The Air Force Research Laboratory/Directed Energy Directorate (AFRL/DE) and NASA/Marshall Space Flight Center (MSFC) are looking at a series of joint laser space calibration experiments using the 12J 15Hz CO2 HIgh Performance CO2 Ladar Surveillance Sensor (HI-CLASS) system on the 3.67 meter aperture Advanced Electro-Optics System (AEOS). The objectives of these experiments are to provide accurate range and signature measurements of calibration spheres, demonstrate high resolution tracking capability of small objects, and precision dray determination for LEO. Ancillary benefits include calibrating radar and optical sites, completing satellite conjunction analyses, supporting orbital perturbations analyses, and comparing radar and optical signatures. In the first experiment, a Global Positioning System (GPS)/laser beacon instrumented micro-satellite about 25 cm in diameter will be deployed from a Space Shuttle Hitchhiker canister or other suitable launch means. Orbiting in low earth orbit, the micro-satellite will pass over AEOS on the average of two times per 24-hour period. An onboard orbit propagator will activate the GPS unit and a visible laser beacon at the appropriate times. The HI-CLASS/AEOS system will detect the micro-satellite as it rises above the horizon, using GPS-generated acquisition vectors. The visible laser beacon will be used to fine-tune the tracking parameters for continuous ladar data measurements throughout the pass. This operational approach should maximize visibility to the ground-based laser while allowing battery life to be conserved, thus extending the lifetime of the satellite. GPS data will be transmitted to the ground providing independent location information for the micro-satellite down to sub-meter accuracies.
We present results of computer simulations of the launch through the atmosphere of a cone-shaped flyer which demonstrate that laser ablation rockets, using a 1MW ground-based laser, can lift 6kg payloads into low earth orbit. We discuss optimization of delivered mass, mass ratio and energy cost.
This report briefly reviews the development, capabilities, and current status of pulsed high-power coherent CO2 laser radar systems at the Maui Space Surveillance System (MSSS), HI, for acquisition, tracking, and sizing of orbiting objects. There are two HICLASS systems, one integrated to the 0.6 m Laser Beam Director and one just integrated Summer 2000 to the 3.7 m Advanced E-O System (AEOS). This new system takes full advantage of the large AEOS aperture to substantially improve the ladar range and sensitivity. These improvements make the AEOS HICLASS system potentially suitable for tracking and characterization experiments of small < 30 cm objects in low-earth-orbits.
In 1995, the NASA Project ORION investigated the feasibility of orbital debris removal using ground-based sensors and lasers (Ref. 1). This study focused on high peak-power pulsed lasers capable of initiating plasma blow-off impulse. The conclusions drawn by this study indicated that a neodymium glass laser might represent the most cost effective and technologically viable solution. Large, repetitively pulsed neodymium glass lasers have been developed by Lawrence Livermore National Laboratory for inertial confinement fusion (ICF). However, the goal of ICF is to focus the high power laser beams on a small, stationary target at very close range. The orbital debris removal problem requires the mating of a high power laser to large diameter optics equipped with laser guide star adaptive optics. The target is a rapidly moving object located many hundreds of kilometers in distance. Since the conclusion of that study, the Air Force Airborne Laser (ABL) program, utilizing a continuous wave Chemical Oxygen- Iodine Laser (COIL), has progressed dramatically. This program integrates a high average power COIL with large diameter optics, which are adaptively controlled to correct for atmospheric turbulence. The target of the Airborne Laser is a rapidly ascending ballistic missile located hundreds of kilometers in range. The similarities between the Airborne Laser and the orbital debris removal mission motivate the examination of ABL COIL technology and its associated optical hardware for the orbital debris removal mission.
Approximately ideal flight paths to low-Earth orbit (LEO) are illustrated for laser-driven flights using a 1-MW Earth-based laser, as well as sensitivity to variations from the optima. Different optima for ablation plasma exhaust velocity VE, specific ablation energy Q*, and related quantities such as momentum coupling coefficient Cm and the pulsed or CW laser intensity are found depending upon whether it is desired to maximize mass m delivered to LEO, maximize the ratio m/M of orbit to ground mass, or minimize cost in energy per gram delivered. A notional, cone-shaped flyer is illustrated to provide a substrate for the discussion and flight simulations. Our flyer design conceptually and physically separates functions of light collection, light concentration on the ablator, and steering. All flights begin from an elevated platform. Flight simulations use a detailed model of the atmosphere and appropriate drag coefficients for sub- and supersonic flight in the continuum and molecular flow regimes. A 6.2-kg payload is delivered to LEO from an initial altitude of 35 km with launch efficiencies approaching vacuum values of about 100 kJ/g.
Results are presented of the first measurements of laser-ablation impulse on structured targets of the modified Fabbro type in which impulse coupling coefficients Cm up to 500 dyne-s/J were obtained for 85-ns duration laser pulses by direct measurement with momentum pendula. Our target design generated these Cm values for single-pulse laser fluence of 1.2 J/cm2 and peak intensity 14 MW/cm2, an order of magnitude below the intensity at which similar coupling has been observed before. This result is important for the ORION demonstration, since it effectively closes a factor- of-20 deficit between presently available projected intensity at 300 km range and the intensity required for optimum coupling to standard materials. The Nd:glass laser employed in these measurements had 1.05 micrometer wavelength and pulse duration from 25 to 100 ns. Ambient pressure was less than 10 millitorr. Impulse coupling data on water ice, stone, carbon phenolic, PMMA and other materials were also obtained, for cross-calibration and because of interest in applications to 'uncooperative' natural bodies in space. We discuss the significance of these results for planning a laser propulsion demonstration in space, as well as possible extensions which could yield appropriate Cm for repetitively- pulsed propulsion of objects into low Earth orbit (LEO).
Orbital debris in low-Earth orbit in the size range from 1 to 10 cm in diameter can be detected but not tracked reliably enough to be avoided by spacecraft. It can cause catastrophic damage even to a shielded spacecraft. With adaptive optics, a ground-based pulsed laser ablating the debris surface can produce enough propulsion in several hundred pulses to cause such debris to reenter the atmosphere. A single laser station could remove all of the 1 - 10 cm debris in three years or less. A technology demonstration of laser space propulsion is proposed which would pave the way for the implementation of such a debris removal system. The cost of the proposed demonstration is comparable with the estimated annual cost of spacecraft operations in the present orbital debris environment. Orbital debris is not the only space junk that is deleterious to the Earth's environment. Collisions with asteroids have caused major havoc to the Earth's biosphere many times in the ancient past. Since the possibility still exists for major impacts of asteroids with the Earth, it shown that it is possible to scale up the systems to prevent these catastrophic collisions providing sufficient early warning is available from new generation space telescopes plus deep space radar tracking.
Orbital debris in low-Earth orbit ranging in size from 1 to 10 cm in diameter can be detected but not tracked reliably enough to be easily avoided by spacecraft. In addition, shielding protection is extremely difficult and costly to accomplish for sizes above 1 - 2 cm. Debris in this size regime traveling at mean velocities on the order of 20000 miles per hour may cause catastrophic damage. Using adaptive optics technologies, a ground-based pulsed laser of sufficient power ablating the debris particle's surface to produce small momentum changes may, in several hundred pulses, lower a target debris particle's perigee sufficiently for atmospheric capture. A single laser facility could remove all of the 1 - 10 cm debris below 1500 km in altitude in approximately three years. A technology demonstration of ground based laser removal is proposed which would pave the way for the implementation of such a debris removal system. The cost of the proposed demonstration is comparable with the estimated annual cost of spacecraft operations in the present orbital debris environment.
Project ULTIMA is an investigation into the feasibility of building ultra-large aperture visible/mid-IR space telescopes. A promising concept found by the study is a freely flying spherical primary mirror, twenty meters or more in diameter, located at the L1 or L2 Earth-Sun libration point. The primary would be passively cooled to 45 K. There would be no metering structure. Instead, using a combination of alignment and steering mirrors, reaction wheels, and microthrusters, the aspherical secondary mirror, active tertiary mirror, and focal plane instruments would be precisely stationed in the correct position above the primary. The primary advanced composition would be either a membrane or ultra-light segmented technology. Preliminary fmdings show that a 20-30 m telescope may be feasible for imaging in the 1-20 µm regime.
Recent advances in adaptive optics support the feasibility of orbital debris removal by laser photoablation impulse, with laser and tracking systems located on the Earth. An in-depth systems analysis shows that individual laser pulses are most effective in lowering perigee and reducing lifetime at a zenith angle of about 40 degrees when the target is approaching the laser. Many pulses are needed to remove debris, and it is important to begin the engagement at the largest zenith angle permitted by tracking and adaptive optics. The 1-10 cm orbital debris hazard will require damage control equipment and procedures as well as on-orbit addition of shielding to the International Space Station. The current risk to satellites in low Earth-orbit is estimated to be $10-100 million per year in replacement costs. These factors suggest an international laser orbital debris removal system could be cost-effective. A demonstration in which radar and optical tracking together with adaptive optics are used to concentrate a laser on a calibrated target in orbit is the next required step in proving the feasibility and cost-effectiveness of such a system.
A proposed space-based test of gravitational theory requires unique performance for thermometry and ranging instrumentation. The experiment involves a cylindrical test chamber in which two free-floating spherical test bodies are located. The test bodies are spheres which move relative to each other. The direction and rate of motion depend on the relative masses and orbit parameters mediated by the force of gravity. The experiment will determine Newton's gravitational constant, G; its time dependence, as well as investigate the equivalence principle, the inverse square law, and post- Einsteinian effects. The absolute value of the temperature at which the experiment is performed is not critical and may range anywhere from approximately 70 to 100 K. However, the experimental design calls for a temperature uniformity of approximately 0.001 K throughout the test volume. This is necessary in order to prevent radiation pressure gradients from perturbing the test masses. Consequently, a method is needed for verifying and establishing this test condition. The presentation is an assessment of the utility of phosphor-based thermometry for this application and a description of feasibility experiments. Phosphor thermometry is well suited for resolving minute temperature differences. The first tests in our lab have indicated the feasibility of achieving this desired temperature resolution.
Fourier telescopes are an effective approach for providing images of hard x-ray and gamma-ray sources found in the sky. Sufficient coverage of the (u,v) plane is a requirement for effective imaging. Unfortunately, this requirement is diametrically opposed to the requirements to minimize instrument weight and cost which translate into minimizing the number of grids. This paper reports on a telescope design incorporating only one real and imaginary set of grid pairs which provides superior imaging performance at a significantly reduced cost.
The Marshall Space Flight Center, Alabama, in a teaming arrangement with the University of Florida, Gainesville, and the Joint Astronomy Center, Hawaii, has completed a comprehensive investigation into the feasibility of a low-cost infrared space astronomy mission. This mission would map the emission of molecular hydrogen in our galaxy at two or three previously inaccessible mid-IR wavelengths, and provide information on the temperatures. The feasibility of the low-cost mission hinged on whether a thermal design could be found which would allow sufficient passive cooling of the telescope to elimiate the need for a large, expensive dewar. An approach has been found which can provide telescope temperatures on the order of 50 K, which makes the mission feasible at low cost in low-Earth orbit.
The Marshall Space Flight Center, Alabama in a teaming arrangement with the Naval Research Laboratory, Maryland has developed the ISIS (impulsive solar imaging spectrometer) mission for viewing the sun and the sky in the EUV, soft, and hard x rays. The soft and hard x ray imaging as well as the gamma-ray spectroscopy will be provided by a three axis pointed Fourier telescope (i.e. a spatial modulation collimator). The EUV imager will be a supporting context instrument. This paper describes the optimized instrument concept and discusses the associated trades made in developing it. For example, the numbers of spatial frequencies measured versus the sensitivity needed for imaging weak sources is discussed in detail. ISIS builds upon the YOHKOH findings in that the telescope is tailored to image compact simple loop sources. Only two spatial frequencies need be measured, allowing substantial gains in sensitivity. In addition, this allows both the real and imaginary Fourier components to be measured, which is a vast improvement over approaches that measure only the real components.
Hard x-rays (10-500 keV) are produced by cosmic sources such as the Crab nebula and solar flares. Imaging these x-rays will allow insight into the processes at work in these energetic sources. Presently, a Fourier telescope design is flying on the Japanese Solar-A satellite providing hard x-ray images of the Sun, and Fourier designs are being considered for the next generation of high energy observing instruments (e.g., High Energy Solar Physics (HESP)). Current solar flare theoretical literature indicates a desire for spatial resolutions down to 1 arcsecond, fields of view greater than the full solar disk (i.e., 32 arcminutes), and temporal resolutions down to 1 second. Although the Sun typically provides relatively high flux levels, the requirement for 1 second temporal resolution raises a question about the viability of Fourier telescopes subject to the aforementioned constraints. Given sufficient sensitive areas, Fourier telescopes are promising concepts for imaging solar hard x-rays. In keeping with this new era of better, faster, cheaper space science missions, a new, virtual grid Fourier telescope approach is discussed. Given an appropriate detector configuration (i.e., 1D imaging detector), one grid may be eliminated completely from this new telescope. For gamma ray and perhaps hard x-ray imaging, this simplification should prove very useful especially in this new era of smaller space science missions.
The S-056 Wolter I soft x-ray mirror used originally on Skylab for x-ray observations of the Sun is still in use today. The mirror has been flow polished to a surface finish of 5 angstroms for spatial frequencies from 1 - 1000 mm-1 and has been tested in the AXAF x- ray calibration facility located at Marshall Space Flight Center, Alabama. The mirror performance was enhanced by a new polishing technique and was found to have an experimental point spread function full width half maximum of less than 1 arc second at 8.3 angstroms. No change in the figure was observed.
Presently, a Fourier telescope design is flying on the Japanese Solar-A satellite providing hard x-ray images of the Sun. Fourier designs are presently being considered for the next generation of high energy observing instruments (e.g., HESP). Hard x-rays (10-500 ke V) are produced by solar flares and cosmic sources such as the Crab nebula. Imaging these x-rays will allow insight to be gained as to processes at work in these energetic sources. Hard xrays, while not imageable by conventional means, may be imaged by Fourier telescopes. In this paper, an advanced rotating modulation collimator (RMC) design using several spatial frequencies is numerically modeled and examined using an end-to-end photon counting simulation. It is then compared to two basic Fourier telescopes measuring only two spatial frequencies, a spatial modulation collimator (SMC) and a RMC. While the more advanced telescope provided better images, diminishing improvement with more spatial frequencies for simple sources is clearly indicated. In addition, a tradeoff was identified for low flux sources in that for simple sources the basic telescope required fewer photons to achieve a stable image than did the more advanced version.
The sun emits hard X-rays (above 10 keV) during solar flares. Imaging hard X-ray sources on the sun with spatial resolutions on the order of 1-5 arcsec and integration times of 1 sec will provide greater insight into the energy release processes during a solar flare. In these events, tremendous amounts of energy stored in the solar magnetic field are rapidly released leading to emission across the electromagnetic spectrum. Two Fourier telescope designs, a spatial modulation collimator and a rotating modulation collimator, were developed to image the full sun in hard X-rays (10-100 keV) in an end-to-end simulation. Emission profiles were derived for two hard X-ray solar flare models taken from the current solar theoretical literature and used as brightness distributions for the telescope simulations. Both our telescope models, tailored to image solar sources, were found to perform equally well, thus offering the designer significant flexibility in developing systems for space-based platforms. Given sufficient sensitive areas, Fourier telescopes are promising concepts for imaging solar hard X-rays.
Several approaches to imaging hard X-rays emitted from solar flares have been proposed or are planned for the nineties including the spatial modulation collimator (SMC) and the rotating modulation collimator (RMC). A survey of current solar flare theoretical literature indicates the desirability of spatial resolutions down to 1 arcsecond, field of views greater than the full solar disk (i.e., 32 arcminutes), and temporal resolutions down to 1 second. Although the sun typically provides relatively high flux levels, the requirement for 1 second temporal resolution raises the question as to the viability of Fourier telescopes subject to the aforementioned constraints. A basic photon counting, Monte Carlo 'end-to-end' model telescope was employed using the Astronomical Image Processing System (AIPS) for image reconstruction. The resulting solar flare hard X-ray images compared against typical observations indicated that both telescopes show promise for the future.
Several approaches to imaging hard x-rays emitted from solar flares have been
proposed for the nineties. These include the fixed modulation collimator, the
rotating modulation collimator (RMC), the spiral fresnel zone pattern, and the
redundantly coded aperture. These techniques are under consideration for use in the
Solar Maximum '91 balloon program, the Japanese Solar-A satellite, the Controls,
Astrophysics, and Structures Experiment in Space (CASES), and the Pinhole/Occulter
Facility (P/OF) and are outlined and discussed in the context of preliminary results
from numerical modeling done at MSFC and the requirements derived from current ideas
as to the expected hard x-ray structures in the impulsive phase of solar flares.
Preliminary indications are that all of the approaches are promising, but each has
its own unique set of limitations.