π Space
How Space telescopes unravel the mysteries of the cosmos

James Webb Space Telescope: the new “Hubble”?

Isabelle Dumé, Science journalist
On November 17th, 2021 |
3 min reading time
Philippe Laudet
Philippe Laudet
Astronomy and Astrophysics Program Manager at CNES
Key takeaways
  • A new space telescope, named James Webb (JWST), will be launched later this year onboard an Ariane 5 rocket – described as being the successor of the renowned Hubble Space Telescope.
  • With a 6.5-metre-diameter segmented mirror, three times the size of Hubble’s and is 400 times more sensitive than current ground-based or space-based infrared telescopes.
  • It will observe infrared waves helping researchers trace them back to the birth of stars and as far back as 200 million after the Big Bang.
  • Data collected by the JWST will also give us more information about the atmosphere of ~10 exoplanets discovered over past decade.

A major new tele­scope, the James Webb Space Tele­scope (JWST)1, is sched­uled for launch a few days before Christ­mas this year. It will leave Earth on an Ari­ane 5 rock­et from the Euro­pean Space Agency’s (ESA) space­port in Kourou, French Guiana. The long-await­ed mis­sion is often described as being the suc­ces­sor of the renowned Hub­ble Space Telescope.

Deep exploration

As the most advanced space obser­va­to­ry ever built, the JWST will oper­ate pri­mar­i­ly at near- and mid-infrared wave­lengths rather than the vis­i­ble-spec­trum used by Hub­ble. As such, it will allow for the most detailed explo­ration yet of the very dis­tant and ancient galax­ies and stars. JWST will also study near­by celes­tial bod­ies, extra­so­lar plan­ets and our own solar sys­tem. It is a tele­scope that should rev­o­lu­tionise our under­stand­ing of exo­plan­ets and how the first stars and galax­ies formed in the Universe.

The JWST is the joint flag­ship project between NASA, ESA and the Cana­di­an Space Agency. It boasts a 6.5‑metre-diameter seg­ment­ed mir­ror – three times the size of Hubble’s – mak­ing it 400 times more sen­si­tive than cur­rent ground-based or space-based infrared tele­scopes. The mir­ror is so large that it has to be fold­ed in three and will be unfold­ed once the tele­scope has reached its destination.

It will span the long-wave vis­i­ble spec­trum and infrared wave­lengths from 0.6 to 28 microns and will car­ry four sci­en­tif­ic instru­ments. Oper­at­ing for at least 5–10 years – hope­ful­ly more – it will be sent to the Lagrange point L2, which is 1.5 mil­lion kilo­me­tres from Earth, behind the orbit of the Moon. Again, this makes it very dif­fer­ent from Hub­ble, which has remained in Earth’s orbit. The JWST is also equipped with a very large 22 x 10 m sun­shield to cool it down and pro­tect it from the Sun’s infrared radiation.

The four science instruments onboard the JWST Integrated Science Instrument Module (ISIM):

- Visible/Near Infrared Cam­era (NIRCAM),

- Near Infrared Spec­tro­graph (NIRSPEC),

- Mid-Infrared Instru­ment (MIRI),

- Fine Guid­ance Sensor/Near InfraRed Imager and Slit­less Spec­tro­graph (FGS/NIRISS).

The main sci­en­tif­ic objec­tives of JWST will be: ‘First light and reion­iza­tion in the ear­ly Uni­verse’, ‘Assem­bly of galax­ies’, ‘Birth of stars and pro­to­plan­e­tary sys­tems’ and ‘Plan­e­tary sys­tems and the ori­gins of life’.

Studying the ‘reionisation’ era

In its first year of oper­a­tion, or “Cycle 1”, JWST will look for atmos­pheres on near­by rocky exo­plan­ets and probe the ear­li­est galax­ies in the Uni­verse – those that formed less than a bil­lion years after the Big Bang. These galax­ies are so faint that they could not be detect­ed by pre­vi­ous tele­scopes, with the excep­tion of a hand­ful dis­cov­ered by Hub­ble. These new obser­va­tions will help us under­stand an impor­tant part of the his­to­ry of the Uni­verse, known as the reion­i­sa­tion (or first light) epoch – a peri­od span­ning about 400,000 to 1 bil­lion years after the Big Bang, when the first stars and galax­ies emerged. It is pos­si­ble that reion­i­sa­tion did not occur every­where at once, but in pock­ets and bub­bles. These bub­bles are relat­ed to the ini­tial large-scale struc­tures of the Uni­verse, and JWST hopes to map this structure.

The JWST will be able to see much fur­ther back in time, to just 200 mil­lion years after the Big Bang, which occurred 13.8 bil­lion years ago. Until now, we have been able to go back as far as 400 to 500 mil­lion years after the Big Bang with exist­ing instru­ments, but JWST will be able to see the ‘first light’ of the Universe.

The total obser­va­tion time in Cycle 1 will be split into sev­er­al sub-cat­e­gories: 32% for galaxy obser­va­tions, 23% for exo­plan­ets, 12% for stel­lar physics and 6% for our own solar sys­tem. With­in these pro­grams, there are small, medi­um, and large pro­grams, some of which are regard­ed as ‘trea­suries’, expect­ed to pro­vide huge amounts of data that will keep future gen­er­a­tions of researchers busy for decades to come.

Studying the atmosphere of target exoplanets

JWST will also study the atmos­pheres of about ten of the thou­sands of exo­plan­ets dis­cov­ered in recent years and observe these worlds as they ‘tran­sit’ in front of their host stars. These obser­va­tions will allow astro­physi­cists to deter­mine whether they have an atmos­phere and to analyse the com­po­si­tion and basic struc­ture of any atmos­phere present using spectroscopy.

The tar­get­ed exo­plan­ets will be between one and three times the size of Earth and are known as ‘super-Earths’ and ‘sub-Nep­tunes’. JWST could trans­form our under­stand­ing of these plan­ets. To be able to detect biosig­na­tures on poten­tial­ly hab­it­able plan­ets, we first need to under­stand the full diver­si­ty of plan­ets that have been dis­cov­ered to date. Super-Earths and sub-Nep­tunes appear to be the most com­mon types of plan­ets in the galaxy, even though we still don’t know what they actu­al­ly are.

The JWST is a ‘titan’, built to trans­form our view of the Uni­verse and to per­form ground-break­ing astron­o­my. It will shed light on the fur­thest reach­es of space ever. But it will also take images to show the world beau­ti­ful objects for the sake of it. These images will kin­dle the imag­i­na­tion and encour­age reflec­tion. What is our Uni­verse? What are we in the midst of all this? And that’s anoth­er rea­son why the JWST is so wonderful.


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