How fast is fast? Much attention lately has centered on a new generation of weapons known as “hypersonic.” The very word conjures up a frightening ultra-speed. China, the United States and Russia are all working on them. They pose special dangers and challenges but need to be better understood for what they really are — and aren’t.

Broadly speaking, hypersonic glide vehicles move through the atmosphere toward a ground-based target at at least five times the speed of sound, or about 3,800 miles per hour. That’s fast, but an intercontinental ballistic missile, such as the U.S. Minuteman III, moves through the airless vacuum of space at about 15,000 miles per hour at burnout. So an ICBM can deliver a nuclear warhead across the oceans in 30 minutes, the nightmare scenario since early in the Cold War. A hypersonic boost-glide vehicle might be lofted by a fast missile and then glide to a target through the atmosphere, while another type would fly strictly within the atmosphere, a speedy cruise missile. But the hypersonic vehicle itself is slower than a ballistic missile.

A ballistic missile basically goes up and then releases a warhead that falls to earth, while the hypersonic glide vehicle takes a lower trajectory and can be maneuvered, making it capable of sneaking under early-warning radar. This is why some analysts believe hypersonics could change the calculations of war — creating incentives to strike an adversary first and without warning. But others point out, correctly, that China, Russia and the United States already have fleets of ICBMs ready to hit targets with exceptional rapidity, so the speed dimension is not so new.

China is known to have an active program to develop hypersonic weapons, and its appetite for weapons modernization is expansive and growing. On Oct. 16, the Financial Times reported that China in August had sent a nuclear-weapons-capable hypersonic glide vehicle into orbit, where it circled the Earth, and then steered it toward a land target, representing a leap in technology. China responded by saying that it tested a reusable civilian space vehicle. It looks as though China might have combined a space orbiter with a hypersonic vehicle. The technology of orbiting a weapons system isn’t new; the Soviet Union built one in the 1960s.

The United States has deployed a limited ground-based antimissile system in Alaska and California, saying it is intended to stop a missile attack from North Korea. But it seems likely that China and Russia are developing asymmetric, exotic weapons systems like hypersonics or Russia’s nuclear-powered cruise missile “Burevestnik” as a potential way to evade U.S. missile defenses. Meanwhile, the United States is developing non-nuclear hypersonic systems that could be used for prompt attacks against targets in regional conflicts.

China’s pursuit of hypersonics is just one more reason the United States ought to continue trying to bring China to the arms control table and keep Russia engaged there. The potential topics should cover all kinds of weapons systems, including hypersonics and missile defenses. The climate is not very promising for negotiations, but the alternative is an arms race, or more than one.