In this paper, the "weapon system" will be narrowly defined as the weapon itself; the platform on which it is carried; and the autonomous but interconnected surveillance, acquisition, tracking, and battle damage assessment (SAT/BDA) system needed to operate the weapon system in the desired "fire and forget" mode. The weapon system is a system-of-systems (weapon-platform-SAT/BDA) embedded in and interconnected with a much larger system-of-systems. Without a national global surveillance and reconnaissance system and associated intelligence system, no target will ever be found, assessed, and handed off. Without a secure, high-bandwidth global command, control, and communication (C3) system, sensor information and command decisions cannot get where they need to go. Without a robust, distributed information system, the many types of raw sensor data can never become the fused all-source information essential to battle management. Without adequate support in the area of readiness and sustainment, a weapon system can not be counted on to do its job. The weapon system concepts described in this white paper must be understood in this context. By 2025, no weapon system will be truly autonomous- to operate most effectively, the weapon systems of 2025 will depend on the smooth, high-speed functioning of the total US military war-making system.

The distributed nature of the system-of-systems described above can be its greatest strength or its greatest weakness. Any critical physical or intangible nodes in the distributed system could be attacked, rendering the entire system useless. The system-of-systems must be designed carefully to minimize or eliminate all critical nodes. Critical nodes that cannot be eliminated must be protected by deception, added defenses (hardening, placement within a secure environment), or redundancy. Ideally, the space weapon system itself should be so well distributed no sensible adversary would contemplate a preemptive strike.

Only the potential weapon system concepts will be discussed in the Space Operations white paper. Some concepts for integrating the weapon system into the global information network are contained in appendix B. The information, C3, and surveillance/reconnaissance and intelligence systems are addressed in other Air Force 2025 white papers.¹⁵

Weapon Platforms

The weapons themselves may be mounted on or fired from a space-based platform (space-based) or they may be mounted on platforms that traverse the space medium, such as an inter-continental ballistic missile (ICBM) or transatmospheric vehicle (TAV) (space-borne). Each scenario has its advantages and disadvantages, which will be detailed for each weapon system.

The space-based platform is the most responsive, because it operates immediately from the high ground of space. Possessing the unique perspective of space, space-based weapons can immediately cover a large theater of operations. This potential advantage grows as the platform's orbital altitude is increased, reaching its peak with platforms placed at geosynchronous orbit, which effectively provides access to almost half the earth's surface from a single platform. Of course, the higher the orbit, the farther the platform is from its targets. Alternatively, if the platform can be placed in low earth orbit (LOE), the range to the target can be minimized at the cost of reduced ground (and time) coverage for each platform. Given the immense volume of near-earth space, a space-based constellation can consist of many platforms, providing reliability through redundancy. A weapon system with enough space-based platforms at the proper orbital altitude(s) can potentially ensure global, full-time coverage and provide the ability to conduct prompt and sustained operations anywhere on the planet.

As hinted above, space-based platforms are not without their limits. The inexorable laws of physics demand that low earth orbit platforms have orbital periods measured in tens of minutes. Global, full-time coverage for low earth-orbiting systems will therefore require numerous platforms and/or new propulsion concepts, such as the "Hoversat," which could potentially, given enough fuel, provide loiter time by installing a jump-jetlike propulsion system on each platform.¹⁶ Since orbits are regular and predictable, any gaps in coverage could easily be exploited by a clever adversary. Each platform must also be lifted into orbit at great cost in energy and money, unless inexpensive space lift is available by 2025. Once in orbit, each platform is automatically difficult to service and maintain. Additionally, a truly effective constellation of platforms could easily become a high-value target in plain sight for a determined adversary. If the US is the only nation possessing such a constellation, this could invite massive active or passive antisatellite (ASAT) countermeasures that would flood near-earth orbit with debris. This debris cloud would threaten the entire world's space assets. By 2025, the ramifications of such a catastrophe would be truly global, affecting every person on the planet. This potential vulnerability could be reduced by miniaturizing and stealthing space-based platforms.¹⁷

The class of platforms called "space-borne" platform is the most flexible, since it can potentially begin its operation under direct human control within the terrestrial environment (on land, sea, or in the air). Servicing and maintenance are less difficult for such platforms, because they are much more accessible to human technicians. Space-borne platforms can be less vulnerable, because they can be held within the confines of sovereign US territory. Their vulnerability is also reduced because they can be made highly maneuverable much more easily than a space-based system. Promising lift concepts for space-borne platforms in 2025 are described in the AF 2025 Space Lift white paper.¹⁸

The most familiar space-borne platform is the ICBM. American ICBMs are currently configured to deliver nuclear weapons to any location on earth within 30 minutes.¹⁹ Given the apocalyptic nature of this weapon, nuclear-tipped ICBMs are generally regarded as the ultimate weapon of deterrence-a weapon no one really wants to use (ever). American ICBMs already exist with a circular error of probability (CEP) measured in feet.²⁰

The debate on the desirability of putting man in space is a long and acrimonious one. No machine can come close to the breadth and depth of mankind's abstract reasoning ability, but it is a very costly task to develop systems to launch and sustain a manned presence in space. A Spacecast 2020 White Paper (section H) makes the argument for a manned space-borne platform called a TAV, the "Black Horse."²¹ The biggest advantage of the manned TAV is that it is probably the most flexible platform yet proposed for space operations simply because it is under the continuous control of a human. Given an appropriate design, the manned TAV could be quickly reconfigured to deliver special operations teams, high-value equipment and supplies, or a wide variety of munitions (in much the same fashion as a high-speed bomber).²² Most important of all-the TAV can put a few well-trained people at the site of a developing conflict anywhere on Earth within 60 minutes from launch.²³

The most important disadvantage of space-borne platforms is their relative lack of responsiveness. A TAV can reach anywhere on earth within 40 minutes once it has reached orbit, but this cannot compare with a speed-of-light attack from a directed energy weapon in orbit above a target. If a space-borne platform is not already hovering "near station," this single disadvantage may be fatal in an era when response times have improved to minutes or even seconds.

Weapon Classes

The potential space-strike weapons can be broadly grouped into four categories: directed energy, projectile, space sortie and information. Information "weapons" are discussed in white papers prepared by other AF 2025 teams.²⁴ The rest of the weapons systems will be described in terms of their capabilities and shortfalls, and countermeasures for each system, will be discussed. Finally, each system is evaluated in light of timeliness, responsiveness, flexibility, precision, survivability, reliability, and selective lethality (desired capabilities described in chapter 2). The final result will be selection of a credible space-force application system-of-systems.

Unfiltered by the atmosphere, the sun provides an enormous flux of natural (incoherent) light in near-earth orbit. Our best measurements of this flux put the available power density at 0.1395 W/cm2.²⁵ Currently, this vast power source is tapped with solar arrays to power satellites. It is conceivable that large focusing mirrors equipped with pointing and tracking and maneuvering systems could be placed in orbit to intercept and redirect solar energy onto the battlefield.²⁶ Single, very large mirrors (on the order of kilometers in diameter) or large arrays of smaller mirrors working in concert would be needed to make this concept useful. Even in LEO orbit, these mirrors would need pointing and tracking accuracies of 10 to 100 nanoradians to qualify as precision aimed weapons.

Optical systems (primarily collecting apertures) currently under study have been limited artificially to a size of four meters for potential launch on the space shuttle.²⁷ The optical substrates are made from ultralow-expansion, rigid glasses such as ZerodurR that are made lightweight with acid-etching techniques.²⁸ Larger, still lightweight structures could potentially be made from advanced aerogel materials, advanced ceramics (such as SiC), engineered composites, structurally supported optically coated plastics, suspended or spun-reflective liquids (a liquid mirror), or inflatable mirrors (reflective films on an inflatable substrate).²⁹ All these approaches have been demonstrated at the earth's surface with structures measured in feet or at most a few meters.³⁰

Capabilities

The most likely incoherent light weapon would consist of an orbiting array of mirrors in the 10-to 100-meter class. With the proper constellation, the orbiting mirrors could intercept and redirect sunlight onto the earth's surface. The simplest use of the system would be to provide battlefield illumination on demand. Depending on the area illuminated, useful illumination could be provided by one to a 100 mirrors operating in concert. By focusing the light from many mirrors onto a single spot or series of spots, battlefield temperature could also be raised (a potential form of weather modification- see the AF 2025 white paper "Weather as a Force Multiplier") and optical sensors (including human eyes) could be temporarily blinded.³¹ Emergency electrical power could be "beamed" to lightweight solar panels erected to intercept the redirected sunlight. To achieve more permanent effects, such as melting, as many as 100 mirrors might need to point and track on a single hardened target for a period ranging from several tens to hundreds of seconds. Spotlight beams from a few mirrors could also be used to aid search and rescue or special operations missions at night. Incoherent light weapon systems are limited in the rate at which they cause permanent damage by the fact that incoherent light, unlike coherent (laser) light, cannot be focused onto extremely small spots.

Countermeasures

Incoherent light is difficult to focus; easy to block with broadband reflective, scattering, or absorptive barriers (such as aerosol clouds); and can be decoupled from target surfaces with reflective coatings. The last two countermeasures can be defeated, however. Reflective coatings tend to degrade naturally, especially in the battlefield environment, and they can be deliberately attacked with abrading materials (sand) or absorptive liquids (paints/dyes). Blocking barriers can be attacked and eliminated by cooperative land, sea, or air forces. In particular, blocking clouds of aerosols (e.g., smoke) can be rapidly eliminated with heavy liquid sprays. A clever adversary can also delay damage to his assets by spreading the absorbed heat through rotating some targets (such as missiles) or by insulating targets with inexpensive materials like cork.³²

Evaluation

The biggest advantage of an incoherent light weapon (if the technology could be adequately developed) is the endlessly available power supply. The range of lethality is also attractive assuming the precision pointing and tracking problems could be conquered. However, the flexibility and survivability of mirrors that may need to be hundreds of meters or even kilometers in size negates this as a viable weapon system. Furthermore, if the constellation were placed in a LEO for better accuracy, sustainment, and reliability, there would have to be many of these very large mirrors just to ensure good timeliness and responsiveness; this is neither practical nor cost-effective.

Lasers can be built as either continuous wave (CW) or pulsed devices. CW laser effects are generally described in terms of power density on target in W/m2; pulsed laser effects are described in terms of energy density on target in J/m2.³⁴ Although significant advances in this technology have been made by both Ballistic Missile Defense Office (SDIO/BMDO) and the USAF Phillips Laboratory Airborne Laser (ABL) organizations, laser technology still needs further development.³⁵ To date, ground-based chemical lasers have been built in the megawatt class (the ALPHA laser).³⁶ Phillips Laboratory is also developing a hundred-kilowatt-class short wave CW chemical laser (SWCL) based on the oxygen-iodine chemical system.³⁷ Weapons-class pulsed lasers have also been built, but primarily for effects and materials research.³⁸

For the space-earth geometry (see fig. 3-1), multimegawatt power is required for a CW weapons laser and hundreds to thousands of joules of energy per pulse is required for a pulsed weapons laser (depends on pulse length and pulse repetition frequency).³⁹ Total power or energy requirements are correspondingly higher for the earth-space-earth geometry. Constellations employing only a few space platforms (e.g., laser stations for the space-earth geometry, laser mirrors for the earth-space-earth geometry) would have to compensate for long slant ranges and correspondingly higher-atmospheric distortion by using even more powerful beams.⁴⁰ Lasers are not all-weather systems. The laser wavelength, and therefore the laser gain medium and optics train, must be carefully chosen to permit good atmospheric propagation. Clouds absorb and scatter laser light, removing power from the beam and distorting the beam's "footprint."

The size of the optics necessary to point and focus a laser beam depends on the frequency of the laser and the range to the target. For visible and near-infrared lasers, the frequencies under study for use at long range, optics in the four to 20 meter diameter should suffice for a system in low earth orbit.⁴¹ For a brief review of research trends in large optics, see the discussion on incoherent light weapons (see page 20).

To achieve the status of a precision-aimed weapon, laser weapon systems will require pointing and tracking accuracies in the 10 to 100 nanoradian range for systems in low earth orbit.⁴² The SDIO/BMDO acquisition, tracking, pointing, and fire control program has already demonstrated a pointing stability to "below the program goal of less than 100 nanoradians."⁴³ It has, however, not yet been proven that large structures in earth orbit can be stabilized to these levels. This is a challenge of particular importance for a distributed laser weapon system consisting of an earth-based laser and a constellation of space-based mirrors. In this scenario, the laser beam must be relayed by several space mirrors before it reaches some targets.

Adaptive optics techniques such as the Guide Star System have been developed to correct atmospheric distortions to low-power laser beams projected from earth to space and back again.⁴⁴ Adaptive optics systems developed to date depend primarily on deformable mirrors-mirrors with small actuators that change the mirror's shape to pre-compensate the beam and correct anticipated or premeasured distortions. Further advances will be required in this technology, both in terms of bandwidth and number/size of actuators, to make this technology work for weapons class lasers. Current advances in microelectromechanical machines and nanotechnology show great promise in this area.⁴⁵ The bandwidth problem on the processing side will probably "handle itself," given the current rate of growth in semiconductor technology and continued commercial/government interest in optical processing techniques.⁴⁶ Advances in high-speed (10 Gbits/sec and up) laser communication systems are also likely to yield solutions of interest to the laser weapon designer.⁴⁷

Capabilities

Lasers are extremely flexible weapons, producing effects that cover the full "spectrum of force." At low power, laser beams can be used as battlefield illumination devices, but with a potential added benefit over incoherent illumination. Using an invisible laser beam (near infrared) at a specifically chosen wavelength and special tuned vision devices similar to night-vision goggles, one could render the battlefield visible only to friendly troops.⁴⁸ At low to medium power, laser beams can be used to designate targets from space, blind sensors in the laser's optical band, ignite exposed flammable objects, raise the temperature in localized regions (possible weather modification effect-see the AF 2025 white paper "Weather as a Force Multiplier"),⁴⁹ perform as an emergency high-bandwidth laser communication system, and serve as a laser probe for active remote-sensing systems.⁵⁰ At slightly higher powers, the enhanced heating produced by the laser can be used to upset sensitive electronics (temporarily or permanently), damage sensor and antenna arrays, ignite some containerized flammable and explosive materials, and sever exposed power and communications lines. The full power beam can melt or vaporize virtually any target, given enough exposure time. With precise targeting information (accuracy of inches) and beam pointing and tracking stability of 10 to 100 nanoradians, a full-power beam can successfully attack ground or airborne targets by melting or cracking cockpit canopies, burning through control cables, exploding fuel tanks, melting or burning sensor assemblies and antenna arrays, exploding or melting munitions pods, destroying ground communications and power grids, and melting or burning a large variety of strategic targets (e.g., dams, industrial and defense facilities, and munitions factories)-all in a fraction of a second.

Pulsed lasers can also produce additional effects based on their ability to deliver rapidly a large amount of energy in a small amount of time. Weapons-class pulsed lasers can vaporize target surfaces so rapidly that an effect very like a rocket firing occurs. In essence, the target experiences a shove or impulse with every laser pulse. If a strong enough impulse is delivered, the laser can discriminate between valid air- or space-borne targets and lightweight decoys (although the details of this process are very difficult to satisfy).⁵¹ If the impulse can be delivered at an object's resonant frequency, cracking and breaking will occur. Similarly, a pulsed laser trained on an object at the proper pulse-repetition frequency can stimulate infrasound vibrations, a potential form of nonlethal force projection that disrupts a target with penetrating, low-frequency oscillations.

Perhaps more significantly, the large space-based mirrors of a distributed laser weapon system (laser is ground based) can also be used as a high-quality, passive remote-sensing system.⁵² By training ground-based, high-power optical telescopes on the mirrors, America's "eyes" can literally be carried to every corner of the earth. Cued by a broader area search, this capability could be the primary surveillance, battle damage assessment, and targeting system for the laser space-strike weapon or a valuable adjunct to America's existing national technical means. With a large constellation of space-based mirrors in LOE, America's opponents could literally never be sure when they are being watched, closing the existing coverage gaps. Rather than depending on a few large, expensive assets that will inevitably become tempting targets, we can protect our surveillance and reconnaissance capability by increasing the number of "eyes" in orbit.

A weapons-class laser is useful only so long as it has fuel. This is a particular problem for a space-based laser, since it can be expensive to lift large quantities of fuel into orbit. This problem could be mitigated by using solid-state or diode laser systems that can be configured to operate on electrical power.⁵³ Such systems are also attractive because of their relatively high efficiency. Diode laser systems have been built with electrical efficiencies as high as 50 percent at room temperature and cooled diodes have demonstrated efficiencies of 90+ percent.⁵⁴ The most powerful contemporary diode laser arrays are still low-power systems (1 - 100 Watts), although the technology appears to be scaleable.⁵⁵ Enormously powerful pulsed glass laser systems have been built as elements of the DOE inertial confinement fusion program, but these systems are huge, inefficient, and quite fragile.⁵⁶ Clearly, further technology work is required to make these systems deployable.

Atmospheric interactions are another challenge for weapons class lasers. Aside from the obvious scattering and absorption problems, high-power CW lasers are known to cause "thermal blooming" (e.g., a severe defocusing of the beam) and "beam steering" (unintended shifts in beam direction) when they pass through the atmosphere.⁵⁷ Pulsed high power lasers, with their attendant powerful electric fields, can stimulate nonlinear optical effects such as "harmonic generation" (e.g., the inadvertent generation of other colors of light) that rob power from the main beam and make it difficult to focus the laser on the target.⁵⁸ Laser beams of higher frequency are more easily focused on the target, requiring smaller control optics and mirrors. Unfortunately, high-frequency lasers are inherently more difficult to develop, usually requiring dangerous exotic fuels and exhibiting much lower efficiencies.⁵⁹ High-frequency lasers, particularly those above the green region of the spectrum, also scatter very strongly in the atmosphere and are increasingly subject to the nonlinear optical effects previously discussed.

Current weapons-class lasers all produce beams in the near infrared (short-wave infrared or SWIR). These frequencies are strongly affected by clouds and suspended particles, and cannot always be depended on to engage targets below the cloud tops at about 30,000 feet.⁶⁰

Countermeasures

Lasers are subject to the same basic countermeasures as incoherent light weapons and can be aided by the counter countermeasures outlined above. An additional phenomenon known as the laser supported combustion (LSC) wave can occur when a high-power laser beam strikes a target surface.⁶¹ As the laser vaporizes surface material from the target, the hot gas can absorb even more energy from the laser beam. If enough energy is present on a short enough timescale, the hot gas is rapidly ionized, producing a hot, dense plasma. The plasma absorbs all the incident energy, essentially shielding the target surface from the direct effect of the beam. This phenomenon is generally a problem for high-power pulsed lasers and represents the upper limit to the amount of laser power one should generally attempt to put on target. At even higher incident powers, the LSC develops into a detonation wave or LSD that swiftly travels back up the laser beam, further decoupling the laser from the target.

Evaluation

The coherent light laser is an extremely attractive space-strike weapon for several reasons. It is highly responsive and timely (e.g., could strike within seconds after a decision is made to take action), it has already demonstrated high-precision capability (especially in recent ABL and SDIO/BMDO tests), and it has inherently high flexibility and selective lethality (from "lighting the battlefield" to temporarily disturbing sensors and electronics to melting or burning large or small targets). Additionally, the ground-based lasers could be relayed to or independently pointed at the space systems of aggressor nations (or organizations), serving as an important US space-control asset as needed. When not needed as a force-application or space-control weapon system, the space-based mirrors can form the basis for a very effective, survivable space-based global surveillance and reconnaissance system.

In a LEO constellation, a 20-meter mirror is certainly not as daunting as a kilometer sized one, but it is still awkwardly large and therefore expensive and less survivable. In fact, each space-based mirror would need a covering until it is used, lest it be damaged by simple antisatellite attacks or by space debris and contamination (e.g., altering the surface and rendering the mirror useless for relaying high-power laser beams). Reliability is also a concern due to the large amounts of power required by the ground-based laser and the lamentable effect of weather (clouds) on the operational availability of the system. However, a distributed laser space-strike system with ground-based lasers could certainly be maintained and even upgraded much more easily than a completely space-based system, thereby increasing the overall reliability. In sum, ground-based CW lasers coupled to space-based mirrors seem a highly effective and feasible option for a space-strike weapon system in 2025.

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