Theory Comets

Comets are frozen balls of dust, gas, and rock. Comets are considered leftovers of the solar system formation, or time capsules containing primitive material left over from the epoch when the Sun and planets formed. There are as many as one hundred billion comets in the outer regions of the solar system. As of 2016, a total of eight comets have been visited by space missions helping to figure out what those cosmic bodies are exactly looking like. Comets are the icy remnants left over from the formation of the planets 4.6 billion years ago. Comets are known to be a mixture of dust and ice, and if fully compact, they would be heavier than water. However measurements have shown that some of them have extremely low densities, much lower than that of water ice, as they should be homogeneously highly porous. Comets' porosity are likely a intrinsic property of dust particles mixed with the ice that make up the interior because cometary dust is typically not a compacted solid, but a fluffy aggregate instead. Typical comet, which are believed to form in the outer Solar System are icy, rather than rocky

Comets may be of the periodic type, that is that they regularly come into the inner solar system. Most of the periodic comet however never were officially seen as such as most weren't seen more than twice at their perihelion. From the thousands of comets seen, 190 only are classified periodic. Short-period comets are orbiting the Sun in less than 200 years and they grossly do in the ecliptic plane. Most such comets are thought to come from the Kuiper Belt, this region of comets and diverse leftovers which lies beyond Neptune's orbit. A comet from the Kuiper Belt gets tossed into the inner solar system through Neptune's gravitation every 40 to 100 years as it is then deflected by planets until Jupiter's gravity sets its definitive orbit. Long-period comets, as far as they are concerned, may have orbits of a duration somewhere between 12,000 to 30 million years, as they have more varied inclinations to the ecliptic. Such comets are originating in a more distant zone, the Oort Cloud, a huge cloud of leftovers dating back to the early solar system. Oort Cloud comets, with their distance from the Sun beginning at 186 billion miles (300 billion kilometers) can have periods of thousands or even millions of years. Long-period comets measuring at least 0.6 miles (1 kilometer) are numerous and on average up to twice as large as 'Jupiter family comets,' whose orbits are shaped by Jupiter’s gravity during close passages and have periods of less than 20 years, mostly about 5 to 7. Long-period comets passing to the Sun are smaller than those spending much more time away from the Sun and researchers also found clustering in the orbits of the long-period comets, suggesting there could have been larger bodies that broke apart to form these groups. It was Comet 2P/Encke -- owing its name to the German astronomer who computed its orbit -- which turned the first short period comet observed, early in the 19th century. With a 3.3-year period and a aphelion inside Jupiter's orbit, its discovery was thrilling, as all comets known until then were of the Halley type with a important period and a aphelion well beyond Uranus. Some comets, at last, may have hyperbolic or parabolic orbits, meaning they are passing just once in the inner solar system. Some comets had being born in a very cold region of the protoplanetary nebula as soon than more than 4.5 billion years ago. Most comets are passing between 0.05 and 2.5 AU from the Sun where they come at their perihelion, that is at their nearest Sun. Comets often begin like objects dwelling in the cold, dark reaches of the solar system as they may be knocked around by the gravitational tug of Jupiter, after that and settle into a much closer orbit to the Sun. Most comets the orbit of which begins like parabolic, are coming from the far edge of the solar system as the orbit is reworked by the tug of giant planets Jupiter and Saturn into very elongated but closed ellipses with orbital periods of hundreds of thousands to millions of years. Comets are eventually more rare than the asteroids in the inner solar system compared to the large band of the Asteroid Belt. The famed Halley comet, which passes each 76 years in the inner solar system, is now heading to its farthest from the Sun. It will not be back until 2061. Comet Halley as a retrograde orbit and a high inclination to the ecliptic, likely due to that it originated into the Oort Cloud, as short-period comets likely had been born in the inner solar system and remained in the plane of it. The gravitational tugs play likely will eventually toss out comets like Halley out of the solar system. A recent concept is developping, the one of 'dead comets', which are comet which have shed the whole of their materials during repeated journeys close to the Sun. They can be darker than coal and hard to find except in infrared studies. New comets, generally, passing close by the Sun for their first time, do break apart about two percent of the time into a trail a chunks of dust and ice, as fragile and loosely held together

Comet Borrelly's nucleus, jets, and coma. As a comet’s nucleus is rotating even when the comet passes to its closest Sun, any jet spewn from, appear to spin like the water jet from a rotating lawn sprinkler. picture NASA/JPL

EPOXI Flyby, the Latest Probe to a Comet!
NASA mission EPOXI, which is the Deep Impact mission prolonged into a mission observing another comet and exoplanets, is bound to comet Hartley 2 for a flyby by Nov. 4 2010, providing the best extended view of a comet in history during its pass through the inner solar system, flying over the nucleus by just 435 miles. The Hubble Space Telescope is helping with studies of the comet from afar as its shows that the nucleus has a diameter of approximately 0.93 miles (1.5 km), which is consistent with previous estimates. The comet is in a highly active state, as it approaches the Sun. The Hubble data show that the coma is remarkably uniform, with no evidence for the types of outgassing jets seen from most "Jupiter Family" comets, of which Hartley 2 is a member. Jets can be produced when the dust emanates from a few specific icy regions, while most of the surface is covered with relatively inert, meteoritic-like material. In stark contrast, the activity from Hartley 2's nucleus appears to be more uniformly distributed over its entire surface, perhaps indicating a relatively "young" surface that hasn't yet been crusted over. On its trajectory to fly past comet Hartley 2, the EPOXI mission self has began imaging the target and scrutinizing it during more than 2 months
103P/Hartley has a orbital period of 6.46 years. It was discovered by Malcolm Hartley in 1986 at the Schmidt Telescope Unit in Siding Spring, Australia

More About Cometary Space Missions!
Since the inception of our site, we have followed several cometary space missions, for which we had a dedicated page. check those at the following links: Deep Impact: The Independence Day Impactor! (comet Tempel 1, 2005); Stardust: Target Comet Wild (comet Wild, 2006); EPOXI Flyby at Comet Hartley 2 (comet Hartley 2, 2010); Stardust-NExT: A Mission At Tempel 1 After Deep Impact! (comet Tempel 1, 2011); Rosetta: Journeying With A Comet To the Sun (comet Churyumov-Gerasimenko, 2014)

The Kuiper Belt is thought to extend beyond Neptune, between 2.8 billion and 28.6 billion miles (4.5 billion and 46 billion kilometers), that is, for its largest, up to 300 Astronomical Units (AU) -300 times the distance Sun-Earth. Important comets, those with a nucleus of about 25-mile (40-kilometer) wide, are thought to be mostly within 50 AU from the Sun. Much of the short-period comets have diameters of about 3,300 ft to 6 miles (about 1 to 10 kilometers). The Kuiper Belt's exact content -and even extent- is still in doubt

a series of comet nucleus as seen by space missions!. NASA

Comets are icy and dusty bodies. As a comet, generally, along the eons, reduces its orbit to Mars and Jupiter, for example, mechanical force let room to sublimation in terms of erosive forces acting upon. Why a comet has a soft interior and a hard crust might be like fluffy ice on the surface of a comet would crystalize and harden as the comet heads toward the Sun and warms up. As the water-ice crystals form, becoming denser and more ordered, other molecules containing carbon would be expelled to the comet's surface. The result is a crunchy comet crust sprinkled with organic dust. As comets come nearer to the Sun, their nucleus is vaporized by heat. Along a solar travel, a comet may display colour cycles in the coma and the surface due to its activity of comet. Certain volatiles on a comet may sublimate earlier on the comet's course towards the Sun. Typically, a comet's water content remains frozen until it comes within about three times Earth's distance to the Sun. More volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances. On a first swingby, exotic ices of nitrogen and carbon dioxide, long preserved in the deep freeze of the outer solar system, vaporize at great distances from the Sun, making the comet appear unusually bright and remarkable. Once those ices are gone and during subsequent passages, the comet may have only a modest amount of water ice remaining for the Sun to vaporize and not brighten as much. Comets may have delivered more than one-fifth of Earth’s atmospheric xenon. A comet the material of which is vaporized by Sun yields a tail which may extend up to millions of miles (millions of km). Such a tail is generally directed away from Sun, no matter whether comet is approaching the Sun or receding from it. Another smaller tail, only made of dust, may exist too, with no preferential direction except that the heaviest dust stay behind the nucleus, along the comet's path (when a comet, on its journey, comes to cross the heliospheric current sheet, a area where the spiral magnetic field of the Sun in the solar system, changes polarity, its dust tail may be disrupted). The "coma" is the nebulous halo which is surrounding the comet's nucleus as created by comet's jets activity which spray dust particles and icy grains into space (most comets typically support jets over only about 3 percent of their surfaces). It's a cloud of dust and gas. Coma, generally, is a huge cloud of atomic hydrogen because water vaporizes from the icy nucleus, and solar ultraviolet light breaks it apart into hydrogen, oxygen, and hydroxyl molecules. Coma may be considered the comet's atmosphere. A well-developed, parabolic shape indicates that the comet had a dense gaseous atmosphere. Comets produce X-ray emission when particles in the solar wind strike the atmosphere of the comet. As the gas flows away from the nucleus, it carries a cloud of tiny dust particles out into space, which slowly expands to create the coma. 3-D maps of a coma has shown that HCN molecules (made of hydrogen, carbon and nitrogen) are released from the nucleus and then spread evenly throughout the coma. Formaldehyde and HNC (made of one hydrogen, one nitrogen and one carbon and breakdown of large molecules and organic dust inside the coma), on a other hand, are produced in the coma self. The warming continues and activity rises as the comet moves ever closer to the Sun and gases coming off the comet's nucleus run into its coma as it is warmed by the Sun. Eventually, pressure from the solar wind causes some of the material to stream out into a long tail. As comets are non-spherical and lumpy, this process is often unpredictable, with activity waxing and waning as they warm. A comet also yields a ion tail when the solar wind made of hot, charged gas, called plasma—sweeps over the comet, capturing the material that has been vaporized into plasma by sunlight, causing it to trail out behind the comet. This tail follows the lines of the magnetic field embedded in the solar wind. The "coma" is produced by the same processes which are yielding the comet's tails. Nuclei are small bodies, with a diameter of about 6 miles (9.5 km), as comae may exceed Jupiter in size! The Ulysses NASA's solar mission developed the concept to gauge the size of the region of space disturbed by a comet's presence, as comets are filling the solar outflow with unusual gases and molecules and slowing the solar wind inside the comet's tail by some 40 percent. A bow shock forms at a comet there where the solar wind collide with, when the comet is nearing the Sun. In its infancy, a bow shock may exist relatively close to a comet. Comet McNaught thus was found remarkably productive in releasing gas and material from its surface and producing the longest tail ever observed in the solar system. Measuring such comet "outgassing" can define the level of activity of a comet, but it does not directly relate to its size however as, in the case of that comet, it would have to be much larger in size to produce such a massive tail. Typically, a comet's water content remains frozen until it comes within about three 3 astronomical units (AU) from the Sun, or the so-called 'snow line.' Some of the comet’s dust also remains on the surface as the ice below sublimates, or falls back on to the nucleus elsewhere, coating it with a thin layer of dusty material and leaving very little ice directly exposed on the surface. A variety of gases, including water vapour, carbon dioxide and carbon monoxide are also originating from frozen reservoirs below the surface. Boulders of, or coated with ice are also seen at comets as they likely result from the comet's passage to the Sun which ejects those out from their original location. Once a comet returning to the depths of interplanetary space, dust is back confined at the surface as the gas continues evaporating at a very low level, coming from ever deeper below the surface. After a passage to the Sun, the surface of a comet is left with grains of dust making. Comets are known to contain other frozen gases than water's, such as carbon monoxide and dioxide (CO and CO2), which sublimate at colder temperatures and much farther from the Sun. Carbon monoxide (CO) and carbon dioxide (CO2) are common molecules found in the environment of the early solar system, and in comets. At larger distances and colder temperatures, other common molecules like CO and CO2 may be the main drivers of the evolution of comets, instead of sublimation due to Sun' heat. Comets may shed up to 400 gallons of water each second, or enough to fill an Olympic-size swimming pool in under 30 minutes as such water given off by the comet is only about half of the dust mass it produces at the same time. Water molecules quickly breaks up into hydrogen atoms and hydroxyl (OH) molecules when exposed to ultraviolet sunlight. Comets are now believed to be carriers of well-preserved materials dating back to the formation of the solar system, some 4.6 billion years ago. Comets now, too, are seen like bodies which, along with asteroids, took part to the Heavy Bombardment Period. Such a period occurred around 3.8 billion years ago when the planets, once formed, just swept, all the debris and various objects which were remaining from the solar system's formation. Such objects heavily struck the planets. Comets and asteroids are thought to have thus brought water, and the fundamental blocks of life! Conspicuously naked-eye comets are representing less than 10 percent of all the comets ever recorded. The term "comet" is originating from a Latin world, "comata", wich means "hairy", refering to the comets' tails. Water, carbon monoxide, ammonia and methanol are four of the most abundant compounds and gases found at a comet. Cyanide, a deadly gas is one of the most easily observed ingredients that is always present in a comet. Comets may release cyanide gas when approaching the Sun as some even do without any dust emission as mostly cometary outbursts always are accompanied by considerable dust. Some cyanide jets are sometimes seen too. Cyanide is a carbon-based molecule as it was brought Earth during the Heavy Bombardment Period

Comet Borrelly's 5-mile (8 km) long nucleus as seen from 2,000 miles (3,200 km). picture NASA/JPL

->What is Composing a Comet?
From the data collected by the Spitzer Telescope's infrared spectrometer during the impact of mission Deep Impact at comet Tempel 1, this particular comet was found made of these following elements: clays, ice, and dry ice. Iron-containing compounds, carbonates, the minerals in seashells, crystallized silicates, such as the green olivine minerals, and polycyclic aromatic hydrocarbons, these carbon-containing compounds found in car exhaust or on burnt toast. Hints of the mineral found in the reddish-brown gem spinel were also observed. Additional molecules include water vapor and carbon dioxide gas. Is such composition typical of any comet? Magnesium-rich olivine in any case, is a constituent of importance of comets, at between 2 to 10 percent

->Much Variety Among the Comets Materials
As comets, until now, were thought to be composed from interstellar dust and ice, no less than 10 percent of comet Wild2, that comet samples of which were taken back Earth by the Stardust mission, originated from the inner solar system. Another finding is that they were likely different stages along which various materials formed and/or aggregated during the solar system formation, which are found back in comets
It might that some comets, on the other hand, formed in the inner, hot, regions of the fledgling solar system, and then were transported to beyond the orbit of Neptune!

The protoplanetary disk from which the planets formed around the Sun might not have extended beyond current Neptune's orbit. Neptune and other gas giants would have formed nearer to the Sun, along with the Kuiper Belt objects. Mostly Uranus and Neptune would have moved outward then, Neptune pushing away some material and the Kuiper Belt objects. Neptune eventually stopped where the protoplanetary disk was ending

->17P/Holmes, the 'Outburst Comet' Better Understood Somewhat
Comet 17P/Holmes is a short-period comet as, twice in the last 116 years, in November 1892 and October 2007, it endured explosions as it approached the Asteroid Belt, and brightened a million-fold overnight. It had a lot of fine or crystallized silicate grains under the form of a shell around its nucleus -the sign of a violent event- at the time of the outburst in 2007. Comet dust is very sensitive, meaning that the grains are very easily destroyed, as fine silicates are thought to be produced in these violent events by the destruction of larger particles originating inside the comet nucleus. That silicate dust then dissipated, leaving only larger particles behind in March 2008, showing that the explosion event is of a kind which fade swiftly, as far as its consequences -except for light- are concerned! Large chuncks of matter further seem to have been ejected too in the event, and solid enough not to be pushed in direction by the solar wind, bringing to streamers in the shell of dust surrounding the comet as they likely escaped from the nucleus. And the same for some larger, 1 mm-wide silicate grains remaining around the comet. The case of the comet 17P/Holmes, thus, is bringing more about the comets, albeit the cause behind such outbursts, which occur on a long span of time, is still unknown

->The Effects of A Cometary Passage Near a Planet
When Oort Cloud comet C/2013 A1 Siding Spring passed by a wooping 87,000 miles to Mars by October 2014, the effects were that a a temporary and very strong layer of ions was added to Mars' ionosphere with also a huge jump in the electron density a few hours after the comet rendezvous at a substantially lower altitude than the normal density peak in the Martian ionosphere, like a result of fine particles from the comet burning up in the atmosphere. Dust from the comet impacted Mars and was vaporized high in the atmosphere, producing what was likely an impressive meteor shower. This debris resulted in significant temporary changes to the planet’s upper atmosphere and possible longer-term perturbations. Intense ultraviolet emission from magnesium and iron ions occurred high in the atmosphere in the aftermath of the meteor shower for several hours after the encounter and then dissipated over the next two days. Eight different types of metal ions, including sodium, magnesium and iron were spotted

How do comets form? Well, the protoplanetary disk resulting from the formation process of a star is, following that, gradually consolidating to form planets, moons, asteroids and comets. As far as comets are concerned, micron-size particles of dust in the disk aresticking together to form centimeter- and millimeter-sized rocks. As the rocks become more massive, gravity takes over, forcing other surrounding pebbles and dust particles to collide with the larger rocks. The process continues until a comet is born. Comets usually form far from their star in the icy regions of the planetary system, molecules, such as water, carbon dioxide and methane, freeze onto the micron-sized dust particles and rocks before they collide. Once a solar system is formed, the gravitational pull from large planets manipulates a comet's orbit and brings it into the inner solar system. As the comet approaches its star, sunlight warms and transforms the frozen gas on and just below the comet's surface directly into vapor, effectively bypassing the liquid phase, a process called sublimation like in nowadays comets. Sublimation forces streams of gas and dust to jet out of the comet, creating an aura or "coma" around the rock as interactions between ingredients in the coma with surrounding sunlight and solar winds eventually create the comet's tail. The gases and rocky particles that make up the coma are the clues that astronomers use to deduce what the core of a comet is made of. The revolution of a comet about itself when it closes Sun yields varying amounts of particles emitted. About how a comet forms, a other view is a hesitation between thin tendrils of dust and ice get drawn slowly inward and pack themselves into a single, uniform mass or a hodge-podge of mini-comets come together to form the core for a comet of substance

Fragmentation might more common like a fate to comets than previously thought with a comet thus, when holding a small nucleus, spinned up by powerful jets and losing chunks of material. Such disintegrating comets have already been observed as their complete disappearance could last, for example, 150 years. A comet may also split into two