“Dear Otto,” wrote former Walker director Martin Friedman to German artist Otto Piene 40 years ago this week. “It’s just possible that having your spectacular work shot down may have hastened its immortalization process.” On Halloween of 1976, a vandal destroyed Piene’s Walker-commissioned artwork Black Stacks Helium Sculpture, which consisted of four 300-foot-long, undulating inflatable tubes ascending from smokestacks on the Minneapolis riverfront. The work of “Sky Art,” as Piene called it, was commissioned as part of the Walker exhibition The River: Images of the Mississippi and was intended to be on view for two weeks.