Things have been heating up in the asteroid belt. A modest telescope trained on the region between Mars and Jupiter would reveal a scene that one could easily mistake for fireflies flashing in a distant, dark wood. The furious activity at the border of the inner and outer solar system is geared toward constructing the Asteroid Belt Astronomical Telescope (ABAT), the most ambitious scientific project ever undertaken. Intense pulses of laser light—the firefly-like flashes that would be viewed from Earth—are transforming billions of 1- to 10-meter asteroids into components of a five-AU-diameter astronomical mirror. (The astronomical unit, AU, is the radius of Earth’s orbit around the Sun.) ABAT’s lasers process thousands of asteroids per hour, cutting and polishing their surfaces to optical flatness and high reflectivity. Last month the telescope passed the 1% mark toward completion and, to celebrate the milestone, released its first image.

Mirror, mirror, on the rock Section: Choose Top of page ABSTRACT Mirror, mirror, on the ro... << First image A devil in the details ABAT’s secure future REFERENCES CITING ARTICLES 1 1 1. A. R. Pillmaier, Space Manuf. Technol. 2, 117 (2020). carbon magnetically levitated in a vacuum—to a high-optical-quality mirror facet. He not only formed a flat optical surface, he also produced a highly reflective hemispherical bump on the back side of the carbon sphere. Using the same laser that formed the facet, Pillmaier was able to reorient the mirror by directing photon pressure to the bump from specific directions. The origins of ABAT, which is shown in figure, go back nearly a century to work by Andy Pillmaier, then a self-described over-the-hill assistant professor at Purdue University.Supported by a small research grant, Pillmaier used a laser to shape and polish an artificial asteroid—actually a 1-centimeter-diameter sphere of pyrolyticmagnetically levitated in a vacuum—to a high-optical-qualityfacet. He not only formed a flat opticalhe also produced a highly reflective hemispherical bump on the back side of thesphere. Using the same laser that formed the facet, Pillmaier was able to reorient theby directing photon pressure to the bump from specific directions. Those early investigations demonstrated the practicality of using laser light to form and manipulate a mirror component in outer space. Pillmaier had always hoped that the techniques he developed would one day lead to an AU-scale telescope, but the realization of his dream had to await the development of sufficiently powerful lasers, optical splitters capable of handling the extreme wattage of asteroid-chopping light, and quantum computers that could keep track of and control tens of millions of differently oriented mirrors spread out over many AU. asteroid. 2 2. V. V. Kurasova et al. , INTP Manuf. 111, 1127 (2090). asteroid mining to shape a 2-meter-diameter asteroid into a large version of Pillmaier’s polished carbon sphere and then steered the facet they created with the accuracy needed for a functioning AU-scale telescope. The success of that effort piqued the interest of the worldwide astronomical community, which raised sufficient public and private funds to start the ABAT project. Progress was slow until 2090. Then, with funding from the global space consortium, Vera Kurasova and her team at the Kharkov Institute of Physics and Technology reproduced Pillmaier’s work with an actualThey employed a pair of fusion-powered lasers used formining to shape a 2-meter-diameterinto a large version of Pillmaier’s polishedsphere and then steered the facet they created with the accuracy needed for a functioning AU-scaleThe success of that effort piqued the interest of the worldwide astronomical community, which raised sufficient public and private funds to start the ABAT project.

First image Section: Choose Top of page ABSTRACT Mirror, mirror, on the ro... First image << A devil in the details ABAT’s secure future REFERENCES CITING ARTICLES Most of ABAT’s observations will be of exoplanets. Project spokesperson Laura Kim notes, “Many great discoveries were made in the past century, and we have learned a lot about the atmospheres of nearby exoplanets. But I and the rest of the astronomical community won’t be satisfied until we see those planets as clearly as we can see Earth from the Moon.” exoplanets far superior to anything achieved before. Last month it released 3 3. L. A. Kim et al. (ABAT collaboration), Exoplanet. Stud. 98, 1709 (2116). 2 exoplanet covered an area of 640 000 m2 on the telescope’s focal plane. The telescope is still learning the locations of its existing mirror facets and the manipulations needed to correctly orient them, so maximum exposure times remain short. Nonetheless, its remarkable release shows that we have entered an entirely new era of observational astronomy. When ABAT approaches its full potential (see the exoplanet like Gliese 832 c, which is 16 light-years away, could be resolved into 2.5 terapixels. That would correspond to a resolution of approximately 10 meters on the planet’s surface. Only 1% finished, ABAT is already able to obtain images of nearbyfar superior to anything achieved before. Last month it releasedits first such image, of Gliese 832 c (figure). So vast is ABAT that the raw image of thecovered an area of 640 000 mon the telescope’s focal plane. Theis still learning the locations of its existingfacets and the manipulations needed to correctly orient them, so maximum exposure times remain short. Nonetheless, its remarkable release shows that we have entered an entirely new era of observational astronomy. When ABAT approaches its full potential (see the table below), a nearbylike Gliese 832 c, which is 16 light-years away, could be resolved into 2.5 terapixels. That would correspond to a resolution of approximately 10 meters on the planet’s Table I. Asteroid Belt Astronomical Telescope imaging parameters Diameter 5 AU Focal length 40 AU Total mirror-facet area Current Ultimate 4 × 109 m2 4 × 1011 m2 Angular resolution Current Ultimate 3 × 10−7 arcsec 2 × 10−11 arcsec

A devil in the details Section: Choose Top of page ABSTRACT Mirror, mirror, on the ro... First image A devil in the details << ABAT’s secure future REFERENCES CITING ARTICLES The lasers that are currently forming ABAT’s optical components can also align tens of millions (and eventually billions) of mirror facets to bring light from celestial sources to a precise focus on imaging arrays that lie above and below the plane of the solar system. The telescope points at different targets by rotating all its asteroid mirrors and maneuvering the imaging arrays to the new focal-point location. Ultimately ABAT will push around millions of imaging arrays as if they were chess pieces, as it sweeps the sky for information about the early universe and for signs of habitable and inhabited worlds. mirror facets has a highly polished, flat reflective surface and an equally polished hemisphere opposite the facet as shown in figure 3 surface ensure that the process does not transfer net momentum. As in Pillmaier’s prototype, each of ABAT’sfacets has a highly polished, flat reflectiveand an equally polished hemisphere opposite the facet as shown in figure. Intense beams of laser light bouncing off the hemispherical reflector serve as actuators by exerting minuscule torques that turn the facet in the desired direction. Coordinated laser pulses directed at the front, flatensure that the process does not transfer net momentum. 4 4. T. M. Hortonson et al. (ABAT collaboration), J. Diatom Eng. 152, 872 (2096). Developing a conventional monolithic sensor array to span ABAT’s focal plane would be impractical, so Kim’s team hit upon a new approach based on diatoms, the microscopic organisms with silica skeletons that hold such a special place in the hearts of nanotechnologists. Group biologists genetically engineered diatoms to produce several specialized organs.Some of the organs convert light to electrical signals. Some store the microwave input power necessary for operating the array. Still others use CPS (celestial positioning system) signals to determine three-dimensional location, measure time, and digitally encode the data for transmission. Trillions of the self-reproducing sensors can simply be grown in a vat. Once they are deployed in space, a celestial spiderweb of crisscrossed laser beams can push around clouds of those microscopic optical sensors to desired locations. Image readout begins when maser beams wake the sensors and provide transmission instructions. Guided by the electromagnetic field of the masers, the sensors align themselves toward a receiving antenna to which they beam their data. The vast data set is then relayed to ABAT’s brain, which puts the information together to form an image. To block sunlight from reaching a sensor swarm, lasers precisely corral asteroid ablation residue into a structure called the Devil’s Footprint. The name comes from a youthful memory Kim has of her student days at the University of Munich. As she recalls, “Munich’s most important cathedral, the magnificent Frauenkirche, has a very interesting feature that you encounter just as you enter the church. There is a lowered area of the floor that vaguely resembles a footprint. When you stand on that part of the floor, no windows are visible.” Many versions of the legend explain the origin of the feature. “As I remember it,” she says, when the Devil walked into the newly completed cathedral, he was overwhelmed by the sunlight streaming in through the church’s many stained-glass windows. In a fit of anger, he stomped his foot, leaving an imprint in the stone, and said, “From this spot, let no man see a single window.” If you stand at the spot, the many columns that support the cathedral ceiling block your view of the windows. “Our Sun-eclipsing swarm of asteroidal detritus plays the same role,” she notes. “But it exists because of our love of the cosmos, not our hatred of sunlight.”