And now . . . dense pack!

So now we have the ''dense pack.'' The Carter administration, you will recall , devised an elaborate scheme for shuttling 200 of the new MX missiles along 4, 600 miles of track in the Utah and Nevada desert. That scheme got caught in the fallout of local politics and--many would say--its own absurdity. Now President Reagan has given a tentative nod to a proposal which would cluster 100 MX missiles in hardened silos in an area 10 to 15 miles square.

Is ''dense pack'' any more feasible a plan for assuring survivability of US ICBMs against Soviet attack? The American public obviously does not know enough to make a quick judgment on so complex an issue. But the scheme does raise questions that need to be looked at closely by the Pentagon and Congress:

* First of all, the plan seems to be based on a shaky theory. This revolves around the concept of ''fratricide.'' It is assumed that the first Soviet warheads attacking the cluster would release enough heat and radiation to destroy the next flight of incoming missiles but--and this is strange--not enough to pin down the MX missiles and prevent firing them through the debris and turbulence. Is such a scenario possible? In any case, say experts, the Russians could burn up the MX missiles in the silos by exploding weapons 200 miles above the ground.

If fratricide is plausible moreover, why would it not serve to protect present missiles and not require a new array? Clearly hard data are needed to put down what is already a fair amount of skepticism.

* Secondly, under the proposal, the MX missiles would be encased in hardened silos capable of withstanding as much as 9,000 pounds per square inch of force from a nuclear blast. This is three times the PSI of the present Minuteman silos. Given the present state of the art, it is not possible to achieve 9,000 PSI.

* Third--and most troubling--the dense pack scheme runs into treaty problems. It might require giving up the ABM agreement with the Soviet Union which does not permit new missile defense sites. It would also be at variance with the SALT II pact (unratified but now being observed by both sides) prohibiting the construction of new missile silos.

The Reagan administration had better think carefully before moving forward with such a development. The ABM treaty contributes to nuclear stability since both sides now know they have an effective deterrence against a first strike (their second-strike missiles, in other words, can get through to the other side). Throwing overboard the treaty would only reduce that deterrence and spur both sides to develop more and more sophisticated offensive weapons to penetrate the ABM net.

In light of these and many other problems, it is far from certain that ''dense pack'' will see the light of day any more than did Mr. Carter's race-track scheme. The administration is going ahead with the MX because the US land-based nuclear forces are thought to be vulnerable to a Soviet first strike. But it is more likely that the missile, whatever basing mode is decided upon, will ultimatley be used as a bargaining chip in the upcoming START negotiations. ''We have to get started on MX so that we will have something to give up'' is how one administration official put it.

That would be the best fate for it.