April 16, 2016

All of the technology is nearly ready for megawatt space based laser systems for science and planetary defense

California Polytechnic State University researchers propose a system capable of probing the molecular composition of cold solar system targets such as asteroids, comets, planets and moons from a distant vantage.

Their concept utilizes a directed energy beam to vaporize or sublimate a spot on a distant target, such as from a spacecraft near the object. With sufficient flux, our published results indicate that the spot temperature rises rapidly, and evaporation of materials on the target surface occurs (Hughes et al., 2015; Lubin and Hughes, 2015; Lubin et al., 2014). The melted spot serves as a high-temperature blackbody source, and ejected material creates a molecular plume in front of the spot. Molecular and atomic absorption of the blackbody radiation occurs within the ejected plume. Bulk composition of the surface material is investigated by using a spectrometer to view the heated spot through the ejected material. They envision a spacecraft that could be sent to probe the composition of a target asteroid, comet or other planetary body while orbiting the targeted object. The spacecraft would be equipped with an array of lasers and a spectrometer, powered by photovoltaics. Spatial composition maps could be created by scanning the directed energy beam across the surface. Applying the laser beam to a single spot continuously produces a borehole, and shallow sub-surface composition profiling is also possible.

Their initial simulations of laser heating, plume opacity, material absorption profiles and spectral detectivity show promise for molecular composition analysis. Such a system has compelling potential benefit for solar system exploration by establishing the capability to directly interrogate the bulk composition of objects from a distant vantage. They propose to develop models, execute preliminary feasibility analysis, and specify a spacecraft system architecture for a hypothetical mission that seeks to perform surface molecular composition analysis and mapping of a near-earth asteroid (NEA) while the craft orbits the asteroid.

This drawing illustrates a system concept for investigating the molecular composition of a distant target, such as an asteroid or comet. A spacecraft is sent to the asteroid, and enters into orbit. Solar cells generate electricity that is used to power a laser, which is directed at the asteroid's surface. The laser will heat a spot on the surface, and very quickly material will begin to evaporate from the spot. The glow from the heated spot is visible at the spacecraft through the plume of evaporated material. Sensors in the spacecraft measure the intensity of light across a span of wavelengths; analysis of light intensity patterns provides information about materials in the plume of evaporated material. Credits: Mark Pryor (Vorticity, Inc.) , Gary B. Hughes (Cal Poly SLO)

Stand-off molecular composition analysis

Conceptual drawing of a proposed laser ablation system for remote composition analysis.

Composition of distant stars can be explored by observing absorption spectra. Stars produce nearly blackbody radiation that passes through the cloud of vaporized material surrounding the star. Characteristic absorption lines are discernible with a spectrometer, and atomic composition is investigated by comparing spectral observations with known material profiles. Most objects in the solar system—asteroids, comets, planets, moons—are too cold to be interrogated in this manner. Material clouds around cold objects consist primarily of volatiles, so bulk composition cannot be probed.

Additionally, low volatile density does not produce discernible absorption lines in the faint signal generated by cold objects. They propose a system for probing the molecular composition of cold solar system targets from a distant vantage. The concept utilizes a directed energy beam to melt and vaporize a spot on a distant target, such as from a spacecraft orbiting the object. With sufficient flux (~10 MW/m2 ) on a rocky asteroid, the spot temperature rises rapidly to ~2500 K, and evaporation of all materials on the surface occurs. The melted spot creates a high-temperature blackbody source, and ejected material creates a molecular plume in front of the spot. Bulk composition is investigated by using a spectrometer to view the heated spot through the ejected material. Spatial composition maps could be created by scanning the surface. Applying the beam to a single spot continuously produces a borehole, and shallow sub-surface composition profiling is possible. Initial simulations of absorption profiles with laser heating show great promise for molecular composition analysis.

Arxiv - Directed Energy Missions for Planetary Defense

ABSTRACT Directed Energy Missions for Planetary Defense

Directed energy for planetary defense is now a viable option and is superior in many ways to other proposed technologies, being able to defend the Earth against all known threats. This paper presents basic ideas behind a directed energy planetary defense system that utilizes laser ablation of an asteroid to impart a deflecting force on the target. A conceptual philosophy called DE-STAR, which stands for Directed Energy System for Targeting of Asteroids and exploRation, is an orbiting stand-off system, which has been described in other papers.

This paper describes a smaller, stand-on system known as DE-STARLITE as a reduced-scale version of DE-STAR. Both share the same basic heritage of a directed energy array that heats the surface of the target to the point of high surface vapor pressure that causes significant mass ejection thus forming an ejection plume of material from the target that acts as a rocket to deflect the object. This is generally classified as laser ablation.

DE-STARLITE uses conventional propellant for launch to LEO and then ion engines to propel the spacecraft from LEO to the near-Earth asteroid (NEA). During laser ablation, the asteroid itself provides the propellant source material; thus a very modest spacecraft can deflect an asteroid much larger than would be possible with a system of similar mission mass using ion beam deflection (IBD) or a gravity tractor. DESTARLITE is capable of deflecting an Apophis-class (325 m diameter) asteroid with a 1- to 15-year targeting time (laser on time) depending on the system design. The mission fits within the rough mission parameters of the Asteroid Redirect Mission (ARM) program in terms of mass and size. DE-STARLITE also has much greater capability for planetary defense than current proposals and is readily scalable to match the threat. It can deflect all known threats with sufficient warning

DE-STARLITE is one component of a more far-reaching philosophy for directed energy planetary defense. A future orbiting system is envisioned for stand-off planetary defense. The conceptual system is called DE-STAR, for Directed Energy System for Targeting of Asteroids and exploRation. Fluctuations in the Earth’s atmosphere significantly hinder ground-based directed energy systems; thus, deploying a directed energy system above Earth’s atmosphere eliminates such disturbances, as the interplanetary medium is not substantial enough to significantly affect the coherent beam.

The DE-STARLITE mission design, which is detailed in this paper, utilizes the same technologies and laser system as the larger standoff directed energy system. Namely, DE-STAR is a modular phased array of lasers that heat the surface of potentially hazardous asteroids to approximately 3000 K, a temperature sufficient to vaporize all known constituent materials. Mass ejection due to vaporization causes a reactionary force large enough to alter the asteroid’s orbital trajectory and thus mitigate the risk of impact. Each DE-STAR system is characterized by the log of its linear size (Lubin et al., 2014). DE-STARLITE is basically a DE-STAR 0, consisting of a laser phased array on the order of 1 meter in diameter. DE-STARLITE utilizes deployable photovoltaic arrays to power the system.

Artistic rendering of a deployed DE-STARLITE spacecraft deflecting an asteroid. The spacecraft is outfitted with two 15 meter diameter MegaFlex PV Arrays, a z-folded radiator deployed up and down, a laser array mounted on a gimbal at the front, and ion engines at the back. The laser array can be either a phased array or a parallel non phased array. A baseline mission includes 1 meter total aperture, with a goal to produce a 10 cm spot from a distance of 10 km. This is an artistic rendering only, demonstrating the overall concept. Note that the optimal thrust vector for orbit deflection is generally parallel or anti-parallel. One advantage of this approach is the ability to target the asteroid from a significant distance, mitigating the effects of the ejecta on the spacecraft. In addition to the ion engines shown in the rear of the spacecraft, there are small ion engines on the sides of the spacecraft for station keeping and maneuvering toward or away from the asteroid.

DE-STARLITE fits into the same basic launch vehicle and mass envelope as the current Asteroid Redirect Mission (ARM) block 1 program, which is designed to capture a 5-10 meter diameter asteroid; however, DE-STARLITE is designed to be a true planetary defense system capable of redirecting large asteroids. It has been designed to use the same ion engines as the ARM program and the same PV system, though due to the reduced mass of DE-STARLITE, a much larger PV array can be deployed within the SLS block 1 mass allocation (70 tons to LEO) if desired. The scaling to megawatt class systems is discussed below. This paper will focus on a 100 kW (electrical) baseline DE-STARLITE as a feasible and fundable option that could pave the way for the ultimate long-term goal of a full standoff planetary defense system. Larger systems are also discussed. This paper details the design of the main elements of the spacecraft, namely, the photovoltaic panels, ion engines, laser array, and radiator as well as the parameters of the launch vehicles under consideration, and details the deflection capabilities of the system.

Conceptual design of the deployed spacecraft with two 15 meter PV arrays that produce 50 kW each at the beginning of life for a total of 100 kW electrical, ion engines at the back, and the laser array pointed directly at the viewer. A 2 meter diameter laser phased array is shown with 19 elements, each of which is 1-3 kW optical output. A 2 meter diameter optical system is one of the possibilities for DE-STARLITE. More elements are easily added to allow for scaling to larger power levels. A 1 - 4.5 meter diameter is feasible; no additional deflection comes from the larger optic, just additional range from the target.

The objective of the laser directed energy system is to project a large enough flux onto the surface of a nearEarth asteroid (via a highly focused coherent beam) to heat the surface to a temperature that exceeds the vaporization point of constituent materials, namely rock, as depicted in Fig. 4. This requires temperatures that depend on the material, but are typically around 2000-3000 K, or a flux in excess of 10^7 W/m2. A reactionary thrust due to mass ejection will divert the asteroid’s trajectory (Lubin et al., 2014). To produce a great enough flux, the system must have both adequate beam convergence and sufficient power. From a distance of 10 km, a spot size on the asteroid of 10 cm provides enough flux to vaporize (sublimate) rock (Hughes et al., 2014). Optical aperture size, pointing control and jitter, and efficacy of adaptive optics techniques are several critical factors that affect beam convergence. As mentioned, the optical power output of the laser is projected to be between 35 kW and 70 kW, depending on technological advancements in laser amplifier efficiency in the coming years. Currently the amplifiers are about 35% efficient but it is expected they will exceed 50% within five years. Similar requirements are sought by power beaming systems (Mankins, 1997; Lin 2002). For the optional (non-phase-locked) fiber focal plane array the lasers are even more efficient and already exceed 50%. Any power level in this range will work for the purpose of this mission, but higher efficiency allows for more thrust on the target for a given electrical input as well as for smaller radiators and hence lower mission mass.

The proposed baseline optical system consists of 19 individual optical elements in a phased array.

Single element of laser phased array, showing fiber-tip actuator for mid-level pointing control and
rough phase alignment

Mounted hexagonal laser phased array with a baseline of 19 elements depicted: (a) at 45 degrees, (b)
face on, and (c) from the back

Both the laser array and the PV arrays are easily extended to larger power levels. The mass per unit power of the laser amplifiers is about 5 kg/kW currently with a strong push to bring this down to 1 kg/kW in the next five years. Similarly the PV is about 7 kg/kW, or similar to the laser amplifiers. Interestingly, it is the radiator panels that are the most difficult to scale up, at about 25 kg/kW. This is an area that needs work, though in all simulations for mission masses the assumption is 25 kW (radiated) for the radiator panels.

ATK has to scale their existing 10 meter diameter design to push the PV arrays to 30 meter diameter which will yield about 225 kW per manufactured unit, or 450 kW per pair and still fit in an SLS PF1B 8.4 m diameter fairing. Fig. 9 and Fig. 10 show the scaling and deployment of the PV arrays to larger sizes for various launch vehicles. Even larger sizes into the megawatt range can be anticipated in the future.

The DE-STARLITE system provides a feasible solution to asteroids and comets that pose a threat to Earth. By
utilizing a directed energy approach with a high powered phase locked laser array to vaporize the target surface the
thrust generated from the mass ejection plume is able to propel the asteroid threat away from the original collision
trajectory towards Earth. DE-STARLITE is a very system at a modest cost. As outlined above, DE-STARLITE employs
laser ablation technologies which use the asteroid as the propellant source for its own deflection, and thus is able to mitigate much larger targets than would be possible with other proposed technologies such as IBD, gravity tractors, and kinetic impactors. With the equivalent mass of an ARM Block 1 arrangement (14 tons to LEO - full SLS block 1 is 70 tons to LEO), designed to capture a 5-10 meter diameter asteroid, DE-STARLITE can mitigate an asteroid larger than Apophis (325 m diameter), even without keyhole effects. Much smaller DE-STARLITE systems could be used for testing on targets that are likely to pass through keyholes. The same technology proposed for DE-STARLITE has
significant long-range implications for space missions, as outlined in other DE-STAR papers. Among other benefits, the DE-STARLITE system utilizes rapidly developing technologies to perform a task previously thought to be mere science fiction and can easily be increased or decreased in scope given its scalable and modular nature. DE-STARLITE is capable of launching on an Atlas V 551, Falcon Heavy, SLS, Ariane V or Delta IV Heavy, among others. Many of the
items needed for the DE-STARLITE system currently have high TRL; however, one critical issue currently being
worked on is the radiation hardening of the lasers, though it appears achievable to raise this to a TRL 6 within 3-5 years.

Laser lifetime also poses an issue, though this is likewise being worked on; a path forward for continuous operation
looks quite feasible, with or without redundancy options for the lasers. Given that the laser amplifier mass is small and the system is designed to take multiple fibers in each configuration, redundant amplifiers can be easily implemented if needed. DE-STARLITE is a critical step towards achieving the long-term goal of implementing a standoff system capable of full planetary defense and many other tasks including spacecraft propulsion. DE-STARLITE represents a practicable technology that can be implemented within a much shorter time frame at a much lower cost. DE-STARLITE will help to establish the viability of many of the critical technologies for future use in larger systems.

Since all asteroids rotate at varying rates, this will cause the average applied thrust to decrease and this must be
taken into account in the system design. A lower limiting rotation period for gravitationally bound objects greater than 150 m is observed to be 2-3 hours consistent with being rubble piles. This effect needs to be taken into account for larger asteroids and for small fast rotators. Since the plume thrust begins within 1 second after the laser is initiated it is possible to compare the time scales of the laser spot motion to the mass ejection time scale to determine the effect of the rotation. In many cases rotation is not a fundamental concern but for those cases where it is, an option is to de-spin the asteroid, since this is an option with the proposed system. Running in a high peak power pulsed mode is one option available to mitigate rotation and allow de-spin. In summary, directed energy is an extremely promising option for true planetary defense. It is modular and scalable and allows for a very cost effective approach that has wide applications beyond planetary defense.

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