Participants in this roundtable display consensus on one point: that the world already possesses enough destructive capacity and there's no need to add more. But whereas Mark Gubrud advocates a ban on hypersonic missile testing, Tong Zhao and I feel that—for better or worse—hypersonic technology is here to stay.
Technology development generally takes the form of an S-curve. Improvements come slowly in the early stages of development. Later, breakthroughs allow rapid improvement. Finally, the technology's physical limits are reached, only modest improvements are possible, and the curve levels off. Ballistic missiles have reached the last stage. Limits on their performance can be overcome only through the development of new technology—such as hypersonic missiles.
Hypersonics provide, with their speed and promptness of delivery, a means to gain and preserve the military high ground. It is inconceivable that nations already invested in gaining that high ground would agree to a ban on hypersonic testing.
But even if they were willing, a test ban would be—despite what Gubrud argues—very difficult to verify. Both hypersonic glide vehicles and hypersonic cruise missiles must be boosted to their release altitudes by ballistic missiles. Consequently, they present an infrared signature (monitored from space) and a radar cross section resembling those of a ballistic missile. As Tong Zhao wrote in Round Two, "Drawing clear distinctions between [terminally guided ballistic missiles] and hypersonic missiles may be technically impossible."
A test ban would also—again, despite what Gubrud argues—discriminate against nations that have not begun testing programs of their own. Gubrud writes that a test ban would not discriminate in the military realm because "all nations would forgo the development of actual, usable weapons" and wouldn't discriminate in the civilian realm because "the knowledge gap between nations would close over time." But in my opinion, the gap wouldn't close. Hypersonic technology is inherently dual-use, so nations wishing to maintain military advantages would jealously guard against the diffusion of civilian hypersonic technology. And after all, what could guarantee that civilian hypersonic technology wouldn't be harnessed for missile applications?
Inevitable solutions. Gubrud wonders in Round Three "just what the advantage [of hypersonic missiles] would be" and writes that "without nuclear warheads they would be incapable of destroying many hard, fixed targets." Without knowing the design specifications of hypersonic missiles, it's difficult to speculate about their payload size, lethality, cost, and the like. But it's true that hypersonics would clearly be limited in certain ways—for example, it appears that only fixed, soft targets would be vulnerable to hypersonic attack. Targets in hardened, deeply buried shelters would be much harder to reach. Hypersonic missiles could achieve deep penetration by virtue of traveling at very high speeds, but in that case their explosive payloads would be smaller—and the damage they could inflict would be lower.
This is just one of many issues that nations developing hypersonic technology must overcome before they deploy these weapons—but as hypersonic technology proceeds inevitably through its S-curve, solutions will be found. The only question is how soon. This in turn depends on political will and funding priorities in nations pursuing hypersonics. What's certain is that countries already developing hypersonic missiles—powerful nations such as the United States, China, and Russia—will not agree to a test ban until they have overcome all the technological hurdles. A test ban simply isn't on the horizon.