Tell me about ATLAST, the proposed successor to the James Webb Space Telescope. I know that at this point ATLAST is just a concept, but in some ways I sort of see it as a wish list for the next flagship telescope, and I'm curious what that wish list looks like?

Mountain: Let me give you a completely different way of thinking about it. The question is what is the future of space science? What questions do we want to answer? I'm a pragmatist when it comes to science, and I think the big question that everyone wants answered, and that we can answer, is whether or not we're alone. And we already know what kind of telescope we need to look for life around another star. This is not a difficult problem; it's a Physics 101 problem. We know where all the closest stars are and we know precisely the right distance a habitable planet will be from its star, and we know how bright a planet is. So now assume you can look at a star, but suppress its light, in order to get a spectrum from a planet that's orbiting it. That spectrum will tell us something about that planet's atmosphere; it might even tell us if there's life there. So how big a telescope do I need to do this?

If I assume that every single star has a planet around and that it's in exactly the right place, then I won't need to look at very many to have a good chance of finding life. With a 4-meter telescope, you can look at 10 systems, the 10 closest systems, in this way (the Hubble has a 2.4-meter mirror and the James Webb Space Telescope will have a 6-meter mirror). If I have an 8-meter mirror, I can observe hundreds of star systems in this way, and if I have a 16-meter mirror I can observe thousands. Those may sound like pretty big numbers, but remember in this scenario I'm assuming that there's an Earth in just the right place around every one of these stars. But we're not sure how many stars have a planet in just the right place, and we obviously don't know how many of those planets have an atmosphere with life in it. Kepler is giving us a handle on the first unknown, and it's looking like the answer is 0.1. It's looking like one in ten stars might have a planet in the habitable zone.

So now let's say I build my 4-meter telescope. That 1 in 10 chance that only gives me one or two habitable planets to look at, which isn't a very big sample size. Certainly not big enough to tell is whether we're alone or not. And so the question becomes how big do you need your sample size to be? From our perspective, the answer is about a thousand. If there's no life in the closest thousand stars, there's a good probability that we're pretty much alone. And that means I need a 16-meter telescope:

What sort of leap in the technology do we need to get there?

Mountain: Well, at first glance, a 16-meter telescope sounds absolutely impossible. But let's think through this. Let's put space aside for a moment and ask ourselves how we got our big ground telescopes. What happened is these astronomers had telescopes with 4-meter mirrors and they realized they weren't big enough to see the faint objects they wanted to look at, objects like distant galaxies. So they tried to build bigger ones, but the problem was that simply scaling up the 4-meter technology didn't work, at least not without spending massive amounts of money. So people like Roger Angel and Jerry Nelson and Ray Wilson came up with brand new technologies; they used active systems and adaptive optics, which eventually earned them the Kavli Prize. These advances allowed them to produce a whole host of new telescopes, facilities like the Gemini Observatory, the Keck Observatory, and the Subaru Telescope.