Astronomers and Earth’s atmosphere are natural enemies. Stargazers want crisp, clear images of their celestial targets, whereas winds and clouds scatter and block starlight in ways that can scuttle even the most careful measurements. Minus the mild inconvenience of lacking air to breathe, many researchers might otherwise prefer our planet had no atmosphere at all—at least during their coveted observing nights at world-class telescopes. The Hubble Space Telescope and other giant off-world observatories can rise above the atmosphere’s complications but at costs that are, for lack of a better word, astronomical.
Now a new preprint study suggests that far from being a bane, Earth’s atmosphere could become astronomy’s boon, serving to amplify starlight in ways that reduce the need for enormous (and enormously expensive) telescopes on the ground and in space. Astronomers badly need such money-saving, performance-boosting approaches as the cost of building new state-of-the-art observatories soars to unsustainable levels.
The most obvious ways to make new discoveries are to look deeper into the heavens or to find fainter objects—which require making ever larger mirrors to collect as much starlight as possible. But that strategy is fast becoming prohibitively expensive as researchers clamor for better, bigger equipment: the nearly 25-meter Giant Magellan Telescope being built in Chile is expected to cost about $1 billion, and the 6.5-meter James Webb Space Telescope currently being prepped for launch in 2021 has a price tag just shy of $10 billion.
Columbia University astronomer David Kipping, author of the paper, which will be published in Publications of the Astronomical Society of the Pacific, says his concept could lead to a “terrascope” that, at only one meter across, could collect as much light as a 150-meter mirror. “The potential of it is huge,” Kipping says. “You could detect mountain ranges on exoplanets. You could detect the faintest sources [of light] in the universe.” A terrascope, Kipping suggests, might even unveil signs of life or even intelligence beyond our solar system.
The key to all this would be atmospheric refraction, which is the way light bends as it enters Earth’s atmosphere from space—a phenomenon you might know best as the cause of our planet’s colorful sunsets. In certain situations, refraction can focus a huge amount of light on a small area, erasing the need for a giant structure to catch it all. In particular, light from far-off sources can refract through the upper atmosphere to form a cone around Earth, projecting rays that come together at a point slightly closer than the moon and then extend outward in a straight line.
An observer on that line would see a distant light source directly behind Earth as a bright ring, amplified some 22,500 times greater than if our planet was not there to refract, Kipping estimates. “This huge [of an] amplification will never be achieved by [a] manufactured telescope,” says Jean Schneider, a physicist at the Paris Observatory. Building, launching and operating a one-meter terrascope at a point of orbital stability slightly beyond the moon would be easy with today’s technology. The only obstacle, Schneider says, is funding.
Kipping is not the first to bring up the concept: so-called atmospheric lensing has been discussed since at least 1979. “In a way, the idea has always been with us,” he says. “The point of my paper was really to highlight this exciting possibility that might deserve further attention.”
“While there are lots of details to work out, this is an example of the innovative thinking that could lead to scientific breakthroughs on a budget where taking a risk makes sense,” says Martin Elvis, a Harvard University astrophysicist who has argued for new ideas to curb the runaway costs of cutting-edge telescopes.
A terrascope could be more than just a telescope, Kipping says. Given a transmitter rather than a detector, the signal-boosting process is reversed: waves of light travel to Earth, refract through the upper atmosphere and refocus on the other side. The result is a narrow beam that can send messages to other planets. Because other planets in our solar system also have light-refracting atmospheres, Kipping says, “you could have an interplanetary communications network—an Internet across the solar system.”
There are pitfalls. For one, Kipping’s calculations are very preliminary; they rely on simplified atmospheric models that do not fully account for real-world variables such as high-altitude clouds. So a terrascope’s performance might fall well short of the estimates offered in his paper. And because atmospheric refraction will only enhance the light of objects precisely aligned to be directly behind Earth as seen by a terrascope, a single device would only be able to image a small fraction of the sky. Launching several detectors would mitigate this problem, but Kipping notes that doing so might also counteract the cost savings that make the idea so appealing.
Some of the difficulties are serious. Slava Turyshev of NASA’s Jet Propulsion Laboratory says Kipping’s estimates for constructing a clear image are overly optimistic. Chief among Turyshev’s concerns is the way unwanted light from Earth, the sun, the moon and even the vicinity of a far-distant target would interfere with imaging. Preventing such “noise” from obscuring any “signal” being received or transmitted by a terrascope, he says, is “very hard, if not impossible.” Another complication is the nature of refraction itself: the degree to which light refracts through Earth’s atmosphere is a function of the light’s wavelength, or color, which can easily lead to scrambled pictures.
Kipping agrees that, at the very least, there is a lot of work to be done. “Questions like this are exactly the sort of thing I hope future research can pursue,” he says. But the terrascope’s potential may be too good to ignore: “The idea of a 100-meter-class—or even larger than that—telescope in space is really tantalizing,” he says.