The word "only" was dropped from a sentence in a Jan. 1 letter from James R. Morrison. The sentence should have read: "However, a joint Mars program is not the only space effort that can achieve these economic and political benefits. (Published 1/6/88)

Roald Sagdeyev, director of the Soviet Space Research Institute, suggested in Outlook {Dec. 13} that the United States and the U.S.S.R. work together in a decades-long, multimission effort to explore the planet Mars. There is an appeal to this suggestion based on probable cost reductions from division of labor in a shared effort and on the mutual confidence-building that can come from successfully accomplishing a difficult and highly visible cooperative task. However, a joint Mars program is not the space effort that can achieve these economic and political benefits.

The real task before the United States in this area is to assess which space goal, or combination of goals, it is in the best interests of our country to pursue. Two of the other possible candidates are an intensive study of the Earth as a planet and man's increasing impact on it (which can be studied globally now for the first time because of the development of satellite technology) and human exploration and -- potentially -- development of the moon. While the moon may not present as captivating an image for the public as Mars, the costs would be less in this resource-constrained world; less time would be required to achieve the goal; and, if successful, a moon project could serve as a proving ground for a longer, larger Mars effort.

In assessing which effort to undertake, it would be well to keep in mind that in recent history major negative strains have occurred in U.S.-Soviet relations about every two years. When they occur, it is the highly visible programs (such as a space program) that are discarded first amidst popular acclaim. The message here is that whatever the United States chooses to do, it should make its decision only after carefully evaluating U.S. priorities and interests. Any rush to judgment could set us off on a non-optimum course, with an impact that could last a decade or more.

Dr. Sagdeyev has made an interesting suggestion -- which, given his official position, his reported closeness to Mikhail Gorbachev and the title of his article, ''To Mars Together -- A Soviet Proposal'' (emphasis supplied), must have political support in the U.S.S.R. The United States should welcome this proposal as an indication of the opening up of the once totally closed Soviet space program and set about evaluating it and the alternatives to it carefully. On the positive side, there is potential for U.S. leadership here. On the negative side, there is potential for America to establish a role as a ''space follower.'' The ball appears to be in our court.

JAMES R. MORRISON Senior Fellow Center for Strategic and International Studies Washington