The James Webb Space Telescope


Photo: Northrop Grumman

The Launch

An idea started in the late 1980s, the James Webb Space Telescope (JWST) is finally ready to launch into space on Friday, December 25th, at 7:20 am EST. The telescope will be launching from Kourou, French Guiana, at the Guiana Space Center on top of the European Space Agency's (ESA) Ariane 5 rocket. The Ariane 5 will launch JWST toward the Sun-Earth Lagrange point (L2), approximately 1 million miles away from Earth, which allows the sun shield to always face the Sun, Earth, and Moon so their light and heat don't reach the telescope's extremely sensitive optics.

After launching from Earth, the telescope will separate from the second stage of the Ariane 5 and begin its 6-month commissioning period. After the course correction maneuver 2.5 days after launch, it starts to unfold as it coasts to the L2 halo orbit. Once it arrives at L2, a final burn is conducted to place the JWST into the targeted orbit. Over the next 4 months, the telescope's instruments begin to cool down to their operational temperatures. When they are sufficiently cooled down, the gold-plated mirrors on Webb will start to align within nanometers. The final months will consist of pointing the telescope at a variety of targets to calibrate and find the limits of the instruments' capabilities.


The Integrated Science Instrument Module before integration on the JWST // Photo: NASA/Chris Gunn

The Instruments

The James Webb Space Telescope has 4 science instruments housed inside the Integrated Science Instrument Module (ISIM), this is referred to as the "Main Payload".

Near-Infrared Camera (NIRCam): This camera provides high-resolution imagery and spectroscopy and is Webb's primary camera which operates on a wavelength (0.6 to 5 microns) where space dust becomes transparent. This helps the telescope look into the details of nebula and star systems otherwise obscured by dust. This camera will also help astronomers investigate planets orbiting nearby stars.

Near-Infrared Spectrograph (NIRSpec): The NIRSpec operates over a similar wavelength as the NIRCam, but is a more versatile tool for near-infrared spectroscopy. This camera has a "micro shutter array", where each shutter cell can be controlled individually, allowing it to be opened/closed or block a portion of the sky.

Near-Infrared Slitless Spectrograph/Fine Guidance Sensor (NIRISS/FGS): Operating at the same wavelength as the last 2 cameras, it provides near-infrared imaging and spectroscopic capabilities. It is also able to capture images of brighter objects at a much higher resolution than the other instruments. The Fine Guidance Sensor is built into the same housing and makes sure Webb is pointing the correct way and staying stable during the observations. The FGS uses stars for accurate tracking.

Mid-Infrared Instrument (MRI): The MRI sees further into the infrared spectrum than the other instruments, operating at a wavelength range of 5 to 28 microns. This allows astronomers to study the redshift of distant galaxies, new stars, faint comets, and other objects deep into space. This instrument is also the coldest of the 4, being cooled down to 7 Kevin (-447 Fahrenheit), or 7 degrees above absolute zero, where atoms theoretically freeze.


The completed mirror // Photo: NASA/Chris Gunn

The Mirrors

The Primary Mirror

The primary mirror on the James Webb Space Telescope will be the largest mirror launched into space, measuring 6.5 meters in diameter. The entire mirror is made up of 18 hexagonal mirrors that are 1.32 meters in diameter that can be moved around individually for proper alignment. The 3 mirrors on each side of the telescope are folded to the side for launch so the telescope can fit inside the fairing of the Ariane 5.

The Secondary Mirror

The secondary mirror is supported by 3 struts that extend from the primary mirror. The struts are folded up for launch and get deployed in front of the primary mirror after the sun shield deploys. It is a smaller mirror (0.74 meters in diameter) that is round instead of hexagonal.

The Materials

The mirrors on Webb are made of beryllium and coated with an extremely thin layer of gold, which is the optimal material for reflecting infrared light to the cameras. The beryllium was selected because of its lightweight and strong characteristics, as well as holding its shape in the extreme temperatures of space. Each mirror for the primary mirror weighs 46 pounds (20 kilograms) each.


Fully deployed sunshield during crucial test // Photo: Northrop Grumman


The Sunshield

The sunshield on the James Webb Space Telescope is made up of 5 layers of Kapton coated in aluminum and the bottom 2 layers additionally coated in silicon to make it electrically conductive. This shield is the size of a tennis court and is made to protect the telescope's instruments from the heat of the Sun, Earth, and Moon that would interfere with infrared observations. The side that faces the sun heats up to 383 Kelvin (230 Fahrenheit) and the side the telescope is on cools down to 36 Kelvin (-394 Fahrenheit). The L2 halo orbit enables the sunshield to always face the Sun, Earth, and Moon while observing. For deployment, 107 pins have to release to properly unfold the entire sunshield and all the layers. There are also ripstops added to the layers to minimize tearing from micrometeorites.


JWST Encapsulation on top of the Ariane 5 rocket on December 17th // Photo: ESA/CNES/Arianespace

After 2 decades, 40 Million Hours of work, and thousands of scientists, engineers, and technicians from 14 countries, the James Webb Space Telescope is now ready for launch. This is truly a feat of human engineering and will open the doors for a plethora of discoveries, and uncover the beginnings of our universe.