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Explained: NASA’s flagship telescope, and its successor

NASA says Webb is not Hubble's replacement — rather, its successor whose science goals were motivated by the results from Hubble.

By: Express News Service | New Delhi |
Updated: December 23, 2021 7:44:54 am
James Webb Space Telescope (Image source:

NASA has announced the launch of the James Webb Space Telescope (JWST) at 7.20 am EST (5.50 pm India time) on December 24. Webb, the world’s premier space science observatory, will succeed the Hubble Space Telescope, NASA’s flagship telescope that has been in service for more than three decades now.

NASA says Webb is not Hubble’s replacement — rather, its successor whose science goals were motivated by the results from Hubble. Webb will primarily study the universe in the infrared, while Hubble looks at it mainly at optical and ultraviolet wavelengths. Webb’s mirror is much larger than Hubble’s; it can, therefore, look farther back into time than Hubble. Also, Hubble is in a much closer orbit around Earth than Webb will be.

WAVELENGTH: Webb’s four instruments to capture images and spectra of astronomical objects will provide wavelength coverage from 0.6 to 28 microns (the infrared part of the electromagnetic spectrum is from about 0.75 microns to a few hundred microns); the instruments on Hubble can observe mainly in the ultraviolet and visible parts of the spectrum from 0.1 to 0.8 microns. Infrared observations are important because light at this wavelength can penetrate the dust that shrouds newly formed stars and planets, and make them visible.

SIZE: Webb’s primary mirror is approximately 6.5 metres in diameter, giving it a significantly larger collecting area than the mirrors of the current generation of space telescopes. Hubble’s mirror has a diameter of 2.4 metres, which means Webb’s collecting area is around 6.25 times that of Hubble’s. Webb will cover more than ~15 times the field of view covered by Hubble’s NICMOS camera. Webb’s sunshield is about 22 m by 12 m, a little less than the size of a tennis court.

ORBIT: Hubble orbits the Earth at an altitude of ~570 km. Webb will not orbit the Earth, instead it will sit at the Earth-Sun L2 Lagrange point, 1.5 million km away. This means that Webb will orbit the Sun along with the Earth, but will stay fixed at the same spot in relation to the Earth and the Sun. At the L2 point, Webb’s solar shield will block the light from the Sun, Earth, and Moon, which will help it stay cool — important for an infrared telescope.

HOW FAR: Because light takes time to travel, the farther away an object is, the farther back in time we are looking. Thus, while Hubble can see the equivalent of “toddler galaxies”, Webb will be able to see “baby galaxies”. This is also because Webb is an infrared telescope, and can see distant objects which are very dim at visible wavelengths of light.

All information: NASA

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