The first significant congressional challenge to the Pentagon plan to protect land missiles by hiding them within fields of decay holes was mounted by 51 representatives yesterday.

They wrote President Carter requesting him to appoint a commission to assess the dangers of the new missile deployment scheme, called MAP for multiple aim point. One missile, under the MAP concept, would be hauled around a field of some 20 holes so Soviet targeters could never be sure which hole held the missile.

Not knowing which hole held the missile, the theory goes, the Soviets would have to shoot at every one of them, using up their nuclear warheads in the process as vast stretches of the United States became a sponge for the deadly fire.

"Our concerns," said the letter, which got its 51st signature yesterday, "span the entire spectrum, including the advisability of deploying a great sponge of targets in the U.S. designed to absorb Soviet warheads; MAP's potential negative impact on the negotiation of a SALT [strategic arms limitation talks] agreement, and the enormous cost and questionable strategic utility of the program."

The lawmakers asked Carter to appoint both civilian and military strategists as well as representatives of the general public to the study commission.

The Joint Chiefs of Staff, the nation's top military body, consider the MAP system as the most promising way to protect U.S. land-based missiles in the 1980s.

Gen. Lew Allen Jr., Air Force chief of staff, told the Air Force Association this week that, "The multiple aim point ICBM basing system appears to be the best of the options for redressing the vulnerability concern.

"To attack us," he continued, "the Soviets would have to exhaust their resources to the extent that they would end the initial wave of attack relative or worse off than when they began it. Therefore, they would not be well advised to attack inthe first place. They would be deterred."

In a counterargument, Herbert Scoville Jr. former CIA executive and an arms control proponent, has said that the Pentagon's "first strike scenario" in justifying MAP "is a fantasy," because the Soviets could not be sure the United States would not launch its missiles before the Soviet warheads arrived. A Soviet first strike could not knock out U.S. missile submarines in any event, he added.

Scoville said MAP would undermine the basic verification feature of arms control agreements - the ability to tell through observation satellites what the other side is doing.

The letter to Carter was circulated for signatures by Rep. Berkley W. Bedell (D-Iowa), Thomas J. Downey (D-N.Y.), Elizabeth Holtzman (D-N.Y.) and Parren J. Mitchell (D-Md.).