Five Billion Years of Solitude

by Lee Billings

  “Astronomers aren’t the only people interested in large space telescopes,” Mountain told me later in his office. “We’ve been talking about NASA, but there is another, much more well-funded government agency which tends to look down rather than up.” He was referring to the secretive U.S. National Reconnaissance Office, the NRO. He arched an eyebrow, and noted that the Hubble had been an offshoot of the NRO’s once-classified “Keyhole” series of spy satellites. “I don’t have the security clearances, and I’d like to keep it that way, but you don’t need security clearances to calculate what aperture size is required if you want Hubble-style image quality and need to avoid someone hiding from you by taking out their watch to time the passage of a satellite overhead.” In a geostationary orbit nearly 36,000 kilometers (22,000 miles) above the Earth’s equator, a satellite would move at the same rate as the Earth turned, effectively hovering in the sky over a fixed point on the planet. To gain useful high-resolution images of the Earth from such altitudes, Mountain implied, would require a mirror on the order of 10 or 20 meters in size. Placed in geostationary orbit over three or four strategic geographic regions, such mirrors could serve as unblinking sentinels, constantly monitoring nearly the entirety of the Earth’s surface.

  From such a system, you could run, but you couldn’t hide, Mountain intimated. The weight reductions possible through active optics would be “what lets you actually launch the bloody things” on existing rockets. In all likelihood, the technologies for active optics and lightweight mirrors in space were more mature than publicly known, and if eventually declassified could greatly benefit science and society. Mountain extolled the possible virtues: besides imaging alien Earths, the light-gathering power of an 8-meter or 16-meter mirror would revolutionize the rest of space-based astronomy, allowing astrophysicists to witness the formation of supermassive black holes and probe the cosmic distribution of dark matter. More generally, he said, large, cheap mirrors could also prove useful for beaming solar power to receiving stations on Earth, or for monitoring our own planet’s changing atmosphere at the resolution of individual clouds to constrain weather forecasts and climate-change projections.

  Some months after my discussion with Mountain, the NRO presented NASA with a smaller but still significant gift: two unused space telescopes and related hardware sitting in a restricted clean room in upstate New York. The NRO considered the telescopes obsolete, and rather than keep them indefinitely in storage, chose to offload them on the nation’s struggling civil space agency. Each telescope was outfitted with a Hubble-size, Hubble-quality 2.4-meter primary mirror—suitable for a host of astronomical observations but too small to be of obvious use in characterizing potentially habitable exoplanets. NASA would need to spend money on launch vehicles and instrumentation to properly utilize the NRO observatories, but the gifts freed up at minimum hundreds of millions of dollars that the cash-strapped agency could, if it chose, devote to developing technologies for larger, life-finding telescopes. Whether NASA would actually make that choice, however, was far from guaranteed.

  “One of the problems we have at the moment is that NASA has yet to make up its mind what it wants to be when it grows up,” Mountain told me during our discussions. “It’s still very much in this mode of boys and their toys, of big rockets as jobs programs. NASA needs a more enduring vision than that, but it can’t change without consultation with Congress and the American people. Ultimately, going out and looking for life, whether around other stars or on other planets in our solar system, that’s an infrastructure that can create powerful partnerships between the agency’s human spaceflight and scientific sides! That same sort of partnership was why the Hubble mission was so successful. Hubble had no peer because we could visit and renew it.”

  As he spoke, Mountain gradually slipped into a more colloquial vernacular, as if leveling over a beer with a skeptical Texas congressman. “So, for example, NASA wants to go to Mars. Well, they ain’t gonna get to Mars before 2030, right? So what else are the astronauts gonna do in the meantime? You’re not gonna get people to Mars building things smaller, you gotta build ’em bigger. Big infrastructures in space for commercial, scientific, and defense applications—that’s the future. Maybe the astronauts should get even better at assembling big systems in space. Maybe the agency should invest in robotically servicing these big structures. Maybe we should better leverage our investment in the International Space Station. Oh, and by the way, we’ve got a great idea you can do all of that with.”

  Conceived in collaboration with three NASA research centers, the Institute’s idea was called OpTIIX, a convoluted acronym for the Optical Testbed and Integration on ISS eXperiment. Proposed for launch to the ISS as early as 2015, OpTIIX would be a low-cost, scalable platform for testing the assembly and active correction of a lightweight, flexible, segmented mirror in space. Its 1.5-meter primary mirror would be composed of six fully actuated 50-centimeter hexagonal segments, each manufactured from sheets of silicon carbide glazed with atom-thin layers of vaporized metal. Collected starlight would bounce from the primary mirror up to a smaller secondary mirror, then back down to a series of tertiary “fast steering” and “pickoff” mirrors that would compensate for jitter and channel the light to cameras for imaging and wavefront control. Star trackers and gyroscopes would work in tandem with a latticework of lasers beamed across the primary mirror to precisely point the telescope and maintain its optimum figure. Thanks to those technologies, OpTIIX would deliver clear images of stars and galaxies despite being bolted to the outside of the ISS, which at any given time would be jostling and bucking in sympathetic frequencies with its cargo of noisy, weighty, rambunctious astronauts. If necessary, the astronauts could perform spacewalks to repair or upgrade the system, but OpTIIX would be designed to allow fully robotic assembly and maintenance as its modular pieces were ferried up to orbit.

  “We are at the limit of what the present paradigm of heavy launch vehicles and restricted folding geometries can do,” Mountain said after a time, reverting to his professorial mode. He gazed out his office windows, past a windowsill lined with five framed pictures of his family. The morning’s fog had boiled off beneath the weak winter sun, revealing a sere landscape of bare trees and dormant grass.

  “Right now we build and test space telescopes on the ground, then fold them up to fit inside a rocket. Right now, without bigger rockets, you can’t get much bigger telescopes. Things like OpTIIX could be the beginning of a scale-invariant process of building bigger and bigger space telescopes, because you take out all the extreme tolerances. If you think about it, for the ground we don’t assemble and test a telescope in a big basement somewhere then cart it up to a mountaintop, do we? No, of course not. We assemble it piece by piece on the mountaintop on the assumption we can then bring its components into alignment. Technical concepts like active optics let you assemble, align, and upgrade your telescope right where it belongs—in space. You can imagine putting things together by robots or astronauts or some combination, and then you just keep on going. You keep on going. You could scale up your telescope almost to infinity.”

  I asked Mountain how likely he thought it was that this vision would come to pass. He furrowed his brow and dragged a hand over his jaw, producing a sound like dry, windblown leaves.

  Americans could well choose to further divest from space science, he finally said, seeming to address his ghostly reflection in the glass windowpane. “The reality is, we’re using federal money, and an awful lot of it. If that money leaves, it doesn’t necessarily come back. It flows elsewhere, into other priorities, which I’m not necessarily objecting to. But you can imagine the discontinuous change. A lot of the capability we’ve built up could go away quite quickly. On the other hand, an argument can be made that investing in science and technology, space science as one part of that, is precisely what has kept this country going and can keep it going into the future as other economies rise—China, India, Europe, and so on. The real issue for me is, is the position we�
��re in part of a natural evolution, or is this just a lucky event?”

  In Mountain’s view, the golden age of Hubble and the other great observatories was a fortunate aberration, something just as much a product of geopolitics and economics as it was of pure technological development and scientific progress. Its genesis could be found in the formative events of the latter half of the twentieth century—the Baby Boom, the Cold War, the Space Race. Astronomers had harnessed that unlikely coalescence of opportunities to create for themselves an almost-mythical dreamtime, a radiant era in which the boundaries of technological capability slipped beyond the mundane realm of Earth, and the horizons of scientific discovery reached the edge of the known universe. And now, perhaps, it was all at an end.

  “Lyman Spitzer came up with the idea of Hubble in 1947, and we finally got the Hubble launched in 1990,” Mountain said. “But if we hadn’t had the space shuttles, if we hadn’t had the Department of Defense developing its spy satellites, making Hubble a reality would probably have taken several decades more. That’s the sort of era we’re returning to, in my opinion. We spent a fortune on Hubble, but it generated its own momentum. It gave us Compton and Chandra and Spitzer and some completely new technology. It gave us JWST, this amazing, huge cryogenic infrared telescope. That was the aberration, that was the Baby Boomers at work. And now they’re going away, and we’ve spent almost every single penny we’ve got, and we’ve got a new generation facing this fundamental shift. It’s hard. . . . What astronomers need to recognize is that once a project’s budget reaches a billion dollars, it enters a whole new realm where other factors besides pure science come into play. The science becomes a necessary but not sufficient condition. That’s really why you and I are talking like this right now.” He turned away from the window to face me.

  “Someone must explain that understanding how the Earth works in fine detail and mastering space technology is actually pretty good for everybody involved. Someone should say that finding life elsewhere could be a humbling experience that would be good for humanity as well. Maybe it could finally give us the kick in the pants we need to fully realize that we could screw up everything if we don’t get our act together. When Galileo lifted that little telescope up to his eye, he didn’t quite know what he was doing, but he unleashed a revolution. Maybe we’re on the verge of another one. We are now beginning to appreciate the complexity of the Earth system, and we are faced with controlling that complexity. We are now realizing that biology and astrophysics are intimately linked. These are hard concepts, but we need to master them as a species to survive. Otherwise, you know, maybe we will find life out there that arose independently, but that would actually be really bad news. Think about it: if extraterrestrial life is everywhere, but sentience and technology are nowhere to be seen, that probably means societies like ours don’t survive very long. Instead, they self-annihilate. If we master all this complexity, we don’t have to be in that position. We should battle this push toward the small, this turning inward.”

  The Institute’s OpTIIX initiative ran out of money in late 2012, just after successfully completing its preliminary design review. Without an estimated additional $125 million, it would never reach the ISS.

  The Order of the Null

  In 1996, when NASA’s administrator, Dan Goldin, unveiled the agency’s plans for a future fleet of space telescopes to image Earth-like planets, the vision he laid out was largely based on a single study, the results of which were published under the title A Road Map for the Exploration of Neighboring Planetary Systems. Goldin had commissioned the study only months before the first discoveries of exoplanets around Sun-like stars, and in the aftermath of those announcements its findings took on new urgency. The study was multitiered, with three separate teams and more than a hundred outside experts offering consultation, but its overall lead was Charles Elachi, a planetary scientist and electrical engineer at the Caltech/NASA Jet Propulsion Laboratory in Pasadena, California. Elachi was overseeing the Laboratory’s space and Earth science programs at the time, and would later ascend to JPL’s directorship. JPL is legendary in space-science circles as the NASA center most responsible for the agency’s greatest robotic explorers—the Pioneer and Voyager probes, the Mars landers, rovers, and orbiters, the Galileo mission to Jupiter, the Cassini mission to Saturn, the Kepler mission, and many others had been designed, built, or managed by JPL. With the exoplanet boom looming, JPL and Elachi saw an opportunity for further prestige and growth: while the Space Telescope Science Institute operated NASA’s space telescopes, JPL and its affiliates would develop and build them. If the new telescopes found any promising planets around nearby stars, JPL might even construct the first robotic probes sent voyaging to other worlds outside the solar system.

  In many of their Road Map presentations, Elachi and his coauthors referenced images like the famous “Blue Marble” photograph of Earth, snapped from a distance of 45,000 kilometers by one of the astronauts of Apollo 17 as they traveled to the Moon in 1972. The whole-hemisphere image reveals the entirety of Africa, covered with jungle, savannah, and desert, as well as the arid Arabian Peninsula and much of ice-covered Antarctica. Whorls and wisps of white cloud stand out against the deep blue seas, and a cyclone can be seen swirling in the Indian Ocean. By showing the Earth as a lonely and fragile oasis in space, the Blue Marble had helped galvanize the environmentalist movement of the 1970s, and it became one of the most widely distributed images in history. What sort of space telescope would it take, the Road Map teams wondered, to reveal such details about a world orbiting another star? Their calculations were sobering: obtaining a Blue-Marble-style optical-wavelength image of an Earth twin orbiting one of the Sun’s nearest neighboring stars would require a single mirror—a “filled aperture”—at minimum some 5,000 kilometers, or 3,000 miles, in diameter. That is, a mirror roughly the same size as the continental United States. Barring humans suddenly developing the technological capability to somehow convert large asteroids into ultra-smooth polished mirrors, such gigantic filled apertures appeared forever out of reach. And even if such a large mirror could be made, the issue of suppressing the 10-billion-to-one glare of starlight loomed as another enormous technical challenge.

  Fortunately, the laws of physics offered a single solution to both problems. When light is emitted from the surface of a star, reflected off the atmosphere of a planet, or absorbed by the material of a detector, it acts like a particle. But as it travels through interstellar space or across a telescope’s mirrors, it behaves more like a wave. Instead of photons pinging against a mirror like drops of rain, imagine a continuous wavefront of light impacting and propagating across every square centimeter of a mirror’s surface simultaneously. This wavelike nature of light allows a curious trick that astronomers call “interferometry”: rather than building, say, a 10-meter mirror, a physics-savvy astronomer could simply place two 1-meter mirrors at a “baseline” of 10 meters apart, combining the light from each mirror to produce a single image with a 10-meter aperture’s resolution. The wavefronts of light propagating from a far-distant source such as a star can equitably fall on any number of interlinked smaller mirrors as if they are a single larger aperture. Place a 1-meter mirror in Los Angeles and another in New York, then link and synchronize them via a computer-controlled beam combiner, and you’ve made an interferometric array with a baseline of 5,000 kilometers and the resolution of a continent-size mirror. Its light-gathering power, however, would still be equivalent to those two meter-size mirrors, and the array’s synchronization would be stymied by the curvature and rotation of the Earth and the overlying atmosphere; gathering enough photons to construct a single high-resolution image of an exoplanet would be entirely infeasible. In deep space, however, an interferometer would be above the atmosphere and could stare uninterrupted by the passage of day or night. Freed from gravity and planetary curvature, in theory it could be made arbitrarily large, with any number of individual mirrors to boost its sensitivity and a baseline of any length to boo
st its resolution.

  Furthermore, when astronomers recombined the disparate waves of light gathered by each mirror, they could align the light waves so that the wave troughs of one beam would precisely overlap with the crests of another beam, splashing against and annihilating each other like out-of-phase ripples on the surface of a pond. The destructive interference would form bands of dark shadow within a resulting image. The shadows would be dark enough, in fact, to null out the bright light of a star, allowing the dim twinkle of accompanying planets to be seen. Short of using the Sun itself as a gravitational lens, an interferometric array offered the greatest hope of obtaining a Blue Marble image of any exoplanet.

  Elachi and his coauthors seized upon the interferometer concept for a TPF, and designed a mission optimized for observing in the infrared, where the star-planet contrast is only 10 million, compared with 10 billion in the optical. Four 1.5-meter cryogenically cooled mirrors on a linear boom forming a 75-meter baseline would operate beyond the orbit of Jupiter, where there is less leftover dust from our solar system’s formation to scatter and corrupt the faint light from nearby stars. If the mission was to operate closer to Earth, each of its mirrors would need to be doubled in size to 3 meters to compensate for the greater density of primordial dust that exists closer to the Sun. TPF-I, as the general mission concept came to be called, would deliver no Blue Marble images of alien Earths, but it would be capable of taking “family portraits” of planetary systems around the nearest thousand stars, with each planet manifesting as a single pixel in the TPF-I’s detectors. Measuring the color of the pixel would hint at whether a world was rocky, ocean-covered, or sheathed in a thick envelope of gas. Cracking its light into a spectrum would allow the detection of atmospheric carbon dioxide, water vapor, and the possible biosignatures of methane and oxygen. Tracking the pixel’s fluctuating brightness over months and years could reveal the planet’s bulk geography—the locations of its continents, oceans, and ice caps—as well as its seasons. The success of the Road Map’s interferometric mission would then pave the way for larger future interferometric arrays that would use formation flying and laser communication to achieve baselines of several thousands of kilometers, missions that could perhaps replicate the Apollo Blue Marble for habitable worlds orbiting other stars. To pave the way for TPF-I itself, a precursor mission called the Space Interferometry Mission (SIM) would be launched. As first conceived, SIM would string seven small mirrors across a large boom, providing as much as a 10-meter interferometric baseline, sufficient to survey more than a hundred nearby stars for the astrometric wobbles of any accompanying Earth-mass planets in their habitable zones.