In 1961, as space fever swept America, a fictional astronaut named Mike Mars made his first appearance. Over the course of eight books, the milk-drinking Air Force pilot does pretty much what you might expect: He defeats his archrival -- another astronaut who's only in it for the money -- gets the better of those dastardly commies, and flies into space and back in pretty much every spacecraft of the period.

Most pieces of hardware in the books, including the Mercury capsule and the Atlas rocket, are well known to history (or at least to then-prepubescent space fans). But the subject of the fifth book, Mike Mars Flies the Dyna-Soar, is a striking exception. Any modern reader would probably assume that, with its swooping hypersonic maneuvers (and its goofy name), the Dyna-Soar must have been pure fiction.

James M. Acton Acton holds the Jessica T. Mathews Chair and is co-director of the Nuclear Policy Program and a senior fellow at the Carnegie Endowment for International Peace. More >

They would be wrong. Originally conceived by the U.S. Defense Department in 1957 as a manned intercontinental bomber, the "dynamic soaring" aircraft -- with its black cylindrical body and delta-shaped wings -- was designed to travel at more than five times the speed of sound. The plan was to use a large rocket to blast the Dyna-Soar into space. Then, rather than arcing high above the Earth, like a ballistic missile, it would re-enter the atmosphere quickly and glide to its target, without power, for vast distances; its creators envisioned a range of up to 12,000 miles.

The concept was simple, but its realization proved fiendishly difficult. In 1963, after spending more than $400 million (about $3 billion today), the Pentagon finally decided that the engineering challenges facing the Dyna-Soar were simply too expensive to overcome, and it canceled the project before the vehicle's first test.

But the interest in boost-glide weapons, as they're called, never completely faded. Some 40 years after the Dyna-Soar's demise, the United States reinvigorated its efforts to develop the technology.

By 2003, U.S. military planners had become worried that the country's long-range conventional weapons, such as cruise missiles, might be too slow to reach hypothetical distant targets that needed to be struck urgently. Although the United States has land- and sea-based ballistic missiles, which can travel much faster and can strike any conceivable enemy in 30 minutes or less, they are all nuclear-armed. So the Pentagon launched the Prompt Global Strike initiative to develop conventional weapons that could reach targets anywhere in the world within "minutes or hours." Boost-glide weapons -- re-envisioned as unmanned missiles that could destroy many types of targets simply by smacking into them at eye-watering speeds -- were an obvious candidate.

The United States has since tested such weapons, but it hasn't actually purchased them. In fact, Washington has not even decided what exactly it would use them for. Although it has already spent an estimated $1 billion on prototypes, the boost-glide weapon remains, as one Pentagon contractor put it, a "missile in search of a mission."

Unfortunately, China and Russia view Washington's interest in the weapons as a done deal. Consequently, both countries have begun their own research and development efforts, potentially sparking a risky new arms race. In his December 2013 annual state-of-the-federation address, Russian President Vladimir Putin railed against U.S. boost-glide development efforts and then boasted of the "advanced weapons" that Russia is developing. The following month, China blasted its first boost-glide missile into the sky. Today, the only Chinese missiles that can reach the continental United States are nuclear-armed, but the use of nuclear weapons would be credible only in the direst of circumstances. Thus, long-range hypersonic conventional weapons could represent a more usable threat -- and render the United States vulnerable to a whole new kind of attack.

The irony? With long-range weapons flying around at high speed, a state could interpret an escalating conflict as needing a nuclear blow. Thus, the technology that some proponents claim would help prevent a nuclear war might conceivably be the very thing that sparks one.

The initial Prompt Global Strike efforts under President George W. Bush were technologically simpler. Recognizing just how much research and development boost-glide weapons would require, his administration first focused on an interim alternative that could be fielded quickly, and in 2006 it announced plans to take the nuclear warheads off some Trident submarine-launched ballistic missiles and replace them with conventional munitions.

Unimpressed, Congress refused funding. Its concern? Russia might mistake the launch of a conventional Trident for its nuclear-armed sibling and retaliate in kind. This fear, however, may have been a convenient excuse for many legislators who by that point, as one former Bush appointee put it, "did not trust the Bush administration with sharp objects."

So boost-glide development was put on a fast track, and in 2008, U.S. military officials indicated that, within four years, the country would have a weapon that could fly 10,000 miles, able to reach almost anywhere on the planet with frightening speed. But this initial effort was a bust. In tests in 2010 and 2011, the so-called Hypersonic Technology Vehicle 2 crashed within three minutes of re-entering the atmosphere -- 20 minutes short of its goal.

The United States then dialed back its ambitions and prioritized what the Pentagon dubs the Advanced Hypersonic Weapon. With an estimated 5,000-mile range, it won't have global reach, but this boost-glide vehicle will still be able to travel farther than any other non-nuclear weapon. In a November 2011 test, the cone-shaped glider slammed into a remote atoll in the middle of the Pacific after traveling about 2,400 miles from its launch site in Hawaii. This success demonstrated to both the United States and its potential adversaries that boost-glide weapons may finally be nearing realization. Another test is expected in August of this year.

Yet despite over a decade of effort, U.S. boost-glide weapons still have no clearly defined mission. The Pentagon seems to be developing them simply to master an abstract technology -- that is, hypersonic weapons just sound cool.

To some extent, this "astrategic" approach has been deliberate. The Bush administration saw the post-9/11 security environment as inherently unpredictable and stated as much in its 2001 Quadrennial Defense Review, which set U.S. defense priorities. It wanted to move away from designing responses to specific threats and toward developing capabilities that would be useful regardless of "who the adversary might be and where a war might occur." Brian Green, then the deputy assistant secretary of defense for strategic capabilities, couldn't have summed up this approach more aptly when, in 2007, he responded to a question about the rationale for Prompt Global Strike weapons: "We prefer actually in our shop not to talk about specific scenarios."

President Barack Obama's Pentagon has not been any more precise. In 2012, Madelyn Creedon, the assistant secretary of defense for global strategic affairs, baldly acknowledged that the United States was "still early in the development of the policy to go with" the technology. The absence of a clear rationale for the weapons, however, has not interfered with the continued funding of research and development efforts. All three flight tests have taken place during the Obama administration.

Although the technology, rather than the strategy, has guided U.S. efforts over the years, this isn't to deny that boost-glide weapons may someday prove useful.

Over the past decade, analysts have suggested that the weapons could be used to eliminate terrorists and destroy nuclear-armed ballistic missiles in North Korea (and, perhaps in the future, in Iran). Two other possibilities popular with military planners include destroying anti-satellite weapons and suppressing advanced defenses. In the abstract, these two missions may seem relatively prosaic compared with assassinating jihadists or preempting a North Korean nuclear attack. However, because China is the potential adversary of the United States with the most advanced anti-satellite and defensive technologies, they are anything but.

Today, probably no threat sparks more concern in the Pentagon than Beijing's so-called anti-access and area-denial capabilities -- "counter-intervention" weapons, to borrow China's term, designed to prevent U.S. forces from entering the Western Pacific during a conflict. These include its fleet of conventional ballistic missiles designed to attack U.S. military bases and aircraft carriers. In the 2010 Quadrennial Defense Review, the Pentagon hinted at the possibility that, in a conflict, boost-glide weapons could be used to suppress Beijing's defenses, presumably by targeting its missiles directly or destroying their command-and-control system.

But this is a potentially dangerous suggestion. The U.S. use of boost-glide weapons to combat advanced Chinese conventional capabilities, including perhaps China's own hypersonic weapons, could dramatically escalate a conflict. It could even prompt the Chinese to use nuclear weapons. The reason is that China is thought to have a single command-and-control system for all its ballistic missiles -- both conventional and nuclear -- so Beijing could misread an incoming U.S. boost-glide attack on this system as one intended to disable its nuclear forces. In response, Beijing might alert, or even employ, its nuclear weapons -- better to use them than lose them. Some Chinese military strategists are so concerned about U.S. conventional weapons that they have called on Beijing to abandon its long-standing pledge never to use nuclear weapons first.

In the paradoxical logic of deterrence, however, escalation risks may actually be desirable. After all, the possibility that things might spiral out of control could make Beijing more reluctant to initiate a conflict. Thus, choosing weapons that increase this danger in ways that neither side can fully control may actually be a sensible strategy for Washington.

American weapon systems have frequently provided models for China. Beijing's interest in cruise missiles surged following their use by the United States in the 1991 Gulf War. Since the late 1970s, the American Pershing II -- the focus of over 50 Chinese studies, including many by government-sponsored institutes -- has inspired Chinese military research into conventional ballistic missiles. And judging by the hundreds of government-funded research papers uncovered by American analyst Lora Saalman, Washington's programs likely catalyzed China's interest in boost-glide weapons as well.

In truth, though, not much information on Chinese efforts has been confirmed. Based on the scant information that's publicly available about China's January test, it's unclear whether the WU-14, the Pentagon's designation for China's hypersonic glider, is simply an improved version of China's notorious "carrier killer" -- the DF-21D anti-ship ballistic missile, which has a range of about 900 miles -- or a much more ambitious design rivaling Washington's Advanced Hypersonic Weapon.

Since the WU-14 test, American pundits have pushed the possibility that the high speed of Chinese boost-glide weapons could defeat U.S. missile defenses in East Asia, which protect both land-based military installations and aircraft carriers at sea. This fear has probably been overstated, however. Although hypersonic gliders re-enter the atmosphere at breakneck speeds, they are slowed by air resistance and are generally not as fast as ballistic missiles by the time they reach their targets. They would, therefore, probably be less effective at breaking through U.S. missile defenses around the Western Pacific than the conventional ballistic missiles that Beijing already has in droves.

It is also highly doubtful that the WU-14 could reach the U.S. mainland. For one, it almost certainly glides at a much slower speed than would be needed to cross the entire Pacific Ocean. That said, the missile is certainly a significant step in this direction. And, to Beijing, these weapons might seem like the perfect tool for correcting a historical asymmetry in vulnerability: From Beijing's vantage point, U.S. conventional weapons, which ring the country from bases in South Korea, Japan, and the United States, have been able to reach its soil for decades, while China's own ballistic missiles, which target deployed U.S. forces, have never been able to reach the United States.

While Chinese boost-glide missiles might struggle to defeat the missile defenses around compact targets, such as U.S. military bases in Asia, they could easily bypass the wide-area defenses based in Alaska and California that are designed to protect the U.S. homeland. Thus, if over the next decade or so China were to develop accurate boost-glide missiles capable of reaching the United States, key military assets -- such as satellite uplinks, communication hubs, and ships in port -- could become vulnerable to conventional attack for the first time. Protecting them through point defenses, burial, or redundancy might be possible, but it would also be extremely expensive.

An even bigger impact might be psychological. The United States has been exposed to Beijing's nuclear weapons for decades, but most Americans never give these forces a passing thought. By contrast, Chinese conventional weapons could represent a much more tangible threat. If U.S. involvement in an East Asian conflict could actually cause China to begin blowing things up in, say, California, Americans might think twice about whether their country's defense commitments in the region -- to Japan and Taiwan, in particular -- were worth the risks.

The United States has spent 10 years and a billion dollars on a weapon that has no defined mission. And in the meantime, American research and development efforts have prompted Russia and China to pursue similar weapons of their own that could be deployed in as little as a decade, starting an arms race that could place the continental United States at risk. In theory, these three powers could agree to avoid such competition. In practice, the prospects for mutual restraint seem extremely dim.

The United States should not risk escalating a conflict with a nuclear-armed power unless it has no other option. But if it doesn't hurry up and find a policy to guide its rapidly advancing technology, it may simply glide into catastrophe.

This article was originally published in Foreign Policy.