Comets: The planetary architects

One wiped out the dinosaurs; others have formed some of the world’s lakes; and the Moon is thought to have formed following a collision between the Earth and a body the size of Mars: comets are much more than lumps of dirty ice journeying through the Universe…

It’s true that comets – when visible from Earth – can be truly spectacular. Take C/2006 P1, perhaps better known as McNaught; it was the first ‘great comet’ of the 21^st Century and was the brightest seen for over 40 years. Comet ISON had a lot to live up to; if McNaught was great, then ISON – billed as the Comet of the Century – had to be spectacular.

Initial indications suggested that the comet was living up to expectations; as it approached the Sun, the tail of the 4.6 billion year old comet was clearly visible. However, it was yet to make its journey through the corona – reaching temperatures of between 1 to 3 million degrees – to slingshot around our star, and emerge unscathed the other side. This never happened; although a fragment of the comet was thought to have survived the scorching temperatures and enormous gravitational pull of the sun, ISON was declared dead in early December.

[caption id="attachment_36644" align="alignright" width="200"] Comet_ISON on 8th October 2013 Credit: Ngc1535/wikipedia[/caption]

During its brief history – brief in terms of how long astronomers were able to study it, as in fact it likely originated in the early Solar System – Comet ISON had the potential to teach scientists about conditions that gave birth to the planetary system we call home. Exactly what secrets ISON was harbouring are yet to be revealed, but it could provide evidence of how the planets formed – and maybe even if life might be found on other planets.

ISON was a sungrazing comet, a classification held by the Great Comet of 1680; the first to be discovered by telescope, and one of the brightest seen in the 17^th Century. Around 83% of sungrazing comets – whose orbits take them incredibly close to the sun at perihelion – come from the Kreutz family, a much larger comet which broke apart several centuries ago. The remaining 17% come from sporadic sungrazers, some of whom are part of three other related groups.

Perhaps the most recognised comet is Halley’s Comet, which last made an appearance in the inner Solar System in 1986. Not only is it the most well-known, it was the first to be observed in detail by spacecraft. During this mission, the comet confirmed many longstanding ideas including the dirty snowball model which predicted a mixture of volatile ices – water, carbon dioxide and ammonia – and lots of dust.

July 1995 saw Comet Hale-Bopp return to our skies and it became one of the most observed comets of the 20^th Century – thanks in part to the World Wide Web. It became the first to be tracked on the internet, with websites posting daily images from around the world. It also doubled the previous record for the longest period of naked eye visibility and was seen slowly making its way across the sky – even in light-polluted cities – for a staggering 18 months.

[caption id="attachment_36645" align="alignleft" width="200"] Comet McNaught in January 2007 Credit: Fir0002/Flagstaffotos[/caption]

It didn’t stop there though; it’s also the farthest comet from the Sun discovered by amateurs, has the largest well-measured nucleus known, and was brighter than magnitude 0 – a measure of brightness – for eight weeks, longer than any other recorded comet.

Astronomers believe that Hale-Bopp has a 15% chance of becoming a sungrazer. As they journey through the Solar System, comets are subjected to the gravitational pull of neighbouring planets which can nudge them from their original trajectory, as happened with ISON. In 1996, Comet Hale-Bopp passed within 0.77 Astronomical Units (AU) of Jupiter, which altered its path and shortened its orbit. Over many orbits spanning several thousands of years, perihelion distance is likely to decrease to very small values before it eventually journeys into the Sun.

These nudges have in the past, sent comets hurtling towards the Earth; the most famous probably being the one that wiped out the dinosaurs 65 million years ago. As asteroid was originally thought to have caused the Earth-altering impact, but in 2013 American scientists suggested it was actually a 10km wide comet which collided with the Earth causing a series of knock-on effects. The comet delivered an estimated energy equivalent to 100 teratons of TNT, and caused megatsunamis, clouds of superheated dust and steam, and huge shockwaves triggering global earthquakes and volcanoes. The resulting crater – partly submerged in the ocean – is a massive 180km in diameter.

[caption id="attachment_36646" align="alignright" width="200"] Comet Hale-Bopp. Credit Vesta/Wikipedia[/caption]

During the course of the Earth’s history, it has been bombarded with comets, and there are many similar but smaller craters confirmed around the world. In fact, many of Canada’s lakes – Deep Bay, Lake Manicouagan, Clearwater Lakes – are impact craters that have filled up with water.

But Earth isn’t the only planet in the danger zone; there are seven other planets in the Solar System that can block its path. In 1994, 20 fragments of Comet Shoemaker-Levy 9 collided with Jupiter – the first direct observation of an extra-terrestrial collision of Solar System bodies. The impact created atmospheric plumes thousands of kilometres high, with hot bubbles of gas and huge scars sweeping across the Jovian sky. Since then several other objects have been observed colliding with Jupiter and astronomers believe that the planet could be hit between 12 and 60 times a year.

And of course, our Moon – our shining beacon in the night sky – is thought to have originated from debris from a collision between the Earth and a body the size of Mars around 4.5 billion years ago, providing a constant reminder of the brutal – yet beautiful – effect a comet could have.

To some comets might just be dirty snowballs travelling at ridiculous speeds through the Universe, but to others they are the astronomical architects which have shaped not only our planet, but the whole solar system.

Naming of comets

Comets are named after the year they were discovered, followed by a letter indicating the half-month of the discovery and a number indicating the order of discovery. Prefixes are added to indicate the nature of the comet:

• P/ indicates a periodic comet (defined for these purposes as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage).
• C/ indicates a non-periodic comet (defined as any comet that is not periodic according to the preceding definition).
• X/ indicates a comet for which no reliable orbit could be calculated (generally, historical comets).
• D/ indicates a periodic comet that has disappeared, broken up, or been lost.
• A/ indicates an object that was mistakenly identified as a comet, but is actually a minor planet.

So for Comet ISON, its designated name - C/2012 S1 - indicates it is a non-periodic comet, the first discovered in the second half of September 2012 (S1).

Comet or asteroid - what's the difference?

Asteroid	A relatively small, inactive, rocky body orbiting the Sun.
Comet	A relatively small, at times active, object whose ices can vaporize in sunlight forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust and/or gas.
Meteroid	A small particle from a comet or asteroid orbiting the Sun.
Meteor	The light phenomena which results when a meteoroid enters the Earth's atmosphere and vaporizes; a shooting star.
Meteorite	A meteoroid that survives its passage through the Earth's atmosphere and lands upon the Earth's surface.