Comet 17P (Holmes)

This is a periodic comet with an orbital period of 6.9 years and a nucleus of about 3.4km. It was discovered in 1892 by the English amateur astronomer Edwin Holmes. Although normally a very faint object, in October 2007 it temporarily brightened by a factor of half a million going from an apparent magnitude of 17 to 2.8 in the space of 42 hours.

It also briefly became the largest object in the Solar system as the coma briefly expanded to a diameter greater than that of the Sun.

The comets discovery in 1892 also coincided with lesser period of brightening, where it reached magnitude 4 or 5, and then faded over a period of a few weeks.

Another outburst of magnitude 3-4 was observed in 2015. The cause of these events however is not definitely known, but may have been due to a build-up of gas in the nucleus.

For more info. see the Wikipedia entry.

Comets

Comets are small, but nonetheless significant visitors to the Solar system.

Typically they have a mass of about one-billionth × that of the Earth and their nucleus is just a few kilometres across. The nucleus is the core of the Comet and consists of a 'dirty snowball' of dust, water ice, carbon dioxide, ammonia and methane.

Comets are classified as either 'long-period' or 'short-period' depending on how often they approach Earth. Long-period comets appear at intervals of over 200 years and originate in the Oort Cloud, a spherical halo of icy bodies that lies about 40 AU's from the Sun. Short-period (or periodic) comets are thought to originate in the Kuiper Belt, a region of icy material that lies between the orbits of Neptune and Pluto. These are also sub-divided into two further categories, with Halley Type having periods of 20-200 years, and Jupiter Type with a period of less than 20 years.

As Comets approach the Sun, at about a ditance of about 3 AU's, they develop an extended region of gas and dust called the coma. When the Comet temperature reaches 215 K (-58° C) sublimation of water ice to vapour occurs. This material flows outward at approx 0.5 km/s and can extend 10⁵ to 10⁶ km in size. The coma then becomes visible from reflected sunlight, and may also emit radiation as the sunlight excites the gas.

If the Comet moves closer to the Sun, it may develop a large visible tail of up to 1 AU long. The tail actually consists of two tails. There is a gas tail or an ion tail which appears to point away from the Sun, and a dust tail which lies behind the Comet in the wake of it's path.

The gas tail (or ion tail) is due to the solar wind hitting the comet and contains a variery of molecular ions. Emission from CO+ is responsible for the blue colour of the tail which can extend 10⁸ km.

The dust tail which trails behind the Comet is from material that is ejected by radiation pressure. These continue to follow a Kepelerian orbit and the dust tail can also be 10⁷ km long.

Comets are brightest when they reach perihelion (their closest distance to the Sun), and they mostly traverse elliptical orbits, although their eccentricity and periodicity vary greatly.

Long-period comets are also of particular interest to scientists, as the Oort cloud is sufficiently far enough away to remain unaffected by heat from the Sun. They are therefore thought to be composed of original material from the formation of the solar nebula, whilst Short-period comets from the Kuiper belt provide information on the the formation of the outer Solar system.

For more info. see the Wikipedia entry on Comets.