Martin Sheen, 59, was sentenced to probation and fined $500 in U.S. District Court in Los Angeles on Wednesday after pleading guilty to a trespassing charge stemming from a protest at Vandenberg Air Force Base. "The West Wing" star was protesting the "Star Wars" missile defense system at the time of his arrest -- along with those of other activists -- last October. Vandenberg is in Central California. Explaining his guilty plea to Reuters, Sheen, who insisted he will still oppose "Star Wars," said, "I have a prior engagement with 'The West Wing,' another year on my contract, and I can't risk losing (at a trial) and going to prison for six months. Morally, I'm bound to fulfill my contract. That was the whole motivation." Besides leveling the fine and probation, Judge Jeffrey Johnson also banned Sheen from returning to Vandenberg without permission. A spokesman for the U.S. Attorney's Office in L.A. said that most of the other protesters arrested with Sheen had pleaded innocent to trespassing charges. They are due to face trial later this year.