The Heliopause

Between the Kuiper Belt and the Oort cloud lies the heliopause, the teardrop-shaped region around the Sun that is filled with solar magnetic fields and the outward-moving solar wind consisting of protons and electrons. Nearer the Sun than the heliopause lies the helio-sheath, a region of transition where the solar wind slows to subsonic speeds, that is, slower than the speed with which disturbances travel through the interstellar medium.

The tail of the heliopause is estimated to be between 110 and 170 AU (17 and 26 billion km [10 and 16 billion miles]) from the Sun. Its shape fluctuates and is influenced by a wind of interstellar gas caused by the Sun's motion through space. The orbits of all the major planets, including Earth's, lie well within the heliopause.

No satellite has yet reached the helio-pause, although the Voyager 1 and 2 probes launched in 1977 are the closest to it at distances from the Sun of 105 and 85 AU (16 and 13 billion km [10 and 8 billion miles]), respectively. (Voyager 1 and 2 crossed into the heliosheath at distances from the Sun of 94 and 84 AU in 2004 and 2007, respectively.) What is known about the heliopause is deduced by its effects on cosmic-ray particles coming into the solar system after passing through it and by the radio emission generated when material thrown off by the Sun in coronal mass ejections crosses it.

Comets

In ancient times, the regularity of the sky was a bedrock of stability in a harsh world. The Sun rose and set. The planets moved along the zodiac. However, sometimes, something new and frightening appeared, a star with a tail trailing behind it, a comet. (The word comet comes from the Greek kometes, meaning "hairy one," a description that fits the bright comets noticed by the ancients.) Made of ice, these objects come from the Kuiper Belt and the Oort cloud and develop diffuse gaseous envelopes and often long luminous tails when near the Sun.

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