Astronomy - Comets and Asteroids

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτηςalready meant "long-haired star, comet" in Greek and was used to mean "the tail of a comet".

The astronomical symbol for comets is ☄, consisting of a small disc with three hair like extensions.

Comets have been observed and recorded since ancient times by many cultures.

From ancient sources, such as Chinese oracle bones, it is known that their appearances have been noticed by humans for millennia.

Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.

Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances.

Bayer Tapestry Halleys Comet

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.

Great Comet 1680

In 1705, Edmond Halley (1656–1742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements.

Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9.

When the comet returned as predicted, it became known as Halley's Comet. It will next appear in 2061.

A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail.

These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across.

Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Lawrence Whipple's (Professor of Astronomy Harvard College) model.

However, some comets may have a higher dust content, leading them to be called "icy dirtballs".

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick.

In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids.

In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission.

In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years.

Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-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 nearest star.



Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide.

Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space.


The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily, and was originally considered to be a new planet.

This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions.

This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής, or asteroeidēs, meaning 'star-like, star-shaped', and derived from the Ancient Greek ἀστήρastēr'star, planet'.

In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably.

Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.

The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans).

However, other orbital families exist with significant populations, including the near-Earth objects.

Asteroid Belt

Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups:

C-type, M-type, and S-type.

C-type asteroids are carbonaceous asteroids their composition includes a large amount of carbon, in addition to rocks and minerals. They are the most common variety, forming around 75% of known asteroids

M-type asteroids are asteroids of partially known composition; they are moderately bright. Some, but not all, are made of nickel–iron, either pure or mixed with small amounts of stone.

S-type asteroids, or siliceous asteroids, are of a stony composition Approximately 17% of asteroids are of this type, making it the second most common after the C-type.

It is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt.

Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt.

Asteroids range in size from Vesta - the largest at about 329 miles (530 kilometres) in diameter  to bodies that are less than 33 feet (10 meters) across. 

The total mass of all the asteroids combined is less than that of Earth's Moon.

Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go.

More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

Several missions have flown by and observed asteroids. The Galileo spacecraft flew by asteroids Gasprain 1991 and Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission studied asteroids Mathilde and Eros; and the Rosetta mission encountered Steins in 2008 and Lutetia in 2010.

Deep Space 1 and Stardust both had close encounters with asteroids.

In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples. On June 3, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.

NASA's Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year. Once it left in September 2012, it headed towards dwarf planet Ceres, with a planned arrival of 2015.

Vesta and Ceres are two of the largest surviving protoplanet bodies that almost became planets. By studying them with the same complement of instruments on board the same spacecraft, scientists will be able to compare and contrast the different evolutionary path each object took to help understand the early solar system overall.