The massive green cargo ship Marshal Konev headed home to the Soviet Union this morning, carrying 1.5 million bushels of yellow corn and Ukrainian sailor Miroslav Medvid, whose ship-jumping adventures along the Mississippi gained more world attention than either the Soviet or U.S. governments desired.

The Medvid incident appeared to end with the U.S. Customs Service allowing the ship to leave despite a congressional subpoena demanding Medvid's presence in Washington Tuesday for questioning about whether he truly wanted to leave. The 120,000-ton ship was expected to enter international waters in the Gulf of Mexico late tonight.

It was unclear who authorized the departure and Customs officials here would say only that they were following instructions "from Washington."

The White House had little to say about Medvid's departure. "The executive branch has carried out its responsibility," said spokesman Peter H. Roussel. "We consider the case closed."

However, Sen. Jesse Helms (R-N.C.), who had raised the stakes in the Medvid incident by asking the Senate Agriculture Committee, which he heads, to subpoena the sailor, said he was "dismayed" that "the State Department clearly decided that it is more important to appease the Soviets than to allow a young man to have an unfettered chance for freedom."

Helms urged President Reagan -- "when he arrives in Geneva" next week for the summit with Soviet leader Mikhail Gorbachev -- "to insist that the Soviets bring Mr. Medvid to Switzerland, where he can have a chance to be examined impartially."

The 25-year-old Medvid, who was returned to his vessel by U.S. Border Patrol agents after jumping ship downriver at Belle Chasse on Oct. 24, was neither seen nor heard from in the past three days as the Marshal Konev loaded corn at the Cargill Inc. elevator here, 3s nearby that the Marshal Konev had permission to leave. The officer, whose directives could be heard over a shortwave monitor, then detailed a plan to avoid a confrontation between a Soviet ship and a barge carrying a group of protesters.

"We'll have a boarding, a very detailed boarding [of the protesters' barge]," the officer said. "Let's go ahead and do a thorough inspection. An hour and a half, or two hours' worth -- that ought to keep them out of the picture for a while."

As the Marshal Konev pulled from its berth at 11:10 a.m. CST, another group of protestors gathered on the shoreline holding U.S. and Ukrainian flags and shouting, "Stop that ship now! Stop that ship now!"

The protestors were members of the Ukrainian American Justice Committee. Most had flown to New Orleans earlier in the week from their hometown of Chicago, and had spent four days here conducting vigils near the loading berth. As the ship pulled away, several insisted that Medvid had been killed.

The Ukrainian-Americans stressed that they were not part of another protest group whose members wanted to harass the Soviet ship by impeding it with tugboats. That group, Saved The Oppressed Peoples (STOP), rented a tugboat here for $100 an hour, hoping to slow the ship as it made its way to New Orleans.

But minutes after the protest boat left shore, the Coast Guard stopped it for a boarding inspection