The Future of Space Telescopes

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Matt O'Dowd (0:48)

The mysteries of the universe seem limitless. However, to unlock them, we're going to need some incredible technologies to peer deeper and more sharply than is currently possible.

Fortunately, the imagination and ingenuity of telescope engineers also seems to be without limit.

We recently talked about some of the most exciting new observatories currently being built. One of those, the James Webb Space Telescope, will succeed the Hubble Space Telescope with more than a factor of five and an increase in collecting area.

Getting such a large telescope into space is a major challenge. And in fact, it may be difficult to go much larger using traditional mirrors. Those things are really hard to get into space in one piece.

Fortunately, NASA isn't constrained by traditional methods. The NASA Innovative Advanced Concepts, NIAC program has some ingenious ideas for overcoming this limitation. The first big idea may revolutionize our study of terrestrial exoplanets. The Kepler mission has determined that terrestrial planets, that is rocky planets like our Earth, are extremely common and may orbit most stars in the Milky Way. But these planets are extremely difficult to directly image because they're dense and small. And in addition, because rock solidifies at a much higher temperature than volatiles like ammonia and water, terrestrial planets tend to form close to their parent star. This brings a special challenge, glare.

Our sun is about 10 billion times brighter than Earth. Train a distant telescope on us and it would be overwhelmed by the sun's rays. So how can we find a terrestrial planet around a star light years away? Maybe it's simple, blot out the star. We've done this for decades with the coronagraph, a disk inside a telescope that occults a star, blocking its light so that any planets can be seen more clearly. However, there's no such thing as a perfect shadow. The wave nature of light causes it to bend or diffract around the edges of a coronagraph back towards the central optical axis. This means it's never possible to completely block the star's light. Good coronagraphs can allow detection of objects from 100,000 to a million times fainter than the central star, but nowhere near the factor of 10 billion difference between the Earth and the sun. In 2005, Dr. Webster Cash proposed a successor to the coronagraph, the Starshade. It's actually a spacecraft outfitted with thrusters to align itself between a space telescope and a star.

A Starshade will be up to 50 meters in diameter and hover 80,000 kilometers in front of the telescope, assuming a four meter diameter telescope mirror. At that distance, the Starshade acts like an artificial eclipse.

The effect of diffraction would be easier to isolate than for a typical internal coronagraph. But the Starshade goes much further. It will not be a simple opaque disk like a standard coronagraph. Instead, it will have a flower-like shape whose cleverly calculated petal geometry is designed to diffract light away from the central axis, not towards it. The number and length of petals optimizes each Starshade for a particular wavelength of light. Each of those petals is articulated to fold into a very compact bud for launch, and then open up like an actual flower once in space. A bit like the James Webb Space Telescope. The main motivation for building Starshades is to suppress the glare of stars enough to see the planets that orbit them. Configured right, glare is suppressed by a factor of 10 billion at 50 milliarchseconds from the star. So, one of these things would allow us to see Earth in orbit around the sun from 60 light years away.

There are a couple of thousand stars within that range and hundreds of sun-like stars, many of which certainly have Earth-like planets. With the Starshade we may soon directly observe terrestrial exoplanets with cameras and spectrographs. We'll be able to detect continents, oceans, ice caps and cloud banks of faraway worlds.

Besides Earth-like exoplanets, the Starshade would also be an enormous help in studying quasars and other high-contrast phenomena. The first Starshade may launch with NASA's WFIRST mission in the 2020s for a budget of around $750 million and a runtime of five years. It's pricey but may ultimately save money as its beneficiary telescope will require no coronagraphs or wavefront detectors or other high-contrast compensators. Oh, and one Starshade could theoretically serve multiple telescopes. Diffraction is a challenge for coronagraphs. But the phenomenon is a challenge for any telescope. The edges of a telescope's primary mirror or lens also cause diffraction. This introduces an unavoidable blur. The diffraction limit defines the best possible resolution of a telescope and it gets smaller or sharper proportional to the size of your telescope's aperture. But that's a tough trade-off. To improve resolution by a factor of 2, you need to double the diameter of the scope, which means the volume and mass, roughly speaking, increase by a factor of 8 And that's a problem when you're trying to launch your telescope into space. Diffraction is expensive to deal with. But what if we could use the phenomenon to our advantage? Enter the Aragascope, another one of Webster Cash's strokes of genius. It's a revolutionary idea to use diffraction optics to focus light instead of what we call geometric optics, so reflection or refraction, like in traditional telescopes.