© N. Melville CC BY-NC-SA 2.0

A meteoroid is a diminutive celestial body composed of rock or metal located in outer space. These entities are characterized by their size, being notably smaller than asteroids, with dimensions ranging from grains to objects measuring up to a meter in width. Entities smaller than meteoroids are categorized as micrometeoroids or space dust. The majority of meteoroids are fragments originating from comets or asteroids, while others consist of debris resulting from collisions impacting celestial bodies like the Moon or Mars.

As a meteoroid penetrates Earth‘s atmosphere, it hurtles at an extremely high velocity, exceeding a minimum of 11 km per second (25,000 miles per hour). This speed is several times faster than a bullet exiting a gun barrel. The intense frictional heating, stemming from the meteoroid’s energetic interaction with atmospheric atoms and molecules, induces the melting and vaporization of its surface. Simultaneously, it raises the temperature of the surrounding air. The outcome of this dynamic process is the luminous spectacle known as a meteor.

Approximately 25 million meteoroids, micrometeoroids, and various space debris make their way into Earth’s atmosphere daily, contributing to an estimated annual influx of 15,000 tonnes of such material. A meteorite, on the other hand, represents the remnants of a meteoroid that has endured the ablation of its surface material while traversing the atmosphere as a meteor and subsequently collided with the Earth’s surface.

CHARACTERISTICS AND DIVERSITY

Under the canopy of a clear night sky, away from the luminous glow of city lights, one can observe several meteors per hour with the unaided eye. These celestial visitors exhibit a range of durations, from a brief fraction of a second to several seconds. Frequently, as the radiant meteoroid traverses the heavens, it undergoes fluctuations in brightness, gives the impression of emitting sparks or flares, and occasionally leaves behind a lingering luminous trail even after concluding its celestial journey.

SIZE AND COMPOSITION

Meteoroids exhibit remarkable diversity in both size and composition. Ranging from minuscule particles to several meters in diameter, meteoroids come in various sizes. In 1995, Beech and Steel, in the Quarterly Journal of the Royal Astronomical Society, introduced a novel definition suggesting that a meteoroid should measure between 100 µm and 10 m (33 ft) in diameter. In 2010, prompted by the identification of asteroids smaller than 10 m, Rubin and Grossman advocated for a revised meteoroid definition encompassing objects ranging from 10 μm (0.00039 in) to one meter (3 ft 3 in) in diameter. This adjustment aimed to uphold a clear distinction between meteoroids and other celestial bodies.

Nearly all meteoroids carry extraterrestrial nickel and iron, and they are categorized into three primary classifications: iron, stone, and stony-iron. Within the category of stone meteoroids, some contain minute grain-like inclusions called chondrules and are referred to as chondrites. Alternatively, stony meteoroids lacking these features are termed “achondrites,” often originating from extraterrestrial igneous processes and containing minimal or no extraterrestrial iron. The composition of meteoroids can be deduced as they traverse Earth‘s atmosphere by analyzing their trajectories and the light spectra emitted during the ensuing meteoric events.

ORIGIN

Meteoroids originate from two primary celestial sources: comets and asteroids. Cometary meteoroids often consist of volatile materials such as ice and dust, and their journeys bring them from the outer reaches of the solar system. These icy bodies release debris as they approach the Sun, creating the meteoroid streams that intersect Earth’s orbit.

Asteroidal meteoroids, on the other hand, stem from the rocky or metallic remnants of asteroids—smaller celestial bodies that orbit the Sun. Collisions between asteroids or the disruption of their surfaces due to various factors can lead to the creation of meteoroids.

VELOCITY

Meteoroids, as they traverse the expanses of space, exhibit astonishing velocities that set them apart in the cosmic ballet. With minimum speeds exceeding 11 kilometers per second (25,000 miles per hour), meteoroids travel at velocities several times faster than a bullet exiting a gun barrel. This remarkable speed is a result of their trajectories through the solar system and their gravitational interactions with celestial bodies.

As meteoroids plunge into Earth’s atmosphere, this extraordinary velocity becomes a key factor in their transformation into meteors or shooting stars. The intense friction generated by their high-speed entry causes the meteoroids to heat up, leading to the captivating luminous displays we witness in the night sky.

COLOUR AND BRIGHTNESS

Meteoroids, as they streak through Earth‘s atmosphere and transform into meteors, often exhibit captivating displays of colour and brightness. The diverse hues and luminosity arise from the interaction between the meteoroid and the atmospheric conditions during its high-speed entry.

COLOUR

The colours of meteors are influenced by the composition of the meteoroid. Different elements present in the meteoroid release distinct colours as they heat up. For example, ionized oxygen produces green and red colours, while calcium can contribute to violet hues. The combination of these elements in the meteoroid determines the vibrant palette seen during a meteor shower.

BRIGHTNESS

The brightness of meteors varies widely, ranging from faint streaks to dazzling fireballs. The size, speed, and composition of the meteoroid all play roles in determining its brightness. Larger meteoroids or those with higher speeds often produce more intense and brighter displays. Some meteors can briefly outshine even the brightest stars in the night sky.

Observing the colour and brightness of meteoroids provides astronomers with valuable insights into their composition and the physical processes occurring during their rapid entry into the Earth’s atmosphere.

DIVERSITY IN ORBITS

Meteoroids, the celestial nomads of our solar system, exhibit a remarkable diversity in their orbits, contributing to the dynamic nature of these cosmic wanderers. Their paths through space are influenced by gravitational interactions with various celestial bodies and their diverse origins.

COMETARY AND ASTEROIDAL ORBITS

Meteoroids originating from comets often follow elongated and eccentric orbits, bringing them from the outer reaches of the solar system. Those from asteroids may have more stable and circular orbits, resembling the paths of their parent bodies.

SHORT-PERIOD AND LONG-PERIOD ORBITS

Some meteoroids have short-period orbits, associated with comets that frequently pass through the inner solar system. Others have long-period orbits, indicating less frequent encounters with the inner planets.

EARTH-CROSSING ORBITS

A significant portion of meteoroids has orbits that intersect with Earth‘s path around the Sun, leading to potential encounters and atmospheric entries.

METEORS

Meteors, often colloquially referred to as “shooting stars” or “falling stars,” are celestial phenomena that captivate observers as they streak across the night sky. These luminous streaks are created when meteoroids, small rocky or metallic fragments in outer space, enter Earth’s atmosphere at high speeds. As meteoroids hurtle through space, they can reach speeds exceeding 11 kilometers per second (25,000 miles per hour) or more. This high velocity is a result of their trajectories and gravitational interactions with celestial bodies. When a meteoroid enters Earth‘s atmosphere, the intense friction generated by its rapid motion causes the air around it to heat up, leading to the brilliant display we observe.

The appearance of a meteor is a dazzling spectacle, lasting for a brief moment up to several seconds. The luminosity of a meteor is influenced by various factors, including the size and composition of the entering meteoroid. Larger and more massive meteoroids tend to produce brighter and more conspicuous meteors, sometimes even casting shadows on the ground.

Meteors exhibit a range of colours, adding to their mystique. The hues are determined by the composition of the meteoroid, as different elements emit specific colours when heated. For instance, ionized oxygen can create green and red colours, while calcium may contribute violet hues. The varying colours contribute to the enchanting and dynamic nature of meteor showers, where multiple meteors can be observed in a short period.

While it may appear that a meteor is only a few thousand feet away from Earth, in reality, meteors commonly occur in the mesosphere, situated at altitudes ranging from 76 to 100 km (250,000 to 330,000 ft). Typically, meteors undergo disintegration within altitudes ranging from 50 to 95 km (160,000 to 310,000 ft). There is approximately a fifty percent probability of meteors colliding with Earth during daylight hours or near daylight. Nevertheless, the majority of meteor observations occur during the night, taking advantage of the darkness to discern fainter celestial objects.

FIREBALL

A fireball, a dazzling type of meteor, graces the night sky with an intense luminosity that sets it apart from the ordinary celestial display, that becomes visible when about 100 km from sea level. Often referred to as a “bolide,” a fireball is characterized by its exceptional brightness and can outshine the full Moon for a brief period. These spectacular phenomena are born from meteoroids that enter Earth’s atmosphere at high speeds, igniting an exhilarating spectacle for observers on the ground.

What sets a fireball apart from a standard meteor is its extraordinary luminosity. Unlike regular meteors, which generate a streak of light as they disintegrate in the atmosphere, a fireball exhibits exceptional brightness, casting a vibrant and sometimes even multicolored illumination. This heightened luminosity is commonly attributed to the larger size of the entering meteoroid or its greater velocity, both factors contributing to heightened frictional heating during atmospheric entry. If observed at the zenith, a fireball would register a magnitude of −3 or brighter.

For instance, a meteor with a magnitude of −1 positioned 5 degrees above the horizon would be categorized as a fireball. This classification stems from the acknowledgment that, if the observer were directly beneath the meteor, it would have presented itself as a magnitude −6 event. This distinction in brightness and classification highlights the unique and striking nature of fireballs in the realm of celestial phenomena.

The origin of fireballs lies in the composition and characteristics of the meteoroid. Typically, fireball-producing meteoroids are larger and more massive than those that result in fainter meteors. These celestial travellers can range in size from a few meters to tens of meters in diameter, making them substantial enough to survive the fiery passage through Earth’s atmosphere and potentially reach the surface as meteorites.

While fireballs can occur at any time, they are more commonly observed during meteor showers. Meteor showers, linked to the debris trails left by comets, provide an increased likelihood of encountering fireballs as Earth passes through these concentrated regions of cosmic material. Notable fireball events may also occur sporadically, capturing global attention and sparking scientific interest.

METEOR SHOWER

The occurrence of a meteor shower arises from the interaction between a celestial body, like Earth, and streams of debris originating from a comet or alternative sources. The periodic passage of Earth through cosmic debris, emitted by comets and various other sources, is a recurring phenomenon in numerous instances. In infrequent instances, these showers can be incredibly dramatic, featuring thousands of falling meteoroids per hour. Conversely, the typical hourly rate of around 5 observed meteors more commonly escalates to approximately 10–50.

Meteor showers are named after the constellation from which the meteors appear to radiate, known as the radiant point. Notable meteor showers, such as the Perseids and Geminids, occur annually and have been observed for centuries. The Perseids, for example, peak in August and are associated with the comet Swift-Tuttle. During their peak, observers may witness dozens of meteors per hour emanating from the constellation Perseus.

EFFECT ON ATMOSPHERE

The entry of meteoroids into Earth’s atmosphere has several notable effects on the atmosphere, creating dynamic and transient phenomena.

IONIZATION OF ATMOSPHERIC GASES

The extreme temperatures generated during entry can ionize the surrounding atmospheric gases. This ionization creates a brief trail of ionized gas behind the meteoroid, contributing to the visible luminosity of the meteor.

SHOCK WAVES

Larger and more massive meteoroids can create shock waves as they move through the atmosphere at supersonic speeds. These shock waves can result in sonic booms, audible on the ground, and are known as “bolides” when associated with particularly bright fireballs.

RELEASE OF MATERIALS

Meteoroids may release materials into the atmosphere as they disintegrate. This can include dust, metal particles, and, in some cases, larger fragments that may reach the Earth‘s surface as meteorites.

CHEMICAL REACTIONS

The high-energy processes involved in meteoroid entry can lead to chemical reactions in the upper atmosphere. For example, the ionization of nitrogen and oxygen can result in the production of nitrogen oxides.

AFFECTING ATMOSPHERIC DENSITY

The presence of meteoroids in the upper atmosphere, even briefly, can influence the local density and temperature conditions. This, in turn, can have subtle effects on the behaviour of radio waves and communication signals passing through that region.

METEORITES

Meteorites are remnants of celestial bodies that survive their journey through Earth’s atmosphere and reach the planet’s surface. Some meteorites are fragments from collisions between asteroids, while others are remnants of comets or other celestial bodies. The journey of a meteoroid begins as it enters Earth’s atmosphere, where it is called a meteor. If it survives the atmospheric entry and lands on Earth, it becomes a meteorite.

Meteorites are composed of a variety of materials, including rock, metal, and sometimes organic compounds. Their composition reflects the diverse nature of their parent bodies, which could be asteroids, comets, or other objects in space.

TYPES

There are three main types of meteorites based on their composition:

• Stony Meteorites: Primarily composed of silicate minerals, these are the most common type.
• Iron Meteorites: Composed mostly of iron and nickel, often displaying a distinctive metallic appearance.
• Stony-Iron Meteorites: Comprising a combination of silicate minerals and metallic elements.