January 16, 2022
Sunday, January 16, 2022
The James Webb Space Telescope is finally in space and all of the complex unfolding “deployment” process is done. All of the major deployment parts were finished over a week ago including the unfurling of the five-layered sunshade and locking the three parts of the primary 6.5-meter mirror into place. The primary mirror is made of 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.
Although the 40-step deployment process of 30 different types of operations had been tested and retested multiple times by a team of extremely competent people, these past several weeks have still been a white-knuckle experience for the Webb team. Heck, it’s been white-knuckle for many of us on the outside too!
Each mirror segment is now being very carefully moved from their launch configuration to a rough alignment (image-stacking) configuration that gets them ready to be exquisitely aligned for the science observations. The launch configuration of the mirrors means that you would see 18 fuzzy separate images of a star. The image-stacking process gets the mirrors aligned enough to put the 18 fuzzy images on top of each other. After that will be the super-precise alignment that will take about three months to achieve.
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 50 K (-370 deg F).
Each of the beryllium mirrors is 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.
The infrared light bounces off the primary mirror to a 0.74-meter secondary mirror that is suspended about 6.5 meters above the primary mirror. The infrared light bounces off the secondary mirror back down toward a central hole in the primary mirror where the light then bounces off a third “tertiary” mirror to the suite of four instruments: the Near-Infrared Camera (NIRCam), Near-Infrared Spectrograph (NIRSpec, that spreads the infrared out into the infrared rainbow of infrared colors or wavelengths), the Mid-Infrared Instrument (MIRI), and the Fine-Guidance Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS). (Say that last instrument five times really fast.) [Slight digression about “infrared colors”: in visible light we see short wavelengths as violet and blue, medium-length wavelengths as green and yellow, and longer wavelengths as orange and red—different colors of visible light have different wavelengths but they’re all forms of light. The infrared is a type of light that has longer wavelengths than red visible light. Our eyes can’t see infrared but if we had infrared-detecting eyes, we’d be able to see the different wavelengths of infrared as different colors in the infrared. We don’t have names for those infrared colors because of our visible light bias.]
While the sunshade gets the mirrors cooled to just 50 K, the NIRCam, NIRSpec, and FGS-NIRISS instruments need to be passively cooled further down to 39 K (-389 deg K) and MIRI needs to operate at just 7 K (-447 deg F) by using a helium cryocooler system. The huge mirror and sophisticated instruments will enable Webb to see objects that are 10 to 100 times fainter than what the Hubble Space Telescope can see.
The next major event for Webb is the final mid-course trajectory burn of the thruster rocket on January 23 that will correct any residual trajectory errors and insert Webb into a looping halo orbit around the L2 gravitational balance point that is 1.5 million kilometers behind Earth. Orbiting around the L2 point will enable Webb’s solar panels to always be in sunlight and not be in Earth’s shadow.
After January 23 will be a five-month process of checking and calibrating the instruments and fine-tuning the alignment of the mirror segments. 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! (Just saying…)
In the evening sky, you’ll just be able to see Jupiter, Saturn, and Mercury low in the southwest just after sunset, though Saturn and Mercury will probably require a pair of binoculars to pick them out in the surrounding twilight glow. Jupiter will be the brighter one higher up. Mercury is dropping quickly down toward the sun now. On January 23, Mercury will pass between us and the sun for the “inferior conjunction”. Saturn will go behind the sun on February 4, so in a few weeks, it’ll just be Jupiter up in the evening sky.
Venus is now climbing up away from the sun in the pre-dawn morning to join Mars in the southeast sky. By the end of the month, Venus will be higher up in the sky than is Mars. The moon tonight is at waning gibbous (looking more than half lit up) and on the night of January 24/25, the moon will be at third (or last) quarter in which it appears half-lit on the left side. On the morning of January 29, you’ll see a thin waning crescent moon next to Mars. You might also be able to spot Mercury very low in the southeast then as well.
I hope that you’ll be able to find a time and place sometime in 2022 to gaze up in wonder at a dark night sky filled with thousands of stars.
—
Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com
