Now You See It, Now You Don’t

Al’s face takes on a far-away smile. “James Webb Space Telescope must look pretty up there — a golden honeycomb on a silver setting, floating in space.”

“Real artistic and all, but how come all that gold? The mirror’s what, 60 feet across. Half of that is 30 for the radius, square it is 900, times 3 plus a little for pi, makes like 3000 square feet. That’s a lot of gold and us taxpayers sent it up to space! Why not make it shiny with something cheaper?”

“Nice on-the-fly math, Mr Feder, but it’s much less gold than you think. Ever play with gold leaf?”

“Yeah, once when my cousin was decorating one of her artsy‑tartsy ceramic pieces. I went to pick up one of her leftover scraps and my breath blew it away.”

“Thin, huh? One of my astronomy magazines had an article, said the gold coating on the JWST‘s dish is only a little thicker than gold leaf, just 1000 atoms top to bottom. They don’t even apply it with a brush like people usually do. Your cousin may have artisan‑level skills but her manual techniques can’t match the precision JWST‘s design demands. The fabricators used a vapor deposition process like the semiconductor people use to make computer chips.”

“So how much gold?”

“Less than three grams per segment. That ring on your finger weighs more. The article said each segment has about $180 worth of gold, pretty small compared to the project’s ten billion dollar price tag.”

“Even so, why gold? Why not something cheaper?”

“I dunno. I see it a lot on different missions. The Insight Mars lander’s below‑deck electronics section was shrouded with gold-covered plastic panels. What’s the story, Sy?”

“Gold’s a special metal, Al. It ranks up with copper and silver for electrical conductivity but unlike them it doesn’t corrode. That’s why so many electrical switches and data cables use a thin layer of it at the contact points. The conductivity comes from the metal’s high concentration of free electrons, like an ocean of them sloshing around the atomic lattice. The free electrons also make gold an efficient reflector for electromagnetic radiation all across the spectrum from UV light way down into the radio range. The engineers for Insight and a lot of other missions put a thin gold layer on those insulation panels to protect the plastic from incoming high‑energy photons that would break up the polymer molecules. JWST needs to send every passing photon into the satellite’s detectors so gold’s high reflectivity made it the obvious candidate for the mirror coating.”

“I wonder if I’ll be able to see that honeycomb through my backyard telescope.”

“We can put numbers to that.” <drawing Old Reliable from its holster and tapping screen‑keys> “Lessee … using the small angle approximation for the sine function, to resolve a grid of 20-foot hexagons a million miles away you’d need an angular resolution of
  (20 feet) / [(106 miles)*(5280 feet/mile)]
   = 3.8×10-9 radians = 784 micro‑arcseconds

For yellow light’s 570‑nanometer wavelength, Rayleigh’s angular resolution formula gives
  aperture diameter
   =1.22*(wavelength)/(angle in radians) = 1.22*570×10-9/3.8×10-9 = 183 meters.
You’d need a telescope with a 550‑foot front end. If I remember right, Al, that’s almost ten times the width of your yard. You’d need at least a 60‑meter mirror just to see the entire dish as anything more than a yellow speckle.”

“Even with a scope like that Al would have a problem, Sy.”

“What’s that, Cathleen?”

“Viewed from the Sun, JWST‘s orbit looks straightforward — a half‑million‑mile half‑year vertical loop around the Sun‑Earth L2 point that itself circles the Sun once a year. Viewed from Earth, though, it’s a far more complicated motion. Earth’s orbit parallels JWST‘s so half the time JWST leads Earth and half it lags behind. At the extremes, JWST scoots along at twice Earth’s speed or else it appears to stop against the background stars. It’d be a challenging target for a star‑tracker program without a minute‑by‑minute computerized ephemeris.”

“He wouldn’t see the honeycomb even with that scope, Moire. JWST‘s always gonna point away from the Sun and us. The only view we’ll ever get is that pink parasol.”

~~ Rich Olcott