Comets are cosmic snowballs made of frozen gases, rocks and dust and are roughly the size of a small city. When a comet's orbit brings it close to the Sun, it heats up and spews out dust and gas, causing it to become brighter than most planets. Dust and gas form a tail that stretches from the Sun for millions of kilometers.

10 facts you need to know about comets

1. If the Sun were as big as a front door, the Earth would be the size of a dime, the dwarf planet Pluto would be the size of a pinhead, and the largest comet of the Kuiper Belt (which is about 100 km across, which is about one twentieth of Pluto ) will be the size of a speck of dust.
2. Short-period comets (comets that orbit the Sun in less than 200 years) live in an icy region known as the Kuiper Belt, located beyond the orbit of Neptune. Long comets (comets with long, unpredictable orbits) originate in the far reaches of the Oort Cloud, which is located at a distance of up to 100 thousand AU.
3. Days on the comet change. For example, a day on Halley's Comet ranges from 2.2 to 7.4 Earth days (the time required for the comet to complete a revolution on its axis). Halley's Comet makes a complete revolution around the Sun (a year on the comet) in 76 Earth years.
4. Comets are cosmic snowballs consisting of frozen gases, rocks and dust.
5. The comet heats up as it approaches the Sun and creates an atmosphere or com. The lump can be hundreds of thousands of kilometers in diameter.
6. Comets do not have satellites.
7. Comets do not have rings.
8. More than 20 missions were aimed at studying comets.
9. Comets cannot support life, but may have brought water and organic compounds - the building blocks of life - through collisions with Earth and other objects in our solar system.
10. Halley's Comet was first mentioned in Bayeux from 1066, which recounts the overthrow of King Harold by William the Conqueror at the Battle of Hastings.

Comets: The Dirty Snowballs of the Solar System

Comets On our journeys through the solar system, we may be lucky enough to encounter giant balls of ice. These are comets of the solar system. Some astronomers call comets "dirty snowballs" or "icy mud balls" because they are made mostly of ice, dust and rock debris. Ice can consist of either ice water or frozen gases. Astronomers believe that comets may be composed of primordial material that formed the basis for the formation of the solar system.

Although most of the small objects in our solar system are very recent discoveries, comets have been well known since ancient times. The Chinese have records of comets that date back to 260 BC. This is because comets are the only small bodies in the solar system that can be seen with the naked eye. Comets that orbit the Sun are quite a spectacular sight.

Comet tail

Comets are actually invisible until they begin to approach the Sun. At this moment they begin to heat up and an amazing transformation begins. Dust and gases frozen in the comet begin to expand and escape with explosive speed.

The solid part of a comet is called the comet's nucleus, while the cloud of dust and gas around it is known as the comet's coma. Solar winds pick up material in the coma, leaving a tail behind the comet that extends several million miles. As the sun illuminates, this material begins to glow. Eventually the comet's famous tail forms. Comets and their tails can often be seen from Earth with the naked eye.

The Hubble Space Telescope captured Comet Shoemaker-Levy 9 as it struck the surface of Jupiter.

Some comets can have up to three separate tails. One of them will consist mainly of hydrogen, and is invisible to the eye. The other tail of dust will glow bright white, and the third tail of plasma will usually have a blue glow. When the Earth passes through these dust trails left by comets, the dust enters the atmosphere and creates meteor showers.

Active jets on Comet Hartley 2

Some comets fly in an orbit around the Sun. They are known as periodic comets. A periodic comet loses a significant portion of its material each time it passes near the Sun. Eventually, after all this material is lost, they will cease to be active and wander around the solar system like a dark rocky ball of dust. Halley's Comet is probably the most famous example of a periodic comet. The comet changes its appearance every 76 years.

History of comets
The sudden appearance of these mysterious objects in ancient times was often seen as a bad omen and a warning of natural disasters in the future. We currently know that most comets reside in a dense cloud located at the edge of our solar system. Astronomers call it the Oort Cloud. They believe that gravity from the stray passage of stars or other objects could knock some of the Oort Cloud comets off and send them on a journey into the inner solar system.

Manuscript depicting comets among the ancient Chinese

Comets can also collide with the Earth. In June 1908, something exploded high in the atmosphere above the village of Tunguska in Siberia. The explosion had the force of 1,000 bombs dropped on Hiroshima and leveled trees for hundreds of miles. The absence of any meteorite fragments led scientists to believe that it may have been a small comet that exploded upon impact with the atmosphere.

Comets may also have been responsible for the extinction of the dinosaurs, and many astronomers believe that ancient comet impacts brought much of the water to our planet. While there is a possibility that the Earth could be hit by a large comet again in the future, the chances of this event happening in our lifetime are better than one in a million.

For now, comets simply continue to be objects of wonder in the night sky.

The most famous comets

Comet ISON

Comet ISON was the subject of the most coordinated observations in the history of comet studies. Over the course of a year, more than a dozen spacecraft and numerous ground-based observers collected what is believed to be the largest collection of data on a comet.

Known in the catalog as C/2012 S1, Comet ISON began its journey to the inner Solar System about three million years ago. It was first spotted in September 2012, at a distance of 585,000,000 miles. This was its very first trip around the Sun, that is, it was made of primordial matter that arose in the early days of the formation of the Solar System. Unlike comets that have already made several passes through the inner Solar System, the upper layers of Comet ISON have never been heated by the Sun. The comet represented a kind of time capsule, which captured the moment of the formation of our solar system.

Scientists from around the world launched an unprecedented observing campaign, using many ground-based observatories and 16 spacecraft (all but four successfully studied the comet).

On November 28, 2013, scientists observed Comet ISON being torn apart by the Sun's gravitational forces.

Russian astronomers Vitaly Nevsky and Artem Novichonok discovered the comet using a 4-meter telescope in Kislovodsk, Russia.

ISON is named after the night sky survey program that discovered it. ISON is a group of observatories in ten countries that work together to detect, monitor and track objects in space. The network is managed by the Institute of Applied Mathematics of the Russian Academy of Sciences.

Comet Encke

Comet 2P/EnckeComet 2P/Encke is a small comet. Its core measures approximately 4.8 km (2.98 mi) in diameter, about one-third the size of the object thought to have killed off the dinosaurs.

The comet's orbital period around the Sun is 3.30 years. Comet Encke has the shortest orbital period of any known comet within our Solar System. Encke last passed perihelion (closest point to the Sun) in November 2013.

Photo of a comet taken by the Spitzer telescope

Comet Encke is the parent comet of the Taurids meteor shower. The Taurids, which peak in October/November each year, are fast meteors (104,607.36 km/h or 65,000 mph) known for their fireballs. Fireballs are meteors that are as bright or even brighter than the planet Venus (when viewed in the morning or evening sky with an apparent brightness value of -4). They can create large explosions of light and color and last longer than the average meteor shower. This is because fireballs come from larger particles of material from the comet. Often, this special stream of fireballs occurs on or around the day of Halloween, making them known as Halloween Fireballs.

Comet Encke approached the Sun in 2013 at the same time that Comet Ison was much talked about and presented, and because of this was photographed by both the MESSENGER and STEREO spacecraft.

Comet 2P/Encke was first discovered by Pierre F.A. Mechain on January 17, 1786. Other astronomers found this comet on subsequent passages, but these observations were not identified as the same comet until Johann Franz Encke calculated its orbit.

Comets are typically named after their discoverer(s) or the name of the observatory/telescope used in the discovery. However, this comet is not named after its discoverer. Instead, it was named after Johann Franz Encke, who calculated the comet's orbit. The letter P indicates that 2P/Encke is a periodic comet. Periodic comets have orbital periods of less than 200 years.

Comet D/1993 F2 (Shoemaker - Levy)

Comet Shoemaker-Levy 9 was captured by Jupiter's gravity, dispersed, and then crashed into the giant planet in July 1994.

When the comet was discovered in 1993, it was already fragmented into more than 20 fragments traveling around the planet in a two-year orbit. Further observations revealed that the comet (believed to have been a single comet at the time) made a close approach to Jupiter in July 1992 and was fragmented by tidal forces as a result of the planet's powerful gravity. The comet is believed to have orbited Jupiter for about ten years before its death.

A comet breaking into many pieces was rare, and seeing a comet captured in orbit near Jupiter was even more unusual, but the biggest and rarest discovery was that fragments crashed into Jupiter.

NASA had a spacecraft that observed - for the first time in history - a collision between two bodies in the solar system.

NASA's Galileo orbiter (then on its way to Jupiter) was able to establish a direct view of the parts of the comet, labeled A through W, that collided with Jupiter's clouds. The clashes began on July 16, 1994 and ended on July 22, 1994. Many ground-based observatories and orbiting spacecraft, including the Hubble Space Telescope, Ulysses and Voyager 2, have also studied the collisions and their consequences.

The trail of a comet on the surface of Jupiter

A “freight train” of fragments crashed on Jupiter with the force of 300 million atomic bombs. They created huge plumes of smoke that were 2,000 to 3,000 kilometers (1,200 to 1,900 miles) high, and heated the atmosphere to very hot temperatures of 30,000 to 40,000 degrees Celsius (53,000 to 71,000 degrees Fahrenheit). Comet Shoemaker-Levy 9 left dark, ring-shaped scars that were eventually worn away by Jupiter's winds.

When the clash happened in real time, it was more than just a show. This gave scientists a new look at Jupiter, Comet Shoemaker-Levy 9, and cosmic collisions in general. Researchers were able to deduce the composition and structure of the comet. The collision also left behind dust that is found at the top of Jupiter's clouds. By observing dust spreading across the planet, scientists were able to track the direction of high-altitude winds on Jupiter for the first time. And by comparing changes in the magnetosphere with changes in the atmosphere after the impact, scientists were able to study the relationship between the two.

Scientists estimate that the comet was originally about 1.5 - 2 kilometers (0.9 - 1.2 miles) wide. If an object of this size struck the Earth, it would have devastating consequences. The impact could send dust and debris into the sky, creating a fog that would cool the atmosphere and absorb sunlight, shrouding the entire planet in darkness. If the fog lasts long enough, plant life will die - along with the people and animals that depend on them to survive.

These types of collisions were more common in the early Solar System. It is likely that comet collisions occurred mainly because Jupiter lacked hydrogen and helium.

Currently, collisions of this magnitude probably occur only once every few centuries - and pose a real threat.

Comet Shoemaker-Levy 9 was discovered by Caroline and Eugene Shoemaker and David Levy in an image taken on March 18, 1993, by the 0.4-meter Schmidt Telescope on Mount Palomar.

The comet was named after its discoverers. Comet Shoemaker-Levy 9 was the ninth short-period comet discovered by Eugene and Caroline Shoemaker and David Levy.

Comet Tempel

Comet 9P/TempelComet 9P/Tempel orbits the Sun in the asteroid belt located between the orbits of Mars and Jupiter. The comet last passed its perihelion (closest point to the Sun) in 2011 and will return again in 2016.

Comet 9P/Tempel belongs to the Jupiter family of comets. Jupiter-family comets are comets that have an orbital period of less than 20 years and orbit near a gas giant. Comet 9P/Tempel takes 5.56 years to complete one full period around the Sun. However, the comet's orbit gradually changes over time. When Comet Tempel was first discovered, its orbital period was 5.68 years.

Comet Tempel is a small comet. Its core is about 6 km (3.73 miles) in diameter, believed to be half the size of the object that killed off the dinosaurs.

Two missions have been sent to study this comet: Deep Impact in 2005 and Stardust in 2011.

Possible impact track on the surface of Comet Tempel

Deep Impact fired an impact projectile onto the surface of a comet, becoming the first spacecraft capable of extracting material from a comet's surface. The collision produced relatively little water and a lot of dust. This suggests that the comet is far from being a “block of ice.” The impact of the impact projectile was later captured by the Stardust spacecraft.

Comet 9P/Tempel was discovered by Ernst Wilhelm Leberecht Tempel (better known as Wilhelm Tempel) on April 3, 1867.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Because Wilhelm Tempel discovered this comet, it is named after him. The letter "P" means that Comet 9P/Tempel is a short-period comet. Short-period comets have an orbital period of less than 200 years.

Comet Borelli

Comet 19P/Borelli Resembling a chicken leg, the small nucleus of Comet 19P/Borelli is about 4.8 km (2.98 miles) in diameter, about a third the size of the object that killed off the dinosaurs.

Comet Borelli orbits the Sun in the asteroid belt and is a member of the Jupiter family of comets. Jupiter-family comets are comets that have an orbital period of less than 20 years and orbit near a gas giant. It takes about 6.85 years to complete one full revolution around the Sun. The comet passed its last perihelion (closest point to the Sun) in 2008 and will return again in 2015.

The Deep Space 1 spacecraft flew close to Comet Borelli on September 22, 2001. Traveling at 16.5 km (10.25 miles) per second, Deep Space 1 passed 2,200 km (1,367 miles) above the nucleus of Comet Borelli. This spacecraft took the best photo of a comet's nucleus ever.

Comet 19P/Borrelli was discovered by Alphonse Louis Nicolas Borrelli on December 28, 1904 in Marseille, France.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Alphonse Borrelli discovered this comet and that is why it is named after him. The "P" means that 19P/Borelli is a short-period comet. Short-period comets have an orbital period of less than 200 years.

Comet Hale-Bopp

Comet C/1995 O1 (Hale-Bopp) Also known as the Great Comet of 1997, Comet C/1995 O1 (Hale-Bopp) is a fairly large comet, with a nucleus measuring up to 60 km (37 miles) in diameter. This is about five times larger than the supposed object that killed the dinosaurs. Due to its large size, this comet was visible to the naked eye for 18 months in 1996 and 1997.

Comet Hale-Bopp takes about 2,534 years to complete one revolution around the Sun. The comet passed its last perihelion (closest point to the Sun) on April 1, 1997.

Comet C/1995 O1 (Hale-Bopp) was discovered in 1995 (July 23), independently by Alan Hale and Thomas Bopp. Comet Hale-Bopp was discovered at an astonishing distance of 7.15 AU. One AU is equal to approximately 150 million km (93 million miles).

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Because Alan Hale and Thomas Bopp discovered this comet, it is named after them. The letter "S" stands for. That Comet C/1995 O1 (Hale-Bopp) is a long-period comet.

Comet Wild

Comet 81P/Wilda81P/Wilda (Wild 2) is a small comet with a flattened ball shape and a size of about 1.65 x 2 x 2.75 km (1.03 x 1.24 x 1.71 mi). Its period of revolution around the Sun is 6.41 years. Comet Wild last passed perihelion (closest point to the Sun) in 2010 and will return again in 2016.

Comet Wild is known as a new periodic comet. The comet orbits the Sun between Mars and Jupiter, but it has not always traveled this orbital path. Initially, the orbit of this comet passed between Uranus and Jupiter. On September 10, 1974, gravitational interactions between this comet and the planet Jupiter changed the comet's orbit into a new shape. Paul Wild discovered this comet during its first revolution around the Sun in a new orbit.

Animated image of a comet

Since Wilda is a new comet (it didn't have as many close orbits around the Sun), it is an ideal sample for discovering something new about the early Solar System.

NASA used this special comet when, in 2004, they assigned the Stardust mission to fly to it and collect coma particles—the first collection of this kind of extraterrestrial material beyond the orbit of the Moon. These samples were collected in an airgel collector as the craft flew 236 km (147 miles) from the comet. The samples were then returned to Earth in an Apollo-like capsule in 2006. In those samples, scientists discovered glycine: a fundamental building block of life.

Comets are typically named after their discoverer(s) or the name of the observatory/telescope used in the discovery. Because Paul Wild discovered this comet, it was named after him. The letter "P" means that 81P/Wilda (Wild 2) is a "periodic" comet. Periodic comets have orbital periods of less than 200 years.

Comet Churyumov-Gerasimenko

Comet 67P / Churyumova-Gerasimenko may go down in history as the first comet on which robots from Earth will land and who will accompany it throughout its orbit. The Rosetta spacecraft, which carries the Philae lander, plans to rendezvous with the comet in August 2014 to accompany it on its journey to and from the inner solar system. Rosetta is a mission of the European Space Agency (ESA), which is provided with essential instruments and support by NASA.

Comet Churyumov-Gerasimenko makes a loop around the Sun in an orbit intersecting the orbits of Jupiter and Mars, approaching but not entering Earth's orbit. Like most Jupiter-family comets, it is believed to have fallen from the Kuiper Belt, the region beyond Neptune's orbit, as a result of one or more collisions or gravitational tugs.

Close-up of the surface of comet 67P/Churyumov-Gerasimenko

Analysis of the comet's orbital evolution indicates that until the mid-19th century, the closest distance to the Sun was 4.0 AU. (about 373 million miles or 600 million kilometers), which is about two-thirds of the way from the orbit of Mars to Jupiter. Because the comet is too far from the heat of the Sun, it has not grown a ball (shell) or tail, so the comet is not visible from Earth.

But scientists estimate that in 1840, a fairly close encounter with Jupiter must have sent the comet flying deeper into the solar system, down to about 3.0 AU. (about 280 million miles or 450 million kilometers) from the Sun. The Churyumov-Gerasimenko perihelion (closest approach to the Sun) was slightly closer to the Sun for the next century, and then Jupiter gave the comet another gravitational shock in 1959. The comet's perihelion has since stopped at 1.3 AU, about 27 million miles (43 million kilometers) beyond Earth's orbit.

Dimensions of comet 67P/Churyumov-Gerasimenko

The comet's nucleus is considered to be quite porous, giving it a density much lower than that of water. When heated by the Sun, the comet is believed to emit about twice as much dust as gas. A small detail known about the comet's surface is that a landing site for Philae will not be selected until Rosetta surveys it at close range.

During recent visits to our part of the solar system, the comet was not bright enough to be seen from Earth without a telescope. This coming year we will be able to see the fireworks close up, thanks to the eyes of our robots.

Discovered on October 22, 1969 at the Alma-Ata Observatory, USSR. Klim Ivanovich Churyumov found an image of this comet while examining a photographic plate of another comet (32P/Comas Sola), taken by Svetlana Ivanova Gerasimenko on September 11, 1969.

67P indicates that it was the 67th periodic comet discovered. Churyumov and Gerasimenko are the names of the discoverers.

Comet Siding Spring

Comet McNaught Comet C/2013 A1 (Siding Spring) heads toward Mars on a low-level flight on October 19, 2014. The comet's nucleus is expected to zip past the planet within a cosmic hair, which is 84,000 miles (135,000 km), about one-third the distance from Earth to the Moon and one-tenth the distance that any known comet has passed Earth. This represents both an excellent opportunity for study and a potential hazard for spacecraft in this area.

Because the comet will approach Mars almost head-on, and because Mars is in its own orbit around the Sun, they will pass each other at a tremendous speed of about 35 miles (56 kilometers) per second. But the comet can be so large that Mars can fly through high-speed particles of dust and gas for several hours. The Martian atmosphere will likely protect rovers on the surface, but spacecraft in orbit will be bombarded by particles moving two or three times faster than the meteorites that spacecraft typically withstand.

NASA spacecraft transmits first photographs of Comet Siding Spring to Earth

“Our plans for using spacecraft on Mars to observe Comet McNaught will be coordinated with plans for how orbiters can stay out of the flow and be protected if necessary,” said Rich Zurek, chief scientist for the Mars program at NASA Jet Propulsion Laboratory.

One way to protect orbiters is to position them behind Mars during the riskiest surprise encounters. Another way is for the spacecraft to “dodge” the comet, trying to shield the most vulnerable equipment. But such maneuvers could cause changes in the orientation of solar panels or antennas in ways that interfere with the vehicles' ability to generate power and communicate with Earth. "These changes will require an enormous amount of testing," said Soren Madsen, chief engineer for the Mars exploration program at JPL. “There are a lot of preparations that need to be made now to prepare ourselves for the eventuality that we learn in May that the demonstration flight will be risky.”

Comet Siding Spring fell from the Oort Cloud, a huge spherical region of long-period comets that circles the Solar System. To get an idea of ​​how far away that is, consider this situation: Voyager 1, which has been traveling in space since 1977, is much further away than any of the planets, and has even emerged from the heliosphere, a huge bubble of magnetism and ionized gas. radiating from the Sun. But it will take the ship another 300 years to reach the inner "edge" of the Oort Cloud, and at its current speed of a million miles a day it will take about 30,000 more years to finish passing through the cloud.

Every once in a while, some gravitational pull - perhaps from passing a star - will push the comet to break free from its impossibly vast and distant vault, and it will fall into the Sun. This is what should have happened to Comet McNaught several million years ago. All this time the fall was directed towards the inner part of the solar system, and it gives us only one chance to study it. According to available estimates, her next visit will be in about 740 thousand years.

"C" indicates that the comet is not periodic. 2013 A1 shows that it was the first comet discovered in the first half of January 2013. Siding Spring is the name of the observatory where it was discovered.

Comet Giacobini-Zinner

Comet 21P/Giacobini-Zinner is a small comet with a diameter of 2 km (1.24 mi). The period of revolution around the Sun is 6.6 years. The last time Comet Giacobini-Zinner passed perihelion (closest point to the Sun) was on February 11, 2012. The next perihelion passage will be in 2018.

Every time Comet Giacobini-Zinner returns to the inner Solar System, its core sprays ice and rocks into space. This shower of debris leads to the annual meteor shower: the Draconids, which occurs every year in early October. The Draconids radiate from the northern constellation Draco. For many years the shower is weak and very few meteorites are visible during this period. However, there are occasional references in the records to Draconid (sometimes called Jacobinid) meteor storms. A meteor storm occurs when a thousand or more meteors are visible within an hour at the observer's location. At its peak in 1933, 500 Draconid meteors were seen within a minute in Europe. 1946 was also a good year for the Draconids, with about 50-100 meteors being seen in one minute in the US.

Coma and nucleus of comet 21P/Giacobini-Zinner

In 1985 (September 11), a re-designated mission called ICE (International Comet Explorer, formally International Sun-Earth Explorer-3) was assigned to collect data from this comet. ICE was the first spacecraft to follow a comet. ICE later joined the famous "armada" of spacecraft sent to Halley's Comet in 1986. Another mission, called Sakigaki, from Japan, was scheduled to follow the comet in 1998. Unfortunately, the spacecraft did not have enough fuel to reach the comet.

Comet Giacobini-Zinner was discovered on December 20, 1900 by Michel Giacobini at the Nice Observatory in France. Information about this comet was later restored by Ernst Zinner in 1913 (October 23).

Comets are typically named after their discoverer(s) or the name of the observatory/telescope used in the discovery. Since Michel Giacobini and Ernst Zinner discovered and recovered this comet, it is named after them. The letter "P" means that Comet Giacobini-Zinner is a "periodic" comet. Periodic comets have orbital periods of less than 200 years.

Comet Thatcher

Comet C/1861 G1 (Thatcher)Comet C/1861 G1 (Thatcher) takes 415.5 years to complete one revolution around the Sun. Comet Thatcher passed its final perihelion (closest point to the Sun) in 1861. Comet Thatcher is a long-period comet. Long-period comets have orbital periods of more than 200 years.

When comets pass around the Sun, the dust they emit spreads into a dust trail. Every year, when Earth passes through this comet trail, space debris collides with our atmosphere, where it breaks up and creates fiery, colorful streaks in the sky.

Chunks of space debris coming from Comet Thatcher and interacting with our atmosphere create the Lyrid meteor shower. This annual meteor shower occurs every April. The Lyrids are among the oldest known meteor showers. The first documented Lyrid meteor shower dates back to 687 BC.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Since A.E. Thatcher discovered this comet, it is named after him. The "C" means that Comet Thatcher is a long-period comet, meaning its orbital period is more than 200 years. 1861 is the year of its opening. "G" denotes the first half of April, and "1" means Thatcher was the first comet discovered during that period.

Comet Swift-Tuttle

Comet Swift-Tuttle Comet 109P/Swift-Tuttle takes 133 years to complete one revolution around the Sun. The comet passed its last perihelion (closest point to the Sun) in 1992 and will return again in 2125.

Comet Swift-Tuttle is considered a large comet - its nucleus is 26 km (16 miles) across. (That is, more than twice the size of the supposed object that killed the dinosaurs.) Chunks of space debris ejected from Comet Swift-Tuttle and interacting with our atmosphere create the popular Perseid meteor shower. This annual meteor shower occurs every August and peaks in the middle of the month. Giovanni Schiaparelli was the first to realize that the source of the Perseids was this comet.

Comet Swift-Tuttle was discovered in 1862 independently by Lewis Swift and Horace Tuttle.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Since Lewis Swift and Horace Tuttle discovered this comet, it is named after them. The letter "P" means that Comet Swift-Tuttle is a short-period comet. Short-period comets have orbital periods of less than 200 years.

Comet Tempel-Tuttle

Comet 55P/Tempel-Tuttle is a small comet whose nucleus is 3.6 km (2.24 mi) across. It takes 33 years to complete one revolution around the Sun. Comet Tempel-Tuttle passed its perihelion (closest point to the Sun) in 1998 and will return again in 2031.

Chunks of space debris coming from the comet interact with our atmosphere and create the Leonid meteor shower. This is typically a weak meteor shower that peaks in mid-November. Every year, the Earth passes through this debris, which, when interacting with our atmosphere, disintegrates and creates fiery, colorful streaks in the sky.

Comet 55P/Tempel-Tuttle in February 1998

Every 33 years or so, the Leonid meteor shower turns into a full-blown meteor storm, during which at least 1,000 meteors per hour burn up in Earth's atmosphere. Astronomers in 1966 observed a spectacular sight: the remains of a comet crashed into the Earth's atmosphere at a rate of thousands of meteors per minute during a 15-minute period. The last Leonid meteor storm occurred in 2002.

Comet Tempel-Tuttle was discovered twice independently - in 1865 and 1866 by Ernst Tempel and Horace Tuttle, respectively.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Since Ernst Tempel and Horace Tuttle discovered it, the comet is named after them. The letter "P" means that Comet Tempel-Tuttle is a short-period comet. Short-period comets have orbital periods of less than 200 years.

Halley's Comet

Comet 1P/Halley is perhaps the most famous comet, having been observed for thousands of years. The comet was first mentioned by Halley in the Bayeux Tapestry, which recounts the Battle of Hastings in 1066.

Halley's Comet takes about 76 years to complete one revolution around the Sun. The comet was last seen from Earth in 1986. That same year, an international armada of spacecraft converged on the comet to collect as much data as possible about it.

Halley's Comet in 1986

The comet will not arrive into the solar system until 2061. Every time Halley's Comet returns to the inner Solar System, its core sprays ice and rock into space. This debris flow results in two weak meteor showers: the Eta Aquarids in May and the Orionids in October.

Dimensions of Comet Halley: 16 x 8 x 8 km (10 x 5 x 5 miles). This is one of the darkest objects in the solar system. The comet has an albedo of 0.03, meaning it reflects only 3% of the light that hits it.

The first sightings of Halley's Comet are lost in time, more than 2,200 years ago. However, in 1705, Edmond Halley studied the orbits of previously observed comets and noted some that appeared to appear again and again every 75-76 years. Based on the similarity of orbits, he proposed that it was in fact the same comet, and correctly predicted the next return in 1758.

Comets are usually named after their discoverer or the observatory/telescope used in the discovery. Edmond Halley correctly predicted the return of this comet - the first prediction of its kind and that is why the comet is named after him. The letter "P" means that Halley's Comet is a short-period comet. Short-period comets have orbital periods of less than 200 years.

Comet C/2013 US10 (Catalina)

Comet C/2013 US10 (Catalina) is an Oort Cloud comet discovered on October 31, 2013 by the Catalina Sky Survey Observatory with an apparent magnitude of 19, using the 0.68-meter (27 in) Schmidt-Cassegrain Telescope. As of September 2015, the comet has an apparent magnitude of 6.

When Catalina was discovered on October 31, 2013, the preliminary determination of its orbit used observations of another object made on September 12, 2013, which gave an incorrect result suggesting an orbital period of only 6 years for the comet. But on November 6, 2013, with a longer observation of the arc from August 14 to November 4, it became obvious that the first result on September 12 was obtained at a different object.

By early May 2015, the comet had an apparent magnitude of 12 and was 60 degrees away from the Sun as it moved further into the southern hemisphere. The comet came to solar conjunction on November 6, 2015, when it was around magnitude 6. The comet approached perihelion (closest approach to the Sun) on November 15, 2015 at a distance of 0.82 AU. from the Sun and had a speed of 46.4 km/s (104,000 mph) relative to the Sun, slightly faster than the Sun's receding velocity at that distance. Comet Catalina crossed the celestial equator on December 17, 2015 and became a northern hemisphere object. On January 17, 2016, the comet will pass 0.72 astronomical units (108,000,000 km; 67,000,000 mi) from Earth and should be magnitude 6, located in the constellation Ursa Major.

Object C/2013 US10 is dynamically new. It came from the Oort Cloud from a loosely coupled, chaotic orbit that could easily be disturbed by galactic tides and traveling stars. Before entering the planetary region (around 1950), Comet C/2013 US10 (Catalina) had an orbital period of several million years. After leaving the planetary region (around 2050), it will be on an ejection trajectory.

Comet Catalina is named after the Catalina Sky Survey, which discovered it on October 31, 2013.

Comet C/2011 L4 (PANSTARRS)

C/2011 L4 (PANSTARRS) is a non-periodic comet discovered in June 2011. It was only noticed with the naked eye in March 2013, when it was near perihelion.

It was discovered using the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) telescope located near the top of Halikan on the island of Maui in Hawaii. Comet C/2011 L4 probably took millions of years to travel from the Oort cloud. After leaving the planetary region of the Solar System, the post-perihelion orbital period (epoch 2050) is estimated to be approximately 106,000 years. Made from dust and gas, this comet's nucleus is about 1 km (0.62 miles) in diameter.

Comet C/2011 L4 was at a distance of 7.9 AU. from the Sun and had a brilliance of 19 stars. Vel., when she was discovered in June 2011. But already at the beginning of May 2012 it revived to 13.5 stars. Vel., and this was visible visually when using a large amateur telescope from the dark side. As of October 2012, the coma (expanding thin dust atmosphere) was about 120,000 kilometers (75,000 mi) in diameter. Without optical assistance, C/2011 L4 was seen on February 7, 2013 and had a magnitude of 6. led Comet PANSTARRS was observed from both hemispheres in the first weeks of March, and it passed closest to Earth on March 5, 2013 at a distance of 1.09 AU. It approached perihelion (closest approach to the Sun) on March 10, 2013.

Preliminary estimates predicted that C/2011 L4 would be brighter, at about 0 magnitude. led (approximate brightness of Alpha Centauri A or Vega). Estimates from October 2012 predicted that it could be brighter, at -4 magnitude. led (roughly corresponds to Venus). In January 2013, there was a noticeable drop in brightness, which suggested that it could be brighter, having only +1 magnitude. led In February the light curve showed a further slowdown, suggesting a perihelion at +2 mag. led

However, a study using a secular light curve indicates that Comet C/2011 L4 experienced a "braking event" when it was at a distance of 3.6 AU. from the Sun and had 5.6 AU. The rate of increase in brightness decreased, and the magnitude at perihelion was predicted to be +3.5. For comparison, at the same perihelion distance, Halley's Comet would have a magnitude of -1.0. led The same study concluded that C/2011 L4 is a very young comet and belongs to the class of “children” (that is, those whose photometric age is less than 4 years of the comet).

Image of Comet Panstarrs taken in Spain

Comet C/2011 L4 reached perihelion in March 2013, and was estimated to have an actual peak of +1 magnitude by various observers around the planet. led However, its low location above the horizon makes it difficult to obtain certain data. This was facilitated by the lack of suitable reference stars and the impossibility of differential atmospheric extinction corrections. As of mid-March 2013, due to the brightness of twilight and its low position in the sky, C/2011 L4 was best visible through binoculars 40 minutes after sunset. On March 17-18, the comet was close to the star Algenib with 2.8 stars. led April 22 near Beta Cassiopeia, and May 12-14 near Gamma Cepheus. Comet C/2011 L4 continued to move north until May 28th.

Comet PANSTARRS bears the name of the Pan-STARRS telescope, with which it was discovered in June 2011.

A comet is a celestial nebulous object with a characteristic bright nucleus-clump and a luminous tail. Comets are composed primarily of frozen gases, ice and dust. Therefore, we can say that a comet is a huge dirty snowball flying in space around the Sun in a very elongated orbit.

Comet Lovejoy, photo taken on the ISS

Where do comets come from?
Most comets come to the Sun from two places - the Kuiper belt (the asteroid belt beyond Neptune) and the Oort cloud. The Kuiper Belt is a belt of asteroids beyond the orbit of Neptune, and the Oort cloud is a cluster of small celestial bodies on the edge of the Solar System, which is farthest from all the planets and the Kuiper Belt.

How do comets move?
Comets can spend millions of years somewhere very far from the Sun, not at all bored among their fellows in the Oort cloud or Kuiper belt. But one day, there, in the farthest corner of the solar system, two comets may accidentally pass next to each other or even collide. Sometimes after such a meeting one of the comets may begin to move towards the Sun.

The gravitational pull of the Sun will only accelerate the movement of the comet. When it flies close enough to the Sun, the ice will begin to melt and evaporate. At this point, the comet will have a tail, consisting of dust and gases that the comet leaves behind. The dirty snowball begins to melt, turning into a beautiful “heavenly tadpole” - a comet.

The fate of the comet depends on the orbit in which it begins to move. As is known, all celestial bodies caught in the gravitational field of the Sun can move either in a circle (which is only theoretically possible), or in an ellipse (this is how all planets, their satellites, etc. move), or in a hyperbola or parabola. Imagine a cone, and then mentally cut a piece from it. If you cut a cone at random, you will probably end up with either a closed figure - an ellipse, or an open curve - a hyperbola. In order to obtain a circle or parabola, it is necessary that the section plane be oriented in a strictly defined manner. If the comet moves in an elliptical orbit, this means that one day it will return to the Sun again. If the comet's orbit becomes a parabola or hyperbola, then the gravity of our star will not be able to hold the comet, and humanity will see it only once. Having flown past the Sun, the wanderer will depart from the solar system, waving her tail at us goodbye.

here you can see that at the very end of the shooting the comet falls apart into several parts

It often happens that comets do not survive their journey to the Sun. If the comet's mass is small, it can completely evaporate in one flyby of the Sun. If the comet's material is too loose, then the gravitational force of our star can tear the comet apart. This has happened more than once. For example, in 1992, Comet Shoemaker-Levy, flying past Jupiter, fell apart into more than 20 fragments. Jupiter was then hit hard. Debris from the comet crashed into the planet, causing severe atmospheric storms. And more recently (November 2013), Comet Ison could not survive its first flyby of the Sun, and its core broke up into several fragments.

How many tails does a comet have?
Comets have several tails. This happens because comets are made not only of frozen gases and water, but also of dust. When moving towards the Sun, the comet is constantly blown by the solar wind - a stream of charged particles. It has a much stronger effect on light gas molecules than on heavy dust particles. Because of this, the comet has two tails - one dusty, the other gaseous. The gas tail is always directed directly from the Sun, the dust tail twists slightly along the trajectory of the comet.

Sometimes comets have more than two tails. For example, a comet may have three tails, for example, if at some point a large number of dust grains are quickly released from the comet's nucleus, they will form a third tail, separate from the first dust tail and the second gas tail.

What will happen if the Earth flies through the tail of a comet?
But nothing will happen. The tail of a comet is just gas and dust, so if the Earth passes through the comet's tail, the gas and dust will simply collide with the Earth's atmosphere and either burn up or dissolve into it. But if a comet crashes into the Earth, it could be hard for all of us.

The content of the article

COMET, a small celestial body moving in interplanetary space and abundantly releasing gas when approaching the Sun. A variety of physical processes are associated with comets, from sublimation (dry evaporation) of ice to plasma phenomena. Comets are the remnants of the formation of the Solar System, a transitional stage to interstellar matter. The observation of comets and even their discovery are often carried out by amateur astronomers. Sometimes comets are so bright that they attract everyone's attention. In the past, the appearance of bright comets caused fear among people and served as a source of inspiration for artists and cartoonists.

Movement and spatial distribution.

All or almost all comets are components of the Solar System. They, like the planets, obey the laws of gravity, but they move in a very unique way. All planets revolve around the Sun in the same direction (which is called “forward” as opposed to “reverse”) in almost circular orbits lying approximately in the same plane (the ecliptic), and comets move in both forward and backward directions along highly elongated ( eccentric) orbits inclined at different angles to the ecliptic. It is the nature of the movement that immediately gives away the comet.

Long-period comets (with orbital periods of more than 200 years) come from regions thousands of times farther than the most distant planets, and their orbits are tilted at all sorts of angles. Short-period comets (periods of less than 200 years) come from the region of the outer planets, moving in a forward direction in orbits lying close to the ecliptic. Far from the Sun, comets usually do not have "tails" but sometimes have a barely visible "coma" surrounding the "nucleus"; together they are called the "head" of the comet. As it approaches the Sun, the head enlarges and a tail appears.

Structure.

In the center of the coma there is a core - a solid body or a conglomerate of bodies with a diameter of several kilometers. Almost all of the comet's mass is concentrated in its nucleus; this mass is billions of times less than the earth's. According to F. Whipple's model, the comet's nucleus consists of a mixture of various ices, mainly water ice with an admixture of frozen carbon dioxide, ammonia and dust. This model is confirmed by both astronomical observations and direct measurements from spacecraft near the nuclei of comets Halley and Giacobini–Zinner in 1985–1986.

When a comet approaches the Sun, its core heats up and the ice sublimates, i.e. evaporate without melting. The resulting gas scatters in all directions from the nucleus, taking with it dust particles and creating a coma. Water molecules destroyed by sunlight form a huge hydrogen corona around the comet's nucleus. In addition to solar attraction, repulsive forces also act on the rarefied matter of a comet, due to which a tail is formed. Neutral molecules, atoms and dust particles are affected by the pressure of sunlight, while ionized molecules and atoms are more strongly affected by the pressure of the solar wind.

The behavior of tail-forming particles became much clearer after direct study of comets in 1985–1986. The plasma tail, consisting of charged particles, has a complex magnetic structure with two regions of different polarity. On the side of the coma facing the Sun, a frontal shock wave is formed, exhibiting high plasma activity.

Although the tail and coma contain less than one millionth of the comet's mass, 99.9% of the light comes from these gas formations, and only 0.1% from the nucleus. The fact is that the core is very compact and also has a low reflection coefficient (albedo).

Sometimes comets are destroyed when approaching planets. On March 24, 1993, at the Mount Palomar Observatory in California, astronomers K. and Y. Shoemaker, together with D. Levy, discovered a comet with an already destroyed nucleus near Jupiter. Calculations showed that on July 9, 1992, comet Shoemaker-Levy-9 (this is the ninth comet they discovered) passed near Jupiter at a distance of half the radius of the planet from its surface and was torn apart by its gravity into more than 20 parts. Before destruction, the radius of its core was approx. 20 km.

Stretching out in a chain, the fragments of the comet moved away from Jupiter in an elongated orbit, and then in July 1994 approached it again and collided with the cloudy surface of Jupiter.

Origin.

Comet nuclei are the remnants of the primary matter of the Solar System, which made up the protoplanetary disk. Therefore, their study helps to restore the picture of the formation of planets, including the Earth. In principle, some comets could come to us from interstellar space, but so far not a single such comet has been reliably identified.

Gas composition.

In table Table 1 lists the main gas components of comets in descending order of their content. The movement of gas in the tails of comets shows that it is strongly influenced by non-gravitational forces. The glow of the gas is excited by solar radiation.

ORBITS AND CLASSIFICATION

To better understand this section, we recommend that you familiarize yourself with the articles: CELESTIAL MECHANICS; CONIC SECTIONS; ORBIT; SOLAR SYSTEM.

Orbit and speed.

The movement of the comet's nucleus is completely determined by the attraction of the Sun. The shape of a comet's orbit, like any other body in the Solar System, depends on its speed and distance from the Sun. The average speed of a body is inversely proportional to the square root of its average distance to the Sun ( a). If the speed is always perpendicular to the radius vector directed from the Sun to the body, then the orbit is circular, and the speed is called circular speed ( v c) on distance a. The speed of escape from the gravitational field of the Sun along a parabolic orbit ( vp) times the circular speed at this distance. If the comet's speed is less vp, then it moves around the Sun in an elliptical orbit and never leaves the Solar System. But if the speed exceeds vp, then the comet passes the Sun once and leaves it forever, moving in a hyperbolic orbit.

The figure shows the elliptical orbits of the two comets, as well as the nearly circular orbits of the planets and a parabolic orbit. At the distance that separates the Earth from the Sun, the circular speed is 29.8 km/s, and the parabolic speed is 42.2 km/s. Near Earth, the speed of Comet Encke is 37.1 km/s, and the speed of Comet Halley is 41.6 km/s; This is why Comet Halley goes much further from the Sun than Comet Encke.

Classification of cometary orbits.

Most comets have elliptical orbits, so they belong to the Solar System. True, for many comets these are very elongated ellipses, close to a parabola; along them, comets move away from the Sun very far and for a long time. It is customary to divide the elliptical orbits of comets into two main types: short-period and long-period (almost parabolic). The orbital period is considered to be 200 years.

SPATIAL DISTRIBUTION AND ORIGIN

Almost parabolic comets.

Many comets belong to this class. Since their orbital periods are millions of years, only one ten-thousandth of them appears in the vicinity of the Sun over the course of a century. In the 20th century observed approx. 250 such comets; therefore, there are millions of them in total. In addition, not all comets come close enough to the Sun to become visible: if the perihelion (the point closest to the Sun) of the comet’s orbit lies beyond the orbit of Jupiter, then it is almost impossible to notice it.

Taking this into account, in 1950 Jan Oort suggested that the space around the Sun at a distance of 20–100 thousand AU. (astronomical units: 1 AU = 150 million km, distance from the Earth to the Sun) is filled with comet nuclei, the number of which is estimated at 10 12, and the total mass is 1–100 Earth masses. The outer boundary of the Oort “comet cloud” is determined by the fact that at this distance from the Sun the movement of comets is significantly influenced by the attraction of neighboring stars and other massive objects ( cm. below). Stars move relative to the Sun, their disturbing influence on comets changes, and this leads to the evolution of cometary orbits. So, by chance, a comet may end up in an orbit passing close to the Sun, but on the next revolution its orbit will change slightly, and the comet will pass away from the Sun. However, instead of it, “new” comets will constantly fall from the Oort cloud into the vicinity of the Sun.

Short-period comets.

When a comet passes near the Sun, its core heats up and the ice evaporates, forming a gas coma and tail. After several hundreds or thousands of such flights, there are no fusible substances left in the core, and it ceases to be visible. For short-period comets that regularly approach the Sun, this means that their populations should become invisible in less than a million years. But we observe them, therefore, replenishment from “fresh” comets is constantly arriving.

Replenishment of short-period comets occurs as a result of their “capture” by planets, mainly Jupiter. It was previously thought that long-period comets coming from the Oort cloud were captured, but it is now believed that their source is a cometary disk called the “inner Oort cloud.” In principle, the idea of ​​the Oort cloud has not changed, but calculations have shown that the tidal influence of the Galaxy and the influence of massive clouds of interstellar gas should destroy it quite quickly. A source of replenishment is needed. Such a source is now considered to be the inner Oort cloud, which is much more resistant to tidal influences and contains an order of magnitude more comets than the outer cloud predicted by Oort. After each approach of the Solar System to a massive interstellar cloud, comets from the outer Oort cloud scatter into interstellar space, and they are replaced by comets from the inner cloud.

The transition of a comet from an almost parabolic orbit to a short-period orbit occurs when it catches up with the planet from behind. Typically, capturing a comet into a new orbit requires several passes through the planetary system. The resulting orbit of a comet typically has low inclination and high eccentricity. The comet moves along it in a forward direction, and the aphelion of its orbit (the point farthest from the Sun) lies close to the orbit of the planet that captured it. These theoretical considerations are fully confirmed by the statistics of cometary orbits.

Non-gravitational forces.

Gaseous sublimation products exert reactive pressure on the comet's nucleus (similar to the recoil of a gun when fired), which leads to the evolution of the orbit. The most active outflow of gas occurs from the heated “afternoon” side of the core. Therefore, the direction of the pressure force on the core does not coincide with the direction of solar rays and solar gravity. If the axial rotation of the nucleus and its orbital revolution occur in the same direction, then the pressure of the gas as a whole accelerates the movement of the nucleus, leading to an increase in the orbit. If rotation and circulation occur in opposite directions, then the comet’s movement is slowed down and the orbit is shortened. If such a comet was initially captured by Jupiter, then after some time its orbit is entirely in the region of the inner planets. This is probably what happened to Comet Encke.

Comets touching the Sun.

A special group of short-period comets consists of comets that “graze” the Sun. They were probably formed thousands of years ago as a result of the tidal destruction of a large core, at least 100 km in diameter. After the first catastrophic approach to the Sun, fragments of the nucleus made approx. 150 revolutions, continuing to fall apart. Twelve members of this family of Kreutz comets were observed between 1843 and 1984. Their origins may be related to a large comet seen by Aristotle in 371 BC.

Halley's Comet.

This is the most famous of all comets. It has been observed 30 times since 239 BC. Named in honor of E. Halley, who, after the appearance of the comet in 1682, calculated its orbit and predicted its return in 1758. The orbital period of Halley's comet is 76 years; it last appeared in 1986 and will next be observed in 2061. In 1986, it was studied at close range by 5 interplanetary probes - two Japanese (Sakigake and Suisei), two Soviet (Vega-1 and Vega-1). 2") and one European ("Giotto"). It turned out that the comet's nucleus is potato-shaped, approx. 15 km and width approx. 8 km, and its surface is “blacker than coal.” It may be covered with a layer of organic compounds, such as polymerized formaldehyde. The amount of dust near the core turned out to be much higher than expected.

Comet Encke.

This faint comet was the first to be included in the Jupiter family of comets. Its period of 3.29 years is the shortest among comets. The orbit was first calculated in 1819 by the German astronomer J. Encke (1791–1865), who identified it with the comets observed in 1786, 1795 and 1805. Comet Encke is responsible for the Taurid meteor shower, observed annually in October and November.

Comet Giacobini–Zinner.

This comet was discovered by M. Giacobini in 1900 and rediscovered by E. Zinner in 1913. Its period is 6.59 years. It was with it that on September 11, 1985, the space probe International Cometary Explorer first approached, which passed through the tail of the comet at a distance of 7800 km from the nucleus, thanks to which data was obtained on the plasma component of the tail. This comet is associated with the Jacobinids (Draconids) meteor shower.

PHYSICS OF COMETS

Core.

All manifestations of a comet are somehow connected with the nucleus. Whipple suggested that the comet's nucleus was a solid body consisting mainly of water ice with dust particles. This “dirty snowball” model easily explains the multiple passages of comets near the Sun: with each passage, a thin surface layer (0.1–1% of the total mass) evaporates and the inner part of the nucleus is preserved. Perhaps the core is a conglomerate of several “cometesimals,” each no more than a kilometer in diameter. Such a structure could explain the disintegration of nuclei, as observed with Comet Biela in 1845 or Comet West in 1976.

Shine.

The observed brightness of a celestial body illuminated by the Sun with a constant surface changes in inverse proportion to the squares of its distances from the observer and from the Sun. However, sunlight is scattered mainly by the gas and dust shell of the comet, the effective area of ​​which depends on the rate of ice sublimation, and that, in turn, on the heat flux incident on the nucleus, which itself varies inversely with the square of the distance to the Sun. Therefore, the brightness of the comet should vary in inverse proportion to the fourth power of the distance to the Sun, which is confirmed by observations.

Kernel size.

The size of the comet's nucleus can be estimated from observations at a time when it is far from the Sun and not shrouded in a gas and dust shell. In this case, light is reflected only by the solid surface of the core, and its apparent brightness depends on the cross-sectional area and reflectance (albedo). The albedo of the nucleus of Comet Halley turned out to be very low - approx. 3%. If this is typical for other nuclei, then the diameters of most of them lie in the range from 0.5 to 25 km.

Sublimation.

The transition of matter from a solid to a gaseous state is important for the physics of comets. Measurements of the brightness and emission spectra of comets have shown that the melting of the main ices begins at a distance of 2.5–3.0 AU, as it should be if the ice is mainly water. This was confirmed by studying comets Halley and Giacobini-Zinner. The gases observed first as the comet approaches the Sun (CN, C 2) are probably dissolved in water ice and form gas hydrates (clathrates). How this "composite" ice will sublimate depends largely on the thermodynamic properties of the water ice. Sublimation of the dust-ice mixture occurs in several stages. Streams of gas and small and fluffy dust particles picked up by them leave the core, since the attraction at its surface is extremely weak. But the gas flow does not carry away dense or interconnected heavy dust particles, and a dust crust is formed. Then the sun's rays heat the dust layer, the heat passes in, the ice sublimates, and gas flows break through, breaking the dust crust. These effects became apparent during the observation of Halley's comet in 1986: sublimation and outflow of gas occurred only in a few regions of the comet's nucleus illuminated by the Sun. It is likely that ice was exposed in these areas, while the rest of the surface was covered with crust. The released gas and dust form the observable structures around the comet's nucleus.

Coma.

Dust grains and gas of neutral molecules (Table 1) form an almost spherical coma of the comet. Usually the coma stretches from 100 thousand to 1 million km from the nucleus. Light pressure can deform the coma, stretching it in an anti-solar direction.

Hydrogen corona.

Since the core ices are mostly water, the coma mainly contains H 2 O molecules. Photodissociation breaks down H 2 O into H and OH, and then OH into O and H. Fast hydrogen atoms fly far from the nucleus before they become ionized, and form a corona, the apparent size of which often exceeds the solar disk.

Tail and related phenomena.

The tail of a comet may consist of molecular plasma or dust. Some comets have both types of tails.

The dust tail is usually uniform and stretches for millions and tens of millions of kilometers. It is formed by dust grains thrown away from the core in the antisolar direction by the pressure of sunlight, and has a yellowish color because the dust grains simply scatter sunlight. The structures of the dust tail can be explained by the uneven eruption of dust from the core or the destruction of dust grains.

The plasma tail, tens or even hundreds of millions of kilometers long, is a visible manifestation of the complex interaction between the comet and the solar wind. Some molecules that leave the nucleus are ionized by solar radiation, forming molecular ions (H 2 O +, OH +, CO +, CO 2 +) and electrons. This plasma prevents the movement of the solar wind, which is permeated by a magnetic field. When the comet hits the comet, the field lines wrap around it, taking the shape of a hairpin and creating two areas of opposite polarity. Molecular ions are captured in this magnetic structure and form a visible plasma tail in its central, densest part, which has a blue color due to the spectral bands of CO +. The role of the solar wind in the formation of plasma tails was established by L. Biermann and H. Alfven in the 1950s. Their calculations confirmed measurements from spacecraft that flew through the tails of comets Giacobini–Zinner and Halley in 1985 and 1986.

Other phenomena of interaction with the solar wind, which strikes the comet at a speed of approx. 400 km/s and forming a shock wave in front of it, in which the matter of the wind and the head of the comet is compacted. The process of “capture” plays a significant role; its essence is that the neutral molecules of the comet freely penetrate the solar wind flow, but immediately after ionization they begin to actively interact with the magnetic field and are accelerated to significant energies. True, sometimes very energetic molecular ions are observed that are inexplicable from the point of view of the indicated mechanism. The capture process also excites plasma waves in the gigantic volume of space around the nucleus. The observation of these phenomena is of fundamental interest for plasma physics.

The “tail break” is a wonderful sight. As is known, in the normal state the plasma tail is connected to the comet's head by a magnetic field. However, often the tail breaks away from the head and lags behind, and a new one is formed in its place. This happens when a comet passes through the boundary of regions of the solar wind with an oppositely directed magnetic field. At this moment, the magnetic structure of the tail is rearranged, which looks like a break and the formation of a new tail. The complex topology of the magnetic field leads to the acceleration of charged particles; This may explain the appearance of the fast ions mentioned above.

Collisions in the Solar System.

From the observed number and orbital parameters of comets, E. Epic calculated the probability of collisions with the nuclei of comets of various sizes (Table 2). On average, once every 1.5 billion years, the Earth has a chance to collide with a core with a diameter of 17 km, and this can completely destroy life in an area equal to the area of ​​North America. Over the 4.5 billion years of Earth's history, this could have happened more than once. Smaller disasters are much more common: in 1908, the nucleus of a small comet probably entered the atmosphere and exploded over Siberia, causing the lodging of forests over a large area.

People watching a falling star in the sky may wonder, what is a comet? This word translated from Greek means “long-haired”. As it approaches the Sun, the asteroid begins to heat up and takes on an effective appearance: dust and gas begin to fly away from the surface of the comet, forming a beautiful, bright tail.

The appearance of comets

The appearance of comets is almost impossible to predict. Scientists and amateurs have been paying attention to them since ancient times. Large celestial bodies rarely fly past the Earth, and such a sight is fascinating and terrifying. History contains information about such bright bodies that sparkle through the clouds, eclipsing even the Moon with their glow. It was with the appearance of the first such body (in 1577) that the study of the movement of comets began. The first scientists were able to discover dozens of different asteroids: their approach to the orbit of Jupiter begins with the glow of their tail, and the closer the body is to our planet, the brighter it burns.

It is known that comets are bodies that move along certain trajectories. Usually it has an elongated shape, and is characterized by its position relative to the Sun.

The comet's orbit may be the most unusual. From time to time, some of them return to the Sun. Scientists say that such comets are periodic: they fly near planets after a certain period of time.

Comets

Since ancient times, people have called any luminous body a star, and those with tails behind them have been called comets. Later, astronomers discovered that comets are huge solid bodies, consisting of large ice fragments mixed with dust and stones. They come from deep space and can either fly past or orbit the Sun, periodically appearing in our sky. Such comets are known to move in elliptical orbits of varying sizes: some return once every twenty years, while others appear once every hundreds of years.

Periodic comets

Scientists know a lot of information about periodic comets. Their orbits and return times are calculated. The appearance of such bodies is not unexpected. Among them there are short-period and long-period.

Short-period comets include comets that can be seen in the sky several times in a lifetime. Others may not appear in the sky for centuries. One of the most famous short-period comets is Halley's Comet. It appears near the Earth once every 76 years. The length of this giant's tail reaches several million kilometers. It flies so far from us that it seems like a stripe in the sky. Her last visit was recorded in 1986.

Fall of comets

Scientists know of many cases of asteroids falling on planets, and not only on Earth. In 1992, the Shoemaker-Levy giant came very close to Jupiter and was torn into numerous pieces by its gravity. The fragments stretched out into a chain and then moved away from the planet’s orbit. Two years later, the chain of asteroids returned to Jupiter and fell on it.

According to some scientists, if an asteroid flies in the center of the solar system, it will live for many thousands of years until it evaporates, flying once again near the Sun.

Comet, asteroid, meteorite

Scientists have identified the difference in the meaning of asteroids, comets, and meteorites. Ordinary people call these names any bodies seen in the sky and having tails, but this is not correct. From a scientific point of view, asteroids are huge blocks of stone floating in space in certain orbits.

Comets are similar to asteroids, but they have more ice and other elements. When approaching close to the Sun, comets develop a tail.

Meteorites are small rocks and other space debris, less than a kilogram in size. They are usually visible in the atmosphere as shooting stars.

Famous comets

The brightest comet of the twentieth century was Comet Hale-Bopp. It was discovered in 1995, and two years later it became visible in the sky with the naked eye. It could be observed in celestial space for more than a year. This is much longer than the radiance of other bodies.

In 2012, scientists discovered Comet ISON. According to forecasts, it should have become the brightest, but, approaching the Sun, it could not meet the expectations of astronomers. However, it was nicknamed in the media “the comet of the century.”

The most famous is Halley's comet. She played an important role in the history of astronomy, including helping to deduce the law of gravity. The first scientist to describe celestial bodies was Galileo. His information was processed more than once, changes were made, new facts were added. One day Halley drew attention to a very unusual pattern of the appearance of three celestial bodies with an interval of 76 years and moving almost on the same trajectory. He concluded that these were not three different bodies, but one. Newton later used his calculations to construct a theory of gravity, which was called the theory of universal gravitation. Halley's Comet was last seen in the sky in 1986, and its next appearance will be in 2061.

In 2006, Robert McNaught discovered the celestial body of the same name. According to assumptions, it should not have glowed brightly, but as it approached the Sun, the comet began to quickly gain brightness. A year later, it began to glow brighter than Venus. Flying near the Earth, the celestial body created a real spectacle for earthlings: its tail curved in the sky.

COMET
a small celestial body moving in interplanetary space and abundantly releasing gas when approaching the Sun. A variety of physical processes are associated with comets, from sublimation (dry evaporation) of ice to plasma phenomena. Comets are the remnants of the formation of the Solar System, a transitional stage to interstellar matter. The observation of comets and even their discovery are often carried out by amateur astronomers. Sometimes comets are so bright that they attract everyone's attention. In the past, the appearance of bright comets caused fear among people and served as a source of inspiration for artists and cartoonists.
Movement and spatial distribution. All or almost all comets are components of the Solar System. They, like the planets, obey the laws of gravity, but they move in a very unique way. All planets revolve around the Sun in the same direction (which is called “forward” as opposed to “reverse”) in almost circular orbits lying approximately in the same plane (the ecliptic), and comets move in both forward and backward directions along highly elongated ( eccentric) orbits inclined at different angles to the ecliptic. It is the nature of the movement that immediately gives away the comet. Long-period comets (with orbital periods of more than 200 years) come from regions thousands of times farther than the most distant planets, and their orbits are tilted at all sorts of angles. Short-period comets (periods of less than 200 years) come from the region of the outer planets, moving in a forward direction in orbits lying close to the ecliptic. Far from the Sun, comets usually do not have "tails", but sometimes have a barely visible "coma" surrounding the "nucleus"; together they are called the "head" of the comet. As it approaches the Sun, the head enlarges and a tail appears.
Structure. In the center of the coma there is a core - a solid body or a conglomerate of bodies with a diameter of several kilometers. Almost all of the comet's mass is concentrated in its nucleus; this mass is billions of times less than the earth's. According to F. Whipple's model, the comet's nucleus consists of a mixture of various ices, mainly water ice with an admixture of frozen carbon dioxide, ammonia and dust. This model is confirmed by both astronomical observations and direct measurements from spacecraft near the nuclei of comets Halley and Giacobini-Zinner in 1985-1986. When a comet approaches the Sun, its core heats up and the ice sublimates, i.e. evaporate without melting. The resulting gas scatters in all directions from the nucleus, taking with it dust particles and creating a coma. Water molecules destroyed by sunlight form a huge hydrogen corona around the comet's nucleus. In addition to solar attraction, repulsive forces also act on the rarefied matter of a comet, due to which a tail is formed. Neutral molecules, atoms and dust particles are affected by the pressure of sunlight, while ionized molecules and atoms are more strongly affected by the pressure of the solar wind. The behavior of the particles that form the tail became much clearer after direct study of comets in 1985-1986. The plasma tail, consisting of charged particles, has a complex magnetic structure with two regions of different polarity. On the side of the coma facing the Sun, a frontal shock wave is formed, exhibiting high plasma activity.

Although the tail and coma contain less than one millionth of the comet's mass, 99.9% of the light comes from these gas formations, and only 0.1% from the nucleus. The fact is that the core is very compact and also has a low reflection coefficient (albedo). The particles lost by the comet move in their orbits and, entering the atmospheres of the planets, cause the formation of meteors ("shooting stars"). Most of the meteors we observe are associated with cometary particles. Sometimes the destruction of comets is more catastrophic. Comet Bijela, discovered in 1826, split into two parts in front of observers in 1845. When this comet was last seen in 1852, the pieces of its nucleus were millions of kilometers away from each other. Nuclear fission usually heralds the complete disintegration of a comet. In 1872 and 1885, when Bijela's comet, if nothing had happened to it, would have crossed the Earth's orbit, unusually heavy meteor showers were observed.
see also
METEOR ;
METEORITE. Sometimes comets are destroyed when approaching planets. On March 24, 1993, at the Mount Palomar Observatory in California, astronomers K. and Y. Shoemaker, together with D. Levy, discovered a comet with an already destroyed nucleus near Jupiter. Calculations showed that on July 9, 1992, the Shoemaker-Levy-9 comet (this is the ninth comet they discovered) passed near Jupiter at a distance of half the radius of the planet from its surface and was torn apart by its gravity into more than 20 parts. Before destruction, the radius of its core was approx. 20 km.

Table 1.
MAIN GAS COMPONENTS OF COMETS

Stretching out in a chain, the fragments of the comet moved away from Jupiter in an elongated orbit, and then in July 1994 approached it again and collided with the cloudy surface of Jupiter.
Origin. Comet nuclei are the remnants of the primary matter of the Solar System, which made up the protoplanetary disk. Therefore, their study helps to restore the picture of the formation of planets, including the Earth. In principle, some comets could come to us from interstellar space, but so far not a single such comet has been reliably identified.
Gas composition. In table Table 1 lists the main gas components of comets in descending order of their content. The movement of gas in the tails of comets shows that it is strongly influenced by non-gravitational forces. The glow of the gas is excited by solar radiation.
ORBITS AND CLASSIFICATION
To better understand this section, we recommend that you read the following articles:
CELESTIAL MECHANICS;
CONIC SECTIONS;
ORBIT;
SOLAR SYSTEM .
Orbit and speed. The movement of the comet's nucleus is completely determined by the attraction of the Sun. The shape of a comet's orbit, like any other body in the Solar System, depends on its speed and distance from the Sun. The average speed of a body is inversely proportional to the square root of its average distance to the Sun (a). If the speed is always perpendicular to the radius vector directed from the Sun to the body, then the orbit is circular, and the speed is called circular speed (vc) at a distance a. The speed of escape from the gravitational field of the Sun along a parabolic orbit (vp) is several times greater than the circular speed at this distance. If the comet's speed is less than vp, then it moves around the Sun in an elliptical orbit and never leaves the Solar System. But if the speed exceeds vp, then it moves around the Sun in an elliptical orbit and never leaves the Solar System. But if the speed exceeds vp, then the comet passes by the Sun once and leaves it forever, moving along a hyperbolic orbit. The figure shows the elliptical orbits of the two comets, as well as the nearly circular orbits of the planets and a parabolic orbit. At the distance that separates the Earth from the Sun, the circular speed is 29.8 km/s, and the parabolic speed is 42.2 km/s. Near Earth, the speed of Comet Encke is 37.1 km/s, and the speed of Comet Halley is 41.6 km/s; This is why Comet Halley goes much further from the Sun than Comet Encke.

Classification of cometary orbits. Most comets have elliptical orbits, so they belong to the Solar System. True, for many comets these are very elongated ellipses, close to a parabola; along them, comets move away from the Sun very far and for a long time. It is customary to divide the elliptical orbits of comets into two main types: short-period and long-period (almost parabolic). The orbital period is considered to be 200 years.
SPATIAL DISTRIBUTION AND ORIGIN
Almost parabolic comets. Many comets belong to this class. Since their orbital periods are millions of years, only one ten-thousandth of them appears in the vicinity of the Sun over the course of a century. In the 20th century observed approx. 250 such comets; therefore, there are millions of them in total. In addition, not all comets come close enough to the Sun to become visible: if the perihelion (the point closest to the Sun) of the comet’s orbit lies beyond the orbit of Jupiter, then it is almost impossible to notice it. Taking this into account, in 1950 Jan Oort suggested that the space around the Sun at a distance of 20-100 thousand AU. (astronomical units: 1 AU = 150 million km, distance from the Earth to the Sun) is filled with comet nuclei, the number of which is estimated at 1012, and the total mass is 1-100 Earth masses. The outer boundary of the Oort “comet cloud” is determined by the fact that at this distance from the Sun the movement of comets is significantly influenced by the attraction of neighboring stars and other massive objects (see below). Stars move relative to the Sun, their disturbing influence on comets changes, and this leads to the evolution of cometary orbits. So, by chance, a comet may end up in an orbit passing close to the Sun, but on the next revolution its orbit will change slightly, and the comet will pass away from the Sun. However, instead of it, “new” comets will constantly fall from the Oort cloud into the vicinity of the Sun.
Short-period comets. When a comet passes near the Sun, its core heats up and the ice evaporates, forming a gas coma and tail. After several hundreds or thousands of such flights, there are no fusible substances left in the core, and it ceases to be visible. For short-period comets that regularly approach the Sun, this means that their populations should become invisible in less than a million years. But we observe them, therefore, replenishment from “fresh” comets is constantly arriving. Replenishment of short-period comets occurs as a result of their “capture” by planets, mainly Jupiter. It was previously thought that long-period comets coming from the Oort cloud were captured, but it is now believed that their source is a cometary disk called the “inner Oort cloud.” In principle, the idea of ​​the Oort cloud has not changed, but calculations have shown that the tidal influence of the Galaxy and the influence of massive clouds of interstellar gas should destroy it quite quickly. A source of replenishment is needed. Such a source is now considered to be the inner Oort cloud, which is much more resistant to tidal influences and contains an order of magnitude more comets than the outer cloud predicted by Oort. After each approach of the Solar System to a massive interstellar cloud, comets from the outer Oort cloud scatter into interstellar space, and they are replaced by comets from the inner cloud. The transition of a comet from an almost parabolic orbit to a short-period orbit occurs when it catches up with the planet from behind. Typically, capturing a comet into a new orbit requires several passes through the planetary system. The resulting orbit of a comet typically has low inclination and high eccentricity. The comet moves along it in a forward direction, and the aphelion of its orbit (the point farthest from the Sun) lies close to the orbit of the planet that captured it. These theoretical considerations are fully confirmed by the statistics of cometary orbits.
Non-gravitational forces. Gaseous sublimation products exert reactive pressure on the comet's nucleus (similar to the recoil of a gun when fired), which leads to the evolution of the orbit. The most active outflow of gas occurs from the heated “afternoon” side of the core. Therefore, the direction of the pressure force on the core does not coincide with the direction of solar rays and solar gravity. If the axial rotation of the nucleus and its orbital revolution occur in the same direction, then the pressure of the gas as a whole accelerates the movement of the nucleus, leading to an increase in the orbit. If rotation and circulation occur in opposite directions, then the comet’s movement is slowed down and the orbit is shortened. If such a comet was initially captured by Jupiter, then after some time its orbit is entirely in the region of the inner planets. This is probably what happened to Comet Encke.
Comets touching the Sun. A special group of short-period comets consists of comets that “graze” the Sun. They were probably formed thousands of years ago as a result of the tidal destruction of a large core, at least 100 km in diameter. After the first catastrophic approach to the Sun, fragments of the nucleus made approx. 150 revolutions, continuing to fall apart. Twelve members of this family of Kreutz comets were observed between 1843 and 1984. Their origins may be related to a large comet seen by Aristotle in 371 BC.

Halley's Comet. This is the most famous of all comets. It has been observed 30 times since 239 BC. Named in honor of E. Halley, who, after the appearance of the comet in 1682, calculated its orbit and predicted its return in 1758. The orbital period of Halley's comet is 76 years; it last appeared in 1986 and will next be observed in 2061. In 1986, it was studied at close range by 5 interplanetary probes - two Japanese (Sakigake and Suisei), two Soviet (Vega-1 and Vega-1). 2") and one European ("Giotto"). It turned out that the comet's nucleus is potato-shaped, approx. 15 km and width approx. 8 km, and its surface is “blacker than coal.” It may be covered with a layer of organic compounds, such as polymerized formaldehyde. The amount of dust near the core turned out to be much higher than expected. See also HALLEY, EDMUND.

Comet Encke. This faint comet was the first to be included in the Jupiter family of comets. Its period of 3.29 years is the shortest among comets. The orbit was first calculated in 1819 by the German astronomer J. Encke (1791-1865), who identified it with the comets observed in 1786, 1795 and 1805. Comet Encke is responsible for the Taurid meteor shower, observed annually in October and November.

Comet Giacobini-Zinner. This comet was discovered by M. Giacobini in 1900 and rediscovered by E. Zinner in 1913. Its period is 6.59 years. It was with it that on September 11, 1985, the space probe "International Cometary Explorer" first approached, which passed through the tail of the comet at a distance of 7800 km from the nucleus, thanks to which data was obtained on the plasma component of the tail. This comet is associated with the Jacobinids (Draconids) meteor shower.
PHYSICS OF COMETS
Core. All manifestations of a comet are somehow connected with the nucleus. Whipple suggested that the comet's nucleus was a solid body consisting mainly of water ice with dust particles. This “dirty snowball” model easily explains the multiple passages of comets near the Sun: with each passage, a thin surface layer (0.1-1% of the total mass) evaporates and the inner part of the nucleus is preserved. Perhaps the core is a conglomerate of several “cometesimals,” each no more than a kilometer in diameter. Such a structure could explain the disintegration of nuclei, as observed with Comet Biela in 1845 or Comet West in 1976.
Shine. The observed brightness of a celestial body illuminated by the Sun with a constant surface changes in inverse proportion to the squares of its distances from the observer and from the Sun. However, sunlight is scattered mainly by the comet's gas and dust shell, the effective area of ​​which depends on the rate of ice sublimation, and that, in turn, on the heat flux incident on the nucleus, which itself varies inversely with the square of the distance to the Sun. Therefore, the brightness of the comet should vary in inverse proportion to the fourth power of the distance to the Sun, which is confirmed by observations.
Kernel size. The size of the comet's nucleus can be estimated from observations at a time when it is far from the Sun and not shrouded in a gas and dust shell. In this case, light is reflected only by the solid surface of the core, and its apparent brightness depends on the cross-sectional area and reflectance (albedo). The albedo of the nucleus of Comet Halley turned out to be very low - approx. 3%. If this is typical for other nuclei, then the diameters of most of them lie in the range from 0.5 to 25 km.
Sublimation. The transition of matter from a solid to a gaseous state is important for the physics of comets. Measurements of the brightness and emission spectra of comets have shown that the melting of the main ices begins at a distance of 2.5-3.0 AU, as it should be if the ice is mainly water. This was confirmed by studying the comets Halley and Giacobini-Zinner. The gases observed first as the comet approaches the Sun (CN, C2) are probably dissolved in water ice and form gas hydrates (clathrates). How this "composite" ice will sublimate depends largely on the thermodynamic properties of the water ice. Sublimation of the dust-ice mixture occurs in several stages. Streams of gas and small and fluffy dust particles picked up by them leave the core, since the attraction at its surface is extremely weak. But the gas flow does not carry away dense or interconnected heavy dust particles, and a dust crust is formed. Then the sun's rays heat the dust layer, the heat passes in, the ice sublimates, and gas flows break through, breaking the dust crust. These effects became apparent during the observation of Halley's comet in 1986: sublimation and outflow of gas occurred only in a few regions of the comet's nucleus illuminated by the Sun. It is likely that ice was exposed in these areas, while the rest of the surface was covered with crust. The released gas and dust form the observable structures around the comet's nucleus.
Coma. Dust grains and gas of neutral molecules (Table 1) form an almost spherical coma of the comet. Usually the coma stretches from 100 thousand to 1 million km from the nucleus. Light pressure can deform the coma, stretching it in an anti-solar direction.
Hydrogen corona. Since the core ices are mainly water, the coma mainly contains H2O molecules. Photodissociation breaks down H2O into H and OH, and then OH into O and H. The fast-moving hydrogen atoms fly far from the nucleus before they become ionized, and form a corona, the apparent size of which often exceeds the solar disk.
Tail and related phenomena. The tail of a comet may consist of molecular plasma or dust. Some comets have both types of tails. The dust tail is usually uniform and stretches for millions and tens of millions of kilometers. It is formed by dust grains thrown away from the core in the antisolar direction by the pressure of sunlight, and has a yellowish color because the dust grains simply scatter sunlight. The structures of the dust tail can be explained by the uneven eruption of dust from the core or the destruction of dust grains. The plasma tail, tens or even hundreds of millions of kilometers long, is a visible manifestation of the complex interaction between the comet and the solar wind. Some molecules that leave the nucleus are ionized by solar radiation, forming molecular ions (H2O+, OH+, CO+, CO2+) and electrons. This plasma prevents the movement of the solar wind, which is permeated by a magnetic field. When the comet hits the comet, the field lines wrap around it, taking the shape of a hairpin and creating two areas of opposite polarity. Molecular ions are captured in this magnetic structure and form a visible plasma tail in its central, densest part, which has a blue color due to the spectral bands of CO+. The role of the solar wind in the formation of plasma tails was established by L. Bierman and H. Alfven in the 1950s. Their calculations confirmed measurements from spacecraft that flew through the tails of comets Giacobini-Zinner and Halley in 1985 and 1986. Other phenomena of interaction with the solar wind, which hits the comet at a speed of approx. 400 km/s and forming a shock wave in front of it, in which the matter of the wind and the head of the comet is compacted. The process of “capture” plays an essential role; its essence is that the neutral molecules of the comet freely penetrate the solar wind flow, but immediately after ionization they begin to actively interact with the magnetic field and are accelerated to significant energies. True, sometimes very energetic molecular ions are observed that are inexplicable from the point of view of the indicated mechanism. The capture process also excites plasma waves in the gigantic volume of space around the nucleus. The observation of these phenomena is of fundamental interest for plasma physics. The “tail break” is a wonderful sight. As is known, in the normal state the plasma tail is connected to the comet's head by a magnetic field. However, often the tail breaks away from the head and lags behind, and a new one is formed in its place. This happens when a comet passes through the boundary of regions of the solar wind with an oppositely directed magnetic field. At this moment, the magnetic structure of the tail is rearranged, which looks like a break and the formation of a new tail. The complex topology of the magnetic field leads to the acceleration of charged particles; This may explain the appearance of the fast ions mentioned above.
Collisions in the Solar System. From the observed number and orbital parameters of comets, E. Epic calculated the probability of collisions with the nuclei of comets of various sizes (Table 2). On average, once every 1.5 billion years, the Earth has a chance to collide with a core with a diameter of 17 km, and this can completely destroy life in an area equal to the area of ​​North America. Over the 4.5 billion years of Earth's history, this could have happened more than once. Smaller disasters are much more common: in 1908, the nucleus of a small comet probably entered the atmosphere and exploded over Siberia, causing the lodging of forests over a large area.