Ezzy Pearson interviews Mark Clampin
At the end of December, the James Webb Space Telescope began its mission to explore the cosmos, but what does it hope to achieve?
What is the James Webb Space Telescope (JWST)?
It’s a 6.5m-aperture infrared telescope, which means it is looking at heat radiation. We have used gold to coat the mirror, which is extremely reflective in infrared wavelengths, so we collect something like 97–98 per cent of all the light that hits the telescope.
How have you fitted a 6.5m-wide telescope in an Ariane 5 rocket, which is only 5m wide?
The size of JWST’s mirror presents a lot of interesting challenges. We’ve made the mirror out of 18 individual, hexagonal segments and have folded everything up for launch, so the central part of the mirror is in 12 segments, with three on each side which we have actually folded around the side of the telescope for launch. Then, once it’s in space, we can deploy it.
Are there any other challenges to building such a big infrared space telescope?
The problem with observing in the infrared region of the electromagnetic spectrum is that you need your telescope to be cold enough so that it’s not just seeing its own heat signature. For the science that we want to do, we actually have to have the JWST as cool as 40 Kelvin, equivalent to 40 degrees above absolute zero. The approach that we’ve taken is something called ‘passive cooling’. The idea is that you fly the telescope with a large, tennis court-sized array of membranes that act as a sunshield, so the telescope cools down to the temperature that we want to operate at. Then, on the other side of those membranes, we’ve got the spacecraft bus (its main structural body) and all the parts that we need to be looking at the sunlight.
Where is the JWST going?
It is going to the second Lagrange point (L2), which is a sort of quasi-gravitationally stable location about 1.5 million kilometres from Earth. We’ve selected somewhere that’s always in the Sun as we need power for the solar arrays, which can operate at L2 for 24 hours a day, seven days a week. It’s also a thermally benign environment out there. As Hubble goes around Earth its focus changes slightly as it goes from day to night to day, something like five microns. You can’t see that in Hubble’s data, but if you tried to do that with JWST’s big mirror, you would see the image quality change. At L2 it will be much more stable.
What science will the JWST do?
It’s an observatory, and can look at a number of different science themes. The original science team that drove the creation of the JWST wanted to look at the very first stars and galaxies in the Universe. We think the first stars and galaxies formed 100 million to 300 million years after the Big Bang. The challenge for us arises because the light from those stars has been shifted so far into the red that they’re actually in the infrared. Then in our own Galaxy we want to study how stars form and evolve. Hubble took beautiful pictures of the big, dark dust clouds, but with the JWST we want to look inside these stellar nurseries and see the stars as they are forming. The final theme that we are excited about is examining exoplanets. We’ve started studying the atmospheres of some of the big gassy planets, but with JWST we’ll start to look at super-Earths and potentially some Earth-like planets. We’ll want to begin looking at their atmospheres and compositions.
How did you decide what to observe?
The first year has been divided up into observations that will be conducted by the teams who built the instruments. Then there’s a set of observations that have been planned for the science community to get an early look at JWST data and understand the telescope’s performance and what it can do scientifically. There’s also a whole set of proposals that were awarded time based on their merit. There is plenty of interest in studying early galaxies for the first time and I think, ultimately, that will be JWST’s legacy.
Mark Clampin is the Director of Sciences and Exploration at NASA’s Goddard Space Flight Center