James Webb Space Telescope Part 2: The Engineering

Last time out I wrote about the incredible scientific goals of the James Webb Space Telescope (JWST), due to launch towards the end of next year. The earliest light in the universe is its main goal along with analysing the birth of new solar systems and the spectra of exoplanets.

Today I am going to write about the amazing engineering challenges that have have been achieved to even get this thing to work. Let’s start at the business end: the mirror. I once read that the primary mirror on Hubble was so perfectly flat and smooth that if it was scaled up to be the size of the Gulf of Mexico then the elevation of its surface would only deviate by about 1 cm. The 2.4 m Hubble mirror was a masterpiece at the time but the mirror on JWST is in a whole other league.

At 6.5 m in diameter the JWST mirror is 6 times larger than Hubble’s and more than 100 times as powerful. It’s so sensitive that at a distance of from earth to the moon the JWST could detect the heat signature of a bee! A mirror this wide would be too big to fit inside even the largest rockets, not to mention being fiendishly difficult to make. For this reason theJWST primary mirror is actually made of 18 smaller, hexagonal mirror that fit together like a honeycomb. This also gives it the ability to fold away some of the hexagons so that it can fit onto the Ariane rocket that will launch it.

jwst-hst-primary-mirrors
Image courtesy of NASA

The JWST detects infrared light, which is basically heat, and it’s so fantastically sensitive that if you were to simply deploy it in space and switch it on all it would be able to do is observe the heat that the device itself gives off; it would just detect itself. To avoid this the operating temperature of the main telescope is below 50 Kelvin, about -220 Celsius.

sunshieldcrosssection
Image credit: STScl

Just being in the vacuum of pace isn’t sufficient to reach such temperatures and so JWST is taking a sunscreen with it – a really very big sunscreen: once fully deployed it will be larger than a tennis court. It is made of 5 layers of a very reflective material with the vacuum of space filling the gap in between, blocking almost all of the heat from the sun from reaching the telescope.

Click here to see a time lapse video of a test deployment of the sunscreen.

To aid with this requirement for the telescope to be as cold as possible it isn’t possible to have it orbiting the earth, it would be too warm. So where to put it? The answer is the second Le Grange point.

l2-3
Image courtesy of NASA

The Lagrange points are positions in an orbital configuration of two large bodies where a small object affected only by gravity can maintain a stable position relative to the two large bodies. The Lagrange points mark positions where the combined gravitational pull of the two large masses provides precisely the centrifugal force required to orbit with them.

The second Lagrange point is about 1 million miles away, that’s 4 times further away than the moon is. There is an important consequence of this. JWST has to work first time. There will be no rescue missions, no service missions. If something goes wrong then that’s it, the telescope is beyond our help.

It will take about one month for JWST to reach its home. During that time the giant hexagonal mirror will fold into place and the enormous sunscreen will deploy. Below you can watch a video that will detail the first 30 days of the mission from the moment of launch to full deployment.

I guess the last thing we need to know is: who is James Webb? Well, he was the head administrator at NASA through the bulk of the space race in the 1960s. He didn’t just ensure that the Americans got to the moon first, however, he was adamant that if he was going to lead NASA it wasn’t going to just be about beating the Russians and getting there first. He wanted the project to be scientifically valid and useful too. That’s an ethos I can work with.

16519914560_17b4482fc9_k
I like this picture because the profile looks quite like a Star Destroyer. Image courtesy of NASA.
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s