USSR's SS-20: mobile, potent

Of all the nuclear missiles Moscow can hurl, none sends a shudder through Europe like the SS-20. The missile is designed to devastate Western Europes's cities, ports, airfields, and command centers. According to the Brookings Institution, the SS- 20 may be the second and third stages of the failed SS-16 intercontinental missile, ''and the Russians may be aiming it at Europe for lack of anything else to do with these components.''

For Europe, with its still vivid memories of World War II bombings, the prospect of suffering much greater damage from an SS-20 attack, against which virtually no defense exists, is alarming.

Essentially, it is the SS-20's mobility that makes it nearly impossible to counter. Mounted on a multiwheeled transporter vehicle and fired from a vertical launch tube, the missile can be shuttled about on roads and wherever suitable surfaces exist.

Jeffrey Record of the Institute for Foreign Policy Analysis (IFPA) says he believes that given the impossibility of targeting the missile, the only defense against it would be a tactical antiballistic missile (ABM) system. But Defense Secretary Caspar W. Weinberger recently conceded that the United States has no ABM system that could effectively tackle incoming Soviet missiles.

Although the Pentagon claims the Soviet Union is replacing its SS-4 medium-range ballistic missiles and SS-5 intermediate-range ballistic missiles with the SS-20, Mr. Record is not convinced that it is. In a just published report, he insists the SS-20, using a solid-fuel rocket, is being deployed ''largely in addition to - rather than as a replacement for - the older SS-4s and SS-5s.''

Each SS-20 missile carries three highly accurate and independently targetable warheads, believed to be in the 150 kiloton range, permitting it to strike three objectives simultaneously.

(A kiloton is the equivalent if 1,000 tons of conventional TNT explosive. The atomic bomb dropped on Hiroshima was in the 12 to 15 kiloton range.)

In its recently published booklet, ''Soviet Military Power,'' the Pentagon claims that each missile carries three warheads and that each launcher vehicle is equipped with a refire missile, similarly equipped with a trio of warheads.

Besides its mobility and firepower, the SS-20 can reportedly hit targets 3, 000-4,000 miles away. Its range ''places at risk all of NATO Europe, including Iceland, the United Kingdom, Portugal, and the Azores,'' says IFPA's Mr. Record.

By contrast NATO'S Pershing IA, 72 of which are deployed in West Germany, has a single warhead and a range of 450 miles.

The Pentagon estimates that as of July 1981, the Soviet Union had deployed 250 SS-20s, 175 aimed at NATO and the remainder at China. Another source puts the figure at 270.

According to the Pentagon SS-20s are being deployed at the rate of one a week. Indeed, it claims that since January 1981 the pace of SS-20 base construction has increased - particularly opposite NATO countries. ''At bases known to be under construction, another 65 launchers with some 195 reloads will be deployed,'' it declares in the booklet ''Soviet Military Power,'' adding that as many as 100-to-150 additional launchers could be deployed before the program is concluded.

The USSR currently has 315 SS-4s and 35 SS-5s in its inventory, the Pentagon notes.

To counter the threat posed by the SS-20 (and the TU-22M Backfire bomber) NATO plans to field 108 Pershing IIs and 464 ground-launched cruise missiles in five NATO countries.

But neither the Pershing II nor the cruise missiles will have anywhere near the range of the SS-20 and Backfire bomber, analysts say.

IFPA says that the deployment of 572 Pershing II and ground-launched cruise missiles ''will do little to arrest the continuing erosion in the theater nuclear balance between NATO and the Warsaw Pact.'' It maintains that the missiles are insufficient to cover the array of critical Warsaw Pact targets.

Indeed, it voices ''serious doubts'' about the prelaunch survivability of the Pershing II and cruise missiles. It would like to see considerably more deployed over a wider area, together with a partial reliance on sea-based platforms, such as submarines. In fact, it holds that NATO should have its own version of the SS-20.