December 28, 2014

Lasers in space could trap and shape reflective dust into telescopes with diameters wider than a planet for detailed imaging of exoplanets

Here we look in detail at the Phase I report for Orbiting Rainbows. NASA provided the funding for a phase 2 study project. They would use several lasers to trap and shape billions of reflective dust particles into single or multiple lenses that could grow to reach tens of meters to thousands of kilometers in diameter. According to Swartzlander, the unprecedented resolution and detail might be great enough to spot clouds on exoplanets. The diameter of the lens would be similar to what hypertelescopes could achieve in space however, the laser shaped dust clouds could more cheaply have a filled in lens.

Swartzlander developed and patented the techniques known as “optical lift,” in which light from a laser produces radiation pressure that controls the position and orientation of small objects.

Ideally, the dielectric particles should have 50% transmissivity and 50% reflectivity, no absorption, and to avoid diffraction they must be smaller than the wavelength of light. This giant, but tenuous, optical assembly has to be maintained either continuously or intermittently via separated free-flying pulsed lasers, which must have enough power, continuous
operation capabilities, and adequate pointing capability to maintain the cloud stably in orbit

Orbiting Rainbows: Optical Manipulation of Aerosols and the Beginnings of Future Space Construction [143 page final Phase 1 report for NASA Innovative Advanced Concepts (NIAC)]

Researchers propose to construct an optical system in space in which the nonlinear optical properties of a cloud of micron-sized particles are shaped into a specific surface by light pressure, allowing it to form a very large and lightweight aperture of an optical system, hence reducing overall mass and cost. Other potential advantages offered by the cloud properties as optical system involve possible combination of properties (combined transmit/receive), variable focal length, combined refractive and reflective lens designs, and hyper-spectral imaging. A cloud of highly reflective particles of micron size acting coherently in a specific electromagnetic band, just like an aerosol in suspension in the atmosphere, would reflect the Suns light much like a rainbow. The only difference with an atmospheric or industrial aerosol is the absence of the supporting fluid medium. This new concept is based on recent understandings in the physics of optical manipulation of small particles in the laboratory and the engineering of distributed ensembles of spacecraft clouds to shape an orbiting cloud of micron-sized objects. In the same way that optical tweezers have revolutionized micro- and nano-manipulation of objects, our breakthrough concept will enable new large scale NASA mission applications and develop new technology in the areas of Astrophysical Imaging Systems and Remote Sensing because the cloud can operate as an adaptive optical imaging sensor. While achieving the feasibility of constructing one single aperture out of the cloud is the main topic of this work, it is clear that multiple orbiting aerosol lenses could also combine their power to synthesize a much larger aperture in space to enable challenging goals such as exo-planet detection. Furthermore, this effort could establish feasibility of key issues related to material properties, remote manipulation, and autonomy characteristics of cloud in orbit. There are several types of endeavors (science missions) that could be enabled by this type of approach, i.e. it can enable new astrophysical imaging systems, exo-planet search, large apertures allow for unprecedented high resolution to discern continents and important features of other planets, hyperspectral imaging, adaptive systems, spectroscopy imaging through limb, and stable optical systems from Lagrange-points. Future micro-miniaturization might hold promise of a further extension of our dust aperture concept to other more exciting smart dust concepts with other associated capabilities.

The evolution of space telescopes, from Hubble, James Webb, inflatable concepts, formation flying, up to hyper-telescopes, where distributed apertures form the primary, naturally leads to the concept investigated in this study. This concept would increase the aperture several times compared to ATLAS, allowing for a true Terrestrial Planet Imager that would be able to resolve exo-planet details and do meaningful spectroscopy on distant worlds. The aperture does not need to be continuous. Used interferometrically, for example, as in a Golay array, imagery can be synthesized over an enormous scale. We leveraged our experience working with large optical systems to consider refractive, reflective and holographic systems. Finding a way to manipulate such distribution of matter in space would lead to a potentially affordable new way of generating very large and potentially re-shapeable optics in space, and indirectly open the way to future technologies for space construction by means of light. It will also enable new astrophysical imaging systems, exo-planet search, hyperspectral imaging, adaptive systems, spectroscopy imaging through limb, and stable optical systems from Lagrange points.

Radiation pressure positions the dust in a coherent pattern oriented toward an astronomical object. The reflective particles form a lens and channel light to a sensor, or a large array of detectors, on a satellite. Controlling the dust to reflect enough light to the sensor to make it work will be a technological hurdle.

Two RIT graduate students on Swartzlander’s team are working on different aspects of the project. Alexandra Artusio-Glimpse, a doctoral student in imaging science, is designing experiments in low-gravity environments to explore techniques for controlling swarms of particle and to determine the merits of using a single or multiple beams of light.

Swartzlander expects the telescope will produce speckled and grainy images. Xiaopeng Peng, a doctoral student in imaging science, is developing software algorithms for extracting information from the blurred image the sensor captures. The laser that will shape the smart dust into a lens also will measure the optical distortion caused by the imaging system. Peng will use this information to develop advanced image processing techniques to reverse the distortion and recover detailed images.

Assuming 10 million grains per aerosol patch, a grain density of 2500 kg/m3, 3 patches of diameter 1 meter, difficulty level 2, cloud thickness 1 micron.

SOURCES - Rochester Institute of Technology, NASA, Wikipedia

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