By Nick Strobel
This essay about the James Webb Space Telescope is based on the October 3, 2021 and October 17, 2021 “Bakersfield Night Sky” columns I wrote for my local newspaper, The Bakersfield Californian. First a little about the L2 point from where Webb does its observing.
There are five gravitational balance points ( “Lagrangian points” for those playing Jeopardy) for the Earth-Sun system. A small satellite can orbit these points in a constant manner. The L1 point is directly between Earth and the sun at 1.5 million kilometers from Earth—much closer to Earth than the sun because of Earth's much weaker gravity. Some of our sun-observing science satellites (e.g., SOHO) have been situated there as well as the Deep Space Climate Observatory (DSCOVR) satellite. The L2 point is located 1.5 million kilometers directly behind Earth with respect to the sun. The L3 point is on the exact opposite point of Earth's orbit around the sun, so we cannot see anything there from Earth because the sun is always in the way.
The L4 and L5 points are gravitationally stable points in Earth's orbit that are 60 degrees ahead and 60 degrees behind Earth, respectively. A gravitationally stable point means that if a satellite starts to drift away from the L4 or L5 point for some reason, the gravity of Earth and the sun would nudge it back toward the L4/L5 point. Back in the 1970s Gerard O'Neill popularized the idea of building giant space stations at the L4 and L5 points in which many thousands of people would live in self-sustaining colonies independent of Earth. I devoured his book, “The High Frontier: Human Colonies in Space” when I was in junior high school (we didn't have “middle school” where I grew up). There are some small asteroids found orbiting the Earth's L4 and L5 points
Earth's L2 point is directly behind the Earth and as such, spacecraft can keep the sun, Earth and moon behind the spacecraft for solar power while having a clear view of deep space. Spacecraft at L2 are close enough to have easy communication with the teams on Earth but are far enough away to not sense the infrared glow of the warm Earth. The Planck mission that mapped the Cosmic Microwave Background to the highest resolution ever, was located at L2 as was the Gaia mission that plotted the positions and velocities of over a billion stars in our home galaxy, the Milky Way.
On Christmas Day 2021, NASA launched another space observatory to the L2 point, the James Webb Space Telescope. With a mirror 6.5 meters across, Webb is the largest observatory put into space. It is the successor to, not a replacement of, the Hubble Space Telescope. Hubble observes primarily in ultraviolet and visible wavelengths, while Webb will observe in the longer wavelengths of infrared, so it can see the first stars that formed in the universe as well as peer into the dusty cocoons surrounding nearby forming stars and planets and take detailed spectra of the atmospheres of nearby exoplanets. Hubble has been able to do imaging and spectroscopy in the “near infrared”—wavelengths slightly longer than the visible band of colors we can see with our eyes—but Webb observes in significantly longer infrared wavelengths. Since the resolution or clarity of a telescope decreases with increasing wavelengths, telescopes need to be correspondingly larger in diameter to achieve the same resolution. That is why radio telescopes are tens of meters across. Webb's mirror needs to be over twice the diameter of Hubble's mirror to get the same resolution in the infrared as Hubble has in the visible.
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 carry-on bag: folding them up and then spreading them out when you get to your destination. Now, the origami exercise was 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. At L2, Webb is 1.5 million kilometers directly behind Earth with respect to the sun. Also because Webb is a large infrared telescope, it needs 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 about the size of a tennis court!
However, one layer of sunshield doesn't work for the highly sensitive Webb. The more than 200 kilowatts of solar energy 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 were 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 operates well below that threshold at just 7 K (–447 deg F).
Each of the beryllium mirrors are 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 was deployed on a telescoping tower about 6.5 meters in front of the primary mirror. There were 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 had to perform flawlessly for Webb to be unfolded and operate correctly. Go to the Webb's Deployment Video playlist on YouTube to see all of the various deployment steps. Also, the JWST team created the Deployment Explorer that gives more details about each step of the complex deployment.
Since Webb is over three times farther than is the moon, it is much, much too far away to service it like we could do with Hubble. To make sure it worked correctly the first time, all of the components and deployment processes were tested and retested multiple times. Webb launched from Kourou, French Guiana (northeastern coast of South America) on a European Space Agency Ariane 5 rocket. This location close to the equator gave the rocket a bigger Earth rotation boost than launching from within the U.S.
Webb took 29 days to get to the L2 point and it unfolded itself during that time. It took 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. The results have been spectacular!
last updated: March 13, 2026