NASA, ESA and the Canadian Space Agency prefer that people think of the James Webb Space Telescope (JWST) as the successor to the Hubble Space Telescope, not its replacement. Why is that, and what’s the difference between the two?
While the JWST is much larger in size, its mission and capabilities are slightly different. As well, let’s not forget that even when it is commissioned six months after launch, that the Hubble telescope will still be continuing its mission.
Here’s the short answer from NASA on the basic differences between the telescopes.
“Hubble’s science pushed us to look to longer wavelengths to ‘go beyond’ what Hubble has already done. In particular, more distant objects are more highly redshifted, and their light is pushed from the UV and optical into the near-infrared. Thus observations of these distant objects (like the first galaxies formed in the Universe, for example) requires an infrared telescope.”
“This is the other reason that Webb is not a replacement for Hubble; its capabilities are not identical. Webb will primarily look at the Universe in the infrared, while Hubble studies it primarily at optical and ultraviolet wavelengths (though it has some infrared capability). Webb also has a much bigger mirror than Hubble. This larger light collecting area means that Webb can peer farther back into time than Hubble is capable of doing. Hubble is in a very close orbit around the earth, while Webb will be 1.5 million kilometers (km) away at the second Lagrange (L2) point.”
The Canadian Space Agency has also chipped in with its own explanation in the following YouTube video.