Webb: Hubble's successor

Since its launch in , the Hubble Space Telescope has had a great impact on our understanding of the universe and the public's interest in astronomy. It has allowed us to peer further into space than ever before, take breathtakingly detailed images of planets and galaxies, and solve many of the universe's great mysteries.

The iconic Butterfly Nebula, taken by the Hubble Space Telescope. (Credit: NASA/HST)

The James Webb Space Telescope will be Hubble's successor, but not its replacement. The two missions have a planned overlap and will work together on new discoveries. Webb will build on Hubble's impressive legacy by helping humanity peer even deeper into the universe.

There are, however, many differences between these two space telescopes. Webb will have unique capabilities that will allow it to perform science Hubble is not able to do.

A comparison of the Hubble Space Telescope and the James Webb Space Telescope. Webb's larger mirror will allow it to capture more light and see farther into the cosmos. (Credit: European Space Agency (ESA)/M. Kornmesser)

Hubble Space Telescope James Webb Space Telescope
Telescope size About the size of a school bus About the size of a tennis court due to its large sunshield; will need to be folded up when it launches on an Ariane 5 rocket
Mirror size A single 2.4 m-wide mirror 18 hexagonal mirror segments, for a total width of 6.5 m
Light observed Ultraviolet, visible and near-infrared light Near- and mid-infrared light
Location Low Earth orbit, at an altitude of 547 km 1.5 million km away from Earth (at a point in space known as Lagrange 2)
Maintenance Due to its location, Hubble can be repaired and upgraded while in orbit. Astronauts famously corrected Hubble's mirror in using Canadarm, Canada's robotic arm on the American Space Shuttle. Webb will be too far from Earth to repair, which is why it has gone through unprecedented testing.
Mission lifetime Hubble was launched in and will remain operational as long as its instruments are functioning. The minimum expected lifetime of Webb is 5 years, but it may go beyond 10 years. This will depend on how long its propellant (needed to keep Webb stable in its orbit) will last.

The top row shows Hubble's famous Ultra Deep Field of thousands of distant galaxies, and a larger image of a single galaxy. The bottom row shows how much more detailed such images are expected to look when captured by Webb. (Credit: Space Telescope Science Institute (STScI))

Why study infrared light?

The Orion Nebula can be seen in visible (left) and infrared (right) light as observed by the Hubble Space Telescope. Observing infrared light allows scientists to penetrate the dust and study otherwise unseen parts of our universe. (Credit NASA/ESA/Hubble Heritage Team (STScI/AURA))

The light we can see with our eyes is composed of many wavelengths, each with its own colour—like those of a rainbow. This is called visible light and is produced by objects like our Sun and artificial lighting in our homes. However, there are many other kinds of light, such as infrared light, that our eyes cannot see. Certain objects, like cold stars and planets, shine most brightly in the infrared. Astronomers must study the infrared universe to understand these intriguing objects.

While Hubble focuses mainly on light that is visible to the human eye, Webb's four scientific instruments were specifically designed to capture infrared light. Webb will be able to peer through cosmic dust to study colder or very distant objects.

Some of the advantages of infrared observations:

Have you ever noticed the sound of an ambulance changing as it drives past you? It is higher pitched as it moves towards you, and lower pitched as it drives away. This effect is known at the Doppler shift: sound waves produced by an object moving towards you are compressed, while they are stretched as the waves move away from you. This same concept applies to light!

Because all galaxies are moving away from us due to the expansion of the universe, the light they produce is being stretched into longer wavelengths. This phenomenon is called cosmological redshift – "red" because stretched light appears redder!

Redshifted light from distant galaxies - infographic

As light travels through our expanding universe, it is stretched into longer wavelengths, meaning that it is moving along the spectrum of light to appear more red. This is known as "redshift." (Credits: STScI, Canadian Space Agency)

Redshifted light from distant galaxies - Text version

Earth is represented on the left of the image. Pointed away from Earth is the James Webb Space Telescope, which is capturing a wave of light emitted from distant galaxies on the right of the image, about a billion light-years away. As ultraviolet light travels from the galaxies to the telescope, the wavelength stretches, becoming less blue and more red.

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