Estimated Read Time: 8 minutes
By Roman C. Lau
Introducing the altitudes between 20 kilometers and 130 kilometers as hypersonic airspace might help frame the operational envelope for hypersonic vehicles and assist with the development of air, space, and joint doctrine. This article first describes the challenge of defining the boundary between airspace and space. As an alternative, it develops and recommends the hypersonic airspace concept as an operational framework to describe the impact of hypersonic weapons on time, space, and forces in the air and space domain. If this airspace is not negotiated and defined, it will continue to be in question and might become subject to misuse and operational disadvantage in forthcoming competing developments.
In a recent Over the Horizon article, Ashley Torkelson concluded that United States Space Command (USSPACECOM) should revise their announcement regarding their area of responsibility (AOR) as “the area surrounding the earth at altitudes equal to or greater than 100 kilometers above mean (average) sea level”. Instead, she states USSPACECOM should expand their lines of the AOR to align “with the joint space doctrine definition of outer space as the area above atmospheric effects on airborne objects become negligible… [and] redefine its geographic AOR to simply outer space.” Torkelson argues for avoiding future international legal obligations that might hamstring America’s ability to utilize highflying objects above enemy territory, or alternatively, how the United States might react to adverse objects. Furthermore, the article touches on the influence of hypersonic weapons within the airspace and space environments and discusses the dispute about the validity of the Kármán Line as the boundary between airspace and space. These arguments are well understood and reasonable. However, “the altitude where atmospheric effects on airborne objects become negligible” depends on many variables and cannot be defined as a clear and distinct boundary.
On the other hand, legal and operational aspects of command responsibilities might not allow leaving such boundaries vague and unprecise. With the advent of hypersonic weapons, analyzing their effects on airspace and space boundaries becomes crucial to provide a solid foundation to adapt international standards and provide safe and secure space and airspace operations.
Space and airspace are regularly perceived as two separated domains. There is space with the orbital physics of gravity on one side, and on the other side, there is airspace with aerodynamic flight. The gradual transition area is often not further differentiated and certainly not included from an operational perspective. Specifically, this dichotomy does not consider the transitional impacts of hypersonic trajectories and the resulting new challenges for managing space and airspace. The characteristics of typical hypersonic weapon trajectories may cross both the higher altitudes of airspace and space.
The nature of space-based objects is entirely different from aerial objects. The laws and forces of aerodynamics do not apply in the near-vacuum of space, neither for aerodynamic steering nor to produce aerodynamic lift. Only gravitational forces and the thrust of rocket motors can influence speeds and directions. Without any air resistance, space objects could theoretically circle in orbits infinitely. However, air resistance goes well beyond the Kármán Line, slows down orbital objects, and draws them back towards the surface of the Earth. Any object built to remain in space on low earth orbits needs boost maneuvers periodically to climb back to the intended orbit. For example, at 400 kilometers (1,300,000 ft), the International Space Station loses approximately 200 m (650 ft) of its altitude every day due to drag. It performs one orbit in 90 minutes at seven km/s (comparable to Mach 21). At a lower orbital altitude of only 200 kilometers (660,000 ft), the higher atmospheric drag pulls an unpowered satellite so significantly further down that it takes less than a day until it plunges toward the Earth’s surface. At an altitude of 130 kilometers (430,000 ft), the drag becomes so significant that a satellite cannot even finish another complete 90-minute orbit before it hits the surface of the Earth. Only at altitudes above 130 kilometers mark the beginning of orbital flight and space conditions. Thus, it is reasonable to refer to the boundary between space and the Earth’s atmosphere at 130 kilometers. While the regular aerial use of the atmosphere practically ceases above an altitude of 20 kilometers, the orbital use of space can only begin at an altitude above 130 kilometers. As the International Association for the Advancement of Space Safety argues that the distance of 18-160 kilometers defines near space in general, hypersonic objects can exploit the characteristics of this zone.
Some civil companies have drawn international attention to these heights through their growing commercial use, e.g., ballistic passenger flights. However, the rapid development of various hypersonic weapons has predominantly conquered this part of the atmosphere. Hypersonic vehicles ideally operate between altitudes of 20 and 130 km. Furthermore, using an Intercontinental Ballistic Missile (ICBM) for launch, a Hypersonic Glide Vehicle (HGV) could reach hundreds of kilometers into space before re-entering the atmosphere. Therefore, hypersonic vehicles blur the lines and fill the seam between outer space and airspace. They highlight a distinct envelope for hypersonic flight and demand a new set of rules, regulations, and doctrines. Introducing the altitudes between 20 kilometers and 130 kilometers as hypersonic airspace might help frame the operational envelope for hypersonic vehicles and assist with developing air, space, and joint doctrine.
In the past, hypersonic engineering focused on two efforts. First is the effective, safe, and smooth as possible atmospheric re-entry of crewed space vehicles. Second, the successful and fast re-entry of the intercontinental nuclear warheads. Today, the development aims to enable hypersonic weapons to survive the physical effects of the extreme aerothermal environments for long durations while maintaining maneuverability. HGV skipping trajectories starting at 120 kilometers allow ranges potentially in excess of 7,000 kilometers; maneuvers with accelerations of up to five g are possible. These new hypersonic trajectories come with blended characteristics between orbital behavior in space and aerial behavior in the airspace environment.
With the advent of hypersonic weapons, regional borders are becoming irrelevant; continental distances are shrinking. Crossing the Atlantic, the Pacific, or the Arctic Oceans at subsonic speed takes many hours, and a fundamental change of direction shortly before the destination is unlikely because of consumed endurance. With hypersonic speeds, such distances pass in minutes, and the target is only evident when the weapon closes in. With maneuvering speeds of Mach 25 (8,3 km/s; 18,500 mph), a hypersonic weapon would need less than a minute to descend from an altitude of 130 kilometers to any target on the ground.
Hypersonic glide vehicles revolutionize the military forces by combining the speed of an ICBM with the maneuverability of a cruise missile and making them a qualitatively new capability. Even a thousand miles away, any hypersonic object is alarming and becomes a potential threat. Any unidentified hypersonic vehicle approaching below an altitude of 130 kilometers is most likely not civilian and probably not friendly or neutral; it might demand a command decision to engage within seconds. These characteristics challenge existing detection systems, enable the attacker to gain surprising effects, and hinder the defender from preparing for active combat. In addition, any international commercial hypersonic air traffic that might arise would have to be integrated and deconflicted. Ensuring military security is one thing, peaceful civilian use of airspace is another.
The hypersonic airspace concept offers an operational framework for both hypersonic weapons’ impact on time, space, and forces in the air and space domain and international civil aviation. The current upper edge of Class A airspace at an altitude of 60,000 ft (18km) could extend to 66,000 ft (20 km), where a newly inaugurated Class H hypersonic airspacecould begin.
The ramifications of the advent of hypersonics are the same for all competitors. These weapons challenge the existing international space and airspace structure. With HGVs blurring the line between airspace and space with their challenging trajectories, the existing set of rules, habits, and practices in airspace and space need a mutual rethinking. Introducing hypersonic airspace as a buffer zone between space and airspace and developing own interests and points of view for applicable rules could help cope with this challenge.
Roman C. Lau is a Colonel in the German Air Force and former air force faculty chair of the Führungsakademie der Bundeswehr in Hamburg, Germany. He is a graduate of Universität der Bundeswehr, Hamburg; Joint Advanced Warfighting School, Norfolk; and a scholar of National Defense University, Washington, DC. Essential parts of this article are based on the content of Colonel Lau’s master’s thesis at the Joint Forces Staff College.
Disclaimer: The views expressed are those of the author and do not necessarily reflect the official policy or position of the Department of the Air Force or the U.S. Government.
Featured Image: https://www.faasafety.gov with author alterations **