James Webb Space Telescope | Ariane 5 ECA

Lift Off Time
(Subject to change)
December 25, 2021 – 12:20 UTC | 9:20 GFT
Mission Name
James Webb Space Telescope (JWST)
Launch Provider
(What rocket company is launching it?)
(Who’s paying for this?)
Led by NASA, in collaboration with the European Space Agency, the Canadian Space Agency and the Space Telescope Science Institute
Ariane 5 ECA
Launch Location
ELA-3, Guiana Space Centre, French Guiana, France
Payload mass
~ 6,500 kg (14,300 lb) 
Where is the spacecraft going?
Earth-Sun L2 Lagrange point
Will they be attempting to recover the first stage?
Where will the first stage land?
It will crash into the Atlantic ocean
Will they be attempting to recover the fairings?
No, this is not possible with the Ariane 5
Are these fairings new?
How’s the weather looking?
This will be the:
– 138th orbital launch attempt of 2021
– 289th Arianespace mission
– 112th Ariane 5 mission
Where to watch
Arianespace livestream

NASA livestream

Tim Dodd, the Everyday Astronaut, will be streaming at T-30ish minutes; come ask questions and join the conversation live!

What Does All This Mean?

The James Webb Space Telescope will launch atop an Ariane 5 rocket from Arianespace’s ELA-3 launch complex at the European Spaceport in French Guiana. After deployment, the spacecraft will make its way to the Earth-Sun L2 Lagrange point. The space observatory is the product of collaboration between NASA, the ESA, and the CSA, and will succeed the Hubble Space Telescope as NASA’s flagship astrophysics mission. It is expected to be operational for at least 5 and a half years, but hopefully stretching to 10.

What Is The James Webb Space Telescope?

The James Webb Space Telescope, or JWST, is the largest space-based observatory ever built. It will be 100 times more powerful than its predecessor, Hubble, and will build on its 30 years of groundbreaking research. The JWST is optimized for infrared wavelengths and will provide unprecedented images and research that will help to expand our understanding of the universe.

The observatory has four primary goals: 

  1. To examine the first light in the Universe and objects that formed shortly after the big bang. 
  2. To investigate how galaxies form and develop.
  3. To observe the formation of stars and planetary systems.
  4. To look at the physical and chemical properties of planetary systems and investigate the potential for life in these systems.

Telescope And Mission Development

Early development for a telescope to proceed Hubble began in 1989, but official development of the JWST began in 1996. It was originally named the Next Generation Space Telescope (NGST), not being renamed to the James Webb Space Telescope until 2002. It was planned to be an 8 m aperture telescope, located at L2, and to cost in the realm of $500 million. At this time, the telescope was initially scheduled to be launched in 2007, but the project has since been subject to 16 launch delays, cost overruns, and major redesigns.

In 2004, construction of the JWST began, and in 2005 the Ariane 5 and the Guiana Space Centre were chosen as the launch vehicle and launch site, respectively. Over the next decade, the individual pieces of the spacecraft were build and tested, before being assembled. By November 2016, final construction of the telescope was finished, and testing could begin.

Launch was delayed in March 2018 after the spacecrafts huge sunshield ripped during a test deployment. In a review following the incident, it was found that the JWST had 344 potential single point failures, any one of which would stop the entire project from working.

Once construction was complete, final tests on the telescope were carried out at the Northrop Grumman factory in Redondo Beach, California. The telescope was then shipped to Kourou, French Guiana aboard the MN Colibri cargo ship, arriving on 12 October, where it then began to be integrated to the rocket. On 22 November NASA announced an incident had occurred, when a clamp band which secured the payload to the adaptor had been accidentally released, causing the telescope to vibrate. Luckily, nothing was found to be damaged as a result of the incident, and preparation could continue. The JWST has now been integrated onto the Ariane 5, and is scheduled to launch on December 25th.

Who Was James Webb?

James Webb was the second administrator of NASA, best known for heading up the Apollo program. Former NASA administrator Sean O’Keefe said of Webb, it’s “thanks to his efforts, we got our first glimpses at the dramatic landscape of outer space… He took our nation on its first voyages of exploration, turning our imagination into reality.”

The naming of the telescope has been the subject of much criticism, due to allegations of Webb’s LGBTQ discrimination. In the months leading up to launch, many have urged NASA to rename the telescope, however in September 2021 they announced that the telescope would not be renamed and denounced the claims of discrimination.

How Does The Telescope Work?

JWST has four main elements: The Spacecraft Bus; the sunshield; the Integrated Science Instrument Module (ISIM); and the Optical Telescope Element (OTE).

JWST, layout
A labeled diagram of the telescope (Credit: NASA)

The Spacecraft Bus

The JWST Spacecraft Bus is the main supporting component of the observatory, containing many of the major systems which allow the telescope to function, such as the computing, communication, and propulsion systems. The bus weighs 350 kg (770 lb) and is made predominantly from graphite composite material. Because of the spacecrafts halo orbit, one side of the observatory will be in continual sunlight. The bus is located on the Sun facing side of the spacecraft, operating at around about 300 kelvins (80 °F, 27 °C).

The bus also has six reaction wheels, two star trackers, and holds the propulsion system. There are ten pairs of thrusters, each pair having one primary and one backup thruster. The thrusters use hydrazine for fuel and dinitrogen tetroxide for oxidizer.

The Sunshield

In order to make observations in the infrared spectrum, the telescope needs to be kept cool, under 50 K (−223.2 °C; −369.7 °F). If hotter than this, the telescopes own infrared radiation would stop the instruments from working correctly. To combat this, the observatory has a huge five layer sun-shielding base, measuring 22 m by 12 m, roughly the same size as a tennis court and twice as big as Hubble. Each of the five layers are as thin a strand of hair and are made of Kapton E. Both sides of each layer is coated with aluminum, and the sun facing side of the the two outer layers are additionally coated with doped silicon, which will reflect the heat from the Sun back out into space. This sunshield is key to blocking all light and heat from the Sun, Earth, and Moon, while maintaining a stable temperature for the instruments on the dark side of the observatory.

JWST, sunshield
A full sized test unit of the sunshield (Credit: NASA/Chris Gunn)

The Optical Telescope Element

The primary mirror of the telescope, the Optical Telescope Element, is comprised of 18 hexagonal mirror segments made of gold plated beryllium. The gold plating is only 1000 atoms thick, and will optimize reflectivity in the infrared. The segments are combined to form a 6.5 meter diameter mirror which collects the light from distant objects. A secondary mirror will then reflect this light to the instruments on board the observatory.

JWST, main mirror, hexagonal segments
The JWST primary mirror (NASA/Chris Gunn)

The Integrated Science Instrument Module (ISIM)

The ISIM is the found at the center of the JWST. It contains the main scientific payload, which has four science instruments and a fine guidance center. The mass of the ISIM is 1400 kg (3086 lb), which is around 23% of the total mass of the JWST. The observatory has 4 primary instruments on board:

NIRCam, or the Near InfraRed Camera, is an infrared imager. Its spectral coverage will range from the edge of the visible to the near infrared, or 0.6 micrometers to 5 micrometers. The imager will have 10 sensors, each 4 megapixels. The instrument will also be the observatory’s wavefront sensor.

NIRSpec, or the Near InfraRed Spectrograph, will perform spectroscopy in the same wavelength range as NIRCam. The design of NIRSpec has three observing modes. The first mode is a low resolution mode which uses a prism. The second, a R~1000 multi-object mode, uses a micro-shutter mechanism, which allows for simultaneous observations of hundreds of individual objects anywhere in the instruments field of view. The last mode is a R~2700 integral field unit, or long-slit spectroscopy mode. The instrument has two sensors, each of 4 megapixels

MIRI, or the Mid-InfraRed Instrument, is going to measure the mid-to-long-infrared wavelength range, going from 5 to 27 micrometers. The instrument has a mid-infrared camera, as well as an imaging spectrometer.

FGS/NIRISS, or the Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph, will stabilize the observatory’s line of sight during science observations. Though FGS and NIRISS are mounted together, they actually have two very different functions. FGS will take measurements which are used to control orientation of the spacecraft and the fine steering mirror. On the other hand, NIRISS is a module for astronomical imaging and spectroscopy, which will work in the 0.8 to 5 micrometer wavelength range.

JWST, ISIM, layout
A diagram of the ISIM (Credit: NASA)

Deployment Timeline

Following launch, Webb will begin its six month commissioning period, which will include unfolding the sunshield and mirror, cooling the telescope to cryogenic operating temperatures, aligning the mirrors and calibrating the scientific instruments. After this, we will start receiving the telescopes first images! You can learn more about the deployment sequence here.

JWST, deployment schedule
JWST launch and deployment sequence (Credit: NASA)


JWST will operate at the second Lagrange point (L2) of the Earth-Sun system, which it will circle around in a halo orbit. Lagrange points are points in space where the gravity of two large orbiting bodies, eg. the Sun and Earth, balances out the centripetal force needed for a spacecraft, such as JWST, to move with them. This means that very little fuel is needed to keep a spacecraft in the desired orbit.

There are five Lagrange points in the Earth-Sun system. L2 is about 1,500,000 km (930,000 mi) away from Earth, and is behind the Earth from the position of the Sun. L2 has been the home of previous astronomy missions, including WMAP, Planck, and Herschel. The observatory’s halo orbit around L2 has a radius of ~800,000 km (500,000 mi), which the spacecraft will take half a year to complete.

L2 orbit, JWST
The L2 Lagrange point (Credit: NASA/WMAP Science Team)

Why Is The James Webb Space Telescope Infrared?

Hubble observed the near ultraviolet, visible and near infrared spectra, but the JWST will observe in a lower frequency range, from long wavelength visible light through to mid infrared. This means it will be able to observe high infrared objects that were too old and too far away for Hubble to see. The JWST will be able to examine objects 13.6 billion light-years away, and because of the time light travels across the universe, this means the telescope will be able to look at objects 13.6 billion years ago. The more distant a cosmic object is, the younger it appears because the light has taken longer to reach Earth. It is expected to be able to see back to the first galaxies forming just a hundred million years after the Big Bang. This is the furthest back in time we will have ever observed!

Observing in the infrared spectrum is also key to seeing further than previous telescopes because of cosmological red-shift. Red-shift is when light shifts into the redder wavelengths, like the near and mid-infrared sections of the light spectrum, as it travels. Because the universe is expanding, light becomes red-shifted as it travels. The further away an object is from us, the faster the light travels, and the larger the red-shift. This means that if we want to view objects further away and older than we have before, we need to view them with highly sensitive infrared telescopes, such as the JWST, which has been designed to see ultraviolet light that has red-shifted into the infrared.

Ground based astronomy is limited in regards to infrared because the water vapor and carbon dioxide present in the Earth’s atmosphere absorb most infrared. Our atmosphere also radiates in the infrared spectrum, which further obscures the objects that we want to observe.

Other space based telescopes, like Hubble, are unable to see these infrared bands because their mirrors are insufficiently cool (Hubble is kept at 15 °C), so the telescope itself radiates in the infrared bands. The JWST’s large sun shield is made of silicon coated and aluminum coated Kaption which will help to keep the mirror and it’s instruments below −223.2 °C. So the JWST is going to be doing work that few other ground and spaced telescopes can do, and do this on a whole new level. 

Infrared observations can also see more easily through regions of cosmic dust which scatters visible light. This means that the telescope will be able to study objects usually hidden by gas and dust in the visible light spectrum, like the active cores of galaxies, and the molecular clouds where stars are born.

In the image below you can see the Carina Nebula, viewed in both visible light and infrared. In the visible light image, you can see the nebula has a large pillar of gas and dust, which is illuminated by massive nearby stars. The radiation and stream of charged particles from these stars means that new stars are born within the pillar, however they cannot be seen because the gas blocks their light. However, we can see all these newly formed stars in the infrared image, as the pillar of gas and dust is not visible.

Carina Nebula, visible light, Infra-red
The Carina Nebula (Credit: NASA, ESA, and the Hubble SM4 ERO Team)

The Ariane 5 ECA

The Ariane 5 ECA is a European heavy-lift launch vehicle developed by Arianespace for the European Space Agency. Regarded as one of the most reliable launch vehicles in the world, the Ariane 5 has launched 109 times since 1996 with a 95.4% success rate. The rocket flew 82 consecutive missions without failure before suffering a partial failure in January 2018. The Ariane 5 launches from the European Spaceport in French Guiana, a spaceport close to the equator, which allows the rocket to take advantage of the Earth’s greater rotation speed there and boost the launch performance. 

The ECA version of the Ariane 5 is capable of launching two large satellites, one on top of the other, using an adapter known as the Système de Lancement Double Ariane (SYLDA). The adapter covers the lower satellite as it supports the higher satellite. When the time comes for satellite deployment, the top satellite is released first, the SYLDA is then jettisoned, and the bottom satellite is released last.

The JWST cannot fit inside any current launch vehicle when fully expanded, so it has been folded to fit inside the Ariane 5’s fairings. It will unfold, segment by segment, over a two week period as it makes its way to its final destination.

Ariane 5, launch
Ariane 5 launch (Credit: ESA/CNES/Arianespace)


Two P241 solid rocket boosters (SRBs) are attached to the sides of the rockets main stage. They are fueled with a mix of ammonium perchlorate (AP) (68%), aluminium fuel (18%), and Hydroxyl-terminated polybutadiene (HTPB) (14%). Each booster provides about 7,080 kN (1,590,000 lbf) of thrust, burning for 130 seconds before crashing into the ocean. They are usually left to sink to the bottom of the ocean, but similarly to the Space Shuttles SRB, it is possible to recover them using parachutes. When this is done it is for post-flight analysis, and the boosters are not reusable.

Main Stage

The Ariane 5 has a cryogenic H173 main stage, called the EPC (Étage Principal Cryotechnique — Cryotechnic Main Stage), which burns for 540 seconds. It is comprised of a main tank, which is 30.5 m tall and has two compartments, one compartment for liquid hydrogen (LH2) and the other for liquid oxygen (LOx). A Vulcain 2 engine sits at the base and provides vacuum thrust of 1,390 kN (310,000 lbf). 

Ariane 5, main stage
The cryogenic main stage (Credit: Arianespace)

Second Stage

The ECA has an upper stage called the ESC-A (Étage Supérieur Cryotechnique — Cryogenic Upper Stage), and uses an HM7B engine, which is fueled by liquid hydrogen (LH2) and liquid oxygen (LOx). The stage provides 67 kN (15,000 lbf) in vacuum, has an ISP of 446 seconds, and will burn for 945 seconds.

Ariane 5, second stage
The cryogenic upper stage (Credit: Arianespace)
  1. How does the ESC-A upper stage have an ISP of 446 but will burn for 945 seconds? Is it a special upgrade to the ESC-A engine?

    1. Hi Omar, ISP is a measurement of how efficient an engine is, it doesn’t mean how long it can burn, that is dependant on how much propellant the rocket has. It’s sometimes a little confusing, but the higher the ISP, the more efficient is the engine. There are some great videos that explain what ISP is way better than what I can do here in the comments.

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