Standing outside on a warm summer's night, you stare up at the dark sky. You observe the glittering stars, watch a satellite drift slowly by, and even steal a glance at the Moon. But in the corner of your eye, you catch something bright whiz by—something fast. Then another. And another. Was it a bird, a plane, Superman? No—it was a meteor. Or was it a meteorite? An asteroid? What's the difference, anyway?
June 30 is International Asteroid Day, so it's the perfect time to dig into a common question of observers of the night sky: What did I just see? From our vantage point on Earth, we are lucky enough to be able to observe many types of celestial objects—so many, in fact, that we have different names for similar objects depending on their sizes, compositions, and interactions with our planet. Join us on an exploration of the various kinds of space rocks!
Asteroids are small celestial bodies made up of rock, metal, and ice that orbit around the Sun. They can vary widely in shape and size, ranging from hundreds of meters to hundreds of kilometers across. At about 1000km, the largest asteroid in the inner solar system is Ceres; it was even thought to be a planet when it was first discovered! Today, it's classified as a dwarf planet, like Pluto.
You may be familiar with asteroids from science-fiction movies in which our intrepid heroes weave their spaceship through a field of giant space rocks. Our solar system has one of these itself: the aptly labeled asteroid belt, which is a large, circular band of variously sized objects—asteroids, dwarf planets, and minor planets—that sits between the planets Mars and Jupiter. The asteroid belt was first observed in 1801 when an Italian astronomer spotted a tiny object in the gap between the two planets. Soon after, more bright dots were spotted and these new mysterious objects were appropriately named "asteroids" after their "star-like" appearance.
Comets have long played prominent roles in legend and folklore. Given that they appear in our skies for a brief period of time at a measurable interval, often glowing colorfully and leaving a bright trail behind them, it's no wonder that comets seem to have a mystical quality about them.
Like asteroids, comets are small celestial bodies that orbit the Sun. However, they differ in their composition: Comets are made up of a combination of rock, water ice, frozen gases, and dust. These latter three ingredients are the cause of a comet's characteristic glow and its trailing tail. Comets typically have highly elliptical orbits around the Sun, which means that they spend some of their time far away from the Sun, where their elements can freeze solid. When they then approach the Sun again, these frozen elements sublimate—turning from solid to gas—causing the bright glow and releasing bits of rock and dust, which make up the tail of the comet.
"Meteoroid" is a general term for a celestial object smaller than an asteroid or a comet. Meteoroids are often chunks of asteroids that have broken off during collisions with other objects in space (mainly each other) or pieces of rock and dust left behind by comets.
When a meteoroid is drawn into Earth's atmosphere, it is then classified as a meteor. Although most meteors are no larger than a grain of sand, meteors carry with them a huge amount of mass and energy when they enter Earth's atmosphere, which is what causes them to burn brightly as the streaks of light—"shooting stars"—that we witness during a meteor shower.
Last but not least, we have meteorites. Most meteors are burned up entirely during their journey, but those that survive and make contact with Earth's surface are then called "meteorites." They are largely made up of metals, as that's the main thing that can survive the journey! Interestingly, scientists have observed that almost a third of meteorites on Earth come from a single asteroid.
So, returning to the question that started this exploration through space rocks: What did we see zip by in the night sky? In most cases, we see the bright trail of a meteor as it burns up in Earth's atmosphere. Whether that meteor stems from an asteroid, a comet, or something else is another question, but being aware of comets and meteor showers, which tend to occur on consistent schedules, is a good way to find out for sure.
Speaking of which, the next major meteor shower is the Perseid meteor shower, active from mid-July to early September. Take a moment to step outside on a dark night and watch for them, especially in early to mid-August. In the right conditions, you may see up to 75 meteors per hour! Don't forget to make a wish.
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