A comet is an icy, small, Solar System body that, when passing close to the Sun, heats up and begins to outgas.
When they are in the inner solar system, comets have a visible atmosphere, and sometimes a tail.
When they have moved back into the outer solar system, they cool, and lose their tail and atmosphere.
Size and composition
Comet nuclei range from a few hundred metres to tens of kilometres across.
The coma and tail can be millions of times larger than the solid/nucleus.
Mostly made of water ice, with some small rocky particles.
Compounds found in comets by spectrographic analysis:
Organic: C, C2, C3, CH, CN, CO, CO2, CS, HCN, CH3CN, HCO, H2CO
Inorganic: H, NH, NH2, O, OH, H2O, S, S2, NH3, NH4
Metals: Na, K, Ca, V, Mn, Fe, Co, Ni, Cu
Ions: C+, CH+, CO+, CO2+, N2+, O+, OH+, H2O+, H3O+, S+, S2+, H2S+, CS2+
Dust: silicates, organic compounds.
Visiblity from Earth
If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope.
Comets have been observed and recorded since ancient times by many cultures.
Comets have a wide range of orbital periods, ranging from several years to several millions of years.
Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune.
Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star.
Long-period comets are directed towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide.
Number of comets
As of 2015 there are over 5,200 known comets, a number which is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion.
Exploration of comets by spacecraft
Comets have been visited by unmanned probes such as the European Space Agency’s Rosetta, which became the first ever to land a robotic spacecraft on a comet, and NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.
Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-earth objects.
10 Need-to-Know Things About Asteroids:
1. If all of the asteroids were combined into a ball, they would still be much smaller than Earth’s moon. If the sun were as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest object in the main asteroid belt) would be as small as a sesame seed.
2. Most Asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.
3. Days and years vary by asteroid. A day on asteroid Ida, for example, takes only 4.6 hours (the time it takes to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid’s time) in 4.8 Earth years.
4. Asteroids are solid, rocky and irregular bodies.
5. Asteroids do not have atmospheres.
6. More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.
7. One asteroid, named Chariklo, is known to have two dense and narrow rings.
8. More than 10 spacecraft have explored asteroids. NEAR Shoemaker even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta).
9. Asteroids cannot support life as we know it.
10. Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi) and the closest dwarf planet to the sun, encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.
Sizes and shapes of asteroids
This composite image shows the comparative sizes of eight asteroids. Up until now, Lutetia, with a diameter of 81 miles (130 kilometers), was the largest asteroid visited by a spacecraft, which occurred during a flyby.
Vesta, which is also considered a protoplanet because it’s a large body that almost became a planet, dwarfs all other small bodies in this image, with its diameter sizing up at approximately 330 miles (530 kilometers).
Image credit: NASA/JPL-Caltech/JAXA/ESA
Where are the asteroids found?
Main asteroid belt
At Lagrangian points around Mars, Jupiter – the Trojans and Greeks, and around Neptune
Many asteroids are also found much further out, in the Kuiper belt; most objects in the Kuiper belt are believed to be more comet-like.