Alison Johnstone, age 15, of Johnson City, Tenn., for her question:

WHAT IS SOLAR WIND?

Solar wind is an extremely thin gas that streams outward from the surface of the sun and through interplanetary space. It is made of electrically charged particles. Solar wind is made up mainly of protons and electrons. About 4 percent of it consists of helium nuclei. All these types of particles travel past the earth at speeds of about 300 miles per second.

In comparison, the earth travels in its orbit at a speed of only 18.5 miles per second.

The streaming gas of the solar winds contains only about five particles per cubic centimeter of space. In contrast, there are more than 1 billion million (that's the number one followed by 18 zeroes) molecules of gas per cubic centimeter of air in the earth's atmosphere at sea level.

The existence of the solar wind had been predicted before it was discovered. In 1962, measurements by the Mariner II space probe confirmed its existence.