How the James Webb Space Telescope is so different from Hubble

Scientists are breathing sighs of relief as the James Webb telescope nears the end of a complex sequence of self-unfurling in space. This new eye in the sky is a completely new instrument from its predecessor, the Hubble Space Telescope.

Other than the fact that both telescopes use curved mirrors to capture the light from the universe, the similarities end there. Webb's primary mirror is 6.5 metres across, involving 18 gold–coated hexagonal sections that fit together like a mosaic.

Hubble uses a single 2.4–metre mirror made of solid glass. The size advantage gives Webb 6.25 times more surface area to capture light. It is the largest space telescope ever built, and in astronomy, size does matter. 

The light it will capture is infrared, which is basically heat, so it will be able to detect radiation from extremely distant objects right back to the early days of the universe.

The telescope's large size meant it could not be simply placed into the cargo bay of a space shuttle and released into space as a single unit like Hubble was. It had to be folded up like a fictional transformer to fit into the nosecone of an Ariane 5 rocket, then unpack itself in space, which was a huge source of anxiety for the scientists and engineers.

There is no room for mistakes and no possibility of repair by astronauts if something goes wrong. It has to be right the first time.

This giant, super-sensitive instrument had to be made as lightweight as possible, yet strong enough to withstand the vibrations and G-forces of a rocket launch.

Once in space, the unfolding sequence involved 50 major deployments with 344 of what the scientists call "single-point failure" items involving latches, hinges, motors, bearings, gears, cables and pulleys. All of them have to work perfectly.

Since it was launched on Christmas Day 2021, that sequence of events has so far proceeded as planned. In fact, the launch was so precise, the telescope will have extra manoeuvring fuel leftover to operate beyond its expected 10–year lifetime. 

The most complex, nail–biting sequence was unfolding a giant sun shield designed to block radiation from the sun, moon and Earth that would overwhelm the telescope's super sensitive instruments.

The shield is made of five layers of thin aluminized plastic that had to be carefully unfolded and stretched tight. Imagine folding a sheet of plastic food wrap the size of a tennis court into a container, then pulling it out in the cold vacuum of space without tearing it. 

The system was tested many times on Earth, but on the ground, gravity is always present. The scientists had to trust that it would work as well in the zero gravity environment of space. Thankfully, that sequence proceeded without a hitch.

The final step is the assembly of the mirror itself. There are two of them, the giant main mirror and a smaller secondary mirror held out in front at the focal point by three long booms. All these pieces have to fit together into a perfect shape to focus light with unprecedented resolution and sensitivity. 

The James Webb telescope is designed to avoid an embarrassing flaw the Hubble experienced right after it was launched in 1990. The first images were blurred because of a flaw in the primary mirror that gave it the wrong shape to focus properly.