As planning for the Pioneer 10 and 11 missions progressed, mission scientists found themselves desiring a third probe. In 1971, a formal mission study was proposed for a spacecraft to be launched to Jupiter in 1974, where it would use the gas giant as a gravitational slingshot to travel outside the ecliptic. This was the first Out-Of-The-Ecliptic mission (OOE) proposed, for Jupiter and solar (Sun) observations.