For the UK Space Agency’s Magazine, Space:UK - Issue 47
Due for launch next year, the giant James Webb Space Telescope will give us a new insight into the Cosmos. For the UK team leading one of the key instruments, the tension is building.
“I’ll be terrified when it launches”, says Gillian Wright. “I’ve spent years of my career working to make this thing happen.”
Wright is leading the development of a crucial instrument for the James Webb Space Telescope (JWST). And the pressure is on.
“It all comes together at the moment of launch,” she says. “It has to be right first time.”
The JWST is the successor to Hubble and, after almost 20 years of construction, is now undergoing a final round of testing before beginning its long-awaited mission in October 2018. The new telescope’s exceptional size, sensitivity and precision instruments will be used to unravel many of the secrets of our universe – from the formation of the earliest galaxies to the detection of atmospheres on planets outside our solar system. Before that can happen, however, scientists and engineers need to fold, like a giant origami model, the 6.5 metre high observatory and its tennis court sized sunshield into an Ariane 5 rocket and blast it more than one and a half million kilometres into space.
Time machine
The JWST is being led by NASA, with significant contributions from the ESA and the Canadian Space Agency. It is designed to study every phase of our cosmic history and will be seven times more powerful than Hubble which, during its 26 years in orbit, has discovered billions of galaxies, stars and planets.
NASA describes the new space telescope as a “powerful time machine with infrared vision”. A “time machine” because, even travelling at the speed of light, some of the electromagnetic radiation that reaches the telescope will have originated billions of years ago, at the very dawn of time.
One of the new capabilities of the JWST is its infrared vision. This will enable the space telescope to detect material through previously impenetrable clouds of cosmic dust. Peering into these clouds should give us the answer to one of astronomy’s biggest mysteries: what were the first luminous objects to form after the Big Bang?
The instrument that will identify these earliest galaxies is the Mid-Infrared Instrument (MIRI). It is fitted with a camera so sensitive it would be able to detect a candle flame on one of Jupiter’s moons.
MIRI is a UK-led project, in partnership with a European Consortium and NASA’s Jet Propulsion Laboratory, and is the result of decades of work headed by Wright, who is also the director of the UK’s Astronomy Technology Centre.
The instrument’s other primary objective will be to reveal, for the first time, planets orbiting stars in other solar systems. “We’ve found lots of these kinds of planets but we don’t know very much about them,” explains Wright. “MIRI offers us the unique opportunity to study them.”
“For the first time ever we’ll have direct images of the planets and we’ll also be able to take spectra,” she adds. “We can look at what the planet is made up of by looking at their chemical signatures from their light.”
Much like shading your eyes from the sun, MIRI will be able to block out star light to examine these planets. “We make a spot that is exactly the size of the image of the star and that stops the light from getting to our detectors,” explains Wright. “You don’t see so much of the light from the star so it’s easier to see the light coming from the planets.”
MIRI is one of four all-important instruments at the heart of the telescope. The others are the Near Infrared Camera (NIRCAM), the Near-Infrared Spectrograph (NIRSpec) and the Fine Guidance Sensor (FGS). Together, they will be able to reveal the universe in a whole new level of detail.
Size Matters
As well as sophisticated instruments, the other thing you need if you want to peer into galaxies 13.5 billions of light years away, is a mirror. A larger mirror will gather more light than a smaller one, much like a bucket will collect more rainwater compared to a teacup.
The primary mirror for the JWST is 6.5 metres in diameter and consists of 18 hexagonal segments made of goldcoated beryllium. Once launched, these segments will unfold and piece together as one immense mirror.
Another engineering feat for the mission is the JWST’s sunshield. Northrop Grumman in Redondo Beach, California, has designed a sunshield the size of a tennis court to shadow the telescope’s instruments to prevent any background heat from interfering with their observations.
The sunshade is made up of five layers of a flexible insulating material called Kapton. The temperature difference between the two sides of the shield is more than 300°C. The sun-facing side has solar panels to power the observatory and the mirror and detectors operate at -233°C on the cold and shaded half.
A Shaky Start
As the launch date approaches, engineers are contending with the challenge of blasting the advanced telescope into space.
In a giant clean room at the Goddard Space Flight Centre in the US state of Maryland, the telescope has been experiencing the violent sounds and vibrations of a rocket launch. This involves forces 10 times stronger than gravity and blasts that rival a rocket explosion. The team has to be certain that nothing disrupts the telescope once it’s inside the Ariane rocket and on its way.
During this whole process, the primary mirror’s precision is continuously assessed to verify that its surface and alignment will not degrade. This ‘Centre of Curvature’ test is fundamental to the telescope’s development.
In early December, during one of the simulated launch tests, one of the restraints on the primary mirror was found to be moving slightly due to the extreme shaking.
“Now that we understand how it happened, we have implemented changes to the test profile to prevent it from happening again,” says Lee Feinberg, an engineer and Optical Telescope Element Manager at Goddard. “We have learned valuable lessons that will be applied to the final pre-launch tests of Webb once it is fully assembled in 2018.
With the schedule back on track, Goddard will up the intensity of the vibrations. Knowing that the JWST can survive conditions more severe than the launch gives Feinburgh and his team “confidence that the launch itself will be fully successful.”
But it’s not just mechanical assessments that are taking place this year. “A lot of the work that is also going on right now is software development,” says Sarah Kendrew, an ESA Instrument Scientist in Baltimore. “We’re making sure we can control the instrument properly.”
“We’ve had successive test campaigns, where more and more hardware gets put together,” says Kendrew. “Now MIRI is just one piece of this enormous telescope and spacecraft.”
In the coming months, the JWST will move to the Johnson Space Flight Centre in Texas, where it will undergo a thermal vacuum test. Using the same chamber that was originally used to test Apollo, the telescope and its integrated instruments will be subjected to chilling temperatures – 40 degrees above absolute zero – to monitor how they will perform in space.
From there, it’s off to California for one final and momentous challenge. The telescope and scientific payload will be attached to the giant sunshield and sailed down the coast to the launch site in French Guiana by mid-2018.
After launch, the scientists will have to wait a few weeks before they know if everything has gone according to plan; the sunshade is cooling and the telescope deploying. With the telescope positioned 1.5 million kilometres from Earth, there is nothing anyone can do to repair any damage. It really does have to be right first time.
With scientific meetings scheduled for later that year, Wright is already looking ahead. “It will probably be about six months after launch that we get the first images back.”
“We’re talking about the plans for early images and how are we going to commission the instruments,” she adds. “It’s a very exciting time.”