Space Rods: Assessing Space-to-Earth Kinetic Weapon Systems

ABSTRACT

In April 2022, the United States announced it would no longer conduct direct-ascent anti-satellite weapons tests resulting in destruction of satellites by kinetic or explosive means, which would usually result in debris fields in space. While this hazard would threaten our modern way of life, what is rarely discussed is the potential and impact of the reverse - weapon systems in space aimed at various locations on Earth. This article performs a first-order physical analysis, as well as reviews tactical and political considerations, for employing space-to-Earth kinetic weapon systems. We offer the following conclusions in evaluating two types of hypothetical kinetic weapons: one staged in low Earth orbit and another in geostationary Earth orbit. First, neither weapon system offers an advantage beyond conventional terrestrial capabilities: this includes the cost effectiveness, the time of flight, and explosive yield of the weapon. Second, deploying such a weapon system incurs a greater international political cost than the strategic benefit it would generate.

Introduction

Even before the establishment of the U.S. Space Force, policy makers, military leaders, and civilian thinkers have openly proposed the deployment of space-to-Earth weapons systems capable of striking targets on Earth, with a lethality only matched by their promptness.[i] In academic discourse,[ii]the press,[iii]and popular media,[iv] two types of orbital weapon systems are often conflated and erroneously interchanged: kinetic weapons deployed at low Earth orbit (LEO) and those deployed at geostationary Earth orbit (GEO). The variable physical nature of the space environment, governed by the predominant force of gravity, necessitates that LEO and GEO based systems each employ different design and tactical considerations. This analysis determined that space-to-Earth kinetic weapons systems are prohibitively expensive, politically counterproductive, and tactically redundant. 

Overview and Preliminary Design Considerations

This analysis discusses general technical design considerations of an orbital weapon deployment system. Specific design considerations and capabilities of these systems are often shrouded by classification. Weapons deployed from orbit operate at hypersonic speeds, around Mach 20 (6.9 km/s), and have parallels to hypersonic glide vehicles (HGVs) under development by the United States, Russia, and China.[v] 

For the sake of comparability, the space-to-Earth kinetic weapons outlined in this analysis are modeled after HGVs currently under development in the United States, principally by the Defense Advanced Research Projects Agency (DARPA). This study assumes a weapon mass of 1,000 kg (2,200 lbs.), comparable to that of the Hypersonic Technology Vehicle 2.[vi]

Additionally, this analysis reviews fuel requirements for deorbiting burns to enable the weapon to strike a terrestrial target. Two orbital configurations are considered: GEO and LEO. An object in GEO takes 24 hours to circle the Earth once, thus allowing a satellite to “hover” over a particular terrestrial location.[vii] This orbit is at an altitude of 35,800 km (22,250 mi).[viii] Alternatively, LEO describes a volume of space roughly a few hundred kilometers of altitude. Objects in LEO take approximately 90 minutes to orbit the Earth.[ix]

Both LEO-based weapons and HGVs have a midcourse phase outside the bulk of the atmosphere.[x] But, unlike terrestrially launched hypersonic weapons,[xi] space-based kinetic weapons are deployed in the near-vacuum of space. This obviates the need for a boost phase to provide the initial speed necessary depart the majority of Earth’s atmosphere and enter the midcourse phase. HGVs’ trajectories are suborbital whereas a LEO weapon would require a deorbiting burn to transition to the terminal phase (atmospheric reentry).[xii] In both systems, trajectory adjustments are made as the weapon penetrates the atmosphere en route to the target.[xiii]

The impact speed of an HGV is subject to two constraints: drag and maneuver. During the midcourse phase, the weapon travels at approximately Mach 20 (6.9 km/s).[xiv] However, as the HGV enters the terminal phase, it encounters atmospheric drag, which slows the object, and performs final maneuvering. The need for course and speed adjustments during the terminal phase is predicated on tactical considerations including avoiding missile defense and early warning systems.[xv]Impact speeds for HGVs have been modeled between Mach 6 and Mach 8 (2.2 km/s to 2.8 km/s).[xvi] Comparably, a weapon deployed from LEO enters the atmosphere at Mach 22 (7.8 km/s),[xvii] similar to an HGV in the midcourse phase. Thus, the weapon encounters similar drag and maneuver considerations during the terminal phase. A GEO weapon, however, would encounter the atmosphere at Mach 29 (10 km/s).[xviii] The implications of this extraordinary re-entry speed will be discussed in detail later.

Throughout this paper, we assume an inert weapon that carries no explosive payload. The latent kinetic energy of the vehicle is the armament, which is liberated explosively upon impact. One kilogram of TNT is defined to have 4.184 megajoules of chemical energy.[xix] According to the classical kinetic energy equation, a 1 kg object traveling at Mach 7 has 2.88 megajoules of kinetic energy—0.688 times the energy of an equivalent mass of TNT.[xx] This energy ratio increases quadratically with the vehicle impact speed. Thus, at Mach 7, a 1,000 kg HGV or LEO weapon has the latent energy equivalence of 688 kg (1,510 lbs) of TNT.[xxi] Adding an explosive warhead would complicate the design, and even without it the yield of the kinetic weapon is comparable to that of modern conventional cruise missiles.

GEO Stationed Weapons

Design Considerations 

For any object in orbit to de-orbit and strike the surface of the Earth, the object’s orbital speed must be reduced.[xxii] For an object in GEO, the speed must be reduced by 3.1 km/s.[xxiii] The cancelling of the orbital speed decreases the eccentricity of the elliptical orbit into a linear path oriented directly at the Earth’s center of mass—thus, the object is in free fall to the Earth’s surface. Cancelling all the orbital velocity is not strictly necessary, though the majority of the object’s velocity must be shed to produce the desired Earth impact. Otherwise, the weapon would encounter the Earth’s atmosphere tangentially, skipping off the atmosphere (expulsion), or the weapon would miss Earth altogether and enter a highly elliptical orbit.[xxiv] 

The relative speed at which the object encounters the atmosphere poses a challenge to the design of a GEO weapon, as the object’s high speed makes it prone to disintegration by explosive friction. This can be mitigated through two factors. First, the geometric design, or ballistic coefficient, affects the weapon’s heating.[xxv] For example, ICBM reentry vehicles use an elongated, sphere-capped cone to reduce the effects of drag.[xxvi] Similar design consideration for the GEO weapon will maximize impact speed, and thus equivalently the explosive yield. Second, the system could incorporate a second burn to reduce speed prior to atmospheric interface.

The necessity to reduce orbital velocity results in significant weight and cost tradeoffs. A typical small rocket engine can have an exhaust velocity upwards of 4.9 km/s.[xxvii] Note that there is a correlation between exhaust velocity and fuel efficiency. Deorbiting a 1000 kg object as described above using a typical small rocket engine requires 970 kg fuel.[xxviii] Thus, the minimum mass of a 1,000 kg reentry mass object in GEO is 1,970 kg. Additional systems such as avionics, communications, power supply, and the deorbiting rocket engine itself would increase the mass of the system, requiring additional payload sent to GEO. 

Most rocket systems list a total mass deliverable to GTO (geosynchronous transfer orbit) rather than GEO.[xxix] The fuel required to maneuver from GTO to GEO is comparatively small but non-negligible. A SpaceX Falcon 9 can deliver 5,500 kg to GTO for $62 million.[xxx] Thus, no more than two 1,000 kg weapons, described above, may be delivered to GEO via a Falcon 9 launch. This places the launch cost alone at $31 million per weapon, not including the non-negligible fuel (and thus economic) cost of transferring the payload from GTO to GEO.

Tactical Considerations

A first order calculation determined a GEO-based weapon’s free fall time and Earth impact speed of 4.1 hours and 10.3 km/s, respectively.[xxxi]  Atmospheric drag, while only an influence for a short distance along the path of travel, will reduce the impact velocity and result in a nominal increase in flight time. After the GEO de-orbiting, minor adjustments in speed relative to the Earth’s surface can be made to expand the weapon’s available strike radius.[xxxii] 

GEO weapon systems have a limited strike radius in terms of longitudinal reach. Three considerations are pertinent to understanding this type of targeting. First, the Earth rotates beneath the weapon as it falls to the surface after completing the 3.1 km/s de-orbit burn.[xxxiii] Thus, the weapon will impact four hours of longitude to the west of the GEO orbital platform position. At the equator, this is roughly 6,680 km (4,150 miles). Second, a retrograde change of speed in addition to de-orbit burn will result in the weapon impacting further to the west (or conversely, a deorbiting burn of less than 3.1 km/s will result in the weapon hitting easterly). Third, although using extra fuel to accelerate the weapon towards the Earth is possible, such a tactic would dramatically increase the mass launched to GEO, and thus the cost of the system, to prohibitive levels. From a practical standpoint, the system is limited to a 4.1 hour minimum fall time.  All of these considerations combine to limit the tactically useable range of a GEO-based weapon.

There are primarily two methods to address the limited longitudinal reach of a GEO weapons system. First, one could station systems at multiple locations along the GEO belt. Second, the weapon could perform hypersonic maneuvering during the terminal phase to expand the available strike radius.

This analysis assumes a loss of speed due to drag, yet still proposes a higher impact speed than an HGV or LEO weapon. We assume a speed of 3.3 km/s, given the in-atmospheric trajectory of the impactor is similar to that of a ballistic missile warhead.[xxxiv] This provides a 1,000 kg weapon with 5.45 gigajoules of kinetic energy, or the equivalent of 1,300 kg (2,860 lbs.) of TNT.[xxxv] While this is a significant explosive yield, it is hardly the nuclear-level explosion often predicted in popular media. Furthermore, the 4.1-hour free fall time to Earth is hardly a “near-instant response” often touted as an advantage to such a system.[xxxvi]

It is unlikely that a single GEO weapon system would be stationed advantageously to strike a potential target unless it occurred by coincidence or was deliberately positioned relative to an adversary. The latter could be viewed as strategically destabilizing. Regardless, the optimal GEO stationing of the system is four hours “ahead” of potential targets (this would be directly above a point 6,680 km east of an equatorial target). For example, if targets are expected to be in the South China Sea, the satellite would orbit above the Marshall Islands. The South China Sea would only be marginally visible from a satellite in GEO above the Marshall Islands. This necessitates additional intelligence, surveillance, and reconnaissance (ISR) assets over the South China Sea to support guidance, target detection, and target identification.

Defending the Weapon System

A GEO weapons system is a floating target. It is inherently vulnerable to adversary counterspace capabilities, both kinetic and non-kinetic.[xxxvii] For example, the Chinese have tested a direct ascent anti-satellite (DA-ASAT) weapon capable of reaching GEO.[xxxviii] Furthermore, U.S. adversaries possess a litany of counterspace systems and capabilities ranging from rendezvous and proximity operations to jamming.[xxxix] These systems have been exercised in operational environments and would likely pose a formidable defense against a GEO weapons system.

Defenses against GEO space-to-Earth Weapons

Even without offensive counterspace capabilities, mobility, dispersal of forces, and hardening provide defenses against a GEO weapons system.[xl] These available defensive strategies are commensurate with defenses already employed against terrestrial conventional and nuclear strikes. A GEO weapon system must also be inherently large due to design constraints and thus an adversary could detect it by using modern space domain awareness (SDA) capabilities. Once an adversary identifies the system as a potential weapons platform, that adversary could monitor the platform to detect changes, including the platform’s repositioning or the launch of a weapon. 

There are three primary means available to detect weapons deployment from GEO: radar, optical, and infrared. A radar system has the benefits of being cost effective, easy to deploy, and allows for rapid data processing. However, once an adversary’s radar begins actively tracking an incoming weapon, the location of that station would become known and subject to electronic warfare (EW) countermeasures. U.S. adversaries, including Russia and China, are developing increasingly capable counterspace radar systems.[xli] It is likely that an adversary would employ multiple radar stations, both fixed locations and mobile platforms, to track space-based weapons. Launching an effective GEO kinetic weapons strike requires a multi-domain approach, in which numerous EW countermeasures are required to suppress adversary SDA capabilities.

Optical systems can be categorized as either passive or active. A passive optical system could consist of small, automated ground-based optical observatories and would appear indistinguishable from civilian telescopes. Detection, identification, and notification of a GEO weapon deployment can be made in far less than the four hours it requires to conduct a strike. Whereas radar requires active tracking, the weapons originator would be unaware of passive optical detection because the system does not emit radiation like a radar system does. An active optical system could employ Light Detection and Ranging (LIDAR) but would be prone to the same counter detection vulnerabilities as traditional radar tracking.

A space weapon could feasibly be coated in stealth and darkening materials to mitigate detection by adversary radar and optical, however these detection mitigation strategies cannot address a GEO weapon’s heat signature. De-orbiting a weapon requires a considerable retrorocket burn, easily detectable by infrared (IR) SDA assets. This would afford an adversary the near total four-hour early warning time. In that time, mobile targets could be dispersed or engaged in continuous movement to foil last-minute terminal guidance. Stationary targets at the surface will still be vulnerable but may be evacuated of personnel and even some equipment. Furthermore, fixed surface targets could be hardened in a fashion similar to protection from “bunker busters” or nuclear strikes.[xlii]

The enormous heat signature from reentry provides a second detection opportunity even if the initial weapon deorbiting burn was not detected. The reentry could easily be detected by an adversary’s rudimentary IR early warning systems.[xliii] Once the weapon has begun reentry, early warning time is less than a minute.[xliv] While a short period of time, it still affords a prepared adversary opportunity to retaliate or employ ballistic missile defense (BMD) systems. Furthermore, the potential for miscalculation is high depending on adversary capability to resolve and track the incoming weapon, because a GEO-launched weapon could be misconstrued as an incoming intercontinental ballistic missile (ICBM), thereby increasing the probability of precipitating a larger scale conflict.

Overall analysis

A GEO weapon system as described here is costly, in that its use would be akin to paying $23,800 per kg of TNT equivalent.[xlv] This compares unfavorably to conventional weapons. A Tomahawk Land Attack Missile has a per-unit cost of $1.87 million and a warhead mass of 450 kg, giving it a cost of $4,156 per kg of explosive.[xlvi] The 4-hour time from deployment to impact is also far worse than many conventional weapons. Furthermore, an initial weapon launch could be detected by electro-optical SDA. That it may be necessary to use EW by the weapon originator to obscure the launch only serves to provide further indication of a pending strike. Therefore, there is no element of surprise with a GEO weapon-deployment system against an adversary that possesses even limited SDA capabilities.

LEO Stationed Weapons

Design Considerations

A weapon deployment in LEO would have a significantly different attack profile than one in GEO, because the LEO deployment bears more similarity to an ICBM or an HGV in mid-course and terminal phases of flight. Implementing a flight path analogous to a GEO weapon, where it is functionally dropped directly downward to the surface, is prohibitive for two reasons: First, the system would require a costly, both in weight and system expense, de-orbiting burn to negate orbital velocity. Second, the effectiveness of a LEO weapon relies on retaining the majority of its kinetic energy that was originally purposed to maintain a stable orbit, whereas a GEO weapon system relies on converting gravitational potential energy to kinetic energy. [xlvii] Instead, a LEO weapon would be deorbited from its host satellite with a minor delta-v burn and attain a flight profile similar to an HGV or ICBM.[xlviii]

We use the same weapon specifications in this presumed LEO deployment system as we discussed in the GEO deployment system. In this analysis, the LEO deployment system is stationed at an altitude of 500 km, above the International Space Station,[xlix]but below the Hubble Space Telescope.[l] The lower altitude of the LEO deployment system inherently limits the effective strike radius on short time scales. Thus, we suggest that a polar-orbiting constellation at evenly spaced ascending nodes (approximately corresponding to longitudes) provides effective global reach of the weapon. 

The LEO satellite must deploy the weapon long before the satellite is above the target itself. The optimal location at which to perform a de-orbit burn is directly opposite the location of the intended perigee (the impact site).[li] After the de-orbit burn, the weapon would gradually lose altitude until it enters the atmosphere half an orbit later. Changing from a circular orbit at 500 km altitude to an elliptical orbit with a perigee of 75 km requires a delta-v of 126 m/s.[lii] Again, the weapon would transit approximately half an orbit before reaching perigee and entering the atmosphere.

A simple (purely retrograde) deorbit burn would not change orbital inclination or ascending node, which means that only targets directly below the path of the satellite are available for attack. Maneuvering the weapon through additional burns during terminal phase can mitigate this constraint (Figure 1: LEO Weapon Deployment - Ground Track). Additionally, fuel may be reserved after a de-orbit burn to allow for a mid-course burn to change the orbital inclination and/or ascending node. This will allow a much wider lateral range for a strike. Table 1 provides estimates on the required delta-V of a mid-course burn to adjust the weapon trajectory to strike a target at longitudes not directly beneath the satellite’s original orbital path.[liii]

We offer the following LEO weapons constellation mass optimization analysis given the correlation between node spacing and delta-v requirements for striking a surface target. With a limited satellite constellation, each LEO weapon must carry a large fuel payload to expand its effective targeting radius (Figure 1: LEO Weapon Deployment - Ground Track). Increasing the number of orbital paths in the constellation reduces the fuel requirement per weapon, however, the overall mass launched to LEO increases with the additional satellites. We determined the optimum constellation is four orbital paths (Table 2: LEO Satellite Constellation – Optimization). Any number of satellites carrying weapons can be launched into each orbital path in the constellation. 

The constellation could then be configured such that the effective strike radius of satellites would ensure global coverage.

Consider the previous example with a LEO weapon system orbiting at 500 km. A deployed weapon provided with an initial deorbit burn resulting in a 75 km perigee requires 46 minutes of lead time from the strike command to target arrival.[liv] This accounts for the half-orbit transit required for the weapon to reach perigee and begin the terminal phase. However, this delay time can reach 92 minutes depending on when the satellite is tasked and the relative location of the target along the orbital path.[lv]

As shown in Table 2: LEO Satellite Constellation – Optimization (Optimal Solution Highlighted), a 1,000 kg weapon in a four-orbit configuration requires 1,758 kg of fuel to ensure full Earth coverage (a total mass of 2,758 kg per weapon).[lvi] A SpaceX Falcon 9 can launch at least 9,600 kg to polar orbit.[lvii] In this analysis, that would accommodate three weapons (plus extra mass for power and avionics, etc.). Comparatively, the Falcon 9 can deploy two weapons of similar characteristics to GEO. Thus, a LEO system is more cost effective than a GEO system.

Tactical Considerations

The shorter deployment time for LEO weapons (46-92 minutes vs. 4 hours for GEO) improves the relative tactical effectiveness. Assuming that the impact velocity of the LEO kinetic weapon is comparable to an HGV (between 2.2 km/s and 2.8 km/s), the explosive yield is on the same order of magnitude as a conventional Tomahawk Land Attack Missile.[lviii]

A constellation-based LEO weapon has, for all intents and purposes, equal effectiveness to a hypersonic missile for two reasons: target acquisition time and strike range. For a LEO system, a time from deployment to surface impact has a minimum range between 46 minutes and 92 minutes. Comparatively, the Advanced Hypersonic Weapon (AHW) has a range of 5600-8000 km and can travel at Mach 8.[lix] Thus, an AHW launched from a fixed site or naval guided missile asset and strike a long-range target in less than an hour. With suitable stationing of AHW capabilities, global coverage is feasible and eliminates the need for a LEO weapons system.

Defenses: For and against LEO Space-to-Earth Weapons

Defending against a LEO weapon system is commensurate to the GEO weapon system. Dispersal and mobility of field assets and hardening of fixed sites are effective tactics. A LEO system dramatically reduces the response time compared to a GEO system, but the response time to a LEO system is comparable to that of an incoming cruise missile or HGV. Tracking a LEO weapon is more complex than a GEO weapon, as a LEO weapon is functionally deployed from the other side of the planet. Monitoring a LEO weapon system, or an incoming weapon, necessitates a global SDA infrastructure.

Both LEO and GEO systems are vulnerable to counterspace operations including kinetic strikes from DA-ASATs. Furthermore, these systems are likely to be targeted in the opening salvo of a conflict, or even prior to the official outbreak of hostilities. The international ramifications of destroying an un-crewed weapons satellite are much less than the destruction of a ballistic missile submarine in “peacetime.” For a peer competitor, an attack on a LEO satellite is likely achieved with less effort than the destruction of a ballistic missile submarine (SSBN or SSGN). 

Overall Analysis

The LEO weapon system delivers no advantage not already offered by conventional terrestrial weapons systems fielded or under development. The explosive yield from the liberated kinetic energy is comparable to a cruise missile or HGV and the response time is worse. Furthermore, the weapon has limited application as a strike option. For example, a bunker buster achieves maximum penetration at speeds around 1 km/s.[lx] A space-to-ground weapon would have to shed the majority of its velocity, and thus its explosive yield, to have an analogous impact profile to a bunker buster weapon. The only benefit of the LEO weapons system beyond existing conventional capabilities is the assurance of global strike coverage with a properly fielded, polar orbiting constellation.

International Policy Considerations and the Weaponization of Space

The Outer Space Treaty (1967) declaration that space will be used for “peaceful purposes” has led to varied interpretations amongst states, particularly when it comes to national security interests and military activity.[lxi] Specifically, the treaty prohibits the deployment of nuclear weapons and weapons of mass destruction in space and the stationing of weapons on celestial bodies, including the Moon.[lxii] Derivatively, conventional weapons are allowed elsewhere throughout the domain.[lxiii] The gradual militarization and weaponization of space has muted historical arguments on preserving the domain as one of exclusively peaceful use. 

For example, the development of the Shuttle Transportation System (STS) was initially perceived by the USSR as an espionage platform capable of seizing satellites and returning them to the surface or a potential orbital nuclear first-strike weapon.[lxiv] The USSR was so threatened by the potential application that it launched its own costly analogue, the Buran Shuttle.[lxv] Furthermore, the USSR made political and diplomatic efforts to stymie the efficacy of early STS launches including a failed attempt to compel the Chilean Government into prohibiting the use of Easter Island in the South Atlantic as an abort landing site.[lxvi] Based on adversary historical perspectives of STS activities, we can reasonably infer that current adversaries view the furtive activities of the X-37B as a dubious expansion of U.S. militarization in space.

Avoiding discovery while fielding a space-to-Earth kinetic weapons system presents a challenge to any nation making the attempt.[lxvii] Furthermore, once identified as a weapons platform, it would be difficult for the observer to determine the payload, be it an inert tungsten rod, an HGV, or a multi-megaton nuclear device. Thus, the mere stationing of such a system in orbit is likely to be perceived as an aggressive militarization and weaponization of space.

Space-to-Earth kinetic weapons do not significantly increase the ability of the U.S. military to strike potential adversaries, especially with globally deployed guided missile submarines (SSGNs) and surface naval missile ships. Conversely, a less-capable nation with launch capability would yield significant benefit from a space-to-Earth kinetic weapons system. If a nation cannot support a viable, globally deployable military, space weapons could afford an asymmetric advantage. An orbital weapons platform expands the ability to project power across the entire terrestrial domain for a substantial but potentially affordable cost. Similarly, United States deployment of space-to-Earth kinetic weapons is likely to invite an arms race with near-peer competitors eager to establish parity. 

This problem is exacerbated when one considers the usual hypothetical targets of hypersonic weapons: naval ships. Many of these weapons are considered first and foremost “ship-killers.”[lxviii] Given the size and dispersal of the U.S. Navy, adversaries have ample targets of opportunity. And given the United States’ reliance on its surface Navy, the inherent vulnerability to hypersonic strikes affords an adversary opportunity to shift a regional balance of power (or at least establish a credible capability).

Advocates for peaceful use of space suggest that the United States should employ a strategy that 1) focuses on space support and force enhancement, 2) retreats from active space control and force application, and 3) codifies space weapons treaties.[lxix] We agree with the first point that the United States should improve its space defense posture. However, the second and third points inadequately address both historical and present-day conditions. During the Cold War, periods of cooperative restraint on space weapons were congruent with détentes in U.S.-USSR relations and subsequently produced the Anti-Ballistic Missile Treaty and the first strategic arms limitation treaty (1972).[lxx] These efforts were not the result of benevolent intent, but an effort by the United States and USSR to mitigate the cost of expensive programs and merely delayed weaponization.[lxxi]

Subsequently, adversaries can be expected to increase aggressive space activities, limited only by their budgets. Exhibition of kinetic force applications, including the 2007 Chinese anti-satellite (ASAT) test, should be viewed as escalations in capabilities.[lxxii]Furthermore, Russian satellite shadowing of U.S. strategic ISR platforms and Russia’s testing of a co-orbital ASAT system indicates increased vulnerability to space-based capabilities.[lxxiii] In 2009, the Prevention of the Placement of Weapons in Outer Space Treaty, drafted by Russia and China, was presented to the U.N. Committee on the Peaceful Uses of Outer Space.[lxxiv] The draft was rejected by the United States due to the narrow scope of the document, which solely limited space-based weapons, and allowed continued development of terrestrial-based weapons capable of striking space assets.[lxxv] U.S. adversaries have continued interests in expanding offensive capabilities in space, both terrestrial and space-based. 

Conclusion

The 2020 Defense Space Strategy explains that “China and Russia have weaponized space as a way to deter and counter a possible U.S. intervention during a regional military conflict.”[lxxvi] Although LEO-based weapons are marginally more effective (in terms of economic cost and response time) than GEO weapons, neither proffer a significant advantage over current terrestrial-based options in the arsenals of the U.S. military. The analysis performed here indicates that space-to-Earth kinetic weapons are not an effective response to the weaponization of space, nor tactically useful in the terrestrial domain. In general, space-to-Earth kinetic weapons are prohibitively expensive, vulnerable, and suffer from latency in response time. Current conventional weapons employed in the terrestrial domain offer comparable effectiveness at lower cost. 

This discussion does not reject the classic military tenet of holding the high ground. Projecting credible combat power in the space domain, as in all terrestrial domains, is critical to safeguarding U.S. national security interests. “When employed against adversaries, military spacepower has deterrent and coercive capacities—it provides independent options for National and Joint leadership but achieves its greatest potential when integrated with other forms of military power.”[lxxvii] Nevertheless, fielding of space-to-Earth kinetic weapons can be destabilizing and provides justification for adversaries to launch their own capabilities. The launch of such weapon systems provide an asymmetric advantage for nations that do not possess forces capable of global strikes. The weaponization of space is maturing from its nascent stage. However, the United States should abstain from the deployment of space-to-Earth kinetic weapon systems. Tactically, little is gained. Strategically, much is lost.

Dr. David W. Harris is an assistant professor in the Department of Aerospace, Physics and Space Sciences at Florida Institute of Technology. Lieutenant Commander James Toomey, USCG, graduated in 2021 as a Schriever Space Scholar from the Air Command and Staff College. This paper represents solely the authors' views and do not necessarily represent the official policy or position of any Department or Agency of the U.S. Government. If you have a different perspective, we’d love to hear from you.

[i] Julian Borger, “Bush Likely to Back Weapons in Space,” The Guardian, 18 May 2005.

“Project Thor: What America’s ‘Rods from God’ Space Based Superweapon Can Do,” Military Watch Magazine, 6 July 2020.

[ii] Bob Preston, Dana J. Johnson, Sean J.A. Edwards, et al., Space Weapons Earth Wars, RAND Corporation, 2011

[iii] Caleb Larson, "'Rods from God': Why Mach 5 Hypersonic Tugeston[sic] Bombs Were Never Dropped," The National Interest, 16 November 2020.

Jared Keller, "The Pentagon’s New Super Weapon Is Basically A Weaponized Meteor Strike," Task & Purpose, 07 June 2017

[iv] Space-to-Earth weapons are portrayed in numerous novels, such as “The Moon is a Harsh Mistress” by Robert A. Heinlein and the Honorverse series by David Weber. Space-to-ground (not necessarily “Earth”) weapons are featured in visual media such as “Babylon 5” and “Stargate SG-1." Computer and video game examples include “Sid Meier’s Alpha Centauri” and “Call of Duty: Ghosts.”

[v] Hypersonic Glide Vehicles are launched by rockets into the upper atmosphere and boundary of space. Richard H. Speier, George Nacouzi, Carrie A. Lee, Richard M. Moore, “Hypersonic Missile Nonproliferation: Hindering the Spread of a New Class of Weapons,” RAND, 2017, xi-xii.

[vi] The Hypersonic Technology Vehicle 2 is carried onboard a Minotaur IV Lite Booster which has a maximum payload capacity of 1452 kg (3200 lb). Thomas Huynh and Joseph Kriz, “Environmental Assessment for Hypersonic Technology Vehicle 2 Flight Tests,” April 2009, 9.

[vii] Arthur C. Clarke, "Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?". Wireless World, October 1945, 305-308.

[viii] Ibid.

[ix] “Catalog of Earth Satellite Orbits.” NASA. NASA, n.d. Accessed September 25, 2021. https://earthobservatory.nasa.gov/features/OrbitsCatalog.

[x] It should be noted that HGVs do not necessarily follow the typical midcourse trajectory of a traditional ballistic missile. HGVs are often designed to follow a depressed trajectory that skims along the tenuous, upper atmosphere.

“Defense Primer: Hypersonic Boost-Glide Weapons” (Washington, D.C.: Congressional Research Service, June 08, 2021), 1.

[xi] The authors assume the HGV utilizes a rocket to initially propel the system. Two U.S. HGVs being developed, the B-52 launched Hypersonic Conventional Strike Weapon and the Land-Based Hypersonic Missile, rely on rocket propulsion during the boost phase. Kelly M. Sayler, “Hypersonic Weapons: Background and Issues for Congress” (Washington, D.C.: Congressional Research Service, August 25, 2021), 6.

[xii] “The Future of the Battlefield,” (Washington, D.C.: Office of the Director of National Intelligence, April 2021), 5.

[xiii] Mark J. Lewis, “Global Strike Hypersonic Weapons,” AIP Conference Proceedings 1898 (020005), November 15, 2017, 1-2.

[xiv] Sayler, “Hypersonic Weapons,” 6.

[xv] Cameron L. Tracy and David Wright, “Modeling the Performance of Hypersonic Boost-Glide Missiles,” Science and Global Security, vol. 28(3), 2020, 17.

[xvi] Ibid, 14.

[xvii] Throughout this document, the authors determine impact speed/atmospheric entry speed using a derivation of energy conservation. The impact speed is integrated over the fall distance to determine free fall time.

[xviii] Eqn 1.

[xix] Ambler Thompson and Barry N. Taylor, “Guide for the Use of the International System of Units (SI),” NIST Special Publication, National Institute of Standards and Technology, July 2008, 59.

[xx] Mahesh C. Jain, Textbook of Engineering Physics (Part I).  ISBN 978-81-203-3862-3., 2009, 9.

[xxi] Ibid.

[xxii] The authors use the term speed and velocity interchangeably for this first order analysis.

[xxiii] The circular orbital velocity is given by

[xxiv] Maren Hülsmann, "Uncertainty Dispersion Analysis and Optimal Control of Atmospheric Re-Entry," PhD dissertation, Universität Bremen, 2017, 6-7.

[xxv] Ibid., 8.

[xxvi] Bob Preston, Dana J. Johnson, Sean J.A. Edwards, et al., Space Weapons Earth Wars, RAND Corporation, 2011, 138-139.

[xxvii] Jonny Dyer, "Rocket Performance and Efficiency," Lecture Notes: Air and Space Propulsion, Stanford Aeronautics and Astronautics, n.d., 3. 

[xxviii] Tsiolkovsky rocket equation,

For this analysis, we assumed the use of an Aerojet Rocketdyne RL10 engine with an exhaust velocity of 4.565 km/s

“Aerojet Rocketdyne RL10 Propulsion System,”Aerojet Rocketdyne, October 2021, 2.

[xxix] “Arianespace begins building final 10 Ariane 5s ahead of Ariane 6 operational debut,” Space Daily, February 2019.

"Delta IV Launch Services User's Guide - June 2013," United Launch Alliance, 4 June, 2013, 2–10; 5–3.

SpaceX, “Capabilities and Services,”n.d.

[xxx] Ibid.

[xxxi] Eqn 1 and Eqn 2, as above.

[xxxii] This is examined in detail in the section “LEO Stationed Weapons” under “Design Considerations”.

[xxxiii] Stephen Thornton and Jerry Marion, Classical Dynamics of Particles and Systems, 5th ed. (Boston, MA: Cengage Learning, 2003).

[xxxiv] Tracy and Wright, “Hypersonic Boost-Glide Missiles,” 17.

[xxxv] Energy can be converted between different unit systems. One such unit system is “kg of TNT,” with a conversion factor of 1 kg TNT = 4.184 million joules.

G. I. Brown, The Big Bang: A History of Explosives, (Thrupp, UK: Sutton Publishing Limited, 1998)

[xxxvi] Blake Stillwell, “The US Air Force's 'rods from god' could hit with the force of a nuclear weapon — with no fallout,” Business Insider, 4 February, 2019.

[xxxvii] “Placing space-based defensive systems in an unprotected, highly vulnerable location – as in near-Earth orbit – without any kind of relative advantage is not the strategy of defense but it the strategy of the ludicrous” (emphasis by author). John J. Klein, Space Warface (Routledge: New York, NY), 2006, 176.

[xxxviii] Todd Harrison, Kaitlyn Johnson, and Thomas G. Roberts, Space Threat Assessment 2018 (Washington, DC: Center for Strategic and International Studies, 2020), 11.

[xxxix] Ibid.

[xl] Headquarters, Department of the Army, “FM 100-30 Nuclear Operations,” 29 October 1996, 18

[xli] Mark Stokes, Gabriel Alvarado, Emily Weinstein, and Ian Easton, “China’s Space and Counterspace Capabilities and Activities,” Project2049 Institute, 30 March, 2020, 57.

[xlii]Orbital kinetic energy weapons are not expected to make good penetrators or bunker busters. Maximum penetration occurs at speeds around 1.0 km/s. In this proposed system, the weapon would impact at much higher speeds. Richard L. Garwin, “Space Weapons: Not Yet,” Pugwash Workshop on Preserving the Non-Weaponization of Space, 14 May 2003, 3.

[xliii] Tracy and Wright, “Hypersonic Boost-Glide Missiles,” 7.

[xliv] The terminal phase of an ICBM, which marks the period from initial atmospheric interaction (reentry) to target, is about 60 seconds. “Three Stages of the Inter-Continental Ballistic Missile (ICBM) Flight,” Airpower Development Centre Bulletin, Issue 305, March 2018, 1-2.

[xlv] Given the aforementioned launch cost of $31,000,000 and the equivalent yield of 1300 kg.

[xlvi] The analysis for this GEO weapon only accounts for the launch costs, which already far exceeds that of a conventional system of similar capability. Office of the Undersecretary of Defense (Comptroller), “Program Acquisition Cost by Weapon System: Fiscal Year 2021 Budget Request,” (Washington, D.C.: Office of the Undersecretary of Defense (Comptroller), February 2020), 5-18.

[xlvii] The orbital velocity of a LEO weapon at 500 km altitude is 7.6 km/s. The delta-v resultant from the potential energy conversion from an object falling from an altitude of 500 km to the Earth’s surface is about 3.0 km/s. Thus, upon impact, the converted potential energy only accounts for 13.5 percent of the explosive yield. This is purely a comparison for academic argument. The drag on the system during reentry is significant, and suggested impact velocities for an HGV at 2.2 km/s to 2.8 km/s. The point here is to express that in a LEO system, the gravitational potential energy is not necessarily a major contributor to the effectiveness of the weapon.

[xlviii] The hypersonic missile system envisioned here utilizes a ballistic missile during the boost phase. Then, the hypersonic weapon skims along the upper atmosphere during the mid-course phase, not necessarily following a true parabolic trajectory, before angling down during the terminal phase to impact the target. Lauren Caston, Robert S. Leonard, Christopher A. Mouton, et al., “The Future of the U.S. Intercontinental Ballistic Missile Force,” RAND

Corporation, 2014, 71.

[xlix] Chris Peat, “Heavens Above: ISS”, n.d.

[l] Michelle Belleville, “Hubble Space Telescope,” NASA, 11 Dec, 2021

[li] Walter Hohmann, “The Attainability of Heavenly Bodies, Technical Translation F-44,” NASA, November 1960.

[lii] Federal Aviation Administration, “Maneuvering In Space,” (Washington, D.C.: Department of Transportation, 11 April 2018), 204-210. 

The authors acknowledge that a more aggressive deorbit burn to reduce target strike time is possible, though not practical. On the first order, it requires an exorbitant increase in fuel payload.

[liii] The delta-V required for a change in orbital inclination can be calculated by

Stephen Thornton and Jerry Marion, Classical Dynamics of Particles and Systems, 5th ed. (Boston, MA: Cengage Learning, 2003).

[liv] The orbital period of a satellite is given by

Where r is the semi-major axis, G is the gravitational constant, and M is the mass of the parent body. The time to deorbit would then be half an orbital period.

[lv] The authors acknowledge that the 46 minutes to 92 minutes range does not include the time the weapon requires to decelerate through the atmosphere and strike the target (terminal phase). This increase to flight time is clearly a further reduction in the tactical utility of the weapon.

[lvi] Any number of weapons can be inserted into each of these four orbits to decrease delay time.

[lvii] Clara Moskowitz, “Largest Commercial Rocket Launch Deal Ever Signed by SpaceX,” Space.com, 16 June 2010. 

Chris Gebhardt, “Iridium boss reflects as final NEXT satellite constellation launches,” NASASpaceFlight.com, 11 January 2019.

[lviii] Gerald G. Brown, Alexandra M. Newman, Richard E. Rosenthal, and Anton A. Rowe, “Optimizing Tomahawk Strikes,” Naval Postgraduate School, January 2001, 2.

[lix] Abel Olguin “Employment of Hypersonic Glide Vehicles: Proposed Criteria for Use,” Sandia National Laboratories, 30 July 2014, 2.

[lx] Garwin, “Space Weapons: Not Yet,” 3. 

[lxi] Ortega, A. A., “Placement of Weapons in Outer Space: The Dichotomy Between Word and Deed,” Lawfare, 28 January, 2021.

[lxii] Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies.

[lxiii] It should be noted that GEO ‘slots’ are limited and managed by the International Telecommunications Union. It is certain that requesting to station a weapons platform in a GEO slot would be met with an adverse reaction from the ITU and international community alike. 

[lxiv] Bart Hendrickx and Bert Vis, Energiya-Buran: The Soviet Space Shuttle, (New York, NY: Springer-Praxis Books in Space Exploration published in Association with Praxis Pub., 2007).

[lxv]Ibid., &

James Oberg, Space Power Theory (Peterson AFB, Colorado: Air Force Space Command, 1999), 57.

[lxvi] Oberg, Space Power Theory, 74.

[lxvii] Assuming that there is secret intent for stationing such a system. This obviously reduces its effectiveness as a deterrent if the enemy is unaware of the capability. Refer to the following quote from the film Dr. Strangelove, “The whole point of the Doomsday Machine is lost if you keep it a secret!”

[lxviii] Paul Bernstein and Harrison Menke, “Russia's Hypersonic Weapons,” Washington Headquarters Services, 13 December 2019.

[lxix] Joan Johnson-Freese, Space as a Strategic Asset (Columbia University Press, 2007), 244.

[lxx] James Clay Moltz, The Politics of Space Security: Strategic Restraint and the Pursuit of National Interests, 2nd ed. (Stanford, 2011), 124-125.

[lxxi] Everett Dolman, Astropolitik: Classical Geopolitics in the Space Age (Routledge, 2001), 160.

[lxxii] “China views space as a critical U.S. military and economic vulnerability, and has fielded an array of direct-ascent, cyber, electromagnetic, and co-orbital counterspace weapons capable of targeting nearly every class of U.S. space asset. The People’s Liberation Army (PLA) has also developed doctrinal concepts for the use of these weapons encouraging escalatory attacks against an adversary’s space systems early in a conflict, threatening to destabilize the space domain. It may be difficult for the United States to deter Beijing from using these weapons due to China’s belief the United States has a greater vulnerability in space.”

“2019 Annual Report to Congress,” U.S.-China Economic and Security Review Commission, 360.

[lxxiii] Hanneke Weitering, “Russia has launched an anti-satellite missile test, US Space Command says,” Space.com, 16 December 2020.

[lxxiv] United Nations, General Assembly, "Report of the Committee on the Peaceful Uses of Outer Space," A/64/20 (1 July 2009)

[lxxv] Moltz, The Politics of Space Security, 310-311.

[lxxvi] Defense Space Strategy Summary (Washington, DC: Department of Defense, June 2020), 3.

[lxxvii] Space Force Capstone (Washington, DC: Headquarters United States Space Force, June 2020), 16.