James Webb telescope and how it works

WEBB TELESCOPE

Looking back in time

How the James Webb Space Telescope will unlock secrets of the universe

It was finished years late at a cost far higher than planned, but NASA’s James Webb Space Telescope due for launch plans to usher in a new era in astronomy as it gathers information on the universe’s earliest stages, star formation and whether planets beyond our solar system may be habitable.

In 1990, NASA launched the Hubble Space Telescope that gave scientists a window into deep space. The Webb launch seeks to succeed the Hubble showing a different side of space.

Valuable payload

The orbiting infrared observatory, designed to be about 100 times more sensitive than its Hubble predecessor, will blast off Dec. 25 at the earliest from a site in French Guiana on South America’s northeastern coast. (Weather and other factors could delay the launch date.)

The telescope will be tightly folded inside the payload bay of an Ariane 5 rocket. The launch vehicle is part of the European Space Agency’s (ESA) contribution to the mission, along with launch services for the U.S. National Aeronautics and Space Administration (NASA) for Webb.

Once in space it will separate from the launch vehicle and begin its journey towards the orbit point.

Fairing

Payload bay

Folded

telescope

Fairing

Payload bay

Folded

James Webb

telescope

Inside

Ariane 5

Fairing

Payload bay

Folded

James Webb

telescope

Inside

Ariane 5

Fairing

Payload bay

Folded

James Webb

telescope

The long deployment

After being released from the rocket, Webb will gradually unfurl as it travels to its destination beyond the Moon, executing one of the most complex deployment sequences ever attempted.

+31 minutes

Solar array deployment

The first step is to power up the Webb by deploying the solar array.

Day 3

Sunshield pallet deployment

Two special pallets holding Webb’s essential sun shields deploy.

Day 4

Tower assembly

A tower holding the instrument package is lifted to its operational location.

Day 5

Momentum flap deployment

As the sun’s solar pressure pushes against the large sun shield this flap helps stabilize the telescope.

Sunshine membrane cover release

On the same day, special covers that hold the tennis court-sized sun shield are released.

Day 6 and 7

Membrane tensioning

Sun shield mid-booms are deployed and the sun shield is slowly tensioned, separating the five layers.

Day 10

Secondary mirror

The support structure for the smaller, secondary mirror is deployed.

Day 13

Primary mirror wings

The side panels of the main mirror are extended, completing the instrument deployment.

One of the goals of the telescope is to look back through time to when galaxies were young. Webb will do this by observing distant galaxies that are over 13 billion light years away from Earth.

When a telescope looks further away, it is also looking back in time. It takes time for the light the telescope is receiving to travel through space. Therefore, we see objects not as they are now but as they were at the time when they released the light that has travelled for billions of years across the universe to reach us.

To see objects so faint and far away, the telescope needs a giant mirror to collect the light.

The size of a telescope’s mirror area determines its sensitivity, or how much detail it can see. Since the Webb telescope has a much bigger mirror than Hubble, it can look further back in time.

James Webb

telescope

Primary mirror

Primary mirror

6.6 m

Hubble

telescope

Mirror

inside

2.4 m

Primary mirror

Primary mirror

6.6 m

JAMES WEBB

TELESCOPE

HUBBLE

TELESCOPE

Primary

mirror inside

2.4 m

Primary mirror

Primary mirror

JAMES WEBB

TELESCOPE

6.6 m

HUBBLE

TELESCOPE

Primary mirror

inside

2.4 metres

Primary mirror

Primary mirror

JAMES WEBB

TELESCOPE

6.6 metres

HUBBLE

TELESCOPE

Primary mirror

inside

2.4 metres

Primary mirror

Primary mirror size

JAMES WEBB

TELESCOPE

6.6 metres

HUBBLE

TELESCOPE

Primary mirror

inside

2.4 metres

Webb’s primary mirror intercepts red and infrared light traveling through space and reflects it onto a smaller secondary mirror. The secondary mirror then directs the light into the scientific instruments where it is recorded.

Because the telescope will be observing very faint infrared signals, it needs to be shielded from any bright, hot sources such as the sun. There is a huge temperature difference between the hot and cold sides of the telescope, separated by a sun shield.

First reflected off

primary mirror

COLD SIDE

Light

-233°C (-388°F)

Instruments

Second

mirror

HOT SIDE

85°C (185°F)

Rays from Sun

First reflected off

primary mirror

COLD SIDE

Light

-233°C (-388°F)

Instruments

Secondary

mirror

HOT SIDE

Rays from Sun

85°C (185°F)

First reflected off

primary mirror

COLD SIDE

Light

-233°C (-388°F)

Instruments

Secondary mirror

HOT SIDE

Rays from Sun

85°C (185°F)

First reflected off

primary mirror

COLD SIDE

Light

-233°C (-388°F)

Instruments

Secondary mirror

HOT SIDE

Rays from Sun

85°C (185°F)

First reflected off

primary mirror

COLD SIDE

Light

-233°C (-388°F)

Instruments

Secondary mirror

HOT SIDE

Rays from Sun

85°C (185°F)

Webb will primarily observe the universe in infrared, while Hubble studies it mostly at optical and ultraviolet wavelengths, although it does have some infrared capability.

Infrared observations are important to astronomy because recently formed stars and planets are often hidden behind masses of dust that absorb visible light. Infrared light can penetrate these obstacles.

“Infrared light will go around the dust grains instead of bouncing off, so we can see that with the Webb telescope. And that’s one of our top goals - to see how stars grow, with their young planets,” said John Mather, Webb senior project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

before after

Images: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

before after

Images: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

This image of the Eagle Nebula’s Pillars of Creation, located 7,000 light years from earth, was taken by the Hubble telescope in visible light.

The same location is pictured in infrared light, revealing a frame dotted with bright stars and replete with more details.

The instruments

The telescope has four science instruments contained within the Integrated Science Instrument Module (ISIM). This section is considered the heart of the telescope and located just behind the primary mirror.

An instrument called a spectrometer can study the atmospheres of exoplanets. Mather said finding one with lots of water - thought to be one of the key ingredients for life - would be “really interesting.” As he put it: “a wet little planet out there that might be a little bit like home.”

An orbit beyond the moon

Unlike Hubble, the telescope will not orbit the Earth. Instead it will orbit the Sun, around 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point, also known as L2.

Lagrange points, named after their discoverer, Joseph-Louis Lagrange, an 18th century mathematician, allow the gravitational pull of two large masses (the Earth and Sun) to equal the force required for a small object, such as a satellite, to move with them.

It will take roughly 30 days for Webb to reach the start of its orbit at the L2 position.

The balance of the combined gravitational pull of the Sun and the Earth allows Webb to keep pace with the Earth as it goes around the Sun.

This allows the satellite’s large sun shield to continuously protect the telescope from the light and heat of the Sun and Earth and Moon.

Webb will not sit stationary at the L2 orbit point. Instead, it will also move in another smaller orbit, roughly similar in size to the Moon’s orbit around the Earth, which takes about 6 months.

This smaller orbit ensures Webb is out of the shadows of both the Earth and Moon so instruments can operate around the clock, unlike Hubble, which goes in and out of shadow every 90 minutes.

“We’re going to look at everything there is in the universe that we can see. We want to know: how did we get here from the Big Bang, how did that work? So, we’ll look,” Mather said.

Video credits

Opening video: Goddard Media Lab, NASA
Deployment sequence video: Conceptual Image Lab, NASA (Animations by Adriana Manrique Gutierrez Released on December 15, 2021)
Instruments video: Goddard Media Lab, NASA
Orbit video: Goddard Media Lab, NASA

Sources

Goddard Space Flight Center, NASA;

Edited by

Christian Schmollinger