WASHINGTON -- The last obstacle in the way of the new U.S.-Soviet nuclear arms deal -- and a Moscow summit meeting -- was a 620-mile difference of opinion about how far a certain Soviet missile would have to travel in multi-thousand-mile tests.
The U.S. position carried the day.
When Soviet technicians forwarded their proposed acceptance Wednesday to President Mikhail S. Gorbachev in London, the Soviet leader wasted no time agreeing. Neither, of course, did President Bush.
He and Mr. Gorbachev had lunch and announced that the final barrier to a nuclear treaty had been surmounted and that they would meet in Moscow at the end of the month, when they would sign the pact.
There were reports in arms control circles here yesterday that Mr. Gorbachev overruled his generals in settling the final technical issue on U.S. terms. Whether he did or not, Bush administration sources confirmed the essential accuracy of the 620-mile issue and its resolution, first recounted at a news conference yesterday by Jack Mendelsohn of the independent Arms Control Association.
After agreeing on hugely significant matters -- reduced limits on nuclear arsenals, cuts in missile forces, intrusive inspections -- negotiators were bogged down over how far a new version of the Soviet SS-25 intercontinental missile must be flown in tests.
The Russians wanted to test it at a 6,200-mile range. The Americans said, no, it must be flown 6,820 miles -- 620 miles farther. Why?
The answer seems full of the arcana chiefly understood by career arms controllers and associated military colleagues seeking to cover every conceivable rat hole. Military men, who look to arms agreements to make opposing forces predictable, say it was a big issue.
What it came down to was that U.S. negotiators insisted on the longer-range test flight as a means of keeping down the "throw-weight" -- the nuclear warhead payload -- of a new version of the SS-25.
The missile as now deployed carries just one nuclear warhead. Under the treaty, it could not carry more. U.S. negotiators believe, however, that the powerful SS-25 rocket has the potential to hurl considerably more firepower, say, three warheads,at intercontinental targets.
They wanted to ensure against the Soviets' making minor changes in the SS-25 (and other missiles, for that matter) and calling it a new type, not bound by the warhead limitation on the existing model.
The insistence on greater-range test flying thus was related to throw-weight, or carrying capacity, which in the details of the treaty is a key factor in defining new-type missiles: The greater the flight distance, the smaller the payload.
In general, as Mr. Mendelsohn underlined at yesterday's briefing, the treaty mandates significant, recognizable changes -- not minor adjustments -- before a missile can be called a new type. It must be able to carry 21 percent more payload and have at least a 5 percent change in length. It may carry any number of warheads up to 10.
Mr. Mendelsohn saw the settlement of the 620-mile issue as "a hedge against a far-fetched scenario, in which a missile may be changed slightly and become the prototype for an upgraded nuclear force."