It carried 45.4 kilograms (100 pounds) of high explosives that were detonated by proximity fuse when the missile was within 9.1 meters (30 feet) of an enemy airplane. X-4 [Illustration: 66. X-4.] The fin-stabilized X-4 air-to-air missile was guided to its target by means of electrical impulses which passed through two wires connecting the rocket to the launch aircraft until detonation. Once the missile was on its way to the target bomber, the fighter pilot directed its course with a separate small control stick in his cockpit. Because the control wires streamed out ahead of the launching aircraft, the pilot was prevented from evasive maneuvering. Launched from German fighter aircraft, usually a FW-190, the X-4 was powered by either a solid-propellant engine or a bi-propellant liquid-rocket engine. It carried a 20-kilogram (44-pound) warhead. Jupiter-C [Illustration: 67. Jupiter-C launches the first American satellite, January 31, 1958.] Jupiter-C carried the first successful American artificial earth satellite, _Explorer 1_, into orbit on January 31, 1958. Jupiter-C launched additional Explorer satellites on March 26 and July 26, 1958. Jupiter-C, or Juno 1, is a modified version of the Redstone Ballistic Missile and a direct descendant of the V-2 (A-4) rocket developed in Germany during the second World War. The vehicle’s main stage is powered by a rocket engine burning liquid oxygen and a hydrazine mixture. The second and third stages are contained in the “tub” on the nose of the rocket. Both use scaled-down Sergeant solid-propellant rockets: eleven in the second stage and three in the third. A final Sergeant motor is attached to the base of the satellite to provide the velocity necessary to place the vehicle in orbit. An electric motor spun the entire “tub” prior to launch and during the climb into space in order to stabilize the satellite. The Jupiter-C was built by the U.S. Army Ballistic Missile Agency. Vanguard [Illustration: 68. Three-stage Vanguard launch vehicle boosts the second American satellite into Earth-orbit, March 17, 1958.] Standing 21.6 meters (70.8 feet) high and weighing more than 10,000 kilograms (20,000 pounds), the Vanguard launch vehicle successfully orbited three satellites. The first was _Vanguard 1_, launched on March 17, 1958. The rocket has three stages. The first-stage motor, burning kerosene and liquid oxygen, operated for 2 minutes and 20 seconds. The second stage carried the vehicle to an altitude of 210 kilometers (130 miles), propelled by white-fuming nitric acid and unsymmetrical dimethylhydrazine (UDMH). With propellants exhausted, the upper stages then coasted to 480 kilometers (300 miles) above the surface of the Earth where the solid-propellant third-stage motor fired to place the satellite into orbit.