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🪞Space8 min read

The James Webb Space Telescope

It had to unfold itself in the dark, a million miles away, through 344 steps that each had to work first time. There was no second attempt, no repair mission, and no way to watch.

In plain English

Hubble orbits about 540 kilometres up. When its mirror turned out to be wrongly ground, astronauts flew up and fitted corrective optics. When its gyroscopes failed, they replaced them. Five servicing missions in total.

The James Webb Space Telescope sits 1.5 million kilometres away, roughly four times further than the Moon. Nobody is going to fix anything.

That single fact shaped the whole project. Webb's primary mirror is 6.5 metres across, which is far too wide for any rocket fairing, so it was built as 18 hexagonal segments on folding wings. The tennis-court-sized sunshield had to pack down into something the size of a rolled rug. The whole observatory launched folded up like origami and then spent about two weeks unfolding itself, autonomously, while coasting away from Earth.

NASA counted 344 single-point failures in that sequence: steps where one mechanism jamming would have ended the mission. No redundancy, no workaround, no engineer with a spanner.

Every single one worked.

Then there is what it looks at. Webb is not really a visible-light telescope. It sees infrared, from about 0.6 to 28 microns, which is the wavelength range where the earliest galaxies now appear — their light stretched into the infrared by billions of years of cosmic expansion. To detect heat that faint, the telescope itself must be extremely cold, around 40 degrees above absolute zero.

That is the reason for the sunshield, and the reason it sits where it does.

Five things to file under "wait, what?"

  • It is not orbiting the Earth. Webb loops around a point in empty space called the second Lagrange point, where the gravity of the Sun and Earth combine so that an object keeps pace with Earth's orbit. That position keeps the Sun, Earth and Moon all in roughly the same direction, so a single shield can block all three at once. It also puts the telescope permanently out of reach.

  • The mirror is coated in about 48 grams of gold. Gold reflects infrared far better than aluminium. The layer is roughly 100 nanometres thick — a few hundred atoms — vacuum-deposited across 25 square metres of beryllium. Beryllium because it barely changes shape as it cools.

  • The sunshield holds a 300-degree temperature difference across five sheets. The Sun-facing layer runs at around 85°C. The far side sits near minus 233°C. Five layers of aluminised Kapton, each thinner than a human hair, separated by vacuum, and the gaps between them do most of the work by letting heat radiate sideways into space.

  • The launch was so accurate it doubled the mission life. Webb carries propellant to hold its position and stay pointed. Ariane 5 placed it so precisely that far less fuel was needed for course corrections than budgeted, taking the expected lifetime from around ten years to twenty or more.

  • A rock hit it within months and the dent is permanent. In May 2022 a micrometeoroid larger than anything the engineers had modelled struck mirror segment C3. The segment can be adjusted to compensate for some of the distortion, but the damage is there for good. It is the cost of a telescope nobody can visit, and the images are still extraordinary.

The full story

Why infrared, and why cold

Light from the most distant galaxies left them when the universe was a few hundred million years old. Expansion has stretched that light on its way here, shifting what began as ultraviolet and visible into the infrared. If you want to see the first galaxies, you need an infrared telescope. There is no choice about it.

The difficulty is that infrared is heat. Everything warm glows in it, including the telescope. A room-temperature mirror is a floodlight at the wavelengths Webb cares about, drowning the signal in its own emission.

So the observatory has a hot side and a cold side and never turns round. Solar panels, antenna and electronics live on the Sun-facing side. The mirror and instruments sit in permanent shade behind the shield, cooling passively to about 40 K. One instrument, MIRI, works at longer wavelengths still and needs an active cryocooler to reach about 7 K.

That architecture is why the observatory can never point anywhere near the Sun, and why its observing schedule is a constant exercise in geometry.

The deployment

Launch was on Christmas Day 2021, from Kourou in French Guiana, on an Ariane 5.

What followed was the part that kept people awake. The solar array unfolded within minutes. Over the next fortnight: the sunshield pallets lowered, a tower extended to separate the warm and cold sections, membrane covers rolled back, five layers separated and tensioned individually, the secondary mirror swung out on its tripod, and finally the two mirror wings folded into place and latched.

Then months of alignment, nudging all 18 segments with actuators until they behaved as one 6.5-metre surface, accurate to tens of nanometres.

The first images were released in July 2022. The engineering achievement had already happened by then, quietly, in the dark.

What it cost, and what it nearly cost

Roughly ten billion dollars and about 25 years. It was years late and vastly over its original budget, and in 2011 the US Congress came close to cancelling it outright.

Those numbers are part of the story rather than a footnote to it. A telescope that cannot be repaired has to be tested to a standard that a serviceable one does not, and testing 344 irreversible steps is where a great deal of that time and money went. The counterfactual — a cheaper, less-tested Webb that jammed halfway through unfolding — would have been the most expensive failure in the history of science.

What it has actually found

The headline results have been about the early universe, and they have been mildly awkward for theory. Webb found galaxies at very high redshift that appear more massive and more mature than models predicted should exist so soon after the Big Bang. Some of those masses have since been revised downward as calibration improved, and the debate is live rather than settled, but the general direction is clear: early galaxy formation happened faster than expected.

It has also done work that has nothing to do with cosmology. Detailed atmospheric chemistry of exoplanets, including detections of carbon dioxide and sulphur dioxide in hot Jupiters. Images of protoplanetary discs with gaps where planets are forming. A photograph of Neptune's rings clearer than anything since Voyager 2 flew past in 1989.

The name

The telescope is named after James Webb, NASA's administrator through most of the 1960s, and that choice has been contested. Critics point to his role in the US State Department and NASA during a period when gay employees were purged from federal government; NASA reviewed the naming in 2022 and did not change it. Many astronomers refer to the observatory as JWST or simply Webb, and some journals have published papers avoiding the full name. It is a live disagreement worth knowing about rather than a settled matter.

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