Solar wind is caused by the expansion of gases in the sun’s outermost atmosphere, the corona. Because of the corona’s extremely high temperature of 1,800,000°F (1,000,000°C), the gases heat up and their atoms start to collide. The atoms lose electrons and become electrically charged ions. These ions create the solar wind. Solar wind has a velocity of 310 miles (500 kilometers) per second, and its density is approximately 82 ions per cubic inch (5 ions per cubic centimeter). Because Earth is surrounded by strong magnetic forces, its magnetosphere, it is protected from the solar wind particles. In 1959, the Soviet spacecraft Luna 2 acknowledged the existence of solar wind and made the first measurements of its properties.