Comet
The solid core of a comet is known as the nucleus. It measures from a few hundred meters to tens of kilometers across. These nuclei are loose collections of ice, dust, and small rocky particles. Fred Whipple described them as dirty snowballs in 1950. Later observations revealed some are actually icy dirtballs with higher dust content. The surface reflects very little light. Halley's Comet reflects about four percent of incoming sunlight. Comet Borrelly reflects less than three percent. Asphalt reflects seven percent. This dark material absorbs heat that drives outgassing processes.
When a comet approaches the Sun, solar radiation warms the nucleus. Volatile materials vaporize and stream out into space. This creates an extended atmosphere called the coma. Water makes up to ninety percent of volatiles released within one astronomical unit of the Sun. The coma can reach fifteen times Earth's diameter. Some comas become larger than the Sun itself. Comet Holmes briefly had a tenuous dust atmosphere larger than the Sun after an October 2007 outburst. The Great Comet of 1811 also had a coma roughly the diameter of the Sun.
Comets follow highly eccentric elliptical orbits around the Sun. Their orbital periods range from several years to millions of years. Short-period comets originate in the Kuiper belt or its scattered disc beyond Neptune. Long-period comets come from the Oort cloud. This spherical cloud extends from outside the Kuiper belt to halfway to the nearest star. Gravitational perturbations from passing stars and galactic tide set long-period comons toward the Sun.
Jupiter exerts the greatest gravitational influence on these objects. It is more massive than all other planets combined. Close encounters with giant planets deflect long-period comets into shorter orbits. Some comets achieve hyperbolic trajectories and leave the Solar System entirely. Three interstellar objects have been identified: 1I/`Oumuamua, 2I/Borisov, and 3I/ATLAS. These never orbited the Sun before entering our system. They move at velocities similar to nearby stars. A rough calculation suggests four hyperbolic comets per century might appear within Jupiter's orbit.
Human cultures have observed comets since ancient times. Chinese oracle bones record sightings from millennia ago. Aristotle believed comets were atmospheric phenomena within the sphere of the moon. He argued they appeared outside the zodiac and varied in brightness over days. Seneca the Younger questioned this logic in the first century CE. He noted their regular movement and imperviousness to wind proved they were not atmospheric.
Tycho Brahe measured parallax for the Great Comet of 1577. His observations showed comets existed beyond Earth's atmosphere. Isaac Newton used his Principia Mathematica to prove gravity creates conic section orbits. Edmond Halley applied Newton's method to twenty-three apparitions between 1337 and 1698. He predicted a return date of 1758-59 for the comet seen in 1531, 1607, and 1682. French mathematicians Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute refined this prediction. A German farmer named Georg Palitzsch actually saw it first on December 25th and 26th of 1758.
Spacecraft missions have visited comets to study them directly. The Halley Armada included probes from Europe and the Soviet Union that flew through Halley's Comet in 1986. Deep Impact blasted a crater on Comet Tempel 1 in July 2005. This mission revealed most water ice lies below the surface. Rosetta orbited Comet Churyumov-Gerasimenko and landed Philae on its surface on the 12th of November 2014. This was the first time a spacecraft ever landed on such an object.
Stardust retrieved materials from Wild 2 showing crystalline grains formed at temperatures over one thousand degrees Celsius. These grains originated in the hot inner Solar System before being redistributed outward. Ulysses unexpectedly passed through the tail of Comet McNaught in 2007. Giotto photographed Halley's nucleus and observed jets of evaporating material. These robotic explorers confirmed Whipple's dirty snowball model while revealing complex organic compounds like glycine and acetamide on comet surfaces.
Fear of comets as acts of God peaked in Europe between 1200 and 1650 CE. Gotthard Arthusius published pamphlets after the Great Comet of 1618 listing disasters like earthquakes and epidemics. Spectroscopic analysis found cyanogen gas in Halley's Comet tail during 1910. Panic buying of gas masks and anti-comet pills followed erroneous reports that the gas might poison millions. Mark Twain correctly speculated he would die with the comet in 1910.
Science fiction often depicts comets as threats overcome by technology. Films like Deep Impact and Armageddon show heroes preventing global apocalypse. The Heaven's Gate cult committed mass suicide triggered by Hale-Bopp's appearance in 1997. Jules Verne wrote Off on a Comet about people stranded on an orbiting body. Sir Arthur C. Clarke sent a crewed expedition to Halley's Comet in his novel 2061: Odyssey Three. These narratives reflect deep-seated anxieties about celestial wanderers bringing doom or change.
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Common questions
What is the solid core of a comet called and what does it contain?
The solid core of a comet is known as the nucleus. It measures from a few hundred meters to tens of kilometers across and consists of loose collections of ice, dust, and small rocky particles.
When did Fred Whipple describe comets as dirty snowballs?
Fred Whipple described comets as dirty snowballs in 1950. Later observations revealed some are actually icy dirtballs with higher dust content.
How much sunlight does Halley's Comet reflect compared to asphalt?
Halley's Comet reflects about four percent of incoming sunlight while asphalt reflects seven percent. This dark material absorbs heat that drives outgassing processes.
Where do short-period comets originate according to current science?
Short-period comets originate in the Kuiper belt or its scattered disc beyond Neptune. Long-period comets come from the Oort cloud which extends from outside the Kuiper belt to halfway to the nearest star.
Who was the first person to see the return of Halley's Comet in 1758?
A German farmer named Georg Palitzsch actually saw it first on December 25th and 26th of 1758. French mathematicians Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute refined this prediction before his observation.
On what date did the Rosetta spacecraft land Philae on a comet surface?
Rosetta orbited Comet Churyumov-Gerasimenko and landed Philae on its surface on the 12th of November 2014. This was the first time a spacecraft ever landed on such an object.