The reported deployment of a second mobile ICBM -- the 10-warhead SS-24 -- by the Soviet Union once again points to the dilemma posed by such systems {news story, Aug. 8}. Mobility is a positive attribute in that it reduces the vulnerability of the weapon to attack. In turn, it adversely affects our ability to monitor these forces under the terms of an arms control regime or to target these weapons in the event of a first-strike nuclear attack.

In considering the challenge of mobile ICBMs, most ignore the potential contribution to peace and stability that could be made by the deployment of strategic defenses. A multitiered, layered defense -- to include a boost-phase defense -- would negate the adverse effect of not being able to target mobile weapons in a retaliatory strike without diminishing the survivability of mobile weapons against preemptive attack. That potential for boost-phase defenses can be found -- sooner, not later -- in current SDI efforts to develop an effective system of space-based kinetic energy weapons.

In the absence of banning the deployment of mobile ICBMs, strategic defenses hold the best hope of meeting the challenge of this new element of the Soviet nuclear arsenal.

JOSEPH T. MAYER

Arlington