Why Gold Is Used in the James Webb Space Telescope (Complete Science Guide)

If you have seen photos of NASA's James Webb Space Telescope, you have seen its most striking feature: a giant hexagonal mirror that gleams brilliant gold against the blackness of space. It is one of the most expensive and complex scientific instruments humanity has ever built — a USD 10 billion telescope launched in December 2021, parked roughly 1.5 million kilometres from Earth, capturing infrared light from the earliest galaxies in the universe. And the entire optical system is coated in gold. Why? The answer is not aesthetic. Gold was chosen for one reason: physics. No other material humans have ever discovered does the specific job JWST needs as well as gold does. This guide explains, step by step, exactly why.

Quick verdict

What is the James Webb Space Telescope?

The James Webb Space Telescope (JWST) is NASA's flagship space observatory of the 2020s. It is named after James Webb, a former NASA administrator who oversaw the Apollo programme in the 1960s. JWST was launched on December 25, 2021, aboard an Ariane 5 rocket from French Guiana, and reached its operating position at the second Lagrange point (L2) about 1.5 million kilometres from Earth in January 2022. Scientific operations began in July 2022. As of 2026, JWST has produced thousands of scientific images and spectra, observed some of the earliest galaxies known, analysed the atmospheres of exoplanets, and rewritten parts of modern cosmology. It was a joint project of NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA), built over roughly 25 years.

The 18 hexagonal mirror segments

JWST's primary mirror is not a single piece — it is a 6.5-metre wide composite mirror made of 18 hexagonal segments arranged in a honeycomb pattern. The entire structure was folded up to fit inside the rocket's nose cone for launch, then carefully unfolded in space over weeks of remote commands. Each hexagonal segment is made of beryllium (a very light, very stiff metal that maintains its shape across enormous temperature swings) and weighs about 20 kg, including its support structure. The 18 segments together provide roughly six times the light-collecting area of the Hubble Space Telescope. And every one of them is coated, on the front face, with a thin layer of pure gold.

Why JWST observes infrared light (and Hubble didn't)

The universe is expanding. The further away an object is from us, the more its light has been stretched (redshifted) by cosmic expansion during its journey through space. Light that left the most distant, earliest galaxies began as ultraviolet or visible light, but by the time it reaches us today it has been stretched all the way into the infrared. The Hubble Space Telescope was designed primarily for ultraviolet and visible light, so it could not see the universe's most distant epochs clearly. JWST is purpose-built for the infrared — specifically the near-infrared and mid-infrared — which lets it peer through cosmic dust to see star formation in progress, analyse the chemical composition of exoplanet atmospheres, and detect the faintest, oldest light in the observable universe.

Why gold is uniquely suited to infrared mirrors

Different materials reflect different wavelengths of light with different efficiencies. In the visible range, aluminum and silver are excellent reflectors, which is why ground-based observatories and the Hubble Space Telescope use aluminum or aluminum-magnesium-fluoride coatings. But in the infrared range, the picture changes completely. Gold has an unusually high reflectivity in the near-infrared and mid-infrared, exceeding 98–99% across most of JWST's working wavelengths. Aluminum drops below 90% in the same range. Silver is comparable to gold in IR reflectivity, but silver tarnishes — even in the near-vacuum of space, microscopic contamination and solar UV radiation can dim its surface over time. Gold is essentially inert; it doesn't tarnish, doesn't react, doesn't degrade. Over JWST's planned multi-decade operational lifetime, that stability matters enormously.

How thin is the gold layer? Just 100 nanometres

Each JWST mirror segment carries a gold coating only about 100 nanometres thick — that is one ten-thousandth of a millimetre, or roughly 1,000 atoms stacked on top of each other. Across all 18 mirror segments and the secondary mirror, the total mass of gold used is around 48 grams — a small bullion coin's worth of metal. At a 24K gold price of roughly $80 per gram, the entire gold content of the telescope is worth a few thousand dollars. The telescope itself cost roughly USD 10 billion. The gold thus represents about 0.00004% of the total mission budget — among the cheapest critical components on board.

How the gold was applied — vapor deposition

Coating the mirror segments was an act of precision engineering. Each beryllium segment was first machined and polished to extraordinary accuracy. Then it was placed inside a vacuum chamber, where pure gold was heated until it vaporised. The gold vapour condensed on the cold mirror surface, building up a uniform layer atom-by-atom. The technique, called physical vapor deposition (PVD), is used widely in semiconductor manufacturing and optics — but coating mirror segments of this size and precision required custom equipment built by Quantum Coating Inc. of Moorestown, New Jersey, and other specialised contractors. After the gold layer was deposited, a thin protective layer of amorphous silicon dioxide (essentially glass) was applied on top to shield the gold from micrometeoroid impacts and surface contamination during launch.

1. 1.Beryllium segments machined to shape and surface-polished to nanometre precision.
2. 2.Segments transported to coating facility and loaded into clean-room vacuum chamber.
3. 3.Pure gold heated under high vacuum until it vaporises.
4. 4.Gold atoms travel in straight lines through the vacuum and condense onto the cool mirror surface.
5. 5.Coating thickness monitored in real time via quartz-crystal microbalance and optical instruments.
6. 6.Thin glass (SiO₂) overcoat applied to protect gold from contamination and abrasion.
7. 7.Coated segments tested for reflectivity, surface uniformity, and structural integrity.

Other places gold appears on JWST

The primary mirror gets the most attention, but it isn't the only place gold appears on JWST. Several other surfaces and structures use gold or gold-coated materials for their own specific reasons.

• Secondary mirror — the smaller hexagonal mirror that reflects light back toward the instruments; also gold-coated.
• Tertiary and fine-steering mirrors — additional optical elements deeper in the instrument package, all gold-coated.
• Sunshield kapton layers — five tennis-court-sized layers of aluminised and silicon-coated kapton; some surfaces use gold-coloured aluminum for solar reflectivity (though the visible 'gold' appearance is mainly the kapton substrate, not pure gold).
• Spacecraft thermal-control surfaces — gold-coated multi-layer insulation (MLI) is used in many areas to manage heat flow.
• Antenna and electronic-interface coatings — gold-plated connectors and waveguides for radio communications and instrument interfaces.

JWST's extreme operating temperature

To detect faint infrared light from distant objects, JWST must itself be extremely cold — otherwise its own thermal emission would drown out the signal it's trying to measure. The telescope operates at about 50 kelvin (−223°C / −370°F), achieved through passive cooling behind the giant five-layer sunshield. At this temperature, the gold coating must remain stable and uniformly reflective without cracking, peeling, or differentially contracting from the beryllium substrate beneath. Beryllium was chosen partly because its thermal expansion coefficient at cryogenic temperatures is nearly zero — the mirror keeps its shape across the brutal temperature drop from Earth assembly (~300 K) to operating temperature (~50 K). Gold's thermal behaviour at these temperatures had to be carefully characterised during testing on Earth before launch.

Hubble vs JWST — different missions, different mirrors

The Hubble Space Telescope, launched in 1990, uses an aluminum-magnesium-fluoride coating on its 2.4-metre primary mirror. Aluminum is the standard choice for visible and near-ultraviolet astronomy because of its high reflectivity in those wavelengths. Hubble was never designed to see deep into the infrared. JWST, by contrast, was designed specifically for infrared work, which is why its mirrors are gold-coated. The two telescopes are complementary, not competitive — Hubble continues to observe in visible and UV while JWST handles infrared. Some objects are observed by both, with each instrument capturing different wavelengths of the same source.

Other space missions that use gold

• Apollo astronaut visors — coated with a thin layer of gold to reflect solar infrared and protect against retinal damage; iconic on Apollo lunar surface images.
• Voyager 1 and Voyager 2 — golden phonograph records carrying greetings from Earth; the records themselves are gold-plated copper.
• Cassini-Huygens — gold-coated thermal blanketing during its mission to Saturn.
• Mars rovers (Curiosity, Perseverance) — gold-plated electronic interconnects and thermal-control surfaces.
• Pioneer 10 and 11 — gold-plated plaques bearing pictograms of humanity for any extraterrestrial finders.
• International Space Station — extensive gold-plated electronics and gold-coated thermal-management surfaces.
• Most communication satellites — gold-plated connectors and electronics throughout.

The poetic full-circle moment

There is a beautiful symmetry in JWST's gold mirrors. Every gold atom on the telescope was originally created in a neutron-star merger billions of years ago. That gold was scattered through space, incorporated into the gas cloud that became our solar system, delivered to Earth by asteroid bombardment, mined from the crust by humans, refined to 99.99% purity, vapor-deposited onto a beryllium telescope, and launched 1.5 million kilometres back into space — where it now reflects infrared light from other galaxies that were forming around the same time the original gold-making kilonovae were happening. The telescope's gold is, in a real sense, gazing back at the cosmic environment in which it was made.

What JWST has actually achieved (highlights)

• Observed some of the earliest galaxies known, dating to within hundreds of millions of years of the Big Bang.
• Detected water, carbon dioxide, methane and other atmospheric components on multiple exoplanets.
• Captured the deepest infrared images of the universe ever recorded.
• Imaged the Pillars of Creation, the Carina Nebula, and other iconic star-forming regions in unprecedented detail.
• Discovered new candidate galaxies that have prompted revisions to standard early-universe cosmology.
• Studied solar-system objects from Mars to the Kuiper Belt with new infrared sensitivity.
• Confirmed that some of the most basic assumptions about early-universe galaxy formation needed updating — galaxies grew faster and earlier than expected.

Common myths — busted

We sent gold back to space. The atoms that made our wedding rings now sit on the most expensive telescope ever built, looking at the same kind of cosmic environment that made them. Few materials have such poetic histories.

Frequently asked questions

How much gold is in the James Webb Space Telescope?

Approximately 48 grams of gold across all 18 primary mirror segments, the secondary mirror, and other optical surfaces. At current gold prices that's worth only a few thousand dollars — a tiny fraction of the telescope's USD 10 billion budget.

Why is gold used instead of silver?

Silver actually has slightly better infrared reflectivity than gold in some ranges — but silver tarnishes from microscopic contamination and solar UV radiation, even in space. Gold is chemically inert and never tarnishes, which is crucial for a multi-decade mission. The combination of high IR reflectivity AND chemical stability makes gold the right choice.

Why not aluminum like the Hubble Telescope?

Aluminum is excellent for visible-light astronomy (which Hubble was designed for) but drops below 90% reflectivity in the infrared wavelengths JWST observes. Gold maintains 98–99% reflectivity across JWST's entire infrared range. For a telescope specifically optimised for infrared, aluminum's lower IR reflectivity was unacceptable.

How thick is the gold coating on JWST?

About 100 nanometres — roughly 1,000 atoms thick. This is the optimal thickness: thick enough to provide full reflectivity, thin enough to minimise mass and material cost. A protective thin layer of silicon dioxide (essentially glass) coats the gold to protect it from contamination and abrasion.

Can you see the gold in JWST photos?

Yes — the unmistakable golden colour of the 18 hexagonal mirror segments is a result of pure gold's intrinsic yellow appearance in visible light. The same physics (relativistic effects on gold's outermost electrons) that gives wedding rings their colour gives JWST mirrors their iconic look in publicity photos.

The bottom line

The James Webb Space Telescope uses gold for a single physical reason: gold reflects roughly 99% of infrared light, beats every alternative material in the infrared range, doesn't tarnish, and stays stable across the brutal cryogenic temperatures of deep-space operation. Only about 48 grams of gold cover all 18 primary mirror segments — applied as a 100-nanometre layer through vapor deposition, then sealed with a protective glass overcoat. The gold is one of the cheapest line items on a USD 10 billion telescope. There was no realistic substitute. Every gold atom on JWST was originally forged in a kilonova billions of years ago — sent back to space to gaze at the same kind of cosmic environment that made it in the first place. Few engineering choices in modern science combine such perfect physics with such poetic symmetry.

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