The James Webb Space Telescope (JWST) is the most advanced and most expensive telescope ever built.

It has often been referred to as the 'successor' to the Hubble Space Telescope, which was launched in 1990. But how similar are they really?

Find out how the James Webb Space Telescope and Hubble Space Telescope compare with astronomers from the Royal Observatory Greenwich.

What type of light do the telescopes see?

Hubble

JWST

Visible and ultraviolet

Hubble’s main focus is on visible and ultraviolet light. Its instruments can observe a small portion of the infrared spectrum from 0.8 to 2.5 microns, but not to the extent that James Webb can. Instead it focuses its unique ultraviolet (0.1 to 0.4 micron) capabilities on work that cannot be done from the ground and its visible (0.4 to 0.8 micron) light instruments on producing the high resolution images we are most familiar with

Red and infrared

JWST has been designed to focus on the infrared part of the spectrum from 0.6 (red light) to 28 microns (infrared). This means it won’t be able to see in ultraviolet light like Hubble, but will be able to focus on infrared bright objects like extremely distant galaxies

Light travels in a range of frequencies along the electromagnetic spectrum. Our eyes have evolved to detect the band of the spectrum which is known as ‘visible light’, which is unsurprising given that our atmosphere blocks out many of the other wavelengths.

However, there are many other forms of light we cannot see, both inside and outside our atmosphere.

 

Electromagnetic spectrum © NASA.

Infrared light has a longer wavelength and can pass through objects in space that visible light is blocked by, such as gas and dust. This is why images taken using telescopes which detect infrared frequencies can pick out objects beyond these clouds, and appear clearer than those taken using other telescopes.

 

Comparison views of Mystic Mountain © NASA/ESA/M. Livio & Hubble 20th Anniversary Team
Comparison views of Mystic Mountain (© NASA/ESA/M. Livio & Hubble 20th Anniversary Team)

So is the James Webb Space Telescope replacing the Hubble Space Telescope?

Because JWST doesn't cover the same types of light that Hubble is capable of, it isn't truly 'replacing' the same capabilities that Hubble has.

However, while we will lose out on the ability to see in ultraviolet light in the same way that Hubble did, by expanding the range of wavelengths out to infrared light, JWST will provide access to a part of the spectrum that Hubble never had.

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How big are the Hubble and James Webb Space Telescopes?

Hubble

James Webb

Hubble is 13.2 metres (43.5 ft.) long and its maximum diameter is 4.2 metres (14 ft.) It is about the size of a large truck.

Hubble's aperture (the part capable of receiving light) is 2.4 metres across

JWST sunshield is about 22 metres by 12 metres (69.5 ft x 46.5 ft). It's about half as big as a 737 aircraft. The sunshield is about the size of a tennis court.

JWST's aperture is 6.5 metres across

 

JWST will have a vast sunshield used to help keep the telescope cool. This is important for all space telescopes but is particularly true for infrared telescopes like JWST as 'warm' objects radiate lots of infrared light.

If the telescope itself isn't kept cool, then the telescope risks blinding itself to the light of any object it is attempting to observe.

The main improvement here though is the telescope aperture.

This is effectively the size of the hole at the end of the telescope, or, in the case of telescopes like these, the size of the mirror that is used to collect the light. It's the equivalent of the pupil at the centre of our eye, the dark 'hole' that lets in light.

The larger the aperture, the more light the telescope can gather in one go and so the fainter an object it can see.

Hubble, with its 2.4 metre aperture, can see objects at least 60,000 times fainter than the human eye (which is then greatly extended by using cameras to take long exposure photographs).

With a mirror almost three times wider, JWST will be able to see objects almost nine times fainter than Hubble, allowing us to peer even further into space.

What are the distances between the telescopes and Earth?

Hubble

JWST

The Hubble Space Telescope orbits around the Earth at an altitude of ~570km

JWST will not actually orbit the Earth. Instead it will sit at the Earth-Sun L2 Lagrange point. This point is around 1.5 million kilometres further away from the Sun than the Earth is

Space-based telescopes have a huge advantage over ground-based ones. By being above the atmosphere they don't have to peer through the shifting air to see into deep space, granting them a clearer view than most ground-based telescopes can achieve. They are also unaffected by the weather, which can be a big problem for astronomers.

However, being in space makes fixing things that go wrong considerably more difficult. Hubble famously had a small flaw in its mirror upon launch that required a mission into space to fix.

JWST will have no such rescue. At 1.5 million kilometres away, far further than any human has ever travelled, if something goes wrong it won't be possible to head into space and fix it.

It is being placed in such a remote place for several reasons. It keeps the telescope far from the reflected radiation of the Earth, one of many choices designed to keep this highly sensitive telescope cool. It will also be in a place where the gravity of the Sun and Earth work together, making it easier to keep the satellite in place. 

How far back in time can JWST see?

The further away an object is, the further back in time we are looking. This is because of the time it takes light to travel from the object to us.

With JWST's larger mirror, it will be able to see almost the whole way back to the beginning of the Universe, around 13.7 billion years ago.

With its ability to view the Universe in longer wavelength infrared light, JWST will be capable of seeing some of the most distant galaxies in our Universe, certainly with more ease than than the visible/ultraviolet light view of Hubble.

This is because light from distant objects is stretched out by the expansion of our Universe - an effect known as redshift - pushing the light out of the visible range and into infrared.

Why has JWST been delayed so long?

Space projects often take longer than expected, but JWST has been more unlucky than most.

It was originally planned to be launched in 2007, but a major redesign, escalating costs and delays led to it being pushed back to around 2018. However, the testing period from 2016 onwards also suffered from long delays, and a further delay was imparted by the global pandemic of 2020.

The launch had been scheduled for 18 December 2021, but this date was pushed back following an 'incident' during launch preparations.

The telescope finally launched on Christmas Day 2021 at 12.20pm GMT (7.20am EST). Over the next six months, the telescope will undergo a series of complex deployment operations, including the unfurling of its huge sunshield. Soon however, it will be ready to deliver its first images and begin a new era in astronomy.

Watch the successful launch back via the NASA live stream below.

James Webb Space Telescope launch

Launch date: 25 December 2021

Launch time: 12.20pm GMT (7.20am EST)

Launch location: Europe's Spaceport, French Guiana