The $10 billion James Webb Space Telescope—which debuts its first batch of show-off images this week—is always talked-up as the successor to the Hubble Space Telescope.
It is not.
They are two completely different telescopes in two completely different places. Yes, Webb has a 6.6 meter mirror compared to Hubble’s 2.4-meter mirror—and so six times more collecting power—but we shouldn’t compare them. While the latter looks at the Universe mainly in visible light, Webb is an infrared telescope. They see the universe differently and the data they collect couldn’t be more different.
Here’s exactly how Webb and Hubble differ—and why the shiny new space telescope will hugely benefit from the work of its ageing colleague:
1. Hubble orbits Earth while Webb orbits the Sun
Unlike Hubble, which orbits Earth and was visited by NASA astronauts for fixes and upgrades, Webb is going a million miles away to Lagrange Point 2—so Webb almost certainly can’t be fixed if anything goes wrong. It’s there primarily so it can operate 24/7 and at as low a temperature as -385ºF-233ºC so it can detect faint heat signals.
2. Hubble can take longer, wider observations than Webb
Webb is in demand. If you’re an astronomer wanting to take long exposure images across vast swathes of space then you’re going to have a hard time convincing anyone that your project is worth spending hundreds of hours on. For example, Mowla’s project—called 3D-DASH, a high-resolution near-infrared survey of galaxies—took 250 hours of Hubble’s time.
With Webb coming online there’s slightly less demand on Hubble’s time, but it also has a wider field of view. “The 3D-DASH program extends Hubble’s legacy in wide-area imaging so we can begin to unravel the mysteries of the galaxies beyond our own,” said Lamiya Mowla, Dunlap Fellow at the Faculty of Arts & Science’s Dunlap Institute for Astronomy & Astrophysics at the University of Toronto, and a member of several James Webb Space Telescope (JWST) teams.
Last month she was co-author on a published paper in The Astrophysical Journal about 3D-DASH. A map of the star-forming regions of the universe that will allow researchers to find rare objects and targets for follow-up observations with Webb. “This image will be a shallow treasure map for Webb to find targets that it can go back and observe deep,” said Mowla.
2. Hubble and Webb see in different wavelengths
While Hubble deals in ultraviolet and visible wavelengths of light, Webb is equipped to detect longer infrared wavelengths at an unprecedented resolution.
“One of the key differences that will keep Hubble relevant is that Hubble absorbs from ultraviolet to the near infrared, whereas Webb observes from near infrared, going slightly towards mid infrared, which means that it does not observe in the ultraviolet and optical at all,” said Mowla. “So if you want to observe something that requires ultraviolet and optical data, then we will still need to go back to Hubble to achieve that.”
UV and optical are for looking at younger parts of the universe, but within in a particular galaxy the young stars are are extremely bright and emit light only in in the shorter wavelengths. “We need the whole spectrum to get a full picture—the both the young and the old part of the galaxy—to see its history and its present so that we can form a picture of what will happen next,” said Mowla.
Most of the images that Mowla will take using Webb of ancient galaxies will use Hubble ancillary data. “All of our targets were chosen based on Hubble observations—and now Webb will add more information to it,” said Mowla. “A Hubble is very, very crucial.”
4. Webb is a successor to Spitzer , not Hubble
Webb is a lot more sensitive to infrared light than Hubble, but there is an overlap. “Webb is about three times more high resolution and about 10 times more sensitive to infrared so it can do what Hubble has been able to do for a long time, but more quickly,” said Mowla. However, it’s not Hubble, but NASA’s Spitzer Space Telescope—in orbit from 2003 until 2022 when it ran out of propellant—that Webb is effectively taking over from.
“It’s mind-boggling how well Webb will do in terms of resolution compared to Spitzer,” said Mowla. “Things that we could see previously as just blobs on the sky will now show up as actual galaxies with spiral arms and all the features that we are used to seeing in our nearby Universe.”
Webb will allow astronomers not only to see galaxies as they existed 10 to 12 billion years ago, but also features within them.
5. Hubble is still needed to make new observations
If it’s already done its work then why do we need Hubble anymore? “There are tons of different things that that we’ll find with Webb that we just haven’t identified before,” said Mowla. “We will want to go back and look at it again with Hubble, maybe deeper for a longer period of time.”
“Hubble is going to remain crucial.”
Wishing you clear skies and wide eyes.