Fast Near the Sun, Slow Far Away: A Comet's Orbit

The Library Telescope Club

1The public library in Elmwood lends small telescopes the same way it lends books, and its teen astronomy club meets on the library's back lawn on clear nights. One March, a faint comet with a short, fuzzy tail hung low in the western sky after sunset. Club member Rosalind Nakagawa wanted to know how a comet moves, so the club's leader, librarian Mr. Ezekiel Frontenac, helped her look up data on the best-known comet of all, Halley's Comet.

2Halley's Comet travels around the Sun in a long, stretched orbit. At its closest, it passes about 0.6 AU from the Sun, inside the orbit of Venus. At its farthest, it is about 35 AU away, beyond the orbit of Neptune. One AU is Earth's average distance from the Sun, about 150 million km. The comet takes about 76 years to go around once. Table 1 shows its speed at different distances on its way out from the Sun.

3Next, Rosalind used an orbit simulator on a library computer. The simulator draws the path of a small object near a star using only the star's gravity. With Halley's numbers entered, it matched Table 1. Then she ran three tests. In test A, she switched gravity off just as the comet passed its closest point. In test B, she doubled the star's mass and started the comet at 35 AU with its usual sideways speed of 0.9 km/s. In test C, she started the comet at 35 AU with no sideways speed at all.

4Gravity also works on much larger scales. The Sun, with its planets and comets, travels around the center of our galaxy, the Milky Way, at about 230 km/s. It is about 26,000 light-years from the center and takes about 230 million years to go around once. The pull comes from the huge mass of stars, gas and dust in the galaxy, much of it closer to the center than the Sun is. Even whole galaxies pull on each other: the Andromeda Galaxy, our large neighbor, is moving toward the Milky Way at about 110 km/s.