How to tell a comet from an asteroid and a meteor from a meteorite

The following essay is reproduced with permission from The conversationan online publication covering the latest research.

Have you ever been outside at night and seen a streak of light cross the sky and disappear? Have you ever wondered where that shooting star came from, or how it came to be in your sky?

Like the director of the Peters Observatory at Hamilton CollegeI have seen many similar streaks in the sky, while spending late nights at the observatory, and I am here to tell you that what you saw is not a star at all. You have observed before your eyes the end of the 4.6 billion year journey of a comet or asteroid.

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Remains of the first solar system

Roughly 4.6 billion years agothe solar system was in its infancy. A vast ball of gas and dust which would become our solar system accumulated matter at its center, forming what would become our Sun. This also condensed the dust into small patches further from the center which would blend into the first pieces of material, called planetesimals.

Asteroids formed from planetesimals in the inner parts of the solar system, near the Sun. This place in the center of the solar system was hot, so the planetesimals were composed mainly of rocks and metals which could resist the heat. The largest of these pieces would freeze with others and form the terrestrial planets: Mercury, Venus, Earth and Mars. The remaining planetesimals that did not form into the terrestrial planets are today’s asteroids, left to orbit the inner part of the solar system.

Comets were formed in the outer parts of the solar system, where it was located quite cold that all water, or any similar hydrogen-based compound, took the form of ice. The planetesimals formed in this region were composed not only of rocks and metals, but also of these ices.

Some planetesimals grew large enough, quickly enough, to exert enough gravitational pull to hold large atmospheres composed of gases that were very abundant in the early solar system, such as hydrogen and helium. These planetesimals became the Jovian planets today: Jupiter, Saturn, Uranus and Neptune. However, planetesimals that did not form in the Jovian planets were left to travel through the solar system as comets.

Origin of meteors

Asteroids are still abundant in the inner solar system, so some will inevitably collide with Earth. When a piece of rock enters the Earth’s atmosphere, it moves at tens of kilometers per second. Upon entering, it can create a thunder-like sonic boom in its wake. When moving through the air at a speed greater than the speed of sound, the asteroid produces a shock wave that can generate this boom.

During its journey through the atmosphere for dozens of kilometers, the asteroid collides with air molecules, and the incredible temperatures and pressures usually vaporize it. This trail of vaporized particles detaching from the asteroid causes a bright line of light across the sky called a meteor, or colloquially shooting star.

Comets, although usually found in the outer solar system, can also cause meteors, and even meteor showers. A few comets take long elliptical paths through the inner solar system each year.

These objects, sometimes called “dirty snowballs” because they are made of dust and ice, tend to melt slowly as they get too close to the Sun, causing the comet to develop a tail of gas and debris left in its wake.

If the comet’s path crosses Earth’s orbit, Earth will collide with these debris fields during its annual orbit around the Sun. When this debris enters the atmosphere, it vaporizes, causing numerous streaks of light called meteor showers. Since it happens every year in the same part of our orbit, meteor showers are annual events. If you find a dark sky you can see dozens of meteors every hour during these annual meteor showers.

Finding meteorites

Meteors large enough to pass through Earth’s atmosphere and crash into the surface are called meteorites. Meteorites usually come from asteroids that were originally bigger than a football field.

Meteorites can be difficult to identify because they resemble Earth rocks. Typically, people collect meteorites in geologically unchanged regions, such as deserts or ice fields, where the meteorites stand out from the landscape.

They are often made of stone, nickel and iron and are probably magnetic. Many have irregular or pockmarked shapes, while others have a smooth crust due to burning in our atmosphere.

Meteorites are quite rare and important for the study of the early solar system. If you think you’ve found one, you need to check your the characteristics of the rock match those of a meteorite and then contact local geologists.

The next time you see a meteor in the night sky, remember that you are witnessing the end of its billions-year journey as it burns up in Earth’s atmosphere.

This article was originally published on The conversation. Read the original article.