THAT WAS a jaw-dropper in Arkansas last Friday. The fuel meant to hurl a Titan nuclear missile 6,000 miles onto a Soviet city exploded in the silo, destroying the missile and catapulting its warhead into a ditch 200 yards away. There was no radiation leakage, no nuclear explosion -- none of the requisite electronic keys was turned -- and no injury to nearby civilians, although one Air Force man was killed. As at Three Mile Island, the episode can fairly be used to illustrate that the system is designed to contain the damage of accident. There is no doubt it will also be used to prove the point that it is perilous to keep anything nuclear, civilian or military, around.

The military has its own perspective. This accident is said to point up the long-known condition of obsolescence of the 54 (now 53) Titan missiles, which carry a third of the megatonnage of all American land-based missles. These ocean-spanning missiles, with nine-megaton warheads, were deployed in the early 1960s and were being described as obsolete -- hard to maintain and fire efficiently -- only a few years later. The United States tried to trade them for something of value in SALT I, but the Soviets were evidently more fearful of the missiles the United States might replace the Titans with and would not deal. With or without SALT II, the official intention now is to replace the Titan with the MX. Meanwhile the United States finds itself with a Titan force disturbingly prone to accidents and breakdowns: there have been hundreds of incidents of one sort or another. What could the Air Force have meant when it gave the Titans a clean bill of health last May?

This accident happened, it is reported, when somebody dropped a wrench socket and punctured the skin of the missile. At Three Mile Island, somebody turned off a valve to a backup cooling system. At Brown's Ferry, somebody lit a candle to find a leak. In the Iranian desert, somebody didn't give helicopter pilots navigation aids. An irreducible, ultimately unavoidable element of human error haunts the complex machines of a "modern" society. It produces this undecipherable equation: even as the country considers buying a new missile for $60 billion or more, a dropped wrench disables an old one. That guy who drops wrenches, is he going to be allowed to work on the MX?