Tail of a comet

After unfulfilled hype that surround­ed the passage of several comets in recent decades, Comet McNaught was a pleasant surprise. It was im­pressively visible from mid-January into early February and can likely still be found with binoculars as it heads away from the Sun towards the outer solar system. Its closest approach to the Sun (perihelion) was on January 12, and like many comets that get fairly near the Sun, it was brighter as it started to move away. This is because the sun’s heat causes ice—the main component of comets—to sublimate from the outer skin of the comet and the closer to the sun it gets, the more material is given off. As the ice sublimates, dust is also released. More particles make for a bigger tail, the most strik­ing part of the spectacle. That said, particles in the tail are very sparsely distributed and it is sunlight reflecting off them—like dust motes in a room—that makes the tail visible.

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Ashley Marles

Unlike the exhaust left in the wake of a smoky car, a comet’s tail doesn’t trail behind it. Pressure from sunlight and radiation—known as the solar wind—keeps the tail more­or-less facing away from the sun.

Gas in the tail is very lightweight, is pushed most readily by the solar wind and has a bluish appearance. Dust particles are heavier, get pushed more slowly and look yel­lowish. Hence the tail spreads wide, bluish on one side, yellower on the other. Sometimes two distinct tails are visible, pointing in divergent directions.

Although dust grains are blown by the solar wind, they are still subject to the sun’s gravity and are effective­ly orbiting the Sun. So, as they are pushed outward, they orbit further out. The farther an orbit is from the Sun, the slower it moves. In consequence, these dust grains lag slightly behind the nucleus of the comet. The farther they are pushed, the more they lag, result­ing in a tail which gently sweeps backward.

The bands in the tail are caused when above-average amounts of material are released from the comet’s head at one time, and the particles are winnowed out as described above. Comet McNaught’s tail length is thought to have reached about 150 million kilometers—roughly the distance between the Earth and the Sun, and the comet is moving at about 60 km/second.

This comet is consid­ered the brightest to have appeared in our skies since Comet Ikeya–Seki in 1965. That comet ap­proached to within 1.2 million km of the centre of the sun and was the brightest comet of last century. Its nucleus broke into several fragments at perihelion. McNaught got to within 15 million km of the Sun but has not fragmented.

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