A step forward for astronomical observation.
After many postponements and years of delay on the initial schedule, the JAMES WEBB telescope is heading to space on board an Ariane rocket from the Kourou Space Centre in French Guiana.
James Webb, the long-awaited replacement of HUBBLE
In order to better understand to what extent the JWST contributed to technological development, we have to focus on its predecessor, HUBBLE.
In 1983, the American congress votes for the implementation of the Large Space Telescope, allocating the funds required for its construction. The LST takes the name of a renowned astronomer, Edwin Hubble (1889-1953).
Launched in 1990, Hubble is a massive telescope: its primary mirror measures 2.4 meters of diameter. The telescope weighs 11 tons and is composed of various instruments such as cameras (Wide Field Camera 3, Advanced Camera for Surveys or the camera-spectrometer Space Telescope Imaging Spectrograph), or spectrometers (Near Infrared Camera and Multi-Object Spectrometer for infra-red light and Cosmic Origins Spectrograph for the ultraviolet light).
This equipment makes the HUBBLE telescope capable of providing highly-detailed images of the space objects it observes.
At that time, it was a huge step forward. Indeed, as the telescope was placed in orbit, observations were no longer submitted to light and atmospheric pollution.
What are the changes brought by JWST?
James Webb is 100 times more powerful than Hubble. It can observe further and more accurately than any other observation instrument ever used before.
Unlike HUBBLE, JWST will not be put into orbit, but will join the second Lagrange point (P2), located 1,500,000 kilometres away from the Earth in the opposite direction of the Sun.
It is a downside if potential repair work is needed. Hubble is only 570 km away from the Earth.
As it is a short distance, the Hubble telescope benefited from many repairs during its 30 years of good and loyal service. Thanks to these repairs, the telescope could stay operational after 2005, which was the deadline year that has been initially given for its use.
Let’s take a closer look at the observation instruments. Hubble and James may be both telescopes, they were not developed to complete the same missions.
Hubble makes observations in the spectrum of visible light (visible spectrum), even if other instruments were added throughout its service life. In particular, in 2009, an instrument to study the radiation field of the near-infrared range was added.
James Webb, on the other hand, was precisely made to observe space in the field of infrared light, invisible to the human eye.
With a longer wavelength coverage and a substantially improved sensitivity, JWST allows to observe further from the beginning of times and to study the formation of first galaxies we could not observe. It also allows to look inside the dust clouds where stars and planetary systems form.
By taking a look at the weight and the size of each telescope, one may be surprised to see that the JWST mirror is less heavy than the one Hubble has, while it is bigger.
Since 1990, which is the year Hubble was created, technology has evolved tremendously.
The weight saving can be explained by the choice of material used to make the mirror. Indeed, Hubble is equipped with a thick glass mirror, whereas JWST’s mirror is made of beryllium divided in 18 light and fine pieces assembled together.
Beryllium is a light metal extremely resistant to any deformation in a very large temperature range. It is an excellent non-magnetic conductor. It is very dangerous to manipulate and breathe beryllium in its pure form. That is why is it one of its oxides which is used: a fine powder called O-30.
The mirrors are coated with a layer of gold to ensure their good reflectivity. This coating has to be thick enough to cover the entire surface of the mirror, but also fine enough to not deform the mirrors when changes in temperature occur.
The shape of the mirror has to be extremely precise for both telescopes (any change from the “ideal” shape of the mirror has to be thinner than the thickness of a human hair).
A risky operation
Send a telescope into space is challenging.
Unlike its predecessor Hubble, which was put into orbit by the space shuttle Discovery, JWST will be send by an Ariane 5 rocket, placed in the rocket fairing.
It is a risky operation indeed, as there is only one JWST.
This project cost 10 billion dollars. It is a cutting-edge telescope, probably one of the most complex instrument ever developed by mankind.
The implementation should last 3 weeks, with more than 180 operations required.
The pressure on NASA teams is huge. The telescope being located 1,500,000 million kilometres away from the Earth, it is inconceivable to resort to human assistance.
Our thoughts go to the Hubble telescope, which once in orbit produced blurry images in 1990.
After some human assistance, 5 spacewalks, a 50 % chance of success and many sleepless nights, everything fell back into place in 1993.
That is also why the lifespan of JWST will be much less than Hubble’s lifespan.
Initially supposed to last for 15 years, thanks to repairs, and probably some luck, Hubble is still operational 30 years after its launch.
To meet the set scientific objectives, JWST was designed to be operational for at least 5 years.