The Physics of Space Security (2005)

May 2005
A few basic laws of physics have important implications for the way satellites, space-based weapons, and anti-satellite weapons can be designed and operated.

This is an excerpt from the introduction to The Physics of Space Security: A Reference Manual, a report published by the American Academy of Arts and Sciences (AAAS). The report was written as part of the AAAS "Reconsidering the Rules of Space" project.

In the nearly fifty years since the Soviet Union launched Sputnik, there has been a steady growth in the number of states that have launched satellites into orbit. Growing even faster is the number of countries that have deployed satellites launched by others. Currently, satellites serve a multitude of civilian and military functions, from facilitating communications and weather forecasting to providing highly accurate navigational information, and many nations envision making future investments in satellites for such uses.

In the U.S. military, there is also a growing interest in broadening the military uses of space to include basing weapons in space, as well as in developing means to attack the satellites of other nations and to protect U.S. satellites from attack. While space has long been home to military systems such as observation, communication, and navigation satellites, these new missions would be a departure from long-held norms. There are currently no known weapons stationed in space that are explicitly designed to apply force. Nor are there any known deployed systems designed explicitly to destroy satellites, either from the ground or from space.

This shift in U.S. military thinking is evident from planning documents released in recent years that envision a restructuring of military commands and the development and deployment of anti-satellite weapons and space-based weapons.¹ These new systems are meant to fulfill four general missions:

  • defending U.S. satellites and ensuring U.S. freedom to operate in space
  • denying adversaries the ability to use space assets
  • intercepting ballistic missiles using space-based interceptors
  • attacking targets on the ground or in the air using space-based weapons

The first two missions reflect the military importance of current U.S. space-based systems. This utility has led to a desire to protect these systems and to deny similar capabilities to potential adversaries, as well as to concern that potential adversaries would seek to disrupt U.S. satellites in a conflict. The third mission is an ongoing interest of many missile defense proponents and is leading toward the deployment of prototype weapons in space as part of a space "testbed." The fourth mission, which has attracted considerable public attention and concern, currently appears to be of less interest to the U.S. military than the other missions.

The U.S. interest in new types of weapons has spawned an emerging international debate. Some key issues include whether the deployment of space-based weapons and anti-satellite weapons is inevitable, what military utility these weapons would have, how their deployment would affect the security of the owner nation and the wider international community, whether their deployment and use would interfere with other military and civilian uses of space, and what normative and legal constraints on the use of space could be useful.

Addressing these issues involves assessing a wide range of political, diplomatic, military, and technical issues. This report is limited to a discussion and analysis of the technical and military issues. This focus sheds light on a number of key questions: What capabilities could anti-satellite weapons and weapons in space realistically provide? Would these capabilities be unique? How do they compare with alternatives? What would they cost? What options would be available to nations seeking to counter these capabilities? The answers are technical realities that must be considered in any policy analysis of space weapons and anti-satellite weapons. Unless debate about these issues is grounded in an accurate understanding of the technical facts underlying space operations, the discussion and policy prescriptions will be irrelevant or, worse, counterproductive.

In assessing proposed military systems, it is important to distinguish between constraints imposed by financial cost, by technology, and by physics. The cost of operating in space is often high relative to the cost of operating in the air or on the ground. While cost will be important in considering development and deployment, it may not be decisive if the system could provide a unique capability that is deemed important. Available technology places important limits on what systems are currently feasible for a country, but those limits can change over time and do not represent fundamental limitations. An example of a technology constraint is the space-based laser. The power levels achieved so far are well below what is required for a usable weapon, but there do not appear to be fundamental limits to increasing the power over time. Physics, on the other hand, places fundamental limits on space operations that will not change with time and these implications must be taken into account when assessing uses of space. An example of a fundamental limit posed by physics is the fact that satellites in low orbits cannot remain stationary over a given location on Earth, so that multiple satellites are required in orbit to ensure that one is always near a given location.

This report provides information on a range of technical issues related to space systems that are important for anyone involved in the debate over space security to understand.² It discusses cost and technology, where appropriate, and attempts to separate these from the fundamental physics issues. It is written for a lay audience but includes appendices that give more detailed information of interest to technical audiences.

The report is intended to familiarize readers with the important technical terminology and concepts related to satellites and operating in space. For example, the behavior of objects traveling at very high speeds in space is much different than the behavior of objects in motion on the ground or in the atmosphere and is largely outside people's day-to-day experience. As a result, most people have not developed intuition about the behavior of satellites, so that attempting to apply lessons from common experience can lead to mistakes and misconceptions.

In addition, the report shows that a few basic laws of physics have important implications for the way satellites, space-based weapons, and anti-satellite weapons can be designed and operated. It explains these underlying physical principles and discusses their implications.

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