ABSTRACT: Military uses of space may lead to actions against space systems, which might be controlled by countermeasures, by the deterrence of such actions, or by unilateral, bilateral, or international measures. The paper reviews potential military uses of space, weapons which might most effectively be used against such space systems, countermeasures, and a program for enhancing security through the use of space, without provoking war in space.
THE MILITARY USES OF SPACE.
Communications.
Navigation.
Observation.
Ballistic Missile Delivery.
Space Delivery of Non-Nuclear Weapons.
Space-based Beam Weapons.
WAR IN SPACE.
Antisatellite Warfare.
The Space Shuttle Is No ASAT Threat.
Ground-based Lasers.
Space-based ASAT Beam Weapons.<br>
Space-based Ballistic Missile Defense.
Counter-ASAT Capabilities.
Military activities are in large part logistic and share many properties with civil operations. Thus, personnel, supplies and arms must be transported, communicated with, find their way, and be monitored. In addition, information is of great value during war; it potentiates military power-- it is a "force multiplier."
Thus, if satellite systems for communications and navigation are of value to the civil economy, they are likely to be of great value to military forces. Of course, there is in this case economy of scale; a small country with military concerns on its borders alone, would probably find it neither necessary nor desirable to have a satellite system which would provide full-earth coverage or hemispheric coverage, when all it needed was the ability to communicate or navigate over an area 1000 km across. Similarly, surveillance satellites which can view the whole earth are of use to those powers or groups of powers with world-wide concerns.
Commercial and peacetime military communications operate primarily via satellites in synchronous equatorial orbit, orbiting the earth in one day so as to remain stationary with respect to antennas on the ground. Such satellites broadcast over a frequency band of width 1-2 gigahertz (GHz), carrying on the order of 250,000 telephone channels or 250 commercial television broadcast channels. An antenna which simultaneously illuminates and receives from the hemisphere of the earth observable by the satellite can provide low-data-rate communications with a fraction of a watt via an omnidirectional antenna on earth. High-data-rate communications require directional antennas (i.e., of size greater than a wavelength), which need to be oriented toward the satellite-- easy for stationary antennas and quite feasible these days for mobile systems. Of course, communication satellite links may be disturbed by jamming of the uplink to the satellite, the downlink at the ground-based receiver, by destruction of the satellite, or by capture of the satellite by one not authorized to control it.
Naturally, the satellite need not be completely destroyed; it would suffice to interfere with some particularly vulnerable aspect of its attitude control, to cover its receiving or transmitting aperture with aluminum foil, or the like.
TRANSIT is a satellite system used world-wide for navigation and position fixing. A single pass of the satellite gives rise to a Doppler shift in received signal frequency, which aside from a bilateral ambiguity determines one's position on earth to an accuracy of some tens of meters. Because the entire Doppler history of the signal must be received, such position fixing requires some minutes of reception, and the opportunities exist only when a TRANSIT satellite passes its distance of closest approach.
There are now deployed some six NAVSTAR satellites (the Global Positioning System-- GPS) which will tell a user within 0.1 sec his position in three dimensions to an accuracy of 10 meters, and vector velocity to about 1 cm/sec. It is clearly much easier to obtain accurate position information from a moving and accelerating vehicle with GPS than with TRANSIT, and the 18 or 24 satellites in the final configuration will provide instantaneous world-wide navigation data via time-difference of arrival (TDOA) and frequency-difference of arrival (FDOA) techniques.
GPS will be valuable for navigation on the surface of the sea, on land, in the air, and in space. It will serve to provide accurate position fixing for satellites and for anti-satellite activities. Periodic receipt of GPS signals will allow missile-firing submarines to update their ship navigation systems, thereby greatly improving the accuracy of SLBMs launched from such submarines. GPS will be useful for enroute navigation of aircraft, in blind landing, and in the delivery of munitions.
Not only the delivery vehicle but the munitions themselves may eventually be equipped with GPS receiver-computers, or with a receiver which could relay the signal from the munition to a station which could do a computation of position and position error, and transmit to the munition either these data or the corrective steering signals.
In particular, GPS signals can be received by ICBMs or SLBMs during boost phase, which will provide SLBMs with accuracy at least as good as those projected for the land-based MX missile. In this connection, the SUM system (for Small Sub Undersea Mobile) for basing the MX missile has been proposed by Dr. Sidney D. Drell and the author to use GPS signals, but to patrol no further than 800 km from U.S. shores in order that a Ground Beacon System (GBS) capable of transmitting NAVSTAR-like signals, should be available for accurate guidance of the ICBM after launch from the small submarine. Thus, destruction of the NAVSTAR satellite constellation would not impair the effectiveness of the MX launched from SUM.
The Ground Beacon System is an example of a backup to a satellite system which can replace satellite capability and which thus helps to preserve the satellites against threat of attack or interference, even during war. Soviet use of GPS (at least in the commercial-- 100 m mode) may help to deter attack on GPS by providing a significant value to the Soviet Union, thus adding to a general reluctance to initiate warfare in space.
Even in such mundane applications as dropping bombs from aircraft, GPS can have the most significant impact on effectiveness, sorties required, and survivability of the aircraft. It is likely to reduce the miss distance by a factor 10, so that only 1% as many weapons are required to destroy a hardened target-- and this with GPS receiver/navigators only on the delivering aircraft and not on the weapon itself.
The Soviet Union has long deployed radar satellites powered by a small nuclear reactor, one of which reentered in Canada. Several nations have launched photographic observation satellites, some of which are recognized in the SALT Agreements as National Technical Means of Verification, and are thus afforded protection by the Soviet Union and the United States.
Similar satellites would clearly be of some utility in military operations, although optical imaging satellites are impaired in utility by night, cloud cover, camouflage, by the vast areas to be covered, and by the constraints of orbital dynamics.
Near space is evidently the medium of delivery of ICBMs and SLBMs. No clearer example of military activity could be advanced, and it has also been observed that such missiles in boost phase are more obvious and clearly locatable than at other times during their flight. They are also a lot "softer" and more easily damaged than after the warheads have been deployed and the essentially inert MIRVs fall through space and reenter toward their targets. Furthermore, a small velocity error induced in the booster will be reflected in a larger miss distance than if the disturbance occurred later in flight.
Thus, boost-phase ballistic missile defense (BMD) is a popular subject for discussion and a goal to be carefully evaluated both by those who would defend their nations and those who would preserve the capability of their strategic offensive forces.
With very good accuracy and guidance to the target, it is conceivable that non-nuclear weapons could have some effectiveness. Their use would be every bit as much an act of war as would the delivery of nuclear weapons, or of destruction by secret agents, but in principle such a weapon could be built. It could be launched from the ground in a ballistic trajectory or called down from low earth orbit. Countermeasures would be much more feasible than against nuclear weapons-- particularly passive defense by steel or concrete spaced at a considerable height above the target.
It is clearly possible eventually to base in space a laser or particle accelerator capable of causing destruction on the ground or particularly to high-flying aircraft. There are some little details regarding detection and identification of these aircraft, the cost of the system, and the like, but the biggest problems arise in asking against whose aircraft the system is to be used. Such satellites might coexist if such a very expensive system is to be deployed by the United States or the Soviet Union (or both) for use only against the aircraft of third nations. However, how would it be guaranteed that they would not be used against one another's aircraft, or against one another's laser-bearing satellites? And in the early stages of construction, how would one be sure that the system would not be thought effective against a ballistic missile attack? Such systems should not be considered unless they are cheaper, more effective, or more durable than other means of attacking aircraft.
We characterize this potential war by the location of the weapons launcher and by the location of the target.
Satellites can be destroyed by co-orbital intercept, by orbital intercept, or by direct ascent from the ground. The Soviet ASAT which has been tested for a good many years is a large booster carrying a rocket-propelled homing vehicle. The ASAT vehicle is placed in orbit and later attacks its quarry in low earth orbit. There is no reason why such activity could not be extended to intermediate and synchronous orbit, requiring a larger booster to provide the greater velocity gain to achieve such orbits. Actual kill from a vehicle which attacks at reasonably high velocity can be achieved with a high-explosive warhead or with pellets. One can foil such an attack by maneuvering, by camouflage, or by physical destruction of the interceptor or by interference with its sensors.
The U.S. has a development program for an ASAT weapon which will be a small rocket to be carried under the wing of an F-15 or other fighter aircraft, and boost it not into orbit but into a direct-ascent intercept of a satellite. Such a system has a shorter response time, since the aircraft can carry the homing rocket anywhere in the world; it is in principle cheaper and has a higher rate of fire and hence a shorter time to sweep all satellites from the sky. The guidance system of the homing vehicle is apparently an autonomous infrared sensor, similar to that which has been advanced for the exoatmospheric "overlay" of a U.S. BMD system. Since the relative velocities of interceptor and satellite may be in the 8 km/sec range, (a kinetic energy of 30 kilojoules (kJ) per gram), small pellets or an open mesh could serve to provide a substantial kill area. Assuming that one needed to provide a one-gram pellet every square meter, a 10-kg warhead could destroy satellites within a circle of diameter 0.1 km. Of course, the timing and guidance problems associated with direct-ascent intercept are substantially more severe than those with orbital intercept, but there is no reason to believe that they are insoluble.
With either the orbital interceptor on a large rocket booster or the small aircraft-carried ASAT, the cost of the destroyer is far less than the cost of a long-lived capable satellite, so that there is essentially no hope of proliferating peacetime satellites to survive by cost-exchange ratio alone against such attack.
The situation is even worse for satellites if the antagonist makes use of special vulnerabilities of the individual satellites.
In the Soviet press and occasionally in the West, the space shuttle has sometimes been portrayed as a threat to satellites, presumably because it will eventually be used to recover and refurbish U.S. satellites. Clearly, however, no one will exchange a shuttle vehicle costing a good fraction of a billion dollars, and the lives of several people, for the destruction of a non-cooperative satellite. Nothing could be simpler than to build into each satellite not intended for recovery a small explosive charge which would be armed some days after launch and which will detonate whenever the satellite is subject to accelerations not induced by its own rocket motors. The very possibility of such (legal) booby traps will keep the space shuttle far away. As for close observation, that can be done far less expensively by unmanned photographic probes.
It is perfectly clear that one can have a ground-based laser which will destroy a satellite in near-earth orbit by overheating it. If the criterion is to provide 10 sols to an unprotected satellite (1 w/cm(2)) at a distance of 500 km, a 10 microradian beam such as might be obtained from a 1-m diam mirror and a 10 micron carbon dioxide laser, would illuminate a region of 10-m diam. A 1 megawatt laser illuminating the satellite for several minutes would provide this hazardous flux.
Naturally, one would have to wait until the satellite orbit carried it sufficiently close to the enemy laser, for good weather, and one would have achieved the destruction of the satellite with a laser which no doubt costs substantially more than the incremental cost of a booster or a direct ascent mechanical satellite killer. If the job were to kill 1000 unprotected satellites in low-earth orbit, clearly the laser would be more cost effective than the rocket approach.
Against attack by ground-based lasers even satellites in low-earth orbit could defend themselves by the deployment of parasols, large-diameter thin metallic foil at the end of a boom, which would shadow the satellite itself from the laser energy, and which would be rotated rapidly enough to spread the laser light over a large receiving area of metal. The front of the metal foil would reflect 99% of the light, and the back would have a highly emissive layer in order to radiate isotropically into space at the black-body rate. A modest boom ensures that the reradiation of some tens of kilowatts of thermal energy from the back of the metal foil will not overheat the satellite. Simultaneous irradiation from 2 lasers on the ground could defeat such a tactic, but additional booms or free-flying defensive shadow shields could be deployed in response.
Because there are relatively few satellites, and these in general very soft because they must be very light, they are natural targets for destruction by energetic particle beams. As you know, the only particle beam useful in space is the hydrogen-atom beam, obtained by accelerating negative hydrogen ions and stripping the second electron after the beam has been accelerated and pointed. The vacuum of space seems an ideal environment for beam weapons. In particular, the H-atom beam might be projected with microradian accuracy and divergence (limited, incidentally by the internal momentum distribution of the second atomic electron, which as transverse momentum of the H-atom beam causes an irreducible divergence diminishing as H-atom velocity). For all beam weapons one must make the choice between synchronous orbit and low-earth orbit. The answer depends upon the required destruction rate, the effective range, the available technology, and whether the targets are concentrated in one portion of the globe more than in another. The H-atom beam from synchronous orbit at 40,000 km would, even with one microradian divergence, illuminate a region 40-m diam, and the energy would be deposited in a depth on the order of 50 g/cm(2). If it were to destroy by overheating, and the requirement were a 100 C temperature rise, an energy of 5 kJ/cm(2) might be required, for a total energy incident of 80 GJ. Some 80 tons of fuel would have to be converted into beam energy at 25% efficiency to provide this energy and at least 8000 tons of rocket fuel would of necessity have been used to transport the fuel to synchronous orbit. Hydrogen atom beams, as such, are easily destroyed. At 500 MeV particle energy, such a beam of hydrogen atoms is reduced in intensity by a factor e in traversing 1 mm of air at normal density. At 100 MeV, the attenuation length is 0.1 mm. However, although the hydrogen atom content of the beam is destroyed, removal of the orbital electron converts the hydrogen atoms to protons, which themselves have 200 or 50 g/cm(2) penetrating power, equivalent to some kilometers of air. Nevertheless, it is the neutral beam which can traverse empty space unbent by the earth's magnetic field, and it is possible to take advantage of the ready strippability of an H-atom beam to protect its potential targets.
The space-based laser fares better in the destruction of satellites, in part because its energy reaching the target is not dissipated in the interior, but only on the surface. Of course, shiny metallic surfaces can be arranged to absorb less than 1% of the incident light, but satellites in general require thermal control and have functions to perform so that a much larger fraction of the incident light is absorbed. In space one has no perturbing atmosphere, and with the use of active optics one might hope to have large mirrors. For space use, an HF laser in the 2.7 micron range might be suitable so that a mirror "only" 3 m diam would be required for 1 microradian divergence. If the incident kill fluence were stated as 1 kJ/cm(2), only 20 GJ of laser energy need be created. Naturally, if the space-based laser were at 400 km from the target, the required energy would be down by a factor 10(4), for the same mirror diameter and tracking accuracy.
For BMD, one must decide whether the system is to work against an occasional ICBM, and only with some probability, or whether it is to handle with high probability mass raids of ICBMs. Assuming that the system is required to cover the entire earth to counter SLBM as well as ICBM launch, and that the orbits are circular and polar, one can determine the optimum number of orbital planes and satellites per orbit, and the cost of the (unopposed) system. Thus, making very optimistic assumptions on laser technology and assuming 400 ICBMs launched uniformly over a period of 600 sec, each with a vulnerable (boost) time of 250 sec, and that the lasers can spend 70% of their time actually illuminating targets (30% switching between targets), a booster hardness of 2 kJ/cm(2) would correspond to a 10-year system cost of approximately $40 B, or about $100 M per booster destroyed. The number of satellites involved ranges from approximately 70 at 500 km altitude to some 8 at 6000 km altitude. A typical spacecraft with a 3-Mw laser and a 7-m diam mirror is assumed to have a dry mass of 3500 kg and a cost of $150 M. These performance goals are far from available. They require an increase in laser brightness of at least a factor 10(6) over that which has been demonstrated in a ground-based system.
Furthemore, this calculation assumes that the laser BMD system is essentially unopposed during operation as well as during construction. Assuming a system at the lowest orbital altitude, the average kill range would be on the order of 2000 km. The boosters might hide behind aluminum foil screens perhaps 100 m square. The satellites can be destroyed by nuclear weapons or by ASATs launched together with a flight of decoys against the satellite systems. Electronic countermeasures can be used against the satellite command and control. The BMD system would either have to have some kind of overall control or would have to fend for itself in selecting targets, multiply illuminating some boosters and allowing others to escape. The boosters themselves by sacrificing one or two MIRVs could harden their surfaces against laser energy by a factor 10 or more.
As an alternative to failure, the ICBM force could be launched simultaneously, thus increasing the cost and number of satellites by a factor 3 or more. The combination of booster hardening and simultaneous launch would require a 10-fold increase in laser BMD cost, without attending to the survivability of the BMD system itself.
Satellites bearing lasers or particle accelerators with the duty to detect and shoot down ICBMs must have sensitive and delicate sensors and computers. The sensors are especially vulnerable to radiation, but the satellites themselves are fragile and visible. Their positions can be known to a few tens of meters, although they might maneuver to avoid attack. Evidently they might be attacked by direct-ascent vehicles with multiple miniature homing vehicles (MHVs), so as to saturate the kill capability of the ASAT. They could be attacked by pellets or by nuclear weapons. Included among the attackers could be very many decoys, which would require the expenditure of fuel from the ASAT. The attackers could have highly refractory metal heat shields at a small angle to the incident radiation, so that they could bore through to the laser itself.
Finally, accelerator-bearing or laser-bearing satellites do not spring fully operative into the sky. Much less does a whole self-defending system come to pass. Is it likely that the Soviet Union will stand by to allow the U.S. to launch large numbers of laser-bearing satellites which might be imagined able to nullify the Soviet ICBM force? Would not the Soviet Union launch small accompanying satellites, with low technology, which would follow the large satellites around in the sky and be detonatable upon radio command? To prevent Soviet contenplation of such "space mines," would the United States proclaim hegemony over space and insist that all vehicles launched into space be inspected?
If the Soviet Union began to launch a series of BMD satellites, would the U.S. stand by without poising itself to attack those satellites? Would the U.S. then accept a declaration of Soviet space hegemony?
Ground-based lasers can be used to attack satellites in low earth orbit, but such a weapon would be more expensive and more readily countered than either an orbital or a direct-ascent intercept.
Satellites would also be sitting ducks for H-atom ASATs based in space or for high-power space-based lasers. Here, however, the ASAT itself would be vastly more expensive than a rocket interceptor or a ground-based laser, and in the ASAT role, its speed of action is unrewarding. Furthermore, relatively few ASATs would do the job, making them vulnerable to rocket attack or other countermeasures.
In the BMD role, H-atom beam accelerators suffer from large energy requirements because of their inefficient deposition of energy. Unless one takes advantage of specific booster vulnerabilities, such as that in certain semiconductor elements of the guidance computer, H-atom BMD seems ineffective. In any case, it is counterable during the deployment phase.
As indicated previously, ICBM boosters evading laser BMD could hide behind large reflective screens, could avoid distinguishing surface markings and rotate in order to spread the heat, could be launched simultaneously rather than spread for optimum effect, or could be hardened against laser light by a factor 10 or considerably more. The combination of simultaneous launch and hardening the second stage of the 3-stage missile would multiply the required number of laser satellites by a factor 10 and increase BMD system costs to the range of hundreds or thousands of billions of dollars.
Even such a proliferated BMD system could still be attacked by space mines or by miniature homing vehicles.
Space-based lasers could threaten high-flying aircraft, but they would be vulnerable to attack by ASAT rocket vehicles.
Space-based weapons, in general, cannot survive against an opponent with a fraction of the resources required to install the space weapons. Logically, their survival and utility (assuming for the moment their effectiveness) is dependent upon outlawing attacks on space weapons. Should we do that? Do we really want an arms race or even a war in space which, paradoxically, would result from pretending that space-based beam weapons can be made survivable?
It is fantasy to argue that one of the superpowers will unilaterally give up the ability to destroy the other, which is tantamount to surrender. A world having known nuclear weapons cannot be free from the threat of destruction of a society by smuggled nuclear weapons (if not by missile warheads, bombs, and more familiar threats). No treaty can enshrine "defense dominance" if such a bypass threat exists.
There is no exit from the present vulnerability of society; we can nevertheless work to ensure that no rational society will destroy another (and hence itself), and that irrational groups do not acquire the means of destruction.
In a world of vast forces, we and our governments must give the most serious concern to world security against a cataclysmic war. A world war should not stem from the assassination of an archduke; it is the instability of the situation, not the instigating event, which is responsible and which must be avoided.
Similarly, given an informed view on both sides of their potential, space weapons should not lead to a breakdown of the present stability of confrontation. But delays, "perceptions," intentional or unintentional distortion, can convert an innocuous (if expensive) playground into a feedback loop which can destabilize the confrontation and precipitate unprecedented destruction.
There are no weapons in space now (unless there is a Soviet ASAT test vehicle up at this moment). We should move with urgency to conclude a bilateral treaty (and then supplement it with an international accord) to ban destructive attacks on satellites, and to forbid the emplacement of instruments in space capable of projecting damaging levels of radiation. While I am not a fan of the solar-power satellite, even such a program could proceed if were open to inspection, as is available in the demilitarization of the Antarctic continent.
Both the United States and the Soviet Union (and other countries with space activities) can be expected to proceed with the development (but not the deployment or the testing in space) of ASAT weapons, but at a level presumed adequate to cope with some future abrogation of the ASAT treaty and the modest space laser capability which might result from a clandestine program.
Rather than welcoming the opportunity to turn space into a boxing ring for a series of fights for which we cannot afford the admission, we should use our human and material resource more effectively to maintain our security and well-being. One might suggest that we let the other side waste its efforts on weapons in space while we counter with feasible, comparatively low-technology rockets, but experience shows that technological optimism, contractor pressure, and fear of asymmetry can result in costly and potentially destabilizing contests, to the detriment of our real military needs.