Vanguard II is an Earth orbiting satellite and the first weather satellite. It was designed to measure cloud-cover distribution over the daylight portion of its orbit for a period of 19 days and to provide information on the density of the atmosphere.

The Vanguard rocket was intended to be the first launch vehicle the United States would use to place a satellite into orbit. Instead, the Sputnik crisis caused by the surprise launch of Sputnik 1 led the U.S., after the failure of Vanguard TV3, to quickly orbit the Explorer 1 satellite using a Juno I rocket, making Vanguard I the second successful U.S. orbital launch.

The Navy was competed with the Army to put the first American satellite into orbit. Their first two Vanguard missions failed, with the second being a highly publicized failure, as it exploded on the launchpad. They went on to put 3 satellites into orbit but did not beat the Army to be the first. Their work in space continues today largely on military satellites.