate in shorter infrared wavelengths and handle the bulk of Webb’s deep-field imaging and chemical analysis of distant galaxies and exoplanet atmospheres. The Mid-Infrared Instrument, MIRI, covers longer wavelengths better suited to studying cooler objects such as newly forming stars still wrapped in dust, which is why it needs the dedicated cryocooler to run at its much colder operating temperature. A fourth instrument package, combining the Fine Guidance Sensor with the Near-Infrared Imager and Slitless Spectrograph, keeps the telescope locked precisely on target while contributing its own imaging and spectroscopy. Splitting the workload this way lets each instrument specialize rather than compromise, which is part of why Webb can resolve detail in both nearby star-forming clouds and galaxies from the universe’s earliest chapters in the same overall mission.
This article was produced with the assistance of AI and reviewed by an editor.
More from Morning Overview
