Date: June 8, 2011
Title: James Webb: Inestimable Expectations
Podcaster: Roz Brown and Dennis Ebbets
Organization: Ball Aerospace
Description: Plans for the Hubble Space Telescope successor – the James Webb Space Telescope – were motivated by continued interest in the formation of stars and galaxies and the early history of our universe. Many important observations have been made at the limits of Hubble’s capabilities but the Webb will observe, in only a few hours, objects that take the Hubble Space Telescope one or more weeks to record. It will explore fundamental issues about the earliest epochs of our universe and the processes by which it evolved.
Bio: Ball Aerospace is the principal optical subcontractor for the Webb, overseeing the construction, polishing and testing of the telescope’s light-reflecting mirrors. Ball Aerospace astronomer Dennis Ebbets talks to Roz Brown.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by David Rossetter on behalf of the Mid-Hudson Astronomical Association specializing in observing and sharing the skies of the Mid-Hudson Region of New York State in the US.
Transcript:
Roz Brown: Plans for the Hubble Space Telescope successor– the James Webb Space Telescope – were motivated by continued interest in the formation of stars and galaxies, and the early history of our universe. Many important observations have been made at the limits of Hubble’s capabilities but Webb will observe, in only a few hours, objects that take the Hubble one or more weeks to record. It will explore fundamental issues about the earliest epochs of our universe and the processes by which it evolved. We’re here with astronomer Dennis Ebbets of Ball Aerospace to learn more about what we can expect from James Webb following its launch, and what’s so different about Webb, compared to Hubble.
Dennis Ebbets: Well, primarily it was designed to observe these small fragments of galaxies in the very early days of the universe.
Roz: We call this telescope a follow-on to Hubble, but it’s very different from Hubble. Tell us about that.
Dennis: Well, it’s a scientific follow-on in the sense that it will observe and study in more detail these infant galaxies that Hubble detected. The big difference is that Webb is larger in both diameter and collecting area than Hubble, and it also works in the infrared part of the spectrum rather than in visible light and ultra violet light.
Roz: What will that mean to the average observer, the difference in infrared red?
Dennis: Well, once you get the data down it’s all digital, so I suppose that you can’t tell too much. The images, after they’re computer processed and color-coated, printed out and displayed will look pretty similar to images that we’re used to seeing. But the content, the fact that you’re looking out in the infrared part of the spectrum at what was normal visible starlight when the light left these galaxies and stars – the red shifts will tell us about how far away they were and when in time these things happened – and we’ll be able to analyze the chemical composition and study the mergers of how these small fragments of galaxies merged together under the influence of gravity.
Roz: The Hubble is one big mirror and James Webb is 18 segmented mirrors – how do you make that work on orbit?
Dennis: That’s one of the most important parts – the deployment and the alignment. You’ve got to make this work as one great big telescope – six-and-a-half meters in diameter rather than 18 smallish telescopes each only one-point-three meters in diameter. And that’s a process of alignment and proper focus and what is called phase control. And you’ve got to get all 18 of these lined up very, very precisely so when their images overlap it makes an image like one big telescope.
Roz: And what will we learn from Webb that we haven’t already learned from Hubble?
Dennis: Well with respect to these distant galaxies, hopefully we will learn the timeframe in which these first stars in the universe first started shining and when the matter in the universe started collecting together under the forces of gravity into the hierarchy of smaller objects that collected into bigger objects and eventually led to the formation and evolution of the big, grand galaxies that we have today. Another thing that I personally think is very, very interesting – and this is something that was discovered since the Webb was first proposed – and that is there are a lot of stars that have planets around them. We have solar systems, planetary systems and sometimes the orbits of those planets around their stars are lined up in such a way that once per year, of the other planet, that planet passes in front of the disc of the star and we see what’s called a transit – kind of analogous to an eclipse that we see from Earth – and when that planet transits in front of the star, for the few hours that it’s in transit – the starlight is passing through the atmosphere of the planet – if it has an atmosphere. And the gases in the atmosphere of that planet can imprint spectral lines – signatures in the spectrum of the star – that are in addition to the star itself and we can actually detect those and determine what kind of gases are present in the atmosphere of the planet. I think that’s very interesting. That’s certainly the whole scientific field of exoplanets that has really blossomed in the last few years. I think it has the potential for being one of the more important and most interesting things we will learn from the Webb – the presence of atmospheres on planets around other stars.
Roz: There was really a lot of technology as well as thought process that was new that went into James Webb.
Dennis: Absolutely. NASA spent a fair amount of effort before they committed to building the telescope to identify what technologies would be needed, and then funding the development. A lot of it had to do with the optics, some of it had to do with detectors and other components for the science instruments, and the big sunshield has a lot of interesting technological challenges.
Roz: James Webb looks totally different than Hubble. Why is that?
Dennis: Well, it has a different configuration, the basic architecture is different. Since it has to work in the infrared, the system has to be very cold, and that includes all of the optical surfaces of the telescope and the science instruments and their detectors. And by cold, I mean 30 or 40 degrees kelvin above absolute zero. And the idea was that if you get it out into space – think of it as a naked telescope – there’s no big tube of baffle around it to hold the heat in – and then there’s the giant, tennis-court-sized star shade. And if you can keep the orientation such that the sun is always on the other side – and the telescope and the instruments are always on the shady side – then they will naturally radiate away a lot of their heat. The calculations are that over the period of a couple months the telescope will cool down and equilibrate at a temperature of 35 kelvin. That’s primarily to allow the infrared wave lengths not to be overwhelmed by the glow of the telescope itself.
Roz: And unlike Hubble, which astronauts were sent to service, we’re not able to service James Webb.
Dennis: That is correct, that is a decision that was made early on, that it would not be serviceable. It is not in a very easy place to get to – by human beings. It is not in orbit around the Earth. The Webb will be in orbit around the sun – what’s called a heliocentric orbit – and it will be about a million miles – one, one-hundredth of an astronomical unit – farther away from the sun than the Earth is. So it’s slightly father out in the solar system, but not by very much. And even though that’s not terribly far away, it’s farther than the moon. No human beings have ever gone there before. So, they (NASA) decided that because it’s going to be in a pretty inaccessible place anyway, they designed it without being serviceable and that saved some costs.
Roz: As an astronomer, what is the most exciting thing about this mission for you?
Dennis: One of the surprises, the discoveries, that has come out of the Hubble work on the distant galaxies is how early in the history of the universe things really got organized and stars and galaxies started to form. We think that the universe is about 14 billion years old. And that is a number that has been refined by Hubble observations over the years. That’s a long time, and one might have thought, naively, that it might have taken many billions of years for the very hot, tenuous gases that came out of the big bang to have cooled to the point where gravity could take over and cause things to coalesce into stars and galaxies. And it appears that actually happened surprisingly early. We see evidence of galaxies out at – what we called red shifts – which equate to distance and look-back time – we see galaxies that look remarkably mature!
They were going concerns very, very early in the history of the universe and personally I find that surprising. It’s cool – a cool development that things got going and got organized so soon in the history of the universe. So I look forward to seeing that refined – if we can understand when the earliest stars and galaxies existed and how it was that things got started so quickly.
Thank you Dennis. Thank you Roz.
End of podcast:
365 Days of Astronomy
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