Five Billion Years of Solitude (31 page)

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

I
n 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 boost 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.

Spurred by Goldin’s enthusiasm and the tacit support of the Clinton administration, NASA quickly greenlighted SIM and convened working groups to solidify plans for TPF-I. Both projects eventually ran into major difficulties. Riding a strong initial pulse of funding, SIM met or surpassed all of its key developmental milestones, but by the mid-2000s the ballooning costs of JWST and of the Bush administration’s new Constellation program had reduced the project’s funding to a dribble. Most astronomers were unconcerned—SIM’s hyperspecialization seemed to offer little to the broader community. Even many planet hunters thought it superfluous, and hoped to simply skip over it to build a much more capable TPF. The mission was repeatedly downgraded and its launch continually delayed, piling on empty expenses until, after consuming more than half a billion dollars, in 2010 SIM was quietly cancelled and its nearly complete flight hardware junked or repurposed.

TPF-I faced a different problem: As its working groups delved deeper into the related technological hurdles, they realized the initial estimates for the mission’s cost and launch date were hopelessly optimistic. Cryogenically cooling all the separate mirrors would be costly and difficult. Reaction wheels required to rotate and point the mirrors on their long boom would cause the entire assembly to vibrate, potentially ruining observations. New designs emerged, including one from the European Space Agency’s own TPF-I project, code-named “Darwin.” Darwin and related concepts would eliminate vibrations by discarding
the long boom in favor of a free-flying array of several mirrors that would gather light and direct it into a central beam-combining hub. Instead of one cryo-cooled spacecraft, the project would now require five or six, each needing to fly in formation with centimeter-scale precision in deep space, drastically increasing mission complexity and the amount of propellant required. The runaway cost growth of the complex, cryogenic JWST suggested that, if anything, TPF-I’s early cost estimates of $1.5 billion would balloon to make it even more ruinously expensive than its predecessor. By 2001, JPL’s notional launch date for a TPF-I had slipped to no earlier than 2014, and mission planners were looking for cheaper alternatives, ideally a single non-cryogenic telescope.

Conventional wisdom held that the very same slippery wavelike behavior of light that enabled interferometry would prevent any single filled-aperture telescope from ever imaging Earth-like exoplanets. To capture the 10-billion-to-one photons emanating from an alien Earth at optical wavelengths, the light must be strictly controlled, the star’s overwhelming glare removed. Yet when starlight falls upon a single mirror it flows in liquescent wavelets, pooling and puddling in frozen ripples and coruscating speckles around the most minuscule surface imperfections. Even a mathematically perfect mirror, of the sort that only exists in computer simulations and the late-night dreams of theorists, would not be immune: light from a point-like distant star striking an ideal circular mirror would still diffract off the mirror edges, forming a central bright disk surrounded by a concentric series of rings. A good number of the disks, ripples, rings, and speckles tended to manifest precisely in the part of a star’s image where one would expect to find any lurking habitable planets. Each aberration would typically only be about a hundredth as bright as a target star, but would still be some eight orders of magnitude brighter than the faint light of any accompanying small, rocky worlds, rendering planetary detections improbable, if not entirely impossible. This was the scientific consensus as presented by any up-to-date optics textbook at the turn of the twenty-first century. And it was totally wrong.

The key to a single-telescope TPF solution was a device called a coronagraph that could, in theory, blot out a star’s diffraction disks and rings. Invented by the French astronomer Bernard Lyot in 1930 to observe the hot, nebulous corona that surrounds the Sun, a coronagraph is any occulting object placed in front of a telescope’s mirror to block out the unwanted light of a target star. To see a coronagraph in action, make one of your own. Hold your right thumb over the Sun’s disk in the sky to prevent most of its glare from reaching your eyes—the principle is the same. You may notice, however, that even if the Sun is entirely blocked, a small amount of sunlight still diffracts around your thumb’s edge. You can dampen some of that extra glare by placing your left thumb a short distance directly behind your right thumb as an extra barrier to block the Sun from your line of sight. In his coronagraphs, Lyot did something similar, crafting a series of “pupil” lenses, partially transparent “masks,” and disk-shaped opaque “stops” that progressively stripped out the residual light scattered off the edges of the initial occulter. Lyot’s instruments were suitable for imaging the Sun’s corona, which is a million times fainter than the Sun itself. But they leaked far too much stray light into a telescope’s optics to allow for the crucial 10-billion-to-one starlight suppression required to image an exo-Earth in visible light.