... тАФ . ... Astronomy for Kids . ... MESSENGER poised for Mercury orbit insertion . On March 17, the spacecraft will become the first to ever orbit the innermost planet. ... Less than 9.5 percent of the usable propellant at the start of the mission will remain after completing the orbit insertion maneuver, but the spacecraft will still have plenty of propellant for future orbit correction maneuvers. ... MESSENGER has been on a 6-year mission to become the first spacecraft to orbit Mercury. ...