Citizen Science by Trisha Muro
Asteroids and Comets and Meteors…Wait, What?
Our Citizen Science column gets a back-to-back dose of astronomy this month. Last month’s article aimed to answer the question, “What’s the difference between a star and a planet?” This month, we’ll stick considerably closer to Earth and tackle asteroids, comets, and meteors. Perfect timing, too, with summertime stargazing season upon us and headlines involving all three objects in the news!
Most asteroids in our solar system orbit the sun in the asteroid belt, between the orbits of Mars and Jupiter. Over 1.5 million asteroids have been characterized, which means that astronomers have been able to observe them closely enough that their orbits and sizes are known with a reasonable degree of accuracy. Both of these factors are crucial, because Earth has been hit by asteroids many times and that threat extends into the future.
Asteroids are fairly rocky objects, but that description can range from a metallic-rocky composition to “a big chunk of rock” to a loose conglomerate of gravel and clay. Sizes can range from the size of a house to the size of a city. Because they’re essentially inert rocks, though, asteroids can only reflect light. They’re extremely dark and cold, which makes them hard to spot.
Comets, on the other hand, really do live up to their nickname as “dusty snowballs.” They’re pretty loosely-packed, carrying abundant water (ice) and various grains of dust. (In astronomy, though, “dust” isn’t like the fuzzy dust bunnies that accumulate when I wait too long to vacuum. Instead, dust refers to small grains like particles of sand that scatter light and are made of varying chemical composition.)
Comets follow highly elliptical orbits around the sun, which means that most of their time is spent far, far away. During this time, the comet is at its most snowball-like. As it swoops back inward toward the sun, the sun’s heat and radiation begin to cause the comet’s water to sublimate, or to transition directly from solid ice to water vapor. This process intensifies as the comet gets closer to the sun, and water vapor trails behind the comet to form its tail. For example, Halley’s Comet remains a comet throughout its 76-year orbit, but it only appears cometlike with its long, bright tail for the brief handful of months when it passes closest to the sun.
Something interesting happens with that tail, however. As the comet streams through space it also begins to shed dust particles as they’re carried away by the sublimating water vapor. It’s not a particularly pretty analogy, but you can visualize this like the trail of slime left behind by a snail in your garden. If an ant walks through that slime, its feet would likely get a little sticky. (I’m no entomologist, but let’s go with this for a minute!) In space, that cometary dust trail exists as a three-dimensional column of dust. But here’s the fun part: just like the ant passing through snail slime, if a comet’s trail of dust intersects Earth’s orbit, we get a meteor shower. Every May our planet passes through the trail left behind by Halley’s Comet: the Eta Aquarid meteor shower. The Perseid meteor shower, occurring every summer in July or August, is caused by dust left behind by Comet Swift-Tuttle.
And that brings us to meteors. A meteor by definition enters (and burns up in) Earth’s atmosphere; a meteorite is the solid chunk that survives and reaches Earth’s surface. Think back to the last time you saw a shooting star—that’s a meteor, scientifically speaking. The object itself could be a small asteroid, a piece of space debris or dust grains from a comet. A meteor flares brightly for the same reason that spacecraft returning to Earth need a heat shield: Its incredibly fast speed generates massive friction, which erupts into heat and light as the object slams through the atmosphere.
Keeping track of asteroids, comets and meteors is a truly astronomical task, requiring the efforts of professionals and amateurs utilizing powerful computers and telescopes. The mathematics behind calculating an object’s orbit in space is incredibly complex, and the influence of gravity is infinite. Astronomical telescopes reveal some of the most visually gorgeous images of our universe. But those telescopes and mathematical computations also alert us to asteroids, comets and even soon-to-be meteors. Next time you make a wish on a shooting star, perhaps also whisper a “thank you” to the astronomers keeping watch on the asteroids, comets and meteors out there.
Trisha Muro is a freelance science writer specializing in physics and astronomy and, although she’s never worked for NASA, she’s been a space nerd for as long as she can remember. Her first book, “It’s (Just) Rocket Science,” was published this spring by Johns Hopkins University Press.
