CONGRESS MISSED a chance this year to pass the most important piece of conservation legislation of the century. Largely because of the obstacles created by one senator, Mike Gravel, the Alaska lands bill never reached the Senate floor.As a result, the fate of more than 100 million acres of public land is up in the air. To protect that land - and to give Congress a second chance - President Carter and Secretary of the Interior Cecil D. Andrus apparently are going to stretch executive authority to the limit between now and Dec. 18.

On that date, the freeze Congress placed on the land in 1971 will expire. In the Native Claims Settlement Act passed that year, Congress' gave itself seven years in which to decide 1) which parts of Alaska should be set aside in national parks, wildlife refuges or other preserves where development is prohibited or limited and 2) which parts of it should be open to mining, timbering and other exploitation. But it failed to meet the deadline when Sen. Gravel refused, at the last minute, to go along with a compromise that had been worked out by Senate and House leaders along with Secretary Andrus and the other senator from Alaska, Ted Stevens. Sen. Garvel then blocked an effort to pass a bill simply extending the deadline by 12 months. That makes the land in question available for development after Dec. 18.

Secretary Andrus has said that he will start the process this week under which he believes that he and President Carter can place all of the land in controversy in national monuments or other special categories to which the deadline would not apply. To do that, they will have to use power granted under the Antiquities Act of 1906 and other pieces of legislation that are rarely hauled off the shelf. But the secretary is right in insisting that all the land Congress might put in federal preserves should be protected until Congress can make up its mind about any of it - though no doubt he will be challenged.

Sen. Gravel blocked the compromise, just as he blocked the original bill, because he and many of his political supporters think more federal land in Alaska ought to be available for commercial use. Conservationists thought the compromise did not sufficiently protect enough land, but they were willing to accept it to get the matter settled. The question of how much is too much - either to develop or to save - has never been easy and has become a major issue in the Alaskan elections.

Sen. Gravel's tactics, however, may work to aid the conservationists next year. If Secretary Andrus is successful, all the land will stay protected until Congress acts. That means the burden of effecting some changes in the status quo in the future would be on those who want to develop the land rather than, as it has been, on those who want to protect it. That is not a bad outcome, given the mess that Sen. Gravel made of the serious efforts of many of his colleagues to produce a bill that would be fair both to Alaskans and to future generations of Americans who, after all, have some stake in the preservation of Alaska in its natural glory.