Why NASA’s new space telescope is pointed at the Big Dipper

NASA’s newest big telescope orbiting Earth, the James Webb Space Telescope, is currently pointed right at a constellation iconic to Alaskans: the Big Dipper.

That, of course, is what’s on the Alaska Flag, and the James Webb is focusing on a star in the Big Dipper to calibrate its ultrasensitive mirrors, which researchers hope will allow them to see almost all the way back to the beginning of the universe.

Lee Feinberg is the Webb Optical Telescope Element Manager with the NASA Goddard Flight Center, and he’s been working on the project for more than 20 years. As Feinberg explains, the Webb is thought of as a replacement for the Hubble Space Telescope.

Listen here:

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The following transcript has been lightly edited for clarity.

Lee Feinberg: The universe expanded rapidly, and light is just getting to us now from the very early universe. Well, Hubble could see back when the universe was a little bit before a billion years old, but it couldn’t see back to some of the first stars and galaxies that formed when the universe was just a few hundred million years old. And that’s because the light from those stars and galaxies has been stretched into the infrared, which Hubble cannot see. And so Webb was really originally designed to go after the very early universe. And to do that it needed a telescope that’s larger than Hubble — it’s six and a half meters versus 2.4 meters for Hubble — but it also needs to be infrared. And that means that it needs to be really cold.

And the reason it needs to be cold is that when mirrors are warm, like the mirrors on Hubble, which are room temperature, they generate infrared light. And it’s really just due to the fact that any object that is warm generates infrared light. That’s how how heat is emitted. And so in the case of the mirrors on a telescope, if those mirrors are warm, they generate a lot of infrared light, which contaminates the images, it prevents you from seeing these very distant, very faint objects.

And the James Webb Space Telescope is actually going to be cooled to about minus 400 degrees Fahrenheit, through this very large sunshield. The telescope’s actually already in space, and it’s cooling down as we speak. And it’s getting closer and closer to this very cold temperature. And at that very cold temperature, we have all sorts of sensitivity and wavelengths, a level of sensitivity that we’ve never had in these infrared wavelengths. And that’s going to not only allow us to study the early universe, but it allows you to study just about every type of astronomy and astrophysics that other space telescopes have studied, including one that’s very exciting, which is exoplanets. And exoplanets have atmospheres, and with Webb, we will be able to look at these exoplanets as they pass in front of stars, and understand the atmospheres of those exoplanets by understanding how those atmospheres absorb different infrared wavelengths. So Webb is really suited for that problem as well, as well as many other problems in our solar system and in galaxies and star formation. So Webb is a very large cryogenic, which means cold, infrared telescope designed to be a successor to Hubble and Spitzer Space Telescope.

Casey Grove: Wow, yeah. So it’s cooling down, and it’s been a long time coming to get to this point. It sounds like there’s still some time to go before it starts doing some of those bigger experiments. But all these years of work kind of also came down to a rocket launch that needed to go well, and that’s a relatively short period of time. What’s that like? It must be nerve racking.

LF: Yeah, you know, it is nerve racking on the one hand. On the other hand, for example, in this case, we actually launched on an Ariane 5, which is a rocket that was a European contribution to the mission. And that rocket, we were tracking it over the many years, it was extremely reliable. So in some ways, we felt pretty confident that it’s a very reliable rocket, it’s launched a lot of times. On the one hand, it’s nerve wracking, but I think what was more nerve wracking, to be honest, was just the fact that we were sort of the largest mission, the heaviest, and we filled it up the most, and the fact, really, that we had to deploy unfolding mirrors and the heat shield in space. So we were probably a little bit more thinking that the deployment was going to be the hardest problem, that there was a lot of reliability in these kinds of launches. But anytime you launch a mission to space, there’s always some level of concern because, you know, nothing is 100% reliable when it comes to launching rockets.

CG: Yeah, no doubt. Well, I just want to be totally clear, I saw that the James Webb was calibrating using a star in the Big Dipper constellation. And, of course, the Big Dipper is very near and dear to Alaskans. It’s the image on our state flag. People have tattoos of it even. And so just very quickly, Lee, do you think that’s a good enough excuse to be talking to you about the James Webb Space Telescope?

LF: I think it’s a good enough excuse for us to pick that star. Yeah, I mean, you know, it’s really neat. There’s so many ways that people are connecting to James Webb Space Telescope all over the world. And actually, to hear that is really interesting to me. And honestly, for me personally, the Big Dipper was always the thing in the sky that I always recognized when I was growing up. I’m learning about sort of what it means in Alaska, but I think it also means a lot to everybody. We didn’t pick this particular star specifically because it was in the Big Dipper. It turns out that we actually just needed a really bright star that is pretty well isolated. So in other words, we didn’t want any other bright stars nearby it. And that is to allow us to do some of the first steps, where we are searching for the 18 primary mirror segments, which we know after deployments are going to start off misaligned. So we just needed a bright star, we needed one that was in the right region of the sky that we could see it continuously. And this is the star that we just happened to pick. But, yeah, you’re right, I mean, it’s in Ursa Major, right near the bowl of the Big Dipper. And I personally agree with you that it’s great that we picked one from Big Dipper because it’s something people can relate to. And if you go on the JWST website, you can actually find the information about the star, it’s HD 84406. But that star is actually bright enough to see with binoculars. So you can actually go outside, and if you know where it is relative to the Big Dipper, you can kind of find it and look at it.

CG: Yeah, I guess we should say, you can’t see it with the naked eye. But if you want to look at the same thing that the James Webb space telescope is pointed at, you can with binoculars.

LF: Exactly, exactly.

CG: What are you looking for during this calibration process? Are there problems that could crop up that you’re trying to avoid or trying to notice?

LF: Well, the James Webb Space Telescope is unique in that that primary mirror is broken into 18 hexagonal segments. And we don’t normally do that with telescopes. Normally with a telescope the big thing is you’ve got a secondary mirror that you need to focus, or in the case of maybe a telescope that you have at home, you just have some little eyepiece, and you just have a little knob that you turn in order to focus it. But here, these 18 primary mirror segments, they have to all get aligned as though they’re a single monolithic mirror. And that means literally to a fraction of a wavelength of light, which, by the way, is about one 20,000th the diameter of a human hair. So we really need to align this extremely well. But because we had to deploy this system, it’s sort of unique, you don’t normally deploy and have something unfold, and then have mirrors deploy, you know, into these positions. Because of all that, from one mirror to the next, they could be separated by as far as a millimeter. So we have to go from a millimeter to what we call a nanometer. And a nanometer is about a millionth of a millimeter. So we’re gonna have to go a factor of a million better in how well these mirrors are aligned. It literally is a process that takes almost three months. But when it’s all said and done, all 18 mirrors are going to be aligned to a fraction of a wavelength of light, as though they’re just a single, big monolithic mirror. And that’s that’s the challenge here.

CG: That’s pretty amazing, how precise it needs to be. So for about three months, it’s going to be pointing at HD 84406, which is just sort of if the Big Dipper is upright, just to the right of the main Big Dipper?

LF: Well, we use that star initially. But we’ll actually be choosing some other stars along the way. Because turns out that for this telescope, which is so large and so big, you don’t normally want to use such a bright star. we consider this a very bright star even though you can only see it with binoculars. But we wanted to start with a very bright star. So we start with it, because initially our mirrors aren’t very well aligned, and therefore we need something that’s brighter so that we can see it. But as we go along, we’ll actually choose stars that are actually dimmer as things get better and better aligned. And so this is the first star we use, but it’s not the only star. But we will be choosing stars that are in the continuous viewing zone and we might choose some other stars in or near the Big Dipper. But this will be the first and then this one gets used for the first couple of weeks and then as it makes sense, we migrate to other stars.