“Take the high ground and hold it!” has been standard combat doctrine for armies since ancient times. Now that people and their machines have entered outer space, it is no surprise that generals the world over regard Earth orbit as the key to modern warfare. But until recently, a norm had developed against the weaponization of space—even though there are no international treaties or laws explicitly prohibiting nonnuclear anti­satellite systems or weapons placed in orbit. Nations mostly shunned such weapons, fearing the possibility of destabilizing the global balance of power with a costly arms race in space.

In war, do not launch an ascending attack head-on against the enemy who holds the high ground. Do not engage the enemy when he makes a descending attack from high ground. Lure him to level ground to do battle.
—Sun Tzu, Chinese military strategist, The Art of War, circa 500 B.C.

That consensus is now in danger of unraveling. In October 2006 the Bush administration adopted a new, rather vaguely worded National Space Policy that asserts the right of the U.S. to conduct “space control” and rejects “new legal regimes or other restrictions that seek to prohibit or limit U.S. access to or use of space.” Three months later the People’s Republic of China shocked the world by shooting down one of its own aging Fengyun weather satellites, an act that resulted in a hailstorm of dangerous orbital debris and a deluge of international protests, not to mention a good deal of hand-wringing in American military and political circles. The launch was the first test of a dedicated antisatellite weapon in more than two decades—making China only the third country, after the U.S. and the Russian Federation, to have demonstrated such a technology. Many observers wondered whether the test might be the first shot in an emerging era of space warfare.

Critics maintain it is not at all clear that a nation’s security would be enhanced by developing the means to wage space war. After all, satellites and even orbiting weapons, by their very nature, are relatively easy to spot and easy to track, and they are likely to remain highly vulnerable to attack no matter what defense measures are taken. Further, developing antisatellite systems would almost surely lead to a hugely expensive and potentially runaway arms race, as other countries would conclude that they, too, must compete. And even tests of the technology needed to conduct space battles—not to mention a real battle—could generate enormous amounts of wreckage that would continue to orbit Earth. Converging on satellites and crewed space vehicles at speeds approaching several miles a second, such space debris would threaten satellite-based telecommunications, weather forecasting, precision navigation, even military command and control, potentially sending the world’s economy back to the 1950s.

“Star Wars” Redux
Since the dawn of the space age, defense planners have hatched concepts for antisatellite and space-based weaponry—all in the interest of exploiting the military advantages of the ultimate high ground. Perhaps the most notable effort was President Ronald Reagan’s Strategic Defense Initiative (SDI)—derided by its critics as “Star Wars.” Yet by and large, U.S. military strategy has never embraced such weapons.

Traditionally, space weapons have been defined as destructive systems that operate in outer space after having been launched directly from Earth or parked in orbit. The category includes antisatellite weapons; laser systems that couple ground-based lasers with airship- or satellite-mounted mirrors, which could reflect a laser beam beyond the ground horizon; and orbital platforms that could fire projectiles or energy beams from space. (It is important to note that all nations would presumably avoid using a fourth kind of antisatellite weapon, namely, a high-altitude nuclear explosion. The electromagnetic pulse and cloud of highly charged particles created by such a blast would likely disable or destroy nearly all satellites and manned spacecraft in orbit [see “Nuclear Explosions in Orbit,” by Daniel G. Dupont; Scientific American, June 2004].)

But virtually no statement about space weapons goes politically uncontested. Recently some proponents of such weapons have sought to expand the long-held classification I just described to include two existing technologies that depend on passage through space: intercontinental ballistic missiles (ICBMs) and ground-based electronic warfare systems. Their existence, or so the argument goes, renders moot any question about whether to build space weapons systems. By the revised definition, after all, “space weapons” already exist. Whatever the exact meaning of the term, however, the questions such weapons raise are hardly new to think tanks and military-planning circles in Washington: Is it desirable, or even feasible, to incorporate antisatellite weapons and weapons fired from orbit into the nation’s military strategy?

The new National Space Policy, coupled with the Chinese test, has brought renewed urgency to that behind-the-scenes debate. Many American military leaders expressed alarm in the wake of the Chinese test, worrying that in any conflict over Taiwan, China could threaten U.S. satellites in low Earth orbit. In April 2007 Michael Moseley, the U.S. Air Force chief of staff, compared China’s antisatellite test with the launch of Sputnik by the Soviet Union in 1957, an act that singularly intensified the arms race during the cold war. Moseley also revealed that the Pentagon had begun reviewing the nation’s satellite defenses, explaining that outer space was now a “contested domain.”

Congressional reaction fell along predictable political lines. Conservative “China hawks” such as Senator Jon Kyl of Arizona immediately called for the development of antisatellite weapons and space-based interceptors to counter Chinese capabilities. Meanwhile more moderate politicians, including Representative Edward Markey of Massachusetts, urged the Bush administration to begin negotiations aimed at banning all space weapons.

International Power Plays
Perhaps of even greater concern is that several other nations, including one of China’s regional rivals, India, may feel compelled to seek ­offensive as well as defensive capabilities in space. The U.S. trade journal Defense News, for instance, quoted unidentified Indian defense officials as stating that their country had already begun developing its own kinetic-energy (nonexplosive, hit-to-kill) and laser-based antisatellite weapons.

If India goes down that path, its archrival Pakistan will probably follow suit. Like India, Pakistan has a well-developed ballistic missile program, including medium-range missiles that could launch an antisatellite system. Even Japan, the third major Asian power, might join such a space race. In June 2007 the National Diet of Japan began considering a bill backed by the current Fukuda government that would permit the development of satellites for “military and national security” purposes.

As for Russia, in the wake of the Chinese test President Vladimir Putin reiterated Moscow’s stance against the weaponization of space. At the same time, though, he refused to criticize Beijing’s actions and blamed the U.S. instead. The American efforts to build a missile defense system, Putin charged, and the increasingly aggressive American plans for a military position in space were prompting China’s moves. Yet Russia itself, as a major spacefaring power that has incorporated satellites into its national security structure, would be hard-pressed to forgo entering an arms race in space.

Given the proliferation of spacefaring entities, proponents of a robust space warfare strategy believe that arming the heavens is inevitable and that it would be best for the U.S. to get there first with firepower. Antisatellite and space-based weapons, they argue, will be necessary not only to defend U.S. military and commercial satellites but also to deny any future adversary the use of space capabilities to enhance the performance of its forces on the battlefield.

Yet any arms race in space would almost inevitably destabilize the balance of power and thereby multiply the risks of global conflict. In such headlong competition—whether in space or elsewhere—equilibrium among the adversaries would be virtually impossible to maintain. Even if the major powers did achieve stability, that reality would still provide no guarantee that both sides would perceive it to be so. The moment one side saw itself to be slipping behind the other, the first side would be strongly tempted to launch a preemptive strike, before things got even worse. Ironically, the same would hold for the side that perceived itself to have gained an advantage. Again, there would be strong temptation to strike first, before the adversary could catch up. Finally, a space weapons race would ratchet up the chances that a mere technological mistake could trigger a battle. After all, in the distant void, reliably distinguishing an intentional act from an accidental one would be highly problematic.

Hit-to-Kill Interceptors
According to assessments by U.S. military and intelligence officials as well as by independent experts, the Chinese probably destroyed their weather satellite with a kinetic-energy vehicle boosted by a two-stage medium-range ballistic missile. Technologically, launching such direct-ascent antisatellite weapons is one of the simplest ways to take out a satellite. About a dozen nations and consortia can reach low Earth orbit (between roughly 100 and 2,000 kilometers, or 60 to 1,250 miles, high) with a medium-range missile; eight of those countries can reach geostationary orbit (about 36,000 kilometers, or 22,000 miles, above Earth).

But the real technical hurdle to making a hit-to-kill vehicle is not launch capacity; it is the precision maneuverability and guidance technology needed to steer the vehicle into its target. Just how well China has mastered those techniques is unclear. Because the weather satellite was still operating when it was destroyed, the Chinese operators would have known its exact location at all times.

Ground-Based Lasers
The test of China’s direct-ascent antisatellite device came on the heels of press reports in September 2006 that the Chinese had also managed to “paint,” or illuminate, U.S. spy satellites with a ground-based laser [see lower box on page 83]. Was Beijing actually trying to “blind” or otherwise damage the satellites? No one knows, and no consensus seems to have emerged in official Washington circles about the Chinese intent. Per­haps China was simply testing how well its network of low-power laser-ranging stations could track American orbital observation platforms.

Even so, the test was provocative. Not all satellites have to be electronically “fried” to be put out of commission. A 1997 test of the army’s MIRACL system (for midinfrared advanced chemical laser) showed that satellites designed to collect optical images can be temporarily disrupted—dazzled—by low-power beams. It follows that among the satellites vulnerable to such an attack are the orbital spies.

The U.S. and the former Soviet Union began experimenting with laser-based antisatellite weapons in the 1970s. Engineers in both countries have focused on the many problems of building high-power laser systems that could reliably destroy low-flying satellites from the ground. Such systems could be guided by “adaptive optics”: deformable mirrors that can continuously compensate for atmospheric distortions. But tremendous amounts of energy would be needed to feed high-power lasers, and even then the range and effectiveness of the beams would be severely limited by dispersion, by attenuation as they passed through smoke or clouds, and by the difficulty of keeping the beams on-target long enough to do damage.

During the development of the SDI, the U.S. conducted several laser experiments from Hawaii, including a test in which a beam was bounced off a mirror mounted on a satellite. Laser experiments continue at the Starfire Optical Range at Kirtland Air Force Base in New Mexico. Pentagon budget documents from fiscal years 2004 through 2007 listed antisatellite operations among the goals of the Starfire research, but that language was removed from budget documents in fiscal year 2008 after Congress made inquiries. The Starfire system incorporates adaptive optics that narrow the outgoing laser beam and thus increase the density of its power. That capability is not required for imagery or tracking, further suggesting that Starfire could be used as a weapon.

Yet despite decades of work, battle-ready versions of directed-energy weapons still seem far away. An air force planning document, for instance, predicted in 2003 that a ground-based weapon able to “propagate laser beams through the atmosphere to [stun or kill low Earth orbit] satellites” could be available between 2015 and 2030. Given the current state of research, even those dates seem optimistic.

Co-orbital Satellites
Recent advances in miniaturized sensors, powerful onboard computers and efficient rocket thrusters have made a third kind of antisatellite technology increasingly feasible: the offensive microsatellite. One example that demonstrates the potential is the air force’s experimental satellite series (XSS) project, which is developing microsatellites intended to conduct “autonomous proximity operations” around larger satellites. The first two microsatellites in the program, the XSS-10 and XSS-11, were launched in 2003 and 2005. Though ostensibly intended to inspect larger satellites, such microsatellites could also ram target satellites or carry explosives or directed-energy payloads such as radio-frequency jamming systems or high-powered microwave emitters. Air force budget documents show that the XSS effort is tied to a program called Advanced Weapons Technology, which is dedicated to research on military laser and microwave systems.

During the cold war the Soviet Union developed, tested and even declared operational a co-orbital antisatellite system—a maneuverable interceptor with an explosive payload that was launched by missile into an orbit near a target satellite in low Earth orbit. In effect, the device was a smart “space mine,” but it was last demonstrated in 1982 and is probably no longer working. Today such an interceptor would likely be a microsatellite that could be parked in an orbit that would cross the orbits of several of its potential targets. It could then be activated on command during a close encounter.

In 2005 the air force described a program that would provide “localized” space “situational awareness” and “anomaly characterization” for friendly host satellites in geostationary orbit. The program is dubbed ANGELS (for autonomous nanosatellite guardian for evaluating local space), and the budget line believed to represent it focuses on acquiring “high value space asset defensive capabilities,” including a “warning sensor for detection of a direct ascent or co-orbital vehicle.” It is clear that such guardian nanosatellites could also serve as offensive weapons if they were maneuvered close to enemy satellites.

And the list goes on. A “parasitic satellite” would shadow or even attach itself to a target in geostationary orbit. Farsat, which was mentioned in an appendix to the [Donald] Rumsfeld Space Commission report in 2001, “would be placed in a ‘storage’ orbit (perhaps with many microsatellites housed inside) relatively far from its target but ready to be maneuvered in for a kill.”

Finally, the air force proposed some time ago a space-based radio-frequency weapon system, which “would be a constellation of satellites containing high-power radio-frequency transmitters that possess the capability to disrupt/­destroy/disable a wide variety of electronics and national-level command and control systems.”

Air force planning documents from 2003 envisioned that such a technology would emerge after 2015. But outside experts think that orbital radio-frequency and microwave weapons are technically feasible today and could be deployed in the relatively near future.

Space Bombers
Though not by definition a space weapon, the Pentagon’s Common Aero Vehicle/Hypersonic Technology Vehicle (often called CAV) enters into this discussion because, like an ICBM, it would travel through space to strike Earth-bound targets. An unpowered but highly maneuverable hypersonic glide vehicle, the CAV would be deployed from a future hypersonic space plane, swoop down into the atmosphere from orbit and drop conventional bombs on ground targets. Congress recently began funding the project but, to avoid stoking a potential arms race in space, has prohibited any work to place weapons on the CAV. Although engineers are making steady progress on the key technologies for the CAV program, both the vehicle and its space plane mothership are still likely decades off.

Some of the congressional sensitivity to the design of the CAV may have arisen from another, much more controversial space weapons concept with parallel goals: hypervelocity rod bundles that would be dropped to Earth from orbital platforms. For decades air force planners have been thinking about placing weapons in orbit that could strike terrestrial targets, particularly buried, “hardened” bunkers and caches of weapons of mass destruction. Commonly called “rods from God,” the bundles would be made up of large tungsten rods, each as long as six meters (20 feet) and 30 centimeters (12 inches) across. Each rod would be hurled downward from an orbiting spacecraft and guided to its target at tremendous speed.

Both high costs and the laws of physics, however, challenge their feasibility. Ensuring that the projectiles do not burn up or deform from reentry friction while sustaining a precise, nearly vertical flight path would be extremely difficult. Calculations indicate that the nonexplosive rods would probably be no more effective than more conventional munitions. Furthermore, the expense of lofting the heavy projectiles into orbit would be exorbitant. Thus, despite continued interest in them, rods from God seem to fall into the realm of science fiction.

Obstacles to Space Weapons
What, then, is holding the U.S. (and other nations) back from a full-bore pursuit of space weapons? The countervailing pressures are threefold: political opposition, technological challenges and high costs.

The American body politic is deeply divided over the wisdom of making space warfare a part of the national military strategy. The risks are manifold. I remarked earlier on the general instabilities of an arms race, but there is a further issue of stability among the nuclear powers. Early-warning and spy satellites have traditionally played a crucial role in reducing fears of a surprise nuclear attack. But if antisatellite weapons disabled those eyes-in-the-sky, the resulting uncertainty and distrust could rapidly lead to catastrophe.

One of the most serious technological challenges posed by space weapons is the proliferation of space debris, to which I alluded earlier. According to investigators at the air force, NASA and Celestrak (an independent space-monitoring Web site), the Chinese antisatellite test left more than 2,000 pieces of junk, baseball-size and larger, orbiting the globe in a cloud that lies between about 200 kilometers (125 miles) and 4,000 kilometers (2,500 miles) above Earth’s surface. Perhaps another 150,000 objects that are a centimeter (half an inch) across and larger were released. High orbital velocities make even tiny pieces of space junk dangerous to spacecraft of all kinds. And ground stations cannot reliably monitor or track objects smaller than about five centimeters (two inches) across in low Earth orbit (around a meter in geostationary orbit), a capability that might enable satellites to maneuver out of the way. To avoid being damaged by the Chinese space debris, in fact, two U.S. satellites had to alter course. Any shooting war in space would raise the specter of a polluted space environment no longer navigable by Earth-orbiting satellites.

Basing weapons in orbit also pre­sents difficult technical obstacles. They would be just as vulnerable as satellites are to all kinds of outside agents: space debris, projectiles, electromagnetic signals, even natural micrometeoroids. Shielding space weapons against such threats would also be impractical, mostly because shielding is bulky and adds mass, thereby greatly increasing launch costs. Orbital weapons would be mostly autonomous mechanisms, which would make operational errors and failures likely. The paths of objects in orbit are relatively easy to predict, which would make hiding large weapons problematic. And because satellites in low Earth orbit are overhead for only a few minutes at a time, keeping one of them constantly in range would require many weapons.

Finally, getting into space and operating there is extremely expensive: between $2,000 and $10,000 a pound to reach low Earth orbit and between $15,000 and $20,000 a pound for geostationary orbit. Each space-based weapon would require replacement every seven to 15 years, and in-orbit repairs would not be cheap, either.

Alternatives to Space Warfare
Given the risks of space warfare to national and international security, as well as the technical and financial hurdles that must be overcome, it would seem only prudent for spacefaring nations to find ways to prevent an arms race in space. The U.S. focus has been to reduce the vulnerability of its satellite fleet and explore alternatives to its dependence on satellite services. Most other space-capable countries are instead seeking multilateral diplomatic and legal measures. The options range from treaties that would ban antisatellite and space-based weapons to voluntary measures that would help build transparency and mutual confidence.

The Bush administration has adamantly opposed any form of negotiations regarding space weapons. Opponents of multilateral space weapons agreements contend that others (particularly China) will sign up but build secret arsenals at the same time, because such treaty violations cannot be detected. They argue further that the U.S. cannot sit idly as potential adversaries gain spaceborne resources that could enhance their terrestrial combat capabilities.

Proponents of international treaties counter that failure to negotiate such agreements entails real opportunity costs. An arms race in space may end up compromising the security of all nations, including that of the U.S., while it stretches the economic capacities of the competitors to the breaking point. And whereas many advocates of a space weapons ban concede that it will be difficult to construct a fully verifiable treaty—because space technology can be used for both military and civilian ends—effective treaties already exist that do not require strict verification. A good example is the Biological Weapons Convention. Certainly a prohibition on the testing and use (as opposed to the deployment) of the most dangerous class of near-term space weapons—destructive (as opposed to jamming) antisatellite systems—would be easily verifiable, because earthbound observers can readily detect orbital debris. Furthermore, any party to a treaty would know that all its space launches would be tracked from the ground, and any suspicious object in orbit would promptly be labeled as such. The international outcry that would ensue from such overt treaty violations could deter would-be violators.

Since the mid-1990s, however, progress on establishing a new multilateral space regime has lagged. The U.S. has blocked efforts at the United Nations Conference on Disarmament in Geneva to begin negotiations on a treaty to ban space weapons. China, meanwhile, has refused to accept anything less. Hence, intermediate measures such as voluntary confidence-building, space traffic control or a code of responsible conduct for spacefaring nations have remained stalled.

Space warfare is not inevitable. But the recent policy shift in the U.S. and China’s provocative actions have highlighted the fact that the world is approaching a crossroads. Countries must come to grips with their strong self-interest in preventing the testing and use of orbital weapons. The nations of Earth must soon decide whether it is possible to sustain the predominantly peaceful human space exploration that has already lasted half a century. The likely alternative would be unacceptable to all.