Every good space enthusiast can sometimes be confused by difficult terms and concepts. It’s not exactly rocket science…but wait. Partly it is! Don’t worry, we got your back. We prepared a small – but very useful – glossary of meteoritic terms. From the basic concepts to the more advanced terms that you need when you turn pro, here is everything you need. Think of it as a starter pack for meteoritics!
Let’s start with the basics.
- Meteoritics: The science of meteors, meteorites, and meteoroids. It is closely connected to similar sciences such as cosmochemistry, mineralogy, and geochemistry.
- Meteor: The flash of light which happens when a meteoroid enters the atmosphere from space. The air molecules trigger reactions with the body of the meteor. Due to ionization, the meteor burns high in the mesosphere, leaving a colored trail behind. Therefore, people call meteors “shooting stars”.
- Meteoroid: All Solar System bodies that are smaller than asteroids. The precise line between these two notions doesn’t exist, but a general rule is that asteroids are larger than 1km in diameter.
- Meteorite: This is a meteoroid that has entered the atmosphere, vaporized (as a meteor) and then landed on the surface of the Earth.
- Lunar Meteorite: A meteorite that originates from the Moon.
- Martian Meteorite: A meteorite that originates from Mars.
- Micrometeorite: A meteorite with a diameter of less than 1mm but bigger than 1 micron. A very, very tiny meteorite. However, it is still bigger than meteoritic dust.
- Meteoritic Dust: Very small particles. Meteoritic dust grains are smaller than micrometeorites.
- Comet: Comets are small Solar System bodies that, when heated, release gasses. Most meteorites (and meteoroids) that we now have originate from comets, i.e. comets are the parent bodies. Most meteoroids are remnants that were left after the disintegration of a comet. Comets usually have two parts – a coma and a tail. The coma is the nebulous circle around the nucleus which forms when the comet is heated while passing close to the Sun. They are mostly made up of ice, dust, and rocky material. Comets, just like asteroids, formed around 4.5 billion years ago. Astronomers employ a general rule of naming comets after the astronomers who were the first to spot them. Halley, however, did not discover the comet, he only made predictions about the upcoming potential sightings of the body. Turns out, he was right – the comet reappears every 75-76 years, and we call it Halley’s Comet.
- Asteroid: Asteroids are large meteoroids, or better yet – minor planets. Scientists have discovered that asteroids are debris from destroyed planetesimals. Most asteroids are located in the Asteroid Belt, right between the orbits of Mars and Jupiter. Some asteroids even co-orbit with Jupiter. In terms of composition, asteroids have three main classes – C-type (rich with carbon), M-type (metallic), and S-type (silicate). Asteroids are often confused with comets and meteoroids. Here is the explanation – the difference between comets and asteroids is in the composition, while we differentiate meteoroids and asteroids by size. The largest asteroid ever discovered is Ceres, which is so big (945km diameter) that it’s also named a dwarf planet.
- Fireball: A light phenomenon brighter than a meteor. Fireballs are essentially the same thing, but they reach magnitude -4.0.
- Bolide: We use this term often as a synonym for fireballs. That’s not a surprise, as both terms describe an extremely bright meteor. Bolide is a fireball that reaches a magnitude of -14 (that’s brighter than the Moon), often exploding in the atmosphere. Superbolide represents a fireball that reaches a magnitude of -17 (or brighter). The latest example of a superbolide airburst was in Chelyabinsk in 2013, when a massive fireball exploded in the air above a densely populated area.
- Radiant: The visible point in the celestial sphere that meteors (during a shower) seem to originate from. For observers, who watch this from a two-dimensional perspective, it might seem that the radiant is the vanishing point for meteors. Their trajectories are almost parallel as they stem from the same parent body, i.e. radiant.
- Parent Body – Each meteorite is a remnant (or piece) that was once part of another bigger celestial body. The larger body from which meteorite originates is called the parent body.
- Shower: Meteor showers are cosmic events during which many meteors can be observed in the sky. They must include a radiant, i.e. the starting point of the shower. Meteor showers are typically named after the constellation in which their radiant is visible. One such example is the Leonid meteor shower. The radiant is located in the constellation of Leo. Perseids were named after Perseus, the apparent constellation in which the radiant lies. Of course, there are sometimes exceptions. The Quadrantids are named after Quadrans Muralis, a constellation that no longer exists, but is rather part of the Boötes constellation.
- Duration: The duration of the shower marks the period during which the shower is most expected to be active. The dates need not necessarily stay fixed each year and they can slightly shift.
- Maximum: The maximum represents the peak of the shower, i.e. the dates when the shower is most prolific. Due to changes in the Civil Calendar in terms of leap years, scientists and astronomers opt for the position of the Sun, better known as Solar Longitude.
- A. (Right Ascension): The angular distance of a certain point measured eastward along the celestial equator from the Sun. It is often used in pair with declination. Essentially, R.A. translates into a space equivalent of Longitude.
- (Declination): Declination represents latitude on the celestial sphere. Together with Right Ascension, these two astronomical coordinates serve for specification of the direction of a point on the celestial sphere in the equatorial coordinate system.
- H.R. (Zenithal Hourly Rate): Zenithal Hourly Rate represents the number of “shooting stars” (meteors) that you would expect to see during the peak of the shower. However, the conditions must be perfect. That includes the radiant which must be directly overhead and conditions for observing included limiting magnitude of +6.5. With higher ZHR, you are more likely to see more meteors. However, the ZHR rate is almost always higher than the actual rate that the observers can see. As the radiant approaches the horizon, the rate decreases.
- Train: The consequence of the ionization that burns up the meteor leaves a visible trail behind the body of the meteor. Scientists call this a train.
- Wake: A very short train that lasts less than a second.
- Stream: Earth can encounter a stream of meteoroids in its orbit. The stream is a massive grouping of meteoroids that possess almost identical orbits. This often signals that they originate from the same parent body.
- Trajectory: The three-dimensional line of motion of a meteor in the atmosphere.
- Path: The projection of the trajectory on the celestial sphere of the observer.
- Fusion Crust: The burnt-like surface of the meteorite after it passed the atmosphere. Due to air molecules and ionization, the body gets burnt surface.
- Ablation – The process through which each meteor loses mass as it hurtles through the atmosphere.
- Strewn Field –During the fall, the meteor typically shatters as soon as it hits the ground. This, in turn, scatters a plethora of fragments in the immediate vicinity. The area around the place of the impact where the fragments land dispersed is the strewn field.
If this is not enough and you yearn to learn more, we prepared a page specifically dedicated to meteor facts. There you’ll see what the best conditions for meteor observing are, and how to prepare for the meteor showers.
If you like history and want to trace the gradual development of meteoritics, you can jump right to the page where we discuss the history and origins of meteors and meteorites. If you have any more questions, please consult FAQ where we provided even more answers!