Comets. Solar system icebergs

The meeting of the European Rosetta probe with comet 67P / Churyumov-Gerasimenko is deservedly considered the most interesting space news of this week. This comet, 3-5 km in size, is far from the only one that received the immediate attention of interplanetary spacecraft. However, there is every reason to consider this meeting to be a landmark one and hopefully be historic.

The mission of the Rosetta probe is a logical consequence of the special, and it can be said mystical, interest of mankind towards the "shaggy" luminaries, as the ancient Greeks called these heavenly bodies. Below we will analyze in a popular form the knowledge accumulated by mankind about the space icebergs, and we will try to understand the great interest in them from the scientific community.

Punctual "mountain lady"

The history of documented observations of comets dates back several thousand years, the most detailed description of the appearance of "shaggy" luminaries can be found in the ancient Chinese chronicles.
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Even then, the appearance of these luminaries was associated with mystical and most often tragic events. So the appearance of a bright comet in 240g. BC. It was interpreted as a sign about the imminent demise of the Chinese empress. The same comet appeared in the sky above Rome in 12g BC. already "prejudged" the fate of Agrippa, a close friend and son-in-law of the 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 the tapestry of 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 coincided with the orbits of the comets of 1531 and 1607. Assuming that we are talking about the same comet, he predicted its appearance in the perigee (the point of orbit nearest the sun) in 1758.

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

The parameters of the orbit of Comet Halley


Again, thanks to Halley's comet, which traveled across the solar disk in 1910, astronomers were able to estimate the approximate dimensions of cometary nuclei, which turned out to be less than 20 km. At the same time, the spectral analysis of the tail of the "shaggy" star, as it turned out to be rich in poisonous cyan and carbon monoxide, was first performed. What caused a great panic in the same year, when the Earth passed through the comet's tail, of course, groundless.

Snapshot of Halley's Comet 1910


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

Comet Halley in 1986


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


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


- Two Japanese probe "Suisei" and "Sakigake", approached the core at 150,000 and 7 million km, respectively.
- ISEE-3 (ICE), who studied the tail of Comet Halley from the Lagrange point L1 (Earth-Sun system).

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


A huge amount of information about the cometary matter was obtained, thousands of images of the nucleus were taken. Estimation of the size of the comet's nucleus confirmed observations of 1910 - the core of irregular shape 15 / 8km A great experience has been gained in the interaction of various space agencies in solving complex technological problems.

Unfortunately, the long-awaited scientific community "year of Halley's comet" was overshadowed by two man-made disasters - the death of the crew of the Challenger and the accident at the Chernobyl nuclear power plant.

In addition to Halley's comet, astronomers have thousands of comets observed over the past 300 years. The cores range in size from a few 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 kernels may deviate to some extent from this model. Thus, the deep impact space probe, which dropped the “projectile” onto Tempel 1 comet, in 2005, made it possible to establish that the comet consists mainly of a porous dust frame.

Bombardment of Comet Tempel with the Deep Impact probe and subsequent flight near the Comet of the Stardust probe

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

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


The size of the 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 Big Comet C / 1996 Hyakutake ... 500 million kilometers behind it!

However, comet tails are not 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" luminary, the tail can be directed both perpendicularly and toward solar radiation. Moreover, in one comet, the tail may consist of several differently directed parts, or even have the appearance of a huge gas-dust envelope.

Comet 17P / Holmes is an example of the atypical structure of a gas-dust envelope (coma) of a comet; the comparative dimensions of its coma with the Sun and Saturn are shown


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 orbit parameters (historical comets). D / - destroyed or "lost" comets and finally class A / - asteroids taken for comets.

The collision of comet Shoemaker-Levy 9 with Jupiter in 1994. Later, the comet was reclassified to the "suicide" class D / 1993 F


Before 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 located, and after that - the year of the discovery. After the year of discovery, the letter is usually designated ½ month and the serial number of the discovery, for example, A for comets discovered in the first half of January and Y respectively for the second half of December. And at the end of 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 in the form of (n) P / the name of the discoverer.

Special attention should be paid to the class of “comets of extremes”, passing extremely close to the Sun. Almost always, they are recorded by space probes studying our star - SOHO and the “twins” Stereo A and B. It is assumed that the main part of these comets are fragments of one giant comet that collapsed thousands of years ago (Kreutz)

"Harem of the King" of the 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 has the most numerous "family"; it is to him that the following comets "belong":

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


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


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


Comet 67P / Churyumov-Gerasimenko , studied by the Rosetta probe (ESA), also belongs to the “family of the king” of the planets.

Further, respectively, follow the family of comets of Saturn, Uranus and Neptune, and the comet Halley 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 most popular version among scientists, “arrive” to us from the outer borders of the Kuiper belt - the Scattered disk (RD). The RD, together with the Kuiper belt, is a huge disk of large ice bodies with a diameter of 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 limits of 50 AU. (or 100 AU with a RD) the belt has an estimated mass of 1–8 Moon masses (the asteroid belt is not more massive than 0.04 Moon masses). Actually the Kuiper belt is generally stable, thanks to orbital resonances with Neptune and with each other.

Kuiper Belt known objects distribution map (distance plot in ae)

The current state of the Kuiper belt and the Oort cloud is associated with the most ancient migration of Neptune to the outer regions of the solar system, under the influence of Jupiter and Saturn resonances. Part of the substance was ejected from the solar system, part, together with the Oort cloud - in its outer parts. Millions of other debris were thrown back 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, it is necessary 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 the 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 a 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 hyperbola (e> 1)


In the latter two cases, it is a trajectory of non-return. A change in orbit parameters is possible at any point, but the changes in velocity in perihelion (increase in apogelium during acceleration and decrease during deceleration) and vice versa affect apogelium most of all. And the stronger the eccentricity, the greater the effect of changing speeds. Moreover, the “sensitivity” of the orbit to disturbances increases with its height, as the orbital velocity of the body decreases inversely with the increase in orbit (people familiar with the Orbiter and KSP simulators know this not by hearsay).

In the inner part of the solar system, in the zone of the terrestrial planets and the asteroid belt, the orbital velocities of bodies are rather high (tens of km / s), and the eccentricities are relatively small. Therefore, for strong orbital perturbations, it is necessary to expend a lot of energy. On the outer edge 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; therefore, even small gravitational perturbations or collisions greatly change the eccentricity. The celestial body significantly increases its apothelium (acceleration), or decreases perihelion (inhibition), heading for the inner parts of the solar system.

Table of the difference in orbital velocities in the solar system? Mercury - Mars (earth group), Jupiter - Neptune (giants) and Pluto (inner part of the Kuiper belt)

Space truckers

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

The comparative size of the orbits of the terrestrial planets on the background of the Kuiper belt, and accordingly the size of the latter on the background of the Oort cloud


The Hills cloud serves as a kind of feeding of an external, spherical cloud, a mass of 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's 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 remnants 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 velocities of comets (meters per second).

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

C / 1996 B2 Hyakutake in the Earth's Sky


C / 2006 P1 McNaught (“Big Comet 2007”) with another example of arched “irregular” coma


In 2010, comet Elenin (C / 2010 X1) intended to do the same, but the gravitational perturbation of Jupiter “prescribed” the comet in the solar system, reducing the eccentricity below 1 (apohelium about 500 AU). The famous Big Comet of 1997, Hale Bopp (C / 1995 01), intended only to give the next lap of honor at the perihelion of its own, almost perpendicular to the plane of the Earth’s orbit. However, the implacable gravity of Jupiter again reduced the comet's perihelion by half — from 600 (a period of 4800 years) to 350 a.e (2400 years).

1997 Big Comet Hale Bopp


And perhaps the most astronomical disappointment of 2013 was comet ISON (C / 2012 S1), moving along a parabolic trajectory (e = 1) from the very edges of the solar system, the celestial body literally collapsed as it passed its perihelion.


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

In conclusion, I would like to recall the words of Mark Twain, like I was born in the year of the emergence of comet Halley (albeit with a difference of 150 years): “I came to this world with a comet and leave, too, with her when she arrives next year” ) 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.

Readers, I sincerely wish to live to that significant time, and let no man-made disasters or the death of idols spoil your impression of admiration for the beauty of the famous cosmic pilgrim.