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Saturday, May 9, 2020

Sunskirters, Sundivers and Sungrazers Are Not Cocktails!: Fundamental Facts About Comets


In a previous post, I presented some information and stamps mostly about Halley's comet. If you are interested in some of the science around comets in general (peppered with some stamps on the subject) this post might be for you.

What is a Comet?

In short, a comet is a cosmic snowball of frozen gases, rock and dust that orbits the sun. A comet goes around the sun in an highly elliptical orbit and can spend hundreds and thousands of years in the outer depths of the solar system before it returns to the Sun at its perihelion (the point in an orbit when a comet or planet is closest to the Sun). As the comet approaches the Sun, the warming of its surface causes its materials to melt and vaporize in a process called "outgassing". The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form. This tail points away from the Sun and even after a comet has passed the Sun, it actually travels tail first! This phenomenon was first noted in the 1530s by Petrus Apianus (Peter Apian), a German mathematician and astronomer. To provide an idea of scale, the tail of a comet can be as long as the distance between the Earth and the Sun (which is about 93 million miles).

This stamp from the Bermuda, a British Territory and an island located in the North Atlantic Ocean honours the discovery of Peter Apian.

A stamp from Grenada Grenadines, showing the 'outgassing' of a comet. Grenada is an island nation member of the British Commonwealth comprising the island of of Grenada and the six smaller islands located at the southern end of the Grenadines in the Southeastern Caribbean Sea. It is reported that Grenada ranks among the countries that have issued the most stamps in the world. Stamps inscribed with the words 'Grenada Grenadines' were issued from 1973 to 1999 for the islands of the Grenadines located to the north of Grenada. Since 1999, stamps for the Grenadines are marked 'Grenada Carriacou & Petite Martinique'.

Kepler's Laws

Like all orbiting bodies, comets follow Kepler’s Laws; i.e. the closer they are to the Sun, the faster they move.

Johannes Kepler, a German astronomer formulated the three laws of planetary motion:

+ Law of Orbits: planets move in elliptical paths with the Sun at one focus;
+ Law of Areas: a line that connects a planet to the Sun sweeps out equal areas in equal times; and
+ Law of Periods: The square of the period of any planet is proportional to the cube of the semi-major axis of its orbit.

Kepler's laws have an important place in the history of astronomy as they mark a key step in the revolution which moved the center of the solar system (and indeed the Universe as it was then thought) from the Earth (geocentric) to the Sun (heliocentric). These laws were published by Kepler between 1609 and 1619.


A stamp honouring Johannes Kepler from the Republic of Sierra Leone. The Republic of Sierra Leone is a country situated on the southwest coast of West Africa. It was a British Colony until 1961.

And, what is the difference between an asteroid and a comet?

The main difference between asteroids and comets is their composition, as in, what they are made of and where there were formed. Asteroids are made up of metals and rocky material, while comets are made up of ice, dust and rocky material. Both asteroids and comets were formed early in the history of the solar system about 4.5 billion years ago, but asteroids formed much closer to the Sun, where it was too warm for ices to remain solid. Comets, on the other hand, formed farther away from the Sun where ices would not melt. Comets which approach the Sun lose material with each orbit because some of their ice melts and vaporizes to form a tail.

Where do comets come from?

Comets are believed to have two sources. Long-period comets (those which take more than 200 years to complete an orbit around the Sun) originate from the Oort Cloud. Short-period comets (those which take less than 200 years to complete an orbit around the Sun) originate from the Kuiper Belt.

Any 'Breaking News' on comets?

In September 2019, Astronomy Magazine reported that astronomers had discovered an orbital region just beyond Jupiter that appears to act as a kind of gateway for some objects entering the inner solar system from the Kuiper Belt. Their discovery could offer a solution to a puzzle that has long confused astronomers: How does a certain class of objects from the Kuiper Belt become comets?Many comets take a straightforward path into the solar system’s inner regions. They come from the Kuiper Belt or the Oort Cloud – regions beyond Neptune containing millions of icy leftovers from the solar system’s beginning. Gravitational nudges send them plunging inward until they swing around the Sun and out again.The puzzle comes from space rocks called Centaurs. First discovered in 1977, these objects from the Kuiper Belt have been gravitationally jostled into unstable orbits between Jupiter and Neptune. Close encounters with one of the giant planets can send a Centaur back to the Kuiper Belt or out to interstellar space, or send it deeper into the inner solar system. As Centaurs come closer to the Sun they begin to develop comas, telltale signs of cometary activity.

Sir Isaac Newton and the discovery of Halley's comet

The stamp below was issued by Ascension Island during the 1985-86 return of Halley's comet. It shows the images of Newton's Reflector Telescope and the comet in the background, indicating a link between the work of Halley and Newton. Indeed there is a link which was best described  in October 1985 when the New York Times reported the following;"it was Halley who encouraged the retiring Newton to write down the principles of gravitation he had developed after years of thought supposedly inspired by the legendary falling apple. Newton recognized that gravity on Earth represented the same law of force as that affecting the motion of planets around the Sun. Halley edited the manuscript and arranged financing for publication in 1687 of Newton's great book, ''The Mathematical Principles of Natural Philosophy.''

Halley's encouragement of Newton was perhaps his greatest contribution to science, according to Alan Cook, a professor of natural philosophy at Cambridge University in England. But in applying Newton's laws of gravitation in making his comet prediction, Halley went an important step further. His correct prediction turned out to be the first direct confirmation of Newton's theories."

Ascension Island is a British Overseas Territory and is an isolated island located  just south of the Equator in the South Atlantic Ocean.

What is special about a comet?

Based on observations from spectroscopy and findings of the Miller-Urey experiments, comets contain all the substances and conditions thought to be necessary for the synthesis of living organisms on Earth. This has led to the hypothesis that life may have originated elsewhere and hitched a ride on a comet to Earth!

How many comets are there?

As of July 2019, Wikipedia reports that there are 6,619 known comets but this number is increasing as they are discovered. Note that it is estimated that more than a trillion potential comets could reside in the Oort Cloud. For those with a deeper level of interest, the Gaia Space Telescope has identified a number of stars in the Milky Way that could cause more frequent comet activity. In 2018, it was announced that a star called Gliese 710 could actually barge into the Oort Cloud itself in 1.3 million years. This would likely dislodge many of the icy rocks in the Oort Cloud and send some of them into cometary orbits around the Sun.

The largest comet

There does not seem to be a consensus on this point as astronomers have yet to decide how the largest comet should be measured! As an example, comet Hyakutake's tail measured 500 million km from its nucleus. Thus it is the longest known tail on a comet. The Hale-Bopp comet had a nucleus of more than 60 miles in diameter. This is the biggest nucleus ever measured. Recently, a new definition has been proposed; i.e. the region of space disturbed by the presence of the comet. In this regard, comet McNaught wins the title of the largest comet ("space disturbed" being a function of the outgassing of a comet).

Near-Sun comets

The orbital paths of some comets take them very close to the Sun and hence they are exposed to conditions that are extreme. Researchers of these type of comets have termed them "near-Sun comets".  Within this broad categorization, there are "sunskirters, sungrazers and sundivers". The orbital path of a sundiver intersects the solar photosphere so they have little chance of surviving their perihelion passage. The sunskirter on the other hand, experiences less violent conditions with a perihelion passage that passes between 3.45 solar radii and 33 solar radii of the Sun's center and depending on its size and physical properties, has a chance of surviving. The sungrazer has a perihelion passage that takes it inside 3.45 solar radii of the Sun's center.  Small sungrazers can completely evaporate during such a close approach to the Sun whilst larger sungrazers can survive many perihelion passages. However, the strong evaporation and tidal forces they experience often lead to their fragmentation. Until the 1880s, it was thought that all bright comets near the Sun were the repeated return of a single sungrazing comet.Then, German astronomer Heinrich Kreutz and American astronomer Daniel Kirkwood determined that, instead of the return of the same comet, each apparition was a different comet, but each was related to a group of comets that had been separated from each other at an earlier passage near the Sun (at perihelion).

When does a comet die?

Once a comet has outgassed or exhausted all available volatile substances, its coma and tail will disappear and the remaining inert nucleus will take on the characteristics of a low albedo asteroid.  Sometimes, the comet completely disintegrates (as in the case of a sundiver or a sungrazer mentioned in the previous section). Although comets seem long-lived from a human perspective, on an astronomical time scale, they evaporate quite rapidly. Bearing in mind that a typical comet nuclei may range in size from a small mountain to a large city, it is not difficult to understand why it takes many years for all that ice to evaporate! The stamp presented below commemorates the recent demise of a comet that was observed by many astronomers on Earth.

Comet Shoemaker-Levy

The most spectacular demise of a comet in recent years occurred in July 1994. Comet Shoemaker-Levy was discovered on 24 March 1993. It had been captured by the gravitational effects of Jupiter (probably 20 or 30 years earlier) and Jupiter's tidal forces had acted to tear the comet apart. The comet disintegrated into a number of fragments, one piece up to 2 kilometers in diameter and approximately 21 fragments collided with Jupiter's southern hemisphere between 16 - 22 July 1994.  The scars from the multiple collisions remained visible on Jupiter's gaseous surface for many months afterwards. The comet was closely observed by astronomers worldwide and this event also provided the first direct observation of an extraterrestrial collision of objects in the Solar System. The stamp below, part of a series of five on a First Day Cover, was issued by Germany in 1999 and shows the collision of the comet with the planet.

All stamps presented in this post are from my personal collection and were photographed using a HUAWEI P30 telephone / camera. 

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