There are a variety of “falling” heavenly bodies that may be observed from the Earth. Among these are comets, meteors, and asteroids. Though they all share certain similarities, their differences have major implications for the study of space. The three bodies are all made of a mixture of materials that often date back to the beginning of the universe, but they are distinguished by the ways in which they orbit, cross, or interact with planets and stars. The work that follows will explore these fascinating phenomena in greater depth.
Comets are small celestial bodies that appear in the night sky as a ball inside a long, thin envelope of visible light. They are composed of water, frozen gasses and dust not incorporated into larger heavenly bodies at the dawn of the universe. Beyond the reach of the sun, comets remain frozen and are relatively difficult to detect. Nearer the sun, “active” comets can be visible in the night sky as their dust tails – composed of particles driven off the nucleus by escaping gasses, and reaching up to ten million kilometers in length – streak behind them. Because of the unusual orbits of most comets, each individual one is seen rarely and will tend to disappear from view for millennia. However, comets have been known since ancient times, and they are now known to be found in large numbers in the Kuiper Belt around Neptune. Comet impacts on Earth are extremely rare, occurring only once every 32 million years, but they are not unknown to science: In 1994, over twenty pieces of the Shoemaker-Levy comet famously collided with Jupiter. However, the possibility of near-Earth asteroid impact tends to be researched more actively by scientists.
- Comets: Basic information and links to a variety of resources, including comet photographs
- Comets 101: Information on comets, their origins, and their composition.
- Educator’s Guide to Comets: Detailed information on a variety of subjects related to comets, including a history of their study and chapters on their motion and fragmentation.
- Exploring Comets: Information on comet discovery and naming, the famous Halley’s Comet, and various other topics, from the Smithsonian National Air and Space Museum.
- Comet’s Dust Holds Basic Ingredients of Life: NPR interview discussing the results of research that followed NASA’s “Stardust” spacecraft mission to collect samples from a comet.
- Comet Impacts Triggered Ice Age Extinctions?: National Geographic article discussing the hypothesis that comet impacts may have created a major extinction event in Earth’s prehistory.
- The Nature of Comets: Articles on comets and their properties derived from lectures at the University of Tennessee’s Department of Physics and Astronomy.
- Comets: A Brief History: Information, photos, diagrams, and a chart of significant comets.
Meteors are what are typically known as “shooting stars.” They appear as a visible streak of light created when a larger body of miscellaneous space debris, properly known as a meteoroid, enters the atmosphere of the Earth. Meteoroids vary in size, but the great majority of meteoroids that pass through the atmosphere each year are no larger than a pebble. Meteors become visible between about 40 and 75 miles above the Earth, and disintegrate when they are only slightly closer to the planet. By definition, meteor impacts occur far more frequently than comet or asteroid impacts. These impacts leave meteorites, the byproducts of meteoroids that survive impact with the Earth’s surface. It is extremely difficult to estimate the number of meteor strikes per year; guesses range from 37,000 to 78,000 tons of material, most of it from tiny pieces. However, larger meteorites are very important for scientific research when they can be recovered. Meteorites come in several types – irons, stones, and “stony-irons” – according to their makeup. By analyzing these fragments, scientists have learned that most meteors come from the Asteroid Belt, though some come from the moon or Mars. Meteors sometimes pass through the Earth’s atmosphere in a stream, causing a meteor shower.
- The American Meteor Society: Information and publications about meteors, meteor showers, how to observe meteors with or without telescopes, and more.
- North American Meteor Network: Nonprofit organization of meteor hunters who track meteor sightings and impacts around North America.
- Meteor Photography Tips: All about capturing meteors on film. Also links to a variety of related resources.
- What Are Meteors and Why Do They Glow? Introductory information about the properties of meteors.
- Comets, Meteors, and Asteroids: In-depth information and Frequently Asked Questions about these three types of heavenly bodies and several other astronomy topics related to them. The same site also includes information on annual meteor impact estimates and how they are determined.
- Classification of Meteors and Meteorites: All about the classification and composition of meteors and meteorites.
- Meteor Shower Photographs: Photographs of meteor showers divided by year. From the University of Michigan.
- Meteors and Meteorites: Information on meteors and a reference chart for major meteor shower dates from the Western Washington University Planetarium.
Asteroids are massive cosmic bodies also known as planetoids. They orbit the sun, and appear somewhat similar to comets, but lack the distinctive tail of the comets. The majority of asteroids are found in the Asteroid Belt between Jupiter and Mars. Historically, it has been difficult to measure asteroids or learn about their composition; scientists were limited to “roundabout” methods such as measuring heat, light reflection, or the apparent size of the asteroid when silhouetted against a star. Recently, however, it has become possible to examine asteroids using space probes. These probes have determined a great deal about asteroids and helped to confirm details that were merely speculative before. Space probes have been able to fly as close as only about a dozen miles from given asteroids. The largest known asteroid, Ceres, is 580 miles in diameter and is estimated to contain one-third the mass of all the asteroids in the Asteroid Belt. Asteroids come in two major types: Asteroids in the outer part of the Asteroid Belt tend to be rich in carbon, while the “younger” asteroids in the inner part of the Belt are rich in metals and were made from melted materials. Many scientists believe that prehistoric asteroid impacts contributed to radical changes on the Earth’s surface about 65 million years ago. Luckily, asteroid impacts – which have the potential to create global disaster – are astronomically rare. Much smaller pieces of asteroids may enter the Earth’s atmosphere as meteors.
- Exploration Planets: Asteroids: Information on the origin and importance of asteroids, from NASA.
- Asteroids: Informational lecture on asteroids, the asteroid belt, and other topics, with a variety of useful diagrams and charts.
- Main Asteroid Belt: Detailed information on asteroids, with photographs and explanatory diagrams.
- Earth-Crossing Asteroids: Information based on NASA-funded research about asteroids that cross the orbit of the Earth.
- Asteroids May Have Accelerated Life on Earth: Informational article about a NASA study shedding light on the role of asteroids in the early formation of life on Earth.
- The Threat to Earth From Asteroids and Comets: Information on potential dangers from asteroids in fact and fiction.
- Killer Asteroid Project: Nonprofit, NASA-funded research project focusing on near-Earth objects, including near-Earth asteroids, their detection and tracking. Variety of information pertaining to the project’s findings.
Conclusion: The Importance of “Traveling” Bodies in Space
Asteroids, meteors, and comets may seem similar, but these three traveling bodies all have their own implications for science. Due to the differing scope of their journeys, each can offer different details about their origin in the universe. Comets and meteors bring material to Earth from distant space that can be analyzed to better understand the building blocks of distant galaxies. Asteroids present a challenge both in piecing together the events of prehistory and preventing disaster in the future. Since the 1990s, space probes have given scientists a completely new way of examining these and other bodies, and more research has gone into predicting and tracking near-Earth objects that may collide with the planet. With time, researchers may glean new information about conditions at the beginning of time. Even mining of the great asteroids may someday become possible, thanks to ongoing efforts in the science of astronomy.