Comets

Comets have been known from the most remote antiquity the sporadic appearance of strange celestial bodies that crossed the beautiful night skies of those times.

Initially they appeared as a luminous point that moved in the middle of the stars.

Then, after a few nights, that point began to develop a tail, which sometimes extended considerably and with a variety of colors that gave them a fantastic appearance.

The Greeks called them “comets”; In Greek, kome means “hair.” Aristotle popularized the name “kometes” to refer to these “stars with hair.”

Their origins are in the outer solar system, and they are intensely affected by the approaches to major planets; Therefore, some are transferred to orbits very close to the Sun that destroys them when they approach, while others are sent forever outside the solar system.

Comet 1577
The Great Comet of 1577, seen over Prague on November 12.  Credit: Wikipedia. Engraving made by Jiri Daschitzky. Web: “en.wikipedia.org/wiki/Great_Comet_of_1577”.

On November 13, 1577, the appearance of a huge comet caused a great sensation.

Then the great astronomer Tycho Brahe, with his instruments, followed its course and calculated its distance from Earth.

He concluded that it was approximately 230 Earth radii, that is, beyond the Moon, which is 60 Earth radii.

The best known comet is Halley’s Comet. Available documents certify that this comet was already observed in the year 239 BC.

Later, the German astronomer Regiomontano, recorded its appearance in the year 1472.

In the year 1531, a large comet was seen again and the astronomer Petrus Apianus wrote down the characteristics that he could perceive.

In 1607, from Prague, Johannes Kepler also saw a large comet and left notes of what he observed.

Halley carefully studied the orbit of this planet

In 1705, the comet made a new appearance. On this occasion, Edmund Halley carefully studied its path and verified that the characteristics of the comet coincided with those described in 1472, 1531 and 1607.

Edmond Halley
Edmond Halley was a British astronomer. Credit: web “wikitree.com/photo/jpg/Halley-148-2”.

Halley concluded that they corresponded to the same celestial object, which described an elliptical orbit around the Sun and returned every 76 years. He predicted its reappearance for the year 1757.

Halley died in 1742 and could not see the return of his comet, which was not seen until December 25, 1758.

The gravitational pull of Jupiter and Saturn was the cause of the delay.

Although Halley did not discover it, this beautiful comet is named after him.

Halley 1986
The passage of comet Halley in 1986. Credit:  web “nostalgia80.com/el-paso-del-cometa-halley-1986/”

What exactly is a comet

In the mid-20th century, the scientific community accepted the idea that a comet is a rocky nucleus mixed with ice and gases.

In colloquial terminology, a comet is said to be a dirty snowball.

In 1986, as Halley’s Comet poked its head over the horizon, five space probes set out to meet it: 2 from the USSR, 2 from Japan, and 1 from Europe.

They flew through the luminous cloud of particles that surrounded the frozen nucleus and checked various data that could also correspond to other comets.

Comets are solid bodies made up of water, dry ice, ammonia, methane, iron, magnesium, and silicates.

Due to the low temperatures of the places where they are, these substances are frozen. In colloquial terminology, a comet is said to be a dirty snowball that reaches a diameter of a few tens of kilometers.

Photons from the solar wind cause the core and envelope to heat up and generate gases that project backward, causing a tail to form that points away from the Sun and extends for millions of kilometers.

This jet of gas is expelled violently and at a great distance. One of the Soviet probes that was 100,000 km from the comet was damaged.

It is estimated that a comet could pass around 2,000 times the Sun before its nucleus is completely exhausted.

The comet 2P Encke

Comet 2P / Encke’s history began on January 17, 1786, when Pierre Méchain, from Paris, sighted it in the vicinity of the star Beta Aquarii.

He immediately communicated this discovery to Charles Messier and to Jacques Cassini son of Giovani Cassini, who also saw it a few days later. It had a shiny core, but it lacked a tail.

Years later, on November 17, 1795, Caroline Herschel discovered, near the star Gamma Cigni, a celestial body barely visible to the naked eye, without a nucleus, but with a slight central condensation of light.

Comet 2 P Encke
Comet 2P / Encke was the second comet to have a periodic orbit discovered. Credit: NASA. web “solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/2p-encke/in-depth/”

Encke’s elliptical path runs from the outskirts of the Martian orbit well into the orbit of Mercury.

It has a period of 3.3 years, the shortest of the known comets and, therefore, the comet that has the highest number of recorded appearances and is one of the best-studied comets.

Its return in 2003 made the 59th of the documented appearances since it was officially discovered in 1786. It is one of the best-studied comets.

It currently has a very faint brightness that barely reaches magnitude 7, so it is not visible to the naked eye and it is almost impossible to distinguish it from an asteroid.

Thousands of years ago it was a bright comet, but because the ice on its surface sublimates all the way around the Sun, it suffers a huge loss of matter. It is estimated that by 2050 it will have exhausted all its ice and will become an asteroid.

Comet encke 2003
Comet Encke’s diffuse spot is seen, captured by a small telescope on November 29, 2003, in Arkansas, USA. Author: Michael Holloway. Web: “apod.nasa.gov/apod/ap031223.html”

That same star was observed during the month of November by the German astronomer Johann Bode, by the Frenchman Alexis Bouvard and by the German amateur astronomer Wilhelm Olbers.

When they calculated its trajectory, they were perplexed to discover that it did not fit a parabola, as a good comet should, but was in an elliptical orbit.

On November 26, 1818, the astronomer’s assistant, Pons, discovered a comet in the constellation of Pegasus.

As it remained visible for about seven weeks, until January 12, 1819, Pons was able to make a long series of observations.

He found that the orbital elements of this comet were very similar to those described by astronomers in 1795.

This made Dominique Arago, Director of the Paris Observatory, suspect that it was the same comet.

At that time, the German astronomer Johann Encke, Director of the Gotha Observatory, through extraordinary computational work, found that the orbit was elliptical and had a period of three and a half years.

Johann Encke
Johann Encke great German astronomer. Credit: web “ztfnews.wordpress.com/tag/johann-franz-encke/

When consulting a catalog, he was struck by the similarities between the elements he had calculated and those of the comets of 1786, 1795, and 1805, and he thought that between 1786 and 1818 this comet would have passed through perihelion seven times without being seen.

Encke was awarded by linking his name to the comet; although Encke himself, modestly, always called it Pons’s comet.

In 1824, the Royal Astronomical Society awarded Encke the gold medal, and sent the silver medal to Pons.

Other news related to comets

On the night of March 24, 1993, the Shoemakers discovered a comet that soon became famous because it was known that its orbit was taking it close to Jupiter and that it would impact the planet.

Of course, this generated great excitement and the media coverage was very intense. Furthermore, at that time the cameras of the Hubble Orbital Telescope were already available.

So on that day, July 6, 1994, all the telescopes were focusing on the same point in the sky where, 650 million km away, Comet Shoemaker was going to meet the huge planet Jupiter.

Comet Shoemacher
Shoemaker impact Jupiter in 1994. Credit: ESA.

The comet disintegrated and its fragments, some of them several kilometers in diameter, were hitting the southern hemisphere of Jupiter.

Comets can have diameters of a few tens of kilometers and are made up of water, dry ice, ammonia, methane, iron, magnesium, and silicates.

Due to the very low temperatures of the places where they move, these substances that make up the comet are frozen.

Some research suggests that the materials that make up comets are organic matter that is decisive for life, and that in the early formation of the Earth, some comets impacted on it and gave rise to living beings.

On January 12, 2005, NASA launched a space probe designed to study the composition of the interior of a comet. It was called Deep Impact (deep impact).

On July 4 of the same year, coinciding with the feast of the independence of the United States, the probe approached the nucleus of Comet 9P / Tempel.

A section of the probe, called the impactor, broke away from it and launched into the core, with which it impacted 34 hours later, opening a 100-meter-diameter crater.

Everything that happened was photographed by the section of the probe that was still flying overhead, by telescopes on Earth, and by some orbital telescopes.

Some astronomers suggest that comets retain, in the form of ice and dust, the composition of the early nebula from which the Solar System formed.

The study of comets can give indications of the characteristics of that primordial cloud.

Mission Deep
An artist’s impression of the Deep Impact spacecraft. Credit: NASA. Web “solarsystem.nasa.gov/missions/deep-impact-epoxi/in-depth/”