Revision summary
JWST launched on 25 December 2021 on Ariane 5. It is an infrared telescope at Sun–Earth L2 with a ~6.5 m gold-coated beryllium mirror and a five-layer sunshield. Hubble is smaller, in low Earth orbit, and strong in visible/UV; Spitzer was a smaller infrared precursor. Goals are first light, galaxy assembly, star-planet formation in dust, and exoplanet spectroscopy. Benefits are astrophysics, technology, and complementarity with Hubble and ground observatories. Fuel at L2 limits life.
Model answer
Copper italics in this answer — like this — are the key facts. Each one is unpacked in the Facts & figures rail.
Introduction
The James Webb Space Telescope (JWST) lifted off on 25 December 2021 on an Ariane 5 from French Guiana. It is the successor in ambition to Hubble, not a copy. Hubble looks mainly in visible and ultraviolet from low Earth orbit. Webb looks in the infrared from a distant cold perch, with a much larger mirror, to see the first galaxies, nearby exoplanet atmospheres, and star nurseries inside dust.
Body
Unique features versus predecessors
- Wavelength: optimised for near- and mid-infrared. Dust-obscured star formation and highly redshifted early galaxies are faint in Hubble’s visible band and bright to Webb.
- Mirror: a deployable beryllium primary about 6.5 metres across (segmented, gold-coated), far larger than Hubble’s 2.4-metre glass, so collecting area and resolution at infrared wavelengths jump.
- Orbit: Sun–Earth Lagrange point L2, about 1.5 million km out, not low Earth orbit. Hubble can be serviced; Webb was designed not to be. L2 lets a huge sunshield keep the optics cold.
- Sunshield: a five-layer tennis-court-scale shade so the telescope stays at cryogenic temperature; mid-infrared instruments need that cold.
- Deployment: folded for launch, then unfolded in space (mirror, shield, tower)—a complexity Hubble did not attempt.
- Partnership: NASA lead with ESA and CSA; Hubble was also NASA–ESA, but Webb’s infrared instrument suite (NIRCam, NIRSpec, MIRI, FGS/NIRISS) is a new generation.
- Spitzer was an infrared predecessor but smaller and warmer in later years; Webb is the leap in aperture and L2 cooling.
Key goals
- First light and reionisation: galaxies and quasars in the early universe, when the first stars lit up.
- Assembly of galaxies over cosmic time.
- Star and planet formation behind dust clouds.
- Exoplanet atmospheres by transit spectroscopy: water, carbon dioxide, and other molecules.
- Solar-system infrared studies (objects too cool or dusty for Hubble’s best band).
Potential benefits
- Science: rewrite galaxy-formation timelines; characterise potentially habitable-zone atmospheres (biosignature claims will still need extreme caution).
- Technology spin-off: cryogenics, lightweight optics, deployable structures, detectors.
- Inspiration and STEM, and a public window on origins questions.
- Complementarity: Hubble still does UV/visible; ground ELTs and ALMA fill other bands. Webb does not retire them.
Caution
- L2 is not a place for astronaut repair. Mission life is limited by station-keeping fuel and instrument health, even if early fuel estimates were generous.
Flow diagram
Conclusion
JWST, launched 25 December 2021, is a cold infrared observatory at L2 with a 6.5-metre deployable mirror—unlike Hubble’s smaller visible/UV telescope in low orbit. Its goals are first galaxies, dusty birth sites, and exoplanet airs. Benefits are knowledge, detectors, and a complement to Hubble, not a replacement of every wavelength.
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Students also ask
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What is the basic principle behind vaccine development? How do vaccines work? What approaches were adopted by the Indian vaccine manufacturers to produce COVID-19 vaccines ?
Next question on this syllabus topic (2022 · Q16). View answer →
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Did Webb replace Hubble?
No. Hubble still uniquely covers UV and much visible work. Webb opens deep infrared.
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Why L2 instead of low Earth orbit?
To keep the telescope behind a sunshield, cold and stable for infrared, away from Earth’s heat and light.
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