James Webb Telescope

On 18 December a Ariane 5 rocket will take off from the  Guiana Space Centre in French Guiana, aboard it will be the $12 billion James Webb Space Telescope (JWST) – the most expensive scientific instrument ever launched into space. In this post I’ll talk about this remarkable telescope and the mission to deploy it.

Who was James Webb?

The telescope is named after James E. Webb (1906-1992). Interestingly, although many  telescopes are named after famous scientists,  Webb wasn’t a scientist. He was the NASA administrator (in effect its CEO) from February 1961 to October 1968.  This was a very exciting time for the organisation. Webb oversaw the development of the crewed space programme from the first American human space flight, Alan B. Shepard’s short suborbital 15-minute hop into space in a Mercury capsule in May 1961, until the dawn of the Apollo programme. He left NASA nine months before Apollo 11’s historic landing on the surface of the Moon.

Webb’s tenure in office was an important time for unmanned space exploration as well. In 1962, Mariner 2 became the first space probe to pass another planet when it went past Venus. In 1965, Mariner 4 took the famous images of Mars which showed its cratered surface.

Image of Mars taken in 1965 by Mariner 4

Why do we need to put a telescope in space?

One question often asked is:

why is it necessary to spend a vast amount of money to put a large telescope into space when it is far simpler and cheaper to build the telescope on the ground?

There are a number of reasons for this. One is if we look at the entire electromagnetic spectrum which runs from gamma rays, which have wavelengths less than 10^-11 metres, through to the longest radio waves, which have wavelengths measured in kilometres, most regions of the spectrum are blocked by the Earth’s atmosphere.

How the atmospheric opacity varies with wavelength. For much of the electromagnetic spectrum the opacity is 100% – meaning that no radiation reaches the Earth’s surface. For those unfamiliar with these units: one nanometre (nm) = one billionth of a metre and one micron (μm) = one millionth of a metre

The two windows, where electromagnetic radiation gets through to the Earth’s surface with relatively little attenuation are as follows.

1. Visible light which has a narrow range of wavelengths between 380 and 750 nanometres and a small range of UV and infrared either side of this. However,  if we consider the infrared region of the spectrum where the JWST will be operating, which has wavelengths up to 28 microns, there is significant attenuation by the Earth’s atmosphere
2. Radio waves  having  wavelengths between 2 cm and 10 m. This is a subset of the entire radio wave spectrum and most Earth-based radio astronomy takes place within this range.

Another good reason for putting a telescope into space is that all astronomical images taken by ground-based telescopes are affected to some degree by turbulence in the Earth’s atmosphere. This is known as seeing and causes a blurring of the image even on the largest and most expensive telescopes. .

A further factor is that if we want to look at  emissions at a single wavelength, i.e. a spectral line  caused by the presence of compounds such as water, methane or carbon dioxide in planets’ atmospheres, these compounds also exist in the Earth’s atmosphere which complicates the analysi

Why is the JWST an infrared telescope? Rather than visible light?

Astronomers will use the JWST to observe over a wavelength range from 0.6 microns, at the red end of the visible light range, up to 28 microns which is well into the infrared. There are three main reasons for this. Firstly, very distant objects such as galaxies are moving away from us at a very high speed. This means the wavelength of the visible light emitted from these objects is increased (or red-shifted) due to the expansion of the Universe into the infrared region of the spectrum. Secondly, objects which are much cooler than stars such as: debris discs, planets, clouds of gas and  objects called brown dwarfs (which are intermediate in mass between the heaviest gas giant planets and the coolest stars) emit mainly in the infrared. Thirdly, taking infrared spectra and observing through infrared filters (which only allow a small range of wavelengths to get through) can tell us a great deal about the chemical composition of astronomical objects. As mentioned before, it is more difficult to study infrared spectral line emissions with ground-based astronomy  because of contamination  by emissions from Earth.

The JWST is often described as a successor to the famous Hubble Space Telescope (shown below) which has been operational since 1990. This isn’t quite true. The primary capabilities of the Hubble Space Telescope are to take observations at wavelengths between  0.1 microns and 0.8 microns, which means that it works at the near ultraviolet and visible wavelength ranges, with only a slight overlap into the infrared ( source https://www.jwst.nasa.gov/content/about/comparisonWebbVsHubble.html). Also, when the JWST starts scientific operations the Hubble Space Telescope will not be switched off. It could well continue operating for another ten years.

Costs of the JWST

One subject for which the JWST has received much negative publicity is its enormous costs. Like many large science projects (and indeed large projects which don’t have much to do with science ) built on innovative technology, the costs have turned out to be much larger than the original estimates and the timescales a lot longer.  When the JWST was first considered back in 1996 the original ball park estimates were that it would cost around $1 billion and would launch in 2007 .  When the development of the telescope started there were numerous slippages and the estimated costs kept escalating. At one stage it looked like that the JWST might be cancelled altogether. However, NASA were committed to completing the project because the scientific value of such a unique instrument was so high. The current costs of the telescope are given below. They also include the first five years of operation.

All expenditure from earlier years has been adjusted upwards for inflation to be  in 2021 dollars.

These costs make it the most expensive astronomical intruments ever constructed. One obvious fact is that, although the JWST is often cited as an example of international cooperation, 92% of the cost is borne by the US taxpayer.

Deployment of the JWST

The diagram below shows how the JWST will appear when it is in operation.

The most noticeable component of the structure (other than the telescope itself) is the sunshield. This measures 22 metres by 10 metres. It would be more precise to call it “the five sunshields” because it has five separate layers to maximise cooling. Each layer is made from a very thin film of Kapton E (a high-performance polymer) coated with aluminium. Because of its sheer size the sunshield needs to be folded up before launch and, via a complex set of manoeuvres, unfolded in space.

The cumulative effect of the five layers of sunshield is to keep the telescope and the instruments attached to it cooled down to a temperature of -233 ^oC. This cooling is  essential because objects around room temperature emit a great deal of infrared radiation at the wavelengths which the JWST will be observing.

The telescope itself has three mirrors in a design known as a three-mirror anastigmat. This is the same design as the LSST being built at the Vera Rubin observatory in Chile .The primary mirror is 6.5 metres in diameter but, because of its sheer size, it consists of 18 separate mirrors. A single 6.5 metre diameter mirror would be difficult to construct and, in any case, would be too large to put into the Ariane launcher. The primary mirrors are made of beryllium, which is one of the lightest metals, and are covered in a thin film of gold, which gives them a yellowish colour. The secondary mirror is 0.74 metres in diameter. To save weight and simplify deployment, unlike the Hubble Space telescope, the JWST has no “telescope tube”. The secondary mirror is held in place by three poles. The telescope will be deployed in a halo orbit around the Earth- Sun L2 Lagrange point. At this location the sunshield will shield it from radiation from both the Sun and the Earth.

An object positioned exactly at the L2 point will always remain on the opposite side of the Earth to the Sun.

In theory, if the Earth and the Sun were the only bodies in the Solar System, an object placed exactly at the L2 Lagrange point would take precisely  one year to orbit the Sun and would always remain on the opposite side of the Earth from the Sun on an imaginary straight line joining the centre of the Earth and the centre of the Sun.

However,  the L2 Lagrange point  isn’t a stable equilibrium. If an object is placed there any  slight perturbation of the object’s position (for example by the approach of another planet) will cause  it to drift away into a different orbit. So, rather than being placed in orbit around the Sun at the L2 Lagrange point,  the JWST is placed in  rather complex trajectory in which the JWST  is in orbit around the Sun, but also appears to be an orbit around the L2. For more details see the simple video below.

This trajectory (known as a halo orbit) is relatively stable. Even so, a small amount of fuel will be  needed to adjust the JWST’s position so that it remains in the halo orbit. This propellant will be a limiting factor of the JWST lifetime. The amount should be sufficient to give the spacecraft a lifetime of more than ten years. When the fuel has been used up the JWST will drift into an elliptical orbit around the Sun and the sunshield will no longer shield the spacecraft from infrared radiation from Earth .

And Finally…

Fingers crossed that the telescope launch is successful on 18 December and that deployment of the telescope goes to plan. It will be a nervous few months for many astronomers! Hopefully, we’ll see some great science from observations taken by the JWST over the coming decade. The image below shows a  Hubble ultradeep field long exposure photograph taken with 22 days of observation. It will be interesting to see what the infrared images taken by the JWST look like!