Bakersfield Night Sky - October 17, 2021

Bakersfield Night Sky — October 17, 2021
By Nick Strobel

I ended my previous column with a question about the James Webb Space Telescope, which will launch at the end of this month. With a mirror 6.5 meters across, Webb will be the largest observatory put into space. It will be the successor to, not a replacement of, the Hubble Space Telescope because Webb will be observing in the longer wavelengths of infrared while Hubble concentrates on the visible and ultraviolet wavelengths. Here's the question: How do you get a telescope with a mirror far larger than can fit in any rocket to operate successfully at a point over three times farther than the moon?

Let's take a look at the first part of the question—fitting Webb in a rocket. Basically, they'll fit it in the same way you can fit your pants and shirts in a small carryon bag: folding them up and then spreading them out when you get to your destination. Now the origami exercise is going to be a lot more difficult and much more precise than folding up your clothes. 

Because Webb is a space infrared telescope, it needs to be far from the warm Earth and moon that glow in the infrared. That's why Webb will be placed at the L2 gravitational balance point I talked about in my previous column. At L2, Webb will be 1.5 million kilometers directly behind Earth with respect to the sun. Also because Webb is a large infrared telescope, it will need a large sunshield, much larger than the telescope itself, so the telescope can observe at a variety of angles in the shadow of the sunshield. The sunshield is going to be about the size of a tennis court.

However, one layer of sunshield won't work for the highly sensitive Webb. The more than 200 kilowatts of solar energy will need to be dampened down to just a fraction of a watt (i.e., by a factor of about a million times). Webb's sunshield consists of five membranes, each about 165 square meters in area and extremely thin to keep the weight down. Each membrane is vapor-coated with an ultra-thin layer of aluminum. Each of those five membranes are folded on a graphite-epoxy frame that has to be strong enough to withstand the shaking and stresses of a rocket launch.

The mirror is actually 18 hexagonal mirrors, each 1.8 meters across that fit together in a honeycomb fashion with an alignment precision of a few millionths of a millimeter. They will be folded in thirds to fit inside the rocket. Each mirror segment is made of beryllium because beryllium is light and stiff and stops changing size (no thermal contraction or expansion) at temperatures below 100 K (-280 deg F). Webb will operate well below that threshold at just 7 K (-447 deg F). 

Each of the beryllium mirrors will be coated in gold to better reflect the infrared light coming from forming stars and planets in our galaxy as well as that coming from the very first stars that formed in the universe billions of years ago. The ultraviolet and visible light from those first stars has been traveling for billions of years and been stretched by the expansion of space to now be in the infrared band. 

There is also a secondary mirror that will be deployed on a telescoping tower about 6.5 meters in front of the primary mirror. There are 178 non-explosive release devices, more than 40 major deployments of 30 different types, 155 motors, more than 600 pulley assemblies, and nearly 100 cables totaling about 400 meters in length that all have to perform flawlessly for Webb to be unfolded and operate correctly. Go to the Webb's Deployment Video playlist on YouTube at https://www.youtube.com/playlist?list=PL691BF261E32A4420 to see all of the various deployment steps.

Since Webb will be over three times farther than is the moon, it will be much, much too far away to service it like we could do with Hubble. To make sure it works correctly the first time, all of the components and deployment processes have been tested and retested multiple times. All of the testing that's possible to do is now done. It's time to fly. Webb will launch from Kourou, French Guiana (northeastern coast of South America) on a European Space Agency Ariane 5 rocket. This location close to the equator will give the rocket a bigger Earth rotation boost than launching from within the U.S.

Webb will take 29 days to get to the L2 point and it will unfold itself during that time. It will take another two months to cool down to the very cold temperatures it needs for its observations and then another three to four weeks to get all of the optics aligned. If everything goes well, scientific operations should start about six months after launch—by about finals week at BC. Those infrared pictures from Webb would make a nice backdrop for commencement! 

If you're vaccinated, you have a MUCH smaller chance of getting so sick that you need to be hospitalized in Kern's already packed hospitals. Check out My Turn vaccination information page for the clinic closest to you and please continue wearing a mask when with large groups in enclosed spaces!

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Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com