Planetary Lensing: Enter the ‘Terrascope’

I’m always fascinated with ideas that do not disrupt the known laws of physics but imply an engineering so vast that it seems to defy practical deployment. Centauri Dreams readers are well aware by now of some of Robert Forward’s vast mental constructions, including lightsails in the hundreds of kilometers and enormous lenses in the outer Solar System as big as some US states. But such notions abound in the realm of interstellar thinking. Thus Clifford Singer’s ideas on pellet propulsion to a receding starship, which from the mathematical analysis require an accelerator 105 kilometers long, an engineering nightmare.

But then, when we reach sizes like these, we might ask ourselves whether we’re not overlooking the obvious. When Cornell’s Mason Peck went to work on wafer-scale spacecraft, one futuristic notion that occurred to him was to charge swarms of tiny ‘sprites’ through plasma interactions and use Jupiter’s magnetic field as a particle accelerator, pushing the chips to thousands of kilometers per second. That gets you to Proxima Centauri at, conceivably, a tenth of lightspeed. And instead of building a vast accelerator, you use the one the Solar System already has.

Columbia University’s David Kipping likewise investigates what we can do with natural objects. Years ago, he became fascinated with Claudio Maccone’s ideas for a FOCAL mission, which would use the Sun’s own mass as the instrument for ‘bending’ starlight to a focus at about 550 AU, one that might be exploited by future deep space telescopes. I put ‘bending’ in apostrophes because actually the light never bends, but flies straight and true through spacetime curved by the presence of mass. The Sun has 300,000 times the mass of the Earth, a useful object!

Image: Writing about Italian physicist Claudio Maccone gives me the chance to tap a favorite memory. Here I’m at the left, having lunch with Claudio ten years ago in the Italian Alps. This was one of many sessions in which Claudio helped me understand the implications of gravitational lensing. Credit: Roman Kezerashvili (City University of New York).

No one in the rich history of gravitational lensing concepts from Einstein through Von Eshleman and on to Maccone, Geoff Landis and Slava Turyshev has done more for the field than Maccone himself, having submitted a proposal to the European Space Agency for FOCAL as far back as 1993, and having authored the key text, Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer/Praxis 2009).

A Lens the Size of the Earth

Of course, in our current stage of technological development, FOCAL itself is quite a reach, given our problems in getting to the outer Solar System — our farthest-flung craft even now are but a third of the way to where the lensing phenomenon could begin to be exploited. Well aware of this, David Kipping wondered if there wasn’t a way to explore the ‘bending’ of light in a different way, one that could help us learn how to untangle complex lensed images and develop near-term technologies at distances much closer to home. And it turns out there is, although it’s not a proposal that relies on gravitational lensing but rather the refraction of light.

Image: Columbia University physicist David Kipping.

Kipping wants us to consider the Earth as the source in a concept he calls the Terrascope. Although Maccone has considered the gravitational lensing properties of individual planets, the effect is small. Kipping reminds us that bending light through refraction has been used since the earliest telescopes. Refraction happens when light moves from a medium like air to a dense medium like glass. The result: Magnification as well as amplification, depending on the size of the lens.

The problem: Keep making larger and larger refracting telescopes and the lenses begin to deform under gravity. Reflector telescopes solve many of the problems of refractors, but we can see how far we’ve pushed their limits when we consider how we’ve had to move to segmented mirrors like the 36-mirror Gran Telescopio Canarias. Segmented mirrors cope with the deformation problems of a single large mirror but demand powerful computing resources. As their size continues to increase, costs skyrocket.

Kipping was originally inspired by the ‘Green Flash’ phenomenon that is the result of the refraction of the Sun’s light at sunset or sunrise. It lasts no more than a second or two, and appears because blue light is attenuated by scattering in the atmosphere as sunlight is spread into its constituent colors. The Columbia physicist, working thirteen years ago on a master’s thesis at Cambridge, realized that at a certain distance from Earth, if the Sun were directly behind our planet, a global green flash would appear, forming a green ring around Earth.

Image: The green flash as seen in Santa Cruz, VA. Credit: Brocken Inaglory CC BY-SA 3.0.

And what happens with more distant starlight? If the Earth is in front of a star, light from the star is deflected by the Earth’s atmosphere by about half a degree as it enters, and another half a degree as it exits — for those light rays that skim the surface. This sets up a focal point at a distance less than the Earth’s distance from the Moon. Even more significantly, a focal line is created as we consider light rays that enter the atmosphere higher up. Here the bending effect is somewhat less, but we also begin to mitigate atmospheric effects that would put noise in our data.

Image: Light from a distant star is deflected by the atmosphere of the Earth by half a degree. After skimming the surface, it is bent again as it exits the atmosphere by one-half a degree. Light entering the opposite hemisphere does the same, creating a focal point. Rays entering the atmosphere higher up bend less because the atmosphere is thinner with altitude, so the result is a focal line. The trick is knowing which light can be effectively sampled. Image credit: David Kipping.

As you can infer from the image above, light that skims the surface of the Earth would encounter too many obstacles to be helpful, but light rays encountering the atmosphere at higher altitudes can give us a focal point at the distance of the Moon. Even here, though, we run into scattering and extinction effects produced by the atmosphere, which depend upon the wavelength of the light we’re looking at.

Moreover, clouds are a factor, meaning we have to choose light that enters the atmosphere higher still. In his paper, Kpping argues that a detector placed at the Hill Sphere distance, about four times the distance between the Earth and the Moon, is positioned to intercept light that would have skimmed the Earth’s atmosphere at an altitude of 14 kilometers. As he notes in a recent email: “So put a detector at the Lagrange point, look back at the Earth, block out the disk of the Earth itself, and around the rim you should see light from distant stars lensed into ring-like structures.”

Image: Extinction effects for a terrascope at the Hill Sphere distance, roughly 4 Earth radii. Here, most of the infrared spectrum becomes usable for lensing. Image credit: David Kipping.

The Hill Sphere distance is benignly close compared to the gravitational lens at 550 AU, which means it offers an interesting observing possibility at a distance we’re experienced at reaching — the James Webb Space Telescope will work within this range. If we can truly exploit this phenomenon, we can get amplifications in the range of 22,500, Kipping estimates. This is actually a conservative estimate, but probably necessarily so, as he explains in his email:

“Now the Earth is much smaller than the Sun, so the amplification is not comparable with FOCAL but still impressive. I compute it is 45,000 after accounting for clouds and extinction losses, and is fairly wavelength independent beyond a micron (refraction doesn’t change much beyond 1micron). I think you might lose up to half of that due to half of the Earth being in daylight, which obviously would be a bright noise source. So removing that, I think 22,500 amplification is more realistic. That means you would turn a 1 meter detector into a ~150 meter effective aperture, amplifying sources by nearly 11 magnitudes.”

A one-meter detector analyzes light in a way that a 150-meter space telescope would otherwise be capable of, without, of course, the vast cost incurred by the latter (remember that even the JWST, a 6.5-meter instrument, has already incurred costs in the range of $10 billion).

But how would we use an effect that allowed us to look only at whatever happens to be behind the Earth during the observing period? I want to talk about this paper more tomorrow, and also alert you to Kipping’s video description of the idea. There are also obvious issues having to do with atmospheric effects and questions about occultation methods in this work. But there are enough serious advantages — we’ll look at several more tomorrow — to make us delve deeper.

The paper is Kipping, “The ‘Terrascope’: On the Possibility of Using the Earth as an Atmospheric Lens,” accepted at Proceedings of the Astronomical Society of the Pacific (abstract).