Technical Implications of Space Weapons and General Conclusions
This is Section 3 of a report published by the American Academy of Arts and Sciences (AAAS), The Physics of Space Security: A Reference Manual, which was authored by UCS scientists David Wright, Laura Grego, and Lisbeth Gronlund.
This report discusses the implications of some of the basic technical issues that govern the use of space. Here we summarize for each section of the report some of the general conclusions that result from analysis of these issues.
Basics of Satellite Orbits (Section 4)
• The speed of a satellite is not arbitrary: it is determined by the satellite's orbit and is closely tied to the satellite's altitude.
• A satellite's orbit does not depend on its mass. All objects with
the same velocity (speed and direction) at a given point in space
follow the same orbit.
• Satellites close to the Earth move faster than those at higher altitudes and, when viewed from the ground, cross the sky faster.
Satellites in low earth orbits (hundreds of kilometers above the
Earth) move rapidly relative to the Earth, completing an orbit in
1.5 to 2 hours.
• Satellites in higher orbits move at slower speeds than those in
lower orbits, and the distance they travel in one orbit is longer.
As a result, the time required for a satellite to orbit (the orbital
period) increases with altitude. Only one altitude (36,000 km)
permits satellites to orbit at the same rate at which the Earth
rotates; such satellites are called geosynchronous.
• Once in orbit, a satellite does not need constant powering to
remain in flight, as airplanes do. Satellites use small onboard
rocket engines to maneuver in space.
• A satellite's orbit always lies in a plane that passes through the
center of the Earth. The angle between that plane and the plane
of the equator is called the orbit's inclination.
Types of Orbits, or Why Satellites Are Where They Are (Section 5)
• Because the Earth rotates underneath the satellite as it orbits, a
satellite in a polar orbit (an orbit that passes over both poles)
travels directly over every point on Earth. Satellites in equatorial
orbits only travel directly over the equator. Satellites may be in
orbits with inclinations between these two extremes; in such cases, the satellite travels directly over points on the Earth with a latitude equal to or less than the satellite's inclination angle.
• Satellites that are in equatorial orbits and that have an orbital
period of 24 hours stay fixed over a point on the equator; they
are called geostationary. While geostationary orbit is useful for
hosting communications and broadcasting satellites, it is not well
suited to such missions as high-resolution imagery or ground
attacks, because such an orbit requires a very high altitude
(36,000 km). Furthermore, because geostationary satellites travel
only in the equatorial plane, they have difficulty communicating
with the Earth's polar regions.
• Satellites that are not in geostationary orbit move with respect to
the ground, and so constant coverage of a particular location on
the Earth requires a constellation of satellites.
• Satellites at high altitudes can see more of the Earth's surface at
one time than can satellites at lower altitudes.
• Satellites that need to be close to the Earth to perform specific
missions, for example, to take high-resolution images of the
ground, must be located in low earth orbits. Being closer to the
Earth's surface makes these satellites more vulnerable to interference from ground-based methods of attack.
Maneuvering in Space (Section 6) and Implications of Maneuvering for Satellite Mass (Section 7)
• Maneuvering a satellite, which requires changing the speed of the
satellite or its direction of travel, can require a large expense of
energy. The mass of propellant a satellite needs to change its velocity increases exponentially with the amount of velocity change.
The difficulty and cost of placing large amounts of propellant in space therefore limit how much maneuvering satellites can do.
• Maneuvers to change the satellite's orbital plane can require large
changes in the satellite's velocity and can therefore require large
amounts of propellant. By contrast, maneuvers that alter the shape or altitude of the orbit but that do not change the orbital plane generally require much less propellant, especially if the satellite moves between low earth orbits.
• Propulsion using new technologies can generate substantially
more velocity change per unit mass of fuel than conventional
chemical propellants do. This reduces the mass of fuel a satellite
needs to carry to perform a given maneuver. While more efficient,
the new propulsion technologies that will be available in the foreseeable future cannot be used to carry out maneuvers quickly, which limits the tactical utility of these technologies.
Getting Things into Space: Rockets and Launch Requirements (Section 8)
• Placing an object in orbit is much more demanding than simply lifting it to a high altitude. Although short- and medium-range ballistic missiles can reach the altitudes of satellites, once there they cannot produce the high speeds necessary to put a satellite into orbit. Even a long-range (10,000 km) ballistic missile cannot put its full payload into orbit.
• A rule of thumb is that a ballistic missile that can deliver a given
payload to a maximum range R on the Earth can lift that same payload vertically to an altitude R/2 above the Earth. Reducing the mass of the payload increases both R and R/2.
• Modern rockets can deliver into low earth orbit a payload that is only 2–4% of the total mass at launch. Roughly 45 tons of propellant
are required for every ton of payload placed in orbit.
• How much mass a launch vehicle can place in orbit depends on the location of the launch site and the intended orbit. Since the rotational speed of the Earth's surface is largest near the equator, launching from sites near the equator allows the launcher to take advantage of that additional speed.
• Reducing the size and mass of satellites can reduce launch costs and may allow satellites to hitch a ride on other launches, which can be cheaper than using a dedicated launcher and may be scheduled more quickly.
Space Basing (Section 9)
• Operating in space has a number of important consequences:
First, placing satellites in orbit is costly. Second, satellites in low earth orbits move relative to the Earth's surface, leading to an intrinsic problem with absenteeism (i.e., low-earth-orbiting satellites
spend most of their time above the wrong part of the Earth), and missions that require low-altitude satellites to be at a specified location require multiple satellites in orbit. Third, repairing, refueling, or updating satellites in orbit is difficult and costly, so it is rarely done. As a result, the reliability of a space-based system decreases with time. Fourth, the space environment is relatively hostile to satellites, with high levels of radiation, large temperature changes from sun to shadow, and fast-moving space debris.
• Space-based ground-attack weapons intended for prompt, on-demand attacks and global reach would have a high absentee ratio, since a large number of satellites would be needed to ensure that one is in place for the mission at all times. Ground-based systems could provide these capabilities on the same timescale, with greater reliability and at a cost many tens of times less.
• Basing a boost-phase missile defense system in space is suggested as a means to cover missile launches anywhere in the world. Because the response time required for boost-phase missile defense is very short, a constellation of hundreds or thousands of space-based interceptors would be required. In addition, the interceptors would need to be able to maneuver significantly when attacking a missile, and the propellant required for these maneuvers would drive up their mass.
• Because of the short response time required, space-based interceptors would be placed in low orbits, where they would be vulnerable to attacks by short-range missiles. Because interceptors
must be close to the launch site of a missile to have time to reach the missile, destroying several interceptors could create a hole in the constellation through which an attacker could fire a long-range
missile.
• Some missions discussed for the military space plane, such as releasing multiple satellites in different orbits, would require significant maneuverability in space and therefore large masses of propellant. For missions other than those within the same orbital plane, it may be more efficient to launch a new vehicle for each satellite rather than to maneuver the vehicle in space to release multiple satellites.
• For missions that require maneuvering and that use satellites with low mass, such as simple inspector satellites, the mass of propellant needed for the mission may not be prohibitively large.
Elements of a Satellite System (Section 10)
• A satellite is made up of a number of different elements, including solar panels, payloads, and communications devices. The system also includes ground stations to control the satellite and communications equipment for linking with the satellite. All these elements can be targeted to interfere with a satellite system.
• Satellites vary greatly in size, with masses from a few kilograms to a few tons.
Overview of Interfering with Satellite Systems (Section 11)
• Interference can range from temporary or reversible effects to permanent disabling or destruction of the satellite. Many methods can be used to interfere with satellites, including electronic interference with communication systems, laser interference with imaging sensors, laser heating of the satellite body, high-power microwave interference with electrical components, collision with another object (kinetic-kill), and nuclear explosions.
• Because satellites can be tracked and their trajectories can be predicted, they are inherently vulnerable to attack. However, a satellite's vulnerability to ASAT attack does not guarantee the effects of an attack will be predictable or verifiable, and this may limit the ASAT attack's usefulness.
• Jamming satellite ground stations (the downlinks) and the satellite's receivers (the uplinks) is relatively simple to do on unprotected systems such as commercial communications satellites.
Jamming protected systems, such as military communications satellites, is much harder. An adversary need not be technologically advanced to attempt a jamming attack.
• Ground-based lasers can dazzle the sensors of high-resolution
reconnaissance satellites and inhibit observation of regions on the
Earth that are kilometers in size. With high enough power, ground- and space-based lasers can partially blind a satellite, damaging relatively small sections of the satellite's sensor.
• A high-power laser can physically damage a satellite if its beam can be held on the satellite for long enough to deposit sufficient energy. This can result in overheating the satellite or damaging its
structure.
• High-power microwave weapons can disrupt or damage the electrical systems of a satellite if enough of their energy enters these systems. Such attacks would be conducted from space rather than from the ground. Microwave attacks could attempt to enter the
satellite through its antennae (a front-door attack) or through other routes, such as seams in the satellite's casing (a back-door attack). The effectiveness of both types of attack would be difficult to predict.
• Satellites in low earth orbits can be attacked by kinetic-kill ASATs carried on short-range missiles launched from the ground. ASATs stationed on the ground or in low earth orbits can be designed to reach targets at higher altitudes in a matter of hours.
• A nuclear explosion at an altitude of several hundred kilometers would create an intense electromagnetic pulse that would likely destroy all unshielded satellites that are in low earth orbit and in the line of sight of the explosion. In addition, persistent radiation created by the explosion would slowly damage unshielded satellites at altitudes near that of the detonation.
Topics in Interfering with Satellites (Section 12)
• Space-based ASATs are likely to be deployed in one of four ways:
co-orbital with and a short distance behind the target satellite (a
trailing ASAT); attached to the target (sometimes called a parasitic ASAT); in a distant part of the same orbit, requiring a maneuver to approach and attack the target; or in a crossing orbit, keeping its distance from the target until the time of engagement. Different interference methods would be suited to different deployment configurations.
• To be covert, a space-based ASAT must elude detection and/or identification during launch, during deployment maneuvers, and while in orbit. No country could assume its deployment of a space-based ASAT would remain covert. At the same time, no country can assume it would be able to detect or identify a space-based ASAT deployed by another country. Detecting a covert weapon may allow the targeted country to publicly protest its presence and to prepare tactical alternatives to the targeted satellite, but may not guarantee the country's ability to defend against the ASAT.
• A simple anti-satellite weapon that could be used by an attacker with relatively low technical sophistication is a cloud of pellets lofted into the path of a satellite by a short- or medium-range ballistic missile. The effectiveness of such an attack would depend on the attacker's ability to determine the path of the target satellite with precision and to control its missile accurately. Unless the attacker can do both, such an ASAT would have limited effectiveness.
• Many systems that rely on satellites can be made to withstand interference that disrupts an individual satellite. The consequences of an attack on a satellite in the system can be reduced by smart design, including building in redundancy, adding backup systems and spares, and developing alternative means to perform vital functions.
