High-flying, Dirty Snowballs

by Morris Yoder | Jan 2, 2024 | 0 comments

Comet NEOWISE II over farm
Comet NEOWISE II. Photo © Morris Yoder.

Comets are nicknamed “dirty snowballs.” That nickname should give you a pretty good idea of what they are—big chunks of ice with dirt, rock, and gases mixed in. In fact, they’re dirty enough that the dirt rains in on us on a regular basis and creates celestial fireworks in the process.

As a result of clouds passing overhead, we get rain showers. As a result of comets passing overhead, we get meteor showers. Meteors, also called falling stars, are sometimes thought to be actual stars falling from the sky, but, in reality, they’re burning rocks from space. Most of them come from the debris that forms a comet’s tail.

Meteors are moving at up to 150,000 miles per hour (240,000 km/h) when they strike the Earth’s atmosphere. This causes lots of friction, which heats the rock enough to make it vaporize into a blazing streak of light in the sky.

When a “dirty snowball” comet passes close to the sun, its ice starts to melt, debris is released, and gases spray out in jets from the nucleus (solid core) of the comet. The nucleus is usually 6 miles (10 km) or less in diameter, although a few are bigger.

The Sun’s radiation and solar wind (charged particles continuously flowing from the sun) push the spray of debris far out into space. As a result, comet tails point away from the Sun. That seems fairly straightforward. We expect to see comet tails straight and long, but oftentimes they’re curved. This happens because comets often have a sharply curved orbit around the sun. The tail looks curved for the same reason that a water stream looks curved coming out of a garden hose nozzle if it’s swept back and forth. Comets often have two tails. One tail is mostly made of dust and is white or yellow. The other is made of gases and ions and is normally blue.

The ion tail extends straight away from the comet since it’s made of gases that are easily pushed by the solar wind. The ions move fast, quickly leaving the nucleus and creating a straight tail, similar to how the light beam from a flashlight looks straight even if the flashlight is swung to the right or left. On the other hand, yellow-white dust tails curve in the direction of the comet’s orbit since the heavier dust particles resist being pushed away from the Sun as fast.

Stranger still, there are times when a comet appears to have what’s called an anti-tail. At those times, the comet doesn’t actually have a tail that stretches out in front of it. This illusion happens only when we view it from the same plane as its orbit. If we could rise above and view from above the orbit plane, we would see the tail spreading out into a fan shape from the nucleus of the comet. We could see a similar effect if we watch a boat coming toward us. The wake only follows the boat, but, depending on how the boat angles toward us, we can see the wake on both the left and right sides of the boat. There are also times that we can be at a vantage point where the ion tail seems to point in one direction and the dust tail in another.

Sometimes the high-flying, dirty snowball tails tell illusory tales, but they always showcase the Creator’s creativity.

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