Asteroids
Asteroids, also called minor planets or planetoids, are rocky, airless remnants left over from the formation of the solar system about 4.6 billion years ago. They are thought to be leftover building blocks of planets that never coalesced due to the strong gravitational influence of Jupiter.
Location and Distribution
- Most asteroids orbit the Sun between Mars and Jupiter in a region known as the Main Asteroid Belt.
- A few are found in other locations:
- Trojans: Share an orbit with a larger planet (notably Jupiter).
- Near-Earth Asteroids (NEAs): Cross or come close to Earth’s orbit.
Size and Mass
- Asteroids vary greatly in size. The largest is Vesta, with a diameter of about 530 km (329 miles).
- Many are much smaller, less than 10 meters across.
- The total combined mass of all known asteroids is less than the mass of Earth’s Moon.
Characteristics
- Asteroids are composed mainly of rock and metals.
- They do not have atmospheres.
- Their irregular shapes are due to their small size and weak gravity.
- Classified into three broad types:
- C-type (Carbonaceous): Rich in carbon, very dark, most common (~75%).
- S-type (Silicaceous): Contain silicate materials and nickel-iron (~17%).
- M-type (Metallic): Composed largely of nickel and iron.
Comets are often described as “cosmic snowballs” made of frozen gases, rock, and dust. They are relics from the early solar system, preserving its primitive material. Unlike asteroids (rocky), comets are icy bodies that become active when they approach the Sun.
Structure of a Comet
- Nucleus – The solid, central core made of ice, dust, and rocky material. When frozen, the nucleus may be only a few kilometers across (comparable to a small town).
- Coma – When a comet nears the Sun, solar radiation causes the nucleus to heat up, releasing gas and dust that form a glowing envelope around it. The coma can be larger than most planets.
- Tails –
- Ion tail: Made of ionized gases, always points directly away from the Sun due to solar wind.
- Dust tail: Made of small solid particles, appears curved and lingers in the comet’s orbit.
- Both tails can extend for millions of kilometers.
Origin and Reservoirs
- Kuiper Belt: A region beyond Neptune containing icy bodies; source of short-period comets (orbiting the Sun in <200 years).
- Oort Cloud: A spherical shell of icy objects far beyond the Kuiper Belt; thought to be the source of long-period comets (orbit >200 years).
- Scientists estimate there may be billions of comets in the Kuiper Belt and trillions in the Oort Cloud.
Examples of Famous Comets
- Halley’s Comet: Most famous short-period comet (period ~76 years); last appeared in 1986, will return in 2061.
- Comet Hale-Bopp: Extremely bright, visible to the naked eye for 18 months (1996–1997).
- Comet Shoemaker-Levy 9: Broke apart and collided with Jupiter in 1994.
- Comet 67P/Churyumov–Gerasimenko: Studied by ESA’s Rosetta Mission.
