CLEVELAND, OHIO—In an underdog city, at an underdog NASA lab, researchers are thinking hard about an undeservedly neglected planet. Venus is Earth’s cousin, closest in composition and size, but for decades it has remained veiled. NASA hasn’t sent a mission there since 1989; more recent European and Japanese orbiters have made halting progress that stops largely at the planet’s thick sulfur clouds. No craft has touched down since 1985, when the last of a series of advanced Soviet landers clad in armored pressure vessels endured a couple hours before succumbing to the deep-ocean pressure and furnacelike temperature of the planet’s surface. The baleful conditions and lack of funding have made Venus, Earth’s closest neighbor, feel more distant than ever. That is, except here.