A NASA spacecraft began orbiting Mercury, becoming the first to fly around the solar system's innermost planet. The space agency said the craft, known as Messenger, began the orbit around 9:00 pm (US time) on Thursday on a mission to circle the planet in an unprecedented study of the tiny, hot planet. The spacecraft began its journey more than six years ago, travelling through the inner solar system and embarking on flybys of Earth, Venus and Mercury. NASA said that by achieving orbit, this marks the first time a spacecraft has accomplished this engineering and scientific milestone at the solar system's innermost planet.