[Diagrams on page 15]

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The elliptical orbit of Halley’s comet

Neptune’s orbit

Uranus’ orbit

Saturn’s orbit

[Diagram]

White box indicates area of the comet’s orbit visible from the earth

Jupiter’s orbit

Mars’ orbit

Earth’s orbit

Sun

Perihelion

A Wanderer in the Solar System

Halley’s comet rides in a high orbit. It is not at all circular but is a long narrow ellipse. It stretches out across the orbits of all the planets from Venus to Neptune. At perihelion it is only 54,000,000 miles from the sun, but at its most distant point, it is more than 3 billion miles away.

At about the distance of Jupiter, the comet comes within range of telescopes, and after it crosses the orbit of Mars, it reaches naked-eye visibility. About this time the tail begins to form. It grows larger as the comet approaches the sun. It always points away from the sun, blown by a solar wind and by solar radiation.

It used to be thought that comets were onetime visitors from interstellar space. Occasionally, one would pass close enough to a large planet, like Jupiter, to be drawn into a closed elliptical orbit in our solar system. However, more recent research seems to imply that comets are gravitationally bound to the Sun like other members of the solar system. At times comets are hurled into a hyperbolic, or open-ended, orbit, and these leave the solar system forever.

Comets are not eternal. They are not even dependable for keeping time. A comet’s schedule is subject to change because of the repeated pull of planets near its path. In fact, a close passage may sling it permanently out of the system, as was done deliberately with the Voyager space vehicles. Also, a periodic comet suffers from aging. Every time it passes the sun, it expends some of its substance to recreate its coma and tail. Some short-period comets have disappeared after repeated circuits, leaving nothing behind but a shower of meteors. Halley’s comet is large enough to have outlived dozens of circuits without marked loss of luster, but it must eventually come to an end.