Comets. Icebergs of the solar system

    The meeting of the European probe Rosetta with comet 67P / Churyumov-Gerasimenko is rightly considered the most interesting space news of this week . This comet, with dimensions of 3-5 km, is far from the only one that has received direct attention from interplanetary spacecraft. However, there is every reason to consider this meeting a landmark and we hope historical.

    The mission of the Rosetta probe is a logical consequence of the special, and we can say mystical, interest of mankind to the "shaggy" (komḗtēs) luminaries, as these ancient bodies called these celestial bodies. Below we will analyze in a popular form the knowledge accumulated by mankind about space “icebergs” and try to understand the huge interest in them from the scientific community.

    Punctual "grieve"

    The history of documented observations of comets dates back several thousand years, the most detailed description of the appearance of "shaggy" bodies can be found in ancient Chinese chronicles.

    Even then, the appearance of these luminaries was associated with mystical and most often tragic events. So the appearance of a bright comet in 240 BC was interpreted as a sign of the imminent demise of the Chinese empress. The same comet that appeared in the sky above Rome in 12 BC already "predetermined" the fate of Agrippa, a close friend and son-in-law of Emperor Augustus. In the 6th century, it also “caused” drought and unrest in Byzantium, and in 1066, according to contemporaries, it definitely doomed England to the invasion of William the Conqueror, Duke of Normandy.

    Comet Halley on a tapestry from Bayeux, 1066

    However, this comet was destined to play a very important role in the history of science. In 1682, the English astronomer Edmund Halley, calculating the orbit of the bright comet he observed, noticed that it coincides with the orbits of comets 1531 and 1607. Assuming that we are talking about the same comet, he predicted its appearance in perigee (the point of orbit closest to the sun) in 1758.

    Its appearance with a monthly delay in 1759 was more than enough to recognize the triumph of Newton's theory of gravitation. Halley's comet is now in the first line of the huge list of comets observed since then. Its 1P / 1682 index indicates that it is the first of the comets to "return" to the Sun, belongs to the group P - short-period comets and was discovered in 1682.

    Halley's comet orbital parameters

    Again, thanks to Halley's comet, which passed through the disk of the sun in 1910, astronomers were able to estimate the approximate sizes of cometary nuclei; it turned out to be less than 20 km. At the same time, for the first time, a spectral analysis of the tail of a “shaggy” body was made, which turned out to be rich in poisonous cyan and carbon monoxide. Which caused a great panic in the same year, when the Earth passed through the tail of a comet, which itself was groundless.

    A photograph of Halley's comet of 1910.

    By the next arrival of the comet in 1986, humanity was no longer limited to observations from the Earth (rather unfavorable that year). A whole flotilla of spacecraft set off to “intercept” the space “iceberg”. The composition of the Halley Armada was as follows:

    Halley's Comet in 1986

    - Two Soviet probes Vega 1 and Vega 2flying over a distance of about 9,000 km from the comet’s nucleus, compiled a 3D map of the nucleus and transmitted 1,500 images (picture below).

    - The European Giotto probe, approaching the core at a distance of 605 km, thanks to the navigation assistance of Soviet devices (photo below).

    - Two Japanese probes “Suisei” and “Sakigake”, which approached the core for 150,000 and 7 million km, respectively.
    - ISEE-3 (ICE) that studied the tail of Halley's comet from Lagrange point L1 (Earth-Sun system).

    Illustration of "Armada Halley", who studied the comet in 86 g

    A huge amount of information about cometary matter was obtained, thousands of shots of the nucleus were taken. Evaluation of the size of the comet's nucleus confirmed the observations of 1910 - the irregularly shaped nucleus 15 / 8km. Extensive experience has been gained in the interaction of various space agencies in solving complex technological problems.

    Unfortunately, the “Halley's comet year”, long expected by the scientific community, was overshadowed by two technological disasters - the death of the Challenger crew and the accident at the Chernobyl nuclear power plant.

    In addition to Halley's comet, astronomers have counted thousands of comets observed over the past 300 years. The cores range in size from several tens of meters to tens of kilometers, and are a mixture of dust and ice, most often water, ammonia and / or methane (the so-called Whipple “dirty snow” model). However, it is obvious that many cores can depart to some extent from this model. So the Deep impact space probe, which dropped the “projectile” onto the Tempel 1 comet, in 2005, made it possible to establish that the comet consists mainly of a porous dust frame.

    “Bombing” of Tempel’s comet with the Deep impact probe and the subsequent passage near the comet of the Stardust probe

    Being the preserved bricks of the primary building material of the solar system, comets are of great interest for geology, chemistry and biology. Presumably, it was the comets that in ancient times delivered to the Earth the main part of the water of its hydrosphere. In the spectral lines of many comets, complex organic compounds were found up to amino acids and urea. Scientists suggest that it is comets, as incubators of complex organic compounds, that could bring to Earth a chemical base for the appearance of life.

    Approaching perihelion, cometary nuclei, under the action of solar radiation, begin to spew huge volumes of gases, bypassing the liquid aggregate state of melting ice (sublimation). Gases, in turn, carry away large masses of dust mixed in ice, which, together with particles of ice, are blown away by solar radiation and wind, in the opposite direction from the star.

    The dimensions of cometary “tails” can reach several hundred million kilometers in length. So, in 1996, the Ulysses space probe (NASA / ESA) unexpectedly passed through the tail of the 1996 Great Comet C / 1996 Hyakutake ... 500 million kilometers behind it!

    However, the tails of comets are far from always “straight” or directed back from the sun. Depending on the orbital features of the comet, its composition, the solar wind, or the interaction of the magnetic field of the sun with the ionized substance of the “shaggy” star, the tail can be directed either perpendicularly or towards solar radiation. Moreover, in a single comet, the tail can consist of several differently directed parts, or even have the appearance of a huge gas-dust shell.

    Comet 17P / Holmes is an example of the atypical structure of the comet’s gas-dust shell (coma), showing the comparative dimensions of its coma with the Sun and Saturn

    Since 1995, all comets are usually divided into classes: P / - Short-period comets, with a circulation period of less than 200 years. C / - long-period comets, with a circulation period of more than 200 years. X / - comets with unknown parameters of the orbit (historical comets). D / - collapsed or "lost" comets, and finally class A / - asteroids mistaken for comets.

    Collision of comet Shoemaker-Levy 9 with Jupiter in 1994. Later, the comet was reclassified to the death row class D / 1993 F

    In front of the class index (most often P /), the ordinal number of the confirmed passage by the comet of the perihelion (the nearest point of the orbit) is usually placed, and after - the year of discovery. After a year of discovery, they usually set a letter denoting ½ month and the serial number of the opening, for example, A for comets open in the first half of January and Y, respectively, for the second half of December. And already at the end are the names of the discoverers. So, the nomenclature name of the comet Churyumov-Gerasimenko would look something like this: 67P / 1969 R1. However, it is most often abbreviated as (n) P / Surname of the discoverer.

    Particularly noteworthy is the class of "comet extremals" passing extremely close to the Sun. Almost always they are fixed by space probes studying our star - SOHO and the “twins” Stereo A and B. It is assumed that the bulk of these comets are fragments of one giant comet that collapsed thousands of years ago (Kreits comets)

    "Harem of the King" of planets

    The main part of short-period comets, in turn, is divided into 4 large families, according to the parameters of the orbit and the gravitational influence of the "master" giant planet. Jupiter possesses the most numerous “family”, it is he who “belongs” to the following comets:

    19Р / Borelli , next to which the Deep Space 1 (NASA) probe worked in 2001;

    103P / Hartley 2, studied by the Deep Impact probe (NASA) in 2010 (animation below), after the aforementioned visit to comet 9P / Tempel (Tempel 1), another typical representative of the "family";

    Comet 81P / Wilda, next to which the Stardust (NASA) probe was able to collect dust samples and deliver them to Earth in 2006;

    Comet 67P / Churyumova-Gerasimenko, studied by the Rosetta probe (ESA), also by its characteristics refers to the "family of the king" of the planets.

    Then come the families of the comets of Saturn, Uranus and Neptune, respectively, with the Halley comet mentioned at the beginning is a typical representative of the short-period comets of the Neptune family.

    "Chaos" in the belt of "stability"

    Some short-period comets, according to the version most popular among scientists, “fly” to us from the outer borders of the Kuiper belt - the Scattered Disk (RD). Together with the Kuiper belt, the taxiway is a huge disk of large ice bodies with diameters from several tens of meters to thousands of kilometers (Pluto and Charon). Stretching from a distance of 35 astronomical units (the orbit of Neptune), to the outer borders of 50 AU (or 100 AU with RD) the belt has an estimated mass of 1-8 masses of the moon (the asteroid belt is not more massive than 0.04 masses of the moon). The Kuiper belt itself is generally stable, thanks to orbital resonances with Neptune and with each other.

    Distribution map of famous Kuiper belt objects (distance graph in ae)

    The current state of the Kuiper belt and the Oort cloud are associated with the ancient migration of Neptune to the outer regions of the solar system, under the influence of the resonances of Jupiter and Saturn. Part of the matter was ejected from the solar system, part, together with the Oort cloud, into its outer parts. Millions of other debris were thrown into the inner part of the solar system, causing a late heavy bombardment 4-3.5 billion years ago.

    The solar system before the "migration" of Neptune (purple orbit) - (a), during (b) and after (c). Green indicates the orbit of Uranus

    To explain the instability of an external, scattered disk, you will have to resort to the basics of celestial mechanics. The two main parameters of the orbit of a celestial body are the apocenter (the point of greatest distance from the surface of a planet or star, in the latter case they speak of apogelia) and the pericenter (the closest point of the orbit, or in the case of revolution around the sun - perihelion). The difference between these values ​​is expressed in the eccentricity of the orbit - the degree of its deviation from the ideal circle (e = 0) to the ellipse (e> 0, but <1) and further to the parabola (e = 1) and the hyperbola (e> 1)

    In the last two cases we are talking about the trajectory of non-return. Changing the parameters of the orbit is possible at any of its points, but the apogelium is most affected by changes in velocities in perihelion (an increase in apogel during acceleration and a decrease during inhibition) and vice versa. And the stronger the eccentricity, the greater the effect of the change in speed. Moreover, the “sensitivity” of the orbit to perturbations increases with its height, since with an increase in the orbit, the speed of the orbital revolution of the body decreases inversely (people familiar with the Orbiter and KSP simulators know this firsthand).

    In the inner part of the solar system, in the zone of the terrestrial planets and the asteroid belt, the orbital velocities of the bodies are quite high (tens of km / s), and the eccentricities are relatively small. Therefore, for strong orbital disturbances, it is necessary to expend a lot of energy. At the outer boundary of the Kuiper belt, in a scattered disk, the orbital velocities of bodies usually lie in the range from several km to several hundred m / s, so even small gravitational perturbations or collisions very much change the eccentricity. The celestial body significantly increases its apogel (acceleration), or reduces perihelion (inhibition), heading into the inner parts of the solar system.

    Difference tableorbital speeds in the solar system? Mercury - Mars (terrestrial group), Jupiter - Neptune (giants) and Pluto (the inner part of the Kuiper belt)

    Space truckers

    But nevertheless, according to the opinion most widespread in the scientific community, most short-period comets of class P / and all comets of class C / come to us from the supposed Oort cloud. The inner part of the Cloud, looks like a toroidal belt, stretching over a distance of 2000 to 20,000 astronomical units (Hills cloud). The mass of this cloud is estimated at least two dozen Earth masses.

    Comparative sizes of the orbits of the terrestrial planets against the background of the Kuiper belt, and accordingly the sizes of the latter against the background of the Oort cloud

    The Hills cloud serves as a kind of fuel for an external, spherical cloud, massive in several earth masses, stretching from a distance of 20,000 AU up to 1 light year, to the gravitational boundary of the solar system (Hill sphere). It is the outer Oort cloud that is considered the main "supplier" of comets to the inner part of the solar system. Presumably these are the remains of the primary “building material” of the solar system, therefore, these objects are of great scientific interest. The braking and acceleration effects described for the Kuiper belt are much stronger here, due to the extremely low orbital speeds of comets (meters per second).

    Of the most famous long-period comets of recent decades, comets C / 1996 B2 Hyakutake, C / 2006 R1 and C / 2009 P1 Maknota should be noted. Having come to us from distant regions of the Oort cloud, both comets for the first and last time, flying perihelion, forever left the solar system along a hyperbolic trajectory (eccentricity is greater than 1).

    C / 1996 B2 Hyakutake in the sky of the sky

    C / 2006 P1 Maknota (“Big Comet 2007”) with another example of an arched “wrong” coma

    In 2010, comet Elenin (C / 2010 X1) intended to do the same, but Jupiter’s gravitational perturbation “prescribed” the comet in the solar system, decreasing the eccentricity below 1 (apogel about 500 AU). The famous "Big Comet of 1997" by Hale Bopp (C / 1995 01) intended only to give another round of honor at its perihelion, almost perpendicular to the plane of the Earth's orbit. However, the inexorable gravity of Jupiter, and this time reduced the perihelion of the comet by half - from 600 (period of revolution 4800 years) to 350 AU (period of revolution of 2400 years).

    1997 Big Comet by Hale Bopp

    And perhaps the biggest astronomical disappointment of 2013 was the comet ISON (C / 2012 S1), moving along a parabolic trajectory (e = 1) from the very outskirts of the solar system, the celestial body literally fell apart during its perihelion.

    Modeling the history of the change in the orbit of our old familiar comet Halley showed that she also came to the solar system from the distant Oort cloud. The gravitational perturbations of the giant planets, as is the case with many other comets, “prescribed” it in the family of Neptune's comets. The apogel of the comet’s orbit barely touches the Kuiper belt (35 AU), and the perihelion passes closer than Venus 88 million km from the Sun. The next time the comet returns to perihelion in 2061.

    In conclusion, I would like to recall the words of Mark Twain, as I was born in the year of the appearance of Comet Halley (albeit a difference of 150 years): “I came to this world with a comet and will also leave with it when it arrives next year” (with ) 1909 Mr. Twain really left in 1910, and with him Leo Tolstoy and the famous Italian astronomer Schiaparelli. Agree, not the most boring company to travel around the solar system.

    To the readers, I sincerely wish to live to that momentous time, and let no technological disasters or the death of idols spoil your impression of admiration for the beauty of the famous space wanderer.


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