> [!key-idea] Key Differences between the following... > - A [[#planetesimal]] is a larger body composed of dust, ice, and rock that later forms planets. > - An [[#asteroid]] has a rockier and more metallic composition. > - A [[#comet]] has an icier composition. > - A [[#meteoroid]] is a smaller piece of a asteroid or comet. > - A [[#meteor]] is a meteoroid that enters a planet's atmosphere. > - A [[#meteorite]] is a meteor that reaches the planet's surface and doesn't burn up in the atmosphere. ## Planetesimals A primitive body composed of loose conglomerates of materials (dust, ice, gas) orbiting in a protoplanetary disk. They eventually grow into kilometre-scale bodies ($\sim 10 \, {\rm km}$) that will combine together to form of the foundation for planetary bodies. ## Asteroid A small, rocky or metallic object ([[#planetesimals|planetesmial]]) that has survived planetary accretion. - They tend to be round and/or elongated. - They can have satellites (smaller gravitationally bound objects). - They are parent bodies of [[#Meteoriod|meteoroids]]. When seen in a telescope, asteroids appear as a points of light. Most are found in a ring between the orbit of Mars and Jupiter called the asteroid belt. > [!info] Types of Asteroids > - *"NEA"* - Near-Earth Asteroid > - *"C-type"* - An asteroid made of material that has largely been unmodified since the formation of the Solar System; the most primitive type of asteroid. > - *"S-type"* - An asteroid made of material that has been modified from its original state, likely as the outer part of a larger, differentiated body that has since broken into pieces. > - *"M-type"* - An asteroid that was once part of the metallic core of a larger, differentiated body that has since been broken into pieces; made mostly of iron and nickel. > [!image] Known Asteroid Orbits > ![[asteroid_orbits.png]] ## Comet A more complex object consisting of dust, rock, and ices; however, it has a significantly large composition of ice than an [[#asteroid]]. - Has small, solid, icy nucleus. - When a comet gets close to a star, its ice and dust content start to vaporize, creating an atmospheric halo and a tail of gas and dust. - When frozen, the solid spans a few miles to tens of miles in size. - When heated, the head of the comet can be larger than a planet and the tail stretches for millions of miles. - The period of an orbit is considered *short* if $P < 200 \, {\rm yr}$. Similarly, the period is *long* if $P > 200 \, {\rm yr}$. When seen in a telescope, a comet appears fuzzy and/or has a tail. > [!image] Anatomy of a Comet > ![[anatomy-of-a-comet.jpg]] > The nucleus of the comet is also known as a *dirty snowball.* ## Meteoroid A small piece of interplanetary debris, usually a piece of [[#asteroid]] or a [[#comet]]. - Typically pebble-sized, but can be larger ($100 \, {\rm \mu m} - 100 \, {\rm m}$). - Often created from a collision. - When a meteoroid enters a planet's atmosphere, it becomes a [[#meteor]]. - If the [[#meteor]] doesn't burn up in the atmosphere and it lands on the surface, it becomes a [[#Meteorite]]. ## Meteor A [[#meteoroid]] that has entered a planet's atmosphere at a very high speed and leaves an incandescent trail. - It is likely to burn up in the atmosphere and produces a streak of light. These are commonly observed as *shooting stars*. > [!info] Types of Meteors > - *"sporadic meteor"* - A meteor that is not associated with a specific meteor shower. > - *"bolide"* - A very bright, exploding meteor. > - *"meteor shower"* - A larger-than-normal display of meteors, occurring when Earth passes through the orbit of a disintegrating comet, sweeping up its debris. (examples: Perseids, Leonids) > - *"radiant"* - The direction in the sky from which the meteors in a meteor shower seem to come. ## Meteorite A [[#meteor]] that survives the atmospheric burning and lands on the planet's surface. > [!info] Categories of Meteorites > 1. *"stony meteorite"* - A meteorite composed primarily of silicate minerals, similar to those found on Earth. > - *"chondrite"* - A stony meteorite containing chondrules > - *"achondrite"* - A stony meteorite that does not contain chondrules. > 2. *"iron meteorite"* - A metallic meteorite composed mostly of iron-nickel alloys. > 3. *"stony-iron meteorite"* - A meteorite consisting of a mixture of silicate minerals and iron-nickel alloys. > [!note] > A *"chondrule"* is a small, crystallized, spherical inclusions of rapidly cooled molten droplets found in some meteorites. ## Trans-Neptunian Object (TNO) A trans-Neptunian object (TNO) is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of $30.1 \, {\rm AU}$.