June 12, 2006

Part IV: Astrobiology

New Worlds Imager
Webster Cash, University of Colorado, Boulder
33 min. (slideshow requires QCShow Player)
Audio only (mp3 format)
View as a webpage (quicktime, real player) (notes)

You have to look and be able to see things that other people looked at and didn't see before. How do you do that? There's two ways. Either you make a new instrument, and it gives you better eyes, like Galileo's telescope. And that's a great way to do it, make such a nice instrument that you don’t have to be so smart, you just look and there it is. Or you try to internalize it is such a way that it really becomes intuitive. Working on the right problem is only part of what it takes to succeed.
— Stephen Chu, Nobel Laureate

Before the time of Galileo, all scientists used the unaided eye to observe the heavens. When Galileo trained his tiny telescope on the sky, its magnification gave him the same view as if he had traveled 90% of the way to his target. With his 10 times improvement in angular scale, he saw a new view of the universe, and revolutionized astronomy.

In this lecture, Webster Cash proposes a new telescope design that will take us much further yet again, and is likely to have as revolutionary an effect. To date, no planet has been seen around another star, even though the gravitational effects of their presences have been measured. The planets are fainter than their host stars by a factor of 1 to 10 billion (1:10,000,000,000). If we are ever going to directly image exoplanets, the glare of the central stars must somehow be suppressed by at least an equivalent amount.

Cash proposes a startlingly simple idea, The New Worlds Observer, an orbiting telescope with an accompanying occulting starshade 50,000 km distant.

Proposing an occulter is not new. The problem is that a simple, round occulter can actually work to exacerbate the problem, making the central star seem brighter than it should be through a phenomenon called "Poisson's Spot."

By 1800, light had been shown to have both particle and wave light natures, and each view had had its champions. Newton argued that light acted as it were composed of particles, while Huygens argued its wave-like nature. In 1818, Augustin Fresnel submitted a paper to a scientific competition sponsored by the French Academy, deriving the equations of diffraction of light as it passes around a solid object, if it were to behave as a wave. His submission led to one of the most famous stories in science.

Siméon-Denis Poisson, one of the judges of the competition, intensely disliked the wave theory and quickly pointed that if Fresnel's equations were true, then there would be conditions in which the diffracted waves would constructively interfere to form a bright spot at the focal plane behind the occulter. Poisson considered this result to be so unreasonable as to disprove Fresnel's equations by itself, with no further proof necessary. François Arago, another judge, disagreed. Arago set up an experimental apparatus and quickly demonstrated the presence of what has now come to be called "Poisson's Spot."

Cash solves the Poisson Spot problem by placing petals of highly constrained shapes around the edge of his occulter. By doing this, he breaks up the natural diffraction pattern and theoretically achieves an astounding 1 in 100 trillion (1:100,000,000,000,000) contrast ratio, more than enough suppression to quickly see Earth-sized planets 100 light-years distant.

More importantly perhaps, The New Worlds Observer is a simple, error-tolerant design that could be launched now, if desired. It also directly leads to second, more extraordinary design, The New Worlds Imager.

In its most advanced form, The New Worlds Imager would be able to capture actual pictures of planets as far away as 100 light-years, showing oceans, continents, polar caps and cloud banks. If extra-terrestrial rainforests exist, they might well be distinguishable from deserts. The New Worlds Imager gives every appearance of being an affordable concept with a very practical technology that should allow us to conduct direct planet imaging in the visible and other wavelengths of light by 2018.

— Wirt Atmar

About the Speaker

Webster Cash writes this about himself:

"My research is in development of new observatories for high energy astrophysics. I am actively involved in providing reflection gratings for the spectrographs on the Constellation-X Mission. I am also developing techniques of x-ray interferometry with the eventual goal of imaging black holes from the MAXIM Mission. I have a new initiative in the area of high contrast imaging for detection of planets outside the solar system."