March 2016 – The James Webb Space Telescope: Our New Eyes in the Sky

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in the night sky. Last month, we examined the origin of our nearest cosmic neighbour, the Moon. In this issue, we will discuss the formal successor to Hubble, the James Webb Space Telescope.

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Fig 1. Mock-up of the Webb at South by Southwest in Austin, Texas. The 18 mirror array is able to be seen attached to the large multi-layer sunshield. Credit: NASA

2018 will mark the beginning of a new era in astronomy. NASA will launch the most powerful telescope ever to work in space, the James Webb Space Telescope. Its mission is simple and diverse: to uncover the universe. From galaxy superclusters to possible life on other planets, the Webb will allow us to see beyond the current horizons of astrophysics.

Why Webb?

Named for the second administrator of NASA, who played a key role in the Apollo missions, the Webb will become our new eyes in space. But not just any old eyes. The Webb will be an infrared-optimised telescope, meaning that it will see best the reddish light just beyond our visible spectrum.

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Fig 2. The Webb is optimised to see in the infrared, just beyond the reach of the Hubble (HST). More energetic light (gamma rays, x-rays) are probed by Swift and RXTE. Less energetic light (Microwave, Radio) are probed by WMAP and VLA. Credit: NASA

The decision to opt for an infrared telescope is the culmination of many years of astrophysical research. The Hubble Space Telescope has provided us with breathtaking and groundbreaking science primarily from the visible region of the spectrum. However, leading areas of reasearch such as extrasolar planets, dust, and the far universe are best studied in the infrared. Previous telescopes such as NASA’s Spitzer and ESA’s Hershel, have probed the infrared universe already (although at slightly different wavelengths of light), but the Webb will reach further still.

The coldest parts of the universe are some of the most interesting – and difficult – to study. This includes interstellar gas, brown dwarfs, and extrasolar planets. The Webb will even be able to study our own solar system in unprecedented detail. This work will lead us to better understand the birthplaces of stars, and the compositions of galaxies in terms of their gas and how it is processed over time. The Kepler telescope has already collected a fabulously large sample of planets outside our own solar system, but the Webb hopes to study these planets directly to gain new insight.

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Fig 3. The Hubble Space Telescope snapped this image, the Ultra-Deep Field, which is one of the deepest images of the universe ever taken. All of these points of lights are galaxies. The Webb will be able to see even further. Credit: NASA/HST

If any lessons are to be learned from the past century of astronomy it is that the universe is a very dusty place. This is not the dust under your couch, but rather microscopic particles of silicon and carbon that pervade whole galaxies. However, this dust is also responsible for introducing serious error into astronomical findings from the visible part of the spectrum, potentially leading to horrifically wrong conclusions. By using the Webb to see in the infrared, we will avoid the effects of this dust and see cleanly through it.

The late and great Carl Sagan once said “telescopes are time machine” – and what he meant was that the further away we look, the earlier in the universe we see, as the early universe is receding away from us due to the expansion of space. What all does this mean? The light from these early galaxies is redshifted – i.e. blue light will appear to be redder. The Webb will be able to capture this light and see back to the first million years after the beginning of the universe.

The Design

The Webb will be unlike any space observatory to date. Instead of a single-cast mirror as with Hubble, the Webb will equipped with a segmented mirror array. It will also weigh just half of the Hubble, but  sport a mirror some 6.5 meters wide. This will give Webb about 7x the light collecting power than Hubble!

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Fig 4. The centre mirror array of the Webb in a clean room environment. Rigorous testing is performed on the mirrors prior to assembly and eventual launch in 2018. Credit: NASA

Each of the Webb’s 18 gold-coated mirror segments have been painstakingly arranged on its primarily mirror assembly in February. Prior to that point, the segments underwent rounds of testing, including ultra-cold cyrogenic freezing. The mirrors themselves are made of beryllium, which is able to withstand very cold temperatures without changing shape. If the mirrors were to flex in any way, the visual clarity and resolution of the Webb would be utterly compromised. Once an initial configuration has been set, the mirror segments will be driven into the correct position using sensitive micro-motors. While ground-based telescopes must contend with the image distorting effects caused by the atmosphere, the Webb will have no such worries.

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Fig 5. The sunshield for the Webb consists of fine super-thin layers of reflective material. This structure will unfold to provide thermal shielding for the sensitive mirrors of the telescope. Credit: C. Gunn/NASA

Instead, the Webb faces a new challenge. Due to the nature of infrared observation, the telescope itself must be kept cool if it is to keep from contaminating its observations with its own heat. To do this, the Webb will be deployed with a protective sunshield as large as a tennis court. This shield will be folded twelve times to fit inside of the launch rocket, but more on that later. The shield is made up of polyimide film, coated with aluminium on one side and silicon on the other. This ensures that the light from the Sun is radiated away from the mirror array. However, the shield will be pushed by solar wind, and will therefore tend to spin. To counter this clearly unwanted affect, a flap located at the rear of the main structure will provide stability.

Instrumentation

The Webb will be equipped with an assortment of high-tech instruments to study the light of distant planets, stars, and galaxies. The instruments have been built by groups in several countries, including the UK.

To observe very dim targets such as extrasolar planets, this instrument will be equipped with a coronagraph to finely block the light of any nearby stars to enhance the light of the dimmer planet. It will also allow the Webb to sense the clarity of its images and inform mission control as to how to adjust the mirror array to obtain a focused image.

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Fig 6. The Near InfraRed Camera mock-up showing the complexity of the instruments aboard the Webb. Credit: NASA

To study the universe in detail, astronomers must study the components of the light emitted by the stars, planets, and galaxies. By breaking up the light using various prism or grating-like elements, the light is broken up into a spectrum. The Near InfraRed Spectrometer will be the backbone of this side of the Webb mission. It will be able to handle many objects at once, collecting many thousands of spectra over its projected lifetime.

The Mid-InfraRed Instrument will be the main imaging device for the deep infrared regime. It is also equipped with a spectrometer. A helium gas cooler will help control the temperature of the instrument. It also features a coronagraph to block starlight. One of it’s lead collaborators is Gillian Wright of the UK Astronomy Technology Centre out of Edinburgh!

The Fine Guidance Sensor and Near InfraRed Imager and Slitless Spectrograph is one of the most important instruments aboard the Webb as it will be used to stabilse the steering of the telescope, and is actually two devices coupled in one mounting. The dual imager and spectrograph will cover the infrared range not included with the Mid-Infrared Instrument.

Launching the Webb

The Webb was originally slated for launch in 2011 with a price tag of $1.6 billion (~£1B). Now it is set to launch in 2018 at a cost nearly 8x the original at $8.8 billion (~£6.2B). The launch vehicle will be provided by ESA’s Ariane 5 rocket, which has flown many satellite and space missions previously. Much of the dimensions of the Webb were dictated by the size of the Ariane rocket nose cone.

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Fig 7. The Webb must be folded up to fit inside the capsule of the Ariane 5 rocket. Once in space, the Webb will slowly deploy over several days. Credit: NASA

Once the Webb is launched, it will begin its journey to the Earth-Sun L2 Lagrange point, some 1.5 million kilometres away from Earth. This point is stable due to a strange quirk of gravity, allowing objects to orbit a point in space which contains literally nothing. The L2 point is favourable as it will keep the Webb on the dark side of Earth, allowing for continual observation not available with the Earth orbit of the Hubble. While on the way to L2, the Webb will deploy its segmented mirrors to unveil the full 18 mirror array. The sunshield will also unfold and stretch out to full-size. Situated below the shield will be a communications dish and solar panels.

However, putting the Webb at L2 also means that there is no chance to fix anything once it is launched- we cannot send a manned space vehicle to L2 with our current technology. It must work the first time.

Success with the James Webb Space Telescope has and will require many risks. But to see the first heartbeats of the cosmos, we must be bold.

Ad Astra Per Aspera.

Next Month: A Conversation with Kate Gould, the Next Author of Heavens Above!

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