Asteroids, comets, meteors — what’s the difference? If you’re new to astronomy, chances are you’re swimming in a sea of new words. And at first glance, asteroids, comets, and meteors might seem quite similar. Is a comet just an asteroid with a tail? And what makes a meteor different from the other two? Let’s take a look at the three terms and gain a general overview of the topic.
What are Asteroids?
Asteroids are often considered synonymous with space rocks, and in some cases this is true. However, many asteroids are nothing more than rubble piles — numerous pieces of rock only loosely bound by gravity, and with low densities.
Although asteroids can be found throughout the solar system, including near or even crossing Earth’s orbit, most of them reside in solar orbit between Mars and Jupiter. These make up the classic main asteroid belt. Don’t be fooled by movies or illustrations, which usually portray this region as densely packed. In fact, the asteroid belt is a substantially roomy area, with a relatively small amount of mass distributed across a band that spans roughly 1 a.u. (astronomical unit — the average distance between Earth and the Sun, equivalent to about 150 million kilometers). The average distance between asteroids in the belt is around 1 million kilometers (600,000 miles), so from any one asteroid’s point of view, no other asteroids would be visible.
Some asteroids are quite large. With names like Vesta, Pallas, and Ceres, these so-called “minor planets” span on the order of hundreds of kilometers. Ceres, the largest asteroid, is massive enough to be rounded by its own gravity. It is classified as a dwarf planet, along with Pluto.
But the asteroid family also includes many more smaller, irregularly shaped rocks, some of which may be fragments from collisions between larger pieces. At the small end, an asteroid could be as wide as a baseball bat is long. Anything under a meter, though, qualifies as a meteoroid (or, if it’s small enough, a micrometeoroid).
Spacecraft have flown by or orbited more than a dozen asteroids: most recently, New Horizons flew by 486958 Arrokoth, Hayabusa 2 orbited and sampled 162173 Ryugu, and Osiris-REX visit to 101955 Bennu is still in progress. Additionally, dozens of asteroids have been mapped via radar during their near-Earth encounters. Many more remain known only as faint points of light. NASA estimates the total number of asteroids is somewhere between 1.1 and 1.9 million.
Searching out and cataloging asteroids has been the fond pursuit of professional and amateur astronomers for centuries. Now, for the amateur astronomer, asteroids represent the chance to try tracking down and observing objects that are a little more challenging to locate than the Moon or bright planets. Telescopes are required, and the only way to be really sure that you’ve observed an asteroid and not just another star is to double-check the next night and see if the point of light has moved in relation to the unchanging background of stars. An exception is if the asteroid is flying by near Earth — then it’s possible for observers to actually watch its movement relative to background stars.
What are Comets?
Comets are a different class of objects, yet if you could visit a comet far from the Sun, you might have a challenging time distinguishing it visually from an asteroid. It would likely look quite similar, just another tumbling, asymmetrical object. Sample a comet’s material, however, and you would discover that it is compositionally quite different. While asteroids are rocky, comets are made of ice, dust, and small amounts of volatile compounds, such as ammonia, methane, and carbon dioxide.
Comets are also traditionally different from asteroids in that they tend to exhibit highly elliptical, or eccentric, orbits. The velocity of a comet varies considerably over the course of its orbit, with the comet speeding up as it approaches and swings by the Sun, and gradually slowing down until it reaches its lowest velocity at the far end of its orbit away from the Sun. This is in stark contrast to asteroid orbits, which, although still ellipses, tend to be closer to circular.
As a comet approaches the Sun, at around 5 astronomical units (a.u.) it begins to form a cloudy halo called a coma in a process known as outgassing. In some cases, solar wind and radiation exert enough pressure on the coma for one or two cometary tails to form. The bluish ion tail, made of ionized gas, points away from the Sun, while the whitish dust tail, made of dust particles pushed out of the coma, points away from the comet’s direction of motion. These tails can stretch across enormous distances (although they are highly diffuse).
Long-tailed comets are highly prized by amateur astronomers and thoroughly enjoyable to observe. On the other hand, it’s also common for comets to disappoint, displaying only a vague fuzzy glow that requires optical aid to see. Predicting comet behavior is notoriously difficult.
We can roughly categorize comets based on the length of their orbits—and therefore by how often they visit the Sun and the inner solar system. Three comet types include:
- Short-period comets – These comets go around the Sun with periods under 200 years. The famed Halley’s Comet fits this classification.
- Jupiter-family Comets – This subset of short-period comets is perturbed by the immense gravitational influence of Jupiter, which flings the comets into periods less than 20 years. Comet 46P/Wirtanen, known in 2018 as the “Christmas Comet” since it reached perihelion in December of that year, is one example.
- Long-period comets – These comets have orbits so enormous that it takes them many centuries—sometimes millennia—to complete a circuit; one example is Comet Hale-Bopp that appeared in night skies in 1997.
What are Meteors?
A meteor is a fragment of an asteroid or a comet that has entered Earth’s atmosphere, burning up as it goes. In fact, meteor showers often occur when Earth encounters debris left behind by a comet passing through the inner solar system.
Sometimes called the poetic but terribly inaccurate term “falling stars,” meteors often appear quite fast and bright. Friction, caused by the fast-moving meteor being slowed by the drag of the air, generates heat as the meteor falls. This friction creates the brief streak of light that we perceive (the same thing happens to spacecraft on reentry). Despite the excitement and enjoyment we feel at the sight of a meteor, most of them are highly insignificant—even a speck may light up as an impressive streak.
Optical aid isn’t very helpful for observing meteors. Even wide-angle binoculars are too limiting. Your best bet is simply to use your eyes and wait — or try capturing meteors with a camera.
Many meteors of this type actually burn up completely in the atmosphere and never make it to the ground. But if a meteor does land on Earth, it becomes a meteorite, and it may eventually be found by humans and collected. Occasionally, meteorites can be of very significant size—take one look at Meteor Crater in Arizona and you can quickly comprehend the size of the meteorite responsible for the impact. Large impacts do happen on Earth from time to time, but these are extremely rare.
(Meteors are often confused with meteoroids and meteorites . . . to learn the difference between these terms, visit our article “About Meteors.”)
How to See Asteroids, Meteors & Comets in the Night Sky
These three types of objects only represent a small fraction of what you can view in the night sky, but all three offer plenty of opportunity to keep you busy for years of observing. Check out out observing guides if you want to learn how to see more of these celestial objects: