Why is the James Webb Space Telescope so important?


The new James Webb Space Telescope took another milestone over the weekend, deploying its main mirror – the crown jewel of this long-awaited observatory.

The mirror measures 6.5 meters in diameter, larger than any mirror previously launched into space. (The main mirror of the Hubble Space Telescope is only 2.4m in diameter.) The size increases the sensitivity of the telescope – the larger the area of ​​the mirror collecting light, the more it can capture the details of a star or d ‘a galaxy.

The mirror is composed of strong and light hexagonal segments assembled by paving. For launch, they were folded into two “wings” to allow the telescope to fit into the launch vehicle. But now they have successfully expanded again.

This follows the recent deployment of the secondary mirror: a small circular mirror that plays an essential role in the reflection of light from the primary mirror to the instruments.

These two mirrors are covered with a thin layer of microscopic gold. It’s not just about looking fancy – it actually optimizes them to reflect infrared light.

The main mirror of the James Webb Space Telescope at NASA Goddard. The secondary mirror is the round mirror at the end of the long poles, which are folded into their launch configuration. Webb’s mirrors are coated with a thin layer of microscopic gold, which optimizes them to reflect infrared light, which is the main wavelength of light this telescope will observe. Credit: NASA / Chris Gunn

Rewrite cosmic history

The most exciting thing about the James Webb Space Telescope (JWST) is its promise to revolutionize infrared astronomy. With a massive mirror and the ability to see light in the infrared portion of the spectrum, it can go back billions of years through history to capture the faint red-shifted light from the very beginning of the universe.

He will be able to observe the first stars and galaxies to sparkle, to probe the mysterious processes which brought the universe out of its dark ages and brought us into the era of light.

Astronomers have been asking burning questions about this first era of the universe for decades – for example, what did those first stars look like? How did magnetism and turbulence play a role in triggering the first stars? How did black holes first form, begin to grow, and become the heart of galaxies?

JWST was specially designed to answer these questions and more.

“Through in-depth observations, James Webb will reveal the very first galaxies formed in the infant universe and how these galaxies evolved over 13 billion years of cosmic time,” says Lisa Kewley, director of ASTRO 3D at the ‘Australian National University.

“We will get an unprecedented picture of the formation and evolution of galaxies like our Milky Way. We will measure how the elements responsible for life: oxygen, carbon and nitrogen, were formed and evolved over 13 billion years of cosmic time. James Webb will also reveal what elements are found in the atmosphere around extrasolar planets.

“The big questions James Webb aims to answer concern all of our origins and our place in the universe: are we unique? Is our Earth unique? Is the Milky Way unique? What are our origins?

The very first chapter in the history of the universe was previously difficult to study, because the only way to learn it is through light. As the universe expands, the light from these early stars has stretched as it travels toward us, shifting it more energetic waves of ultraviolet or visible light toward us. red end of the electromagnetic spectrum.

“We can’t see a fractured bone without an X-ray machine,” says Nicha Leethochawalit, ASTRO 3D Fellow at the University of Melbourne. Likewise, we need near infrared wavelengths to detect galaxies at the start of cosmic time, and mid infrared wavelengths to determine their compositions.

Unfortunately for us, this infrared light is the same as heat, and we have a lot of it on Earth, drowning out weaker signals from galaxies far away.

But by launching a huge infrared-sensitive telescope into the icy expanses of space, we can capture the twinkles of these ancient stars.

“In the visible to mid-infrared wavelength range, the diameter of JWST is unprecedented,” said Leethochawalit. “It will detect the weakest and most distant source known to mankind.”

The Hubble Space Telescope has been orbiting the planet for over 30 years, but because it is not optimized to study the universe in the infrared, it has not been able to answer all of the questions that JWST will address. In fact, the Hubble results inspired the design of the JWST.

Hubble discovered that stars, galaxies and supermassive black holes existed much earlier in the history of the universe than we thought, and that they have evolved considerably over time. We also learned about the role dark matter and dark energy play in the evolution of the universe, and we discovered thousands of exoplanets orbiting stars. JWST will tell us more about each of these things.

JWST’s larger mirror – along with its specific infrared capability and greater distance from heat from Earth – means this new telescope will take infrared astronomy beyond what Hubble has been able to do.

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Comparison of the Carina Nebula in visible (left) and infrared (right) light, two images from Hubble. In the infrared image, we can see more stars that were not visible before. Credit: NASA / ESA / M. 20th Anniversary Livio & Hubble Team (STScI)

So what’s the next stop on the journey?

Unlike Hubble, JWST will not be orbiting Earth – in fact, it is still en route to its final place of work, the second Lagrange Point (L2).

Here, at this particular point about 1.5 million kilometers from Earth, the telescope will actually orbit the Sun – but still remain aligned with Earth. It will essentially follow our planet in orbit, held in place by the gravity of the Earth and the Sun, so it will need relatively little rocket fuel.

Orbit of the James Webb Space Telescope as seen from the North Pole of the Sun and seen from the point of view of the Earth.

It also allows its sun visor – which was also successfully deployed last week – to still protect the delicate mirror and instruments from the heat of the Earth and the Sun. The telescope should operate at around -225 ° C, so this protective configuration is essential.

It will take a total of 30 days for JWST to complete its journey to L2, so there are still a few weeks left before it is inserted into orbit.

Meanwhile, the mirrors continue to prepare. They have already started the necessary cooling process in the shade of the sun visor, but it will take several weeks for them to reach stable operating temperatures. It’s a finely controlled process, with electric heat bands managing the rate of cooling so everything carefully retracts.

As everything cools down, each individual mirror segment will be carefully moved from the launch configuration and into position.

All the pieces must be perfectly aligned in order to properly focus on distant galaxies. This work is done by tiny mechanical motors called actuators attached to the back of each mirror piece, which can move the pieces until they act together as one large cohesive piece, producing crisp images.

“Aligning the primary mirror segments as if they were a single large mirror means that each mirror is aligned 1 / 10,000th the thickness of a human hair,” says Lee Feinberg, head of parts of the Webb Optical Telescope at NASA’s Goddard Space Flight Center, Maryland. . “What’s even more amazing is that the engineers and scientists working on the Webb Telescope literally had to invent how to do this.”

Then, once JWST is in orbit and the mirror is ready, the scientific instruments will be brought online.

But it will take another five full months before JWST looks up to the sky for the very first time, as it will take time for the optical systems to be properly aligned and for the instruments to be meticulously calibrated.

“JWST is a remarkable feat of engineering, one of the most complex instruments ever built, and will demonstrate our ability to harness new space technologies more than 1.5 million km from Earth,” said Simon Driver of the International Center for Research in Radio Astronomy. (ICRAR) and the University of Western Australia.

“Australia will play a critical role in tracking the Ariane 5 launcher as it enters space, and later in routinely downloading scientific data eight hours a day from the deep space tracking station in NASA in Tidbinbilla, ACT. “

To look forward

Astronomers around the world are eagerly awaiting JWST to begin its science program in mid 2022.

“It’s like waiting for Christmas for 20 years,” says Daniel of Monash University. “The first round of sightings will be especially special for our group at Monash, as we have time to image a baby planet that we discovered in 2018. We hope that an image of a child planet orbiting around a young sun will prove to be one of James Webb’s first spectacular results.

But while some people have been waiting for decades, for young scientists it will be the start of exciting careers.

“Over the past few years as a young researcher, I have learned many current questions and challenges in astrophysics,” says Juan Manuel Espejo, doctoral student at the Center for Astrophysics and Supercomputing at Swinburne University of Technology.

“What I have found as a common theme in almost every one of them is that JWST will play a crucial role in proving answers and alleviating our curiosity for understanding the cosmos.”

Claudio Lagos Urbina, of ASTRO 3D, adds: “I’m so excited to see this – as a computational astrophysicist we’ve been making predictions about what JWST will see for a few years and now it’s time to test those predictions and hopefully turn these new observations into a transformational understanding of galaxy formation and, in particular, the birth of our home, the Milky Way.

“We really expect a breakthrough in our ability to understand the formation of the Universe.”


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