快猫短视频

When the James Webb Space Telescope unfurls its mirror a million and a half kilometres out in space four years from now, it will be the culmination of nearly two decades of planning by John Mather. He tells Anil Ananthaswamy about the challenges of building an heir to the stunningly successful Hubble Space Telescope

Why do we need the James Webb Space Telescope, when Hubble is still up there?

The short answer is that Hubble has tantalised us by showing us signs of things that would be really exciting to know about, but are just beyond its reach.

What do you expect we will see?

The first luminous objects in the universe that formed a few hundred million years after the big bang. We have good evidence for them, but can鈥檛 yet see them directly. That鈥檚 because, as the universe expands, it stretches out the wavelength of their light beyond what Hubble can pick up. With the James Webb Space Telescope (JWST) we think we will be able to see back to a couple of hundred million years after the big bang. We鈥檒l see stars made from primordial material that has not already been cycled through previous generations of stars.

How will the JWST differ from Hubble?

The first requirement is that it should be an infrared telescope, while Hubble operates mainly in visible and UV wavelengths. This means it will have to be colder than Hubble so as not to swamp the cold infrared light.

Where are you going to station it to ensure that it stays cold enough?

We hunted for a long time to see if we could keep it near Earth like Hubble, and the answer was no. No Earth orbit, no combination of shielding could do the job. The best place turns out to be the Lagrangian point L2, which is 1.5 million kilometres away, on the opposite side of the Earth from the sun. A one-sided sun shield can cast a shadow over the telescope to block out any solar radiation and keep its temperature down to about 50 kelvin.

Are there any other key requirements?

The JWST needs to be as big as we can manage. A report for NASA, released in 1996, by astronomer Alan Dressler called for it to have a mirror diameter or 鈥渁perture鈥� of at least 4 metres, because they knew that there were rockets that could launch a telescope of this size. Yet even then we were already thinking: 鈥淒oes it have to stop there? How about building an even bigger segmented telescope that you fold up during launch?鈥�

It turned out that the head of NASA at the time, Dan Goldin, knew about this technique, because he came from a company that had worked on segmented telescopes. Goldin was convinced that this is what we had to do. When he announced it to the American Astronomical Society, they gave him a standing ovation.

So it鈥檚 a telescope that can be folded to fit into the rocket and unfurled once it鈥檚 in space. That sounds frighteningly challenging.

Well, it is challenging. And if you are a serious engineer, you know that it鈥檚 frightening.

What are the other technically difficult bits about the JWST?

First, we had to choose the right material for the mirrors. The JWST鈥檚 aperture is much larger than Hubble鈥檚 and a rocket can鈥檛 carry that much payload to L2. Obviously we have to have a very lightweight mirror. We chose beryllium as the material for this. Each segment, which is 1.3 metres across, is only a couple of millimetres thick, with a honeycomb-like structure milled into it for strength and stiffness.

Then we had to figure out how to adjust the telescope once it was unfurled in space. There were big problems with Hubble鈥檚 mirror, and it turned out that the people who solved that problem had done the maths to calculate how to adjust the mirror in space. This will help us adjust the JWST鈥檚 mirror once it鈥檚 deployed.

What was the problem with Hubble?

It had been polished perfectly to focus at the wrong place. The problem was noticed only after it had been launched. To fix it they had to develop all the mathematics to measure the curvature of the mirror in space, without touching it. We can use the same formulae to adjust the JWST to the right shape after launch. All 18 mirror segments are on motors, so they can all be adjusted to focus correctly.

What else did you learn from NASA鈥檚 experience with Hubble?

Not to repeat the mistakes that were made when measuring Hubble鈥檚 mirror curvature. If you have something that matters, you had better have two competing sets of analysis, and if they don鈥檛 agree, you鈥檇 better find out why. Two competing tests were carried out on the Hubble mirror, but no one figured out why they didn鈥檛 agree. They decided that they believed one.

Hubble did not have a test on the ground to show that it would focus correctly: it was deemed too expensive and difficult. We decided that we needed some kind of test to determine that the JWST will focus properly. We have arranged to use the giant vacuum tank at the Johnson Space Flight Center, where the Apollo astronauts trained. It is big enough to hold our telescope, and we will take the temperature down to about 40 kelvin to test it.

In terms of the science that will come out from the JWST, what are you particularly looking forward to?

There are four major areas that people are going to work on, ranging from the formation of the first luminous objects, probably supermassive stars, to the origin of galaxies, which people are guessing were much smaller than present-day galaxies and merged later to make bigger ones.

As of now, almost all galaxies are thought to have a black hole in the middle. Which came first? Did the black hole make the galaxy, or vice-versa? Also, we don鈥檛 understand the influence of dark matter on this process. Closer to home, we are certainly hoping to understand how stars are made. Almost all of them are made inside dusty clouds and we can鈥檛 see through the dust. Infrared light will go around the dust grains, so we can see into the clouds with the JWST. And finally, really close to home, we would like to understand how solar systems evolved, by looking at the remains of the early parts of our solar system 鈥� the little bodies in the outer solar system.

鈥淲e want to understand how stars are made and solar systems form鈥�

Having supervised the building of such a complex instrument, you must be keen to use it. Do you have any particular project in mind?

One of these days, I鈥檒l have to write a proposal to get observing time on the telescope, just like everybody else. Right now, I don鈥檛 have the time.

Don鈥檛 you get your own observing time for having built it?

No, I don鈥檛. Observing time on one of these things is so precious that I don鈥檛 think any person should be allowed to say, 鈥淲ell, I鈥檒l do what I want.鈥�