Comets

A comet is a minor body composed mainly of ice, typically embedded with solids. Comets revolve around the Sun in highly elliptical orbits. The Oort Cloud is a vast cloud of cometary bodies extending billions of kilometers from the Sun.

Overview

Comets are familiar to nearly everyone as majestic, starlike objects with long tails stretching across a wide band of the sky. The most famous comet, Halley’s comet, makes its periodic return to the night skies every seventy-five years. The word comet is derived from a Greek word meaning “long-haired.” Comets were once greatly feared as bad omens, but they have since been identified and cataloged as objects that come into the inner solar system from deep space. Most of them occupy orbits that carry them far away from the Sun. Many comets make only a single approach to the Sun and never return, while others exist in stable but highly elliptical orbits that allow them to return after an extended period.

Astronomer Fred L. Whipple proposed one of the first theories to explain comets' makeup. Whipple suggested that comets were dirty snowballs, essentially bodies of water ice incorporating dust and perhaps volatiles other than water. This remained the primary theory through the first four decades of the space age. Only when spacecraft began visiting comets could it be put to the test.

Until the space age, comets were studied only in visible light through optical telescopic images. The first comet to be analyzed using Earth-orbital instruments, which permitted ultraviolet and visible observations, was the much-heralded Comet Kohoutek in 1973 and early 1974. Comet Kohoutek turned out to be visually disappointing from Earth, but images and data collected by the orbiting Skylab 4 astronauts advanced the understanding of comets.

In 1986, the European space probe Giotto passed about 600 kilometers from Halley’s comet as the comet made its close approach to the Sun. The investigation verified existing theories that comets comprise ice covered by black dust or soil. In other words, the spacecraft confirmed the dirty snowball model, at least for this comet. Using data taken by the spacecraft, scientists determined that the dust is composed of carbon, hydrogen, oxygen, and nitrogen. Other metals, such as iron, calcium, nickel, potassium, copper, and silicon, have also been discovered in comets. Halley’s comet was one of the darkest objects ever seen in the solar system; it has virtually no albedo. Only one other major body in the solar system, Saturn’s satellite Iapetus, is known to be this low in albedo.

As a comet approaches the Sun, it absorbs solar radiation and becomes warmer. The main body of the comet is called the nucleus. As the nucleus warms, the ice beneath the comet’s soil evaporates. Because the comet has no atmosphere, evaporated substances, also called volatiles, escape into the vacuum of space. This gaseous envelope that surrounds the comet is called the coma. As the coma grows, it forms a plume of vapor that carries away some of the comet’s surface dust. This mixture of evaporated volatiles and dust is carried away from the comet by the solar wind, is ionized by high-energy particles, and creates the spectacular tail of the comet. The comet’s tail, glowing in the solar wind, can stream behind the comet for millions of kilometers. Cometary nuclei consist primarily of volatile ice and dust. That ice is nearly all water ice, but there is also evidence of ice composed of carbon dioxide and methane. More elementary nitrogen, oxygen, and carbon monoxide compounds may exist as volatile ices.

Comets are typically small bodies. Halley’s comet is an irregular potato-shaped object, fourteen by seventeen kilometers. Some noted that images of Halley’s comet captured during the Giotto mission suggested that the famous comet resembled the cartoon character Felix the Cat. The largest known comet is Bernardinelli-Bernstein, with a nucleus of 80 miles. Comets are thought to have formed as the solar system evolved. Comets were accreted out of material at the outer edge of the solar nebula that ultimately condensed to become the Sun and planets. Because cometary material was fashioned at the solar system's outer edge, the Sun did not evaporate comets’ volatiles. At the same time, the solar system's giant planets formed at what would become the outer orbits of the solar system. These massive planets encountered the newly formed comets, and the comets not engulfed by the giant planets were, over the first billion years, ejected into interstellar space by the planets’ massive gravitational fields. Not all comets met that fate, however. Some were gently nudged into stable orbits closer to the Sun. Others were flung into the inner solar system, eventually impacting the inner planets. There are strong reasons to believe that Earth’s oceans came from cometary ice delivered to the planet during the early bombardment era, but that is not universally accepted.

What remained after billions of years of planetary encounters was an extraordinarily large cloud of comets extending outward from orbits beyond Pluto in all directions. A virtual spherically shaped cloud of comets surrounds the Sun at a distance from one thousand to one hundred thousand astronomical units (AU). This cloud, which may contain as many as two trillion comets of all shapes and sizes, is called the Oort Cloud. It is named after the Dutch astronomer Jan Hendrik Oort, who proposed its existence in 1950. The spherically shaped Oort Cloud is not the only source of comets in the solar system. There is also a disk-shaped source of comets that extends from about thirty-five to forty AU out from the Sun to about one thousand AU. This source, the Kuiper Belt, was named for the astronomer Gerard Peter Kuiper, who theorized its possible existence in 1951. The disk-shaped Kuiper Belt blends with the spherical Oort Cloud at about one thousand AU.

The Oort Cloud is the source for long-period comets with orbital periods greater than two hundred years. The Kuiper Belt is most likely the primary source for short-period comets, with orbital periods of less than two hundred years, such as Halley’s comet. Comets have definite life spans, unlike planets. Each time a comet streaks in toward the Sun, volatile gases stream off the comet and form a beautiful cometary tail while depleting the comet’s total mass. The comet melts away with each pass toward the Sun. When Halley’s comet streamed past the Sun in 1986, the Giotto spacecraft measured a loss of forty tons of mass per second from the comet. If the supply of comets had not steadily replenished from deep space, they would have all been lost long ago.

The Sun is one among billions of stars in the Milky Way galaxy. In the relatively nearby region of the galaxy, there are hundreds of local stars, which are all revolving around the galactic center and are moving relative to one another. Because stars are so far apart on average, the chance of one star colliding with another is relatively low. However, the possibility of a local star passing near or through the Oort Cloud (which extends up to one hundred thousand AU away from the Sun) is very high over millions of years. It is estimated that since the solar system formed, about five thousand stars have passed within one hundred thousand AU of the Sun. If an object as massive as another star passed close to the Oort Cloud, it could easily cause enough gravitational perturbations to direct comets toward the Sun.

Since the Oort Cloud is spherical, long-period comets can appear to approach the Sun from any point in space. Short-period comets, originating from the Kuiper Belt, always appear to emanate from a band along the ecliptic plane (the plane that contains the planetary orbits). After careful study of where comets originate, their orbits were analyzed. It has been discovered that areas of the sky are richer in comets than others, and others appear to be practically devoid of comets. Four different theories have been advanced to explain the source of these emerging comets. The first theory postulates that the passage of stars in or near the Oort Cloud may affect the gravitational balance of comets that they are sent falling in toward the Sun. The second theory involves brown dwarfs, which are massive objects—about thirty times Jupiter's mass—not quite planets or stars. They do not have enough mass to create the conditions for thermonuclear ignition at their core. They predominantly radiate infrared energy and cannot be readily seen from Earth’s surface. Estimates approximate the number of brown dwarfs near the Sun to be sixty times greater than that of ordinary stars. A brown dwarf should pass through the Oort Cloud every seven million years. Such an object would travel very slowly compared to the Sun and would gravitationally release large swarms of comets into the solar system. These two stellar mechanisms, the action of a passing star or a brown dwarf, are estimated to have been the source of about one-third of the observed comets.

According to the third theory, huge molecular clouds in interstellar space (much more massive than a single star) may pass at very large distances (tens of light-years) and may cause a release of comets through gentle perturbations of their orbits. The final theory for the source of newly appearing comets is galactic tidal action. Each galaxy has a gravitational field, which causes an attraction toward the midplane of the galaxy of all bodies (comets and stars). As these bodies orbit the galaxy, they are gravitationally influenced by one another. The galactic tide is the difference between the galactic forces acting on the Sun and the comet. Because the force of the galactic tide is very specific with respect to direction, it cannot act toward the poles of the Sun or the equator. Observations of cometary tracks confirm that comets from deep space do not seem to approach the Sun from these segments of the celestial sphere. This mechanism explains the approach of most long-period comets entering the solar system from the Oort Cloud.

In the aftermath of the US decision to be the only spacefaring nation not to dispatch a spacecraft to investigate Halley’s comet on its appearance in the inner solar system, the National and Space Administration (NASA) proposed the Comet Rendezvous and Asteroid Flyby (CRAF). CRAF was a sister ship to the Cassini spacecraft. Because of budget cuts, NASA was able to save the Cassini mission, but CRAF was canceled. If it had been adopted, CRAF would have rendezvoused with Comet Kopff and remained in its vicinity for thirty-two months to observe variations in that comet’s activity during different portions of its orbit. CRAF would also have dropped penetrometers into the comet to ascertain information about the internal structure and chemically analyze surface materials.

Aspects of the ambitious CRAF concept were recycled into cheaper comet missions, such as Deep Space 1, Stardust, and Deep Impact. Also, the European Space Agency (ESA) developed the Rosetta spacecraft to visit a comet.

Launched on October 24, 1998, the Deep Space 1 spacecraft began tests of an ion propulsion system and autonomous navigation system. Deep Space 1’s targets were the asteroid Braille and Comet Borrelly. Flying by the comet at a relatively close distance, Deep Space 1 captured images of a comet’s nucleus that had higher resolutions than any of those captured by the probes that had visited Halley’s comet. Comet Borrelly was shaped like a bowling pin and displayed emission jets not distributed uniformly across its irregular nucleus. Deep Space 1 was not outfitted with debris shields, but it survived the close encounter nevertheless. In time, Deep Space 1 ran out of propellant, but the mission showed that ion propulsion could be used on a spacecraft designed to visit multiple targets, such as comets.

The Stardust mission was launched on February 7, 1999, and was directed toward Comet Wild 2, where it opened up special sample collectors incorporating aerogel to capture interplanetary and cometary dust. After the spacecraft’s encounter with the comet on January 2, 2004, its sample collectors were sealed for a two-year journey back to Earth. On January 15, 2006, Stardust's sample collection unit safely reentered Earth’s atmosphere and was recovered intact in Utah.

In June 2008, researchers studying comet 26P/Grigg-Skjellerup material collected by the Stardust spacecraft announced that they had discovered new mineral grains. This mineral, named brownleeite after Donald Brownlee of the University of Washington, was a variety of manganese silicides not previously predicted by models of comets or the of material from the early protosun’s nebula.

DeepImpact was designed, as its name suggests, to fly to a comet and strike it to excavate material from deep below its surface. The spacecraft launched on January 12, 2005, and its onboard navigation steered it toward the comet Tempel 1, released an impactor payload made largely of copper (an element not expected to be found naturally within the comet), and then veered out of the way to observe the resulting impact of the payload on the comet. The impact was observed by Deep Impact as well as by the Hubble Space Telescope, the Chandra X-Ray Observatory, the Spitzer Space Telescope, the Swift spacecraft, and ESA’s XMM-Newton observatory and Rosetta spacecraft. This coordinated effort permitted time-evolution studies of the plume and debris cloud created by the high-speed impact of the copper payload on Tempel 1.

Rosetta was launched on March 2, 2004. This was the European Space Agency’s second attempt at a comet study and incorporated both a flyby craft and a lander named Philae. The mission was designed to rendezvous with the comet 67P/Churyumov-Gerasimenko in May 2014, orbit it for many months while mapping the surface, and observe changes in the comet’s activity as its distance to the Sun changed. The lander was then scheduled to touch down on the comet on or about November 2014, where it would secure itself to the surface in the comet’s weak gravity field and then begin studies of the chemical composition and physical characteristics of the comet’s surface. The Rosetta spacecraft entered into orbit around a comet on August 6, 2014, and Philae touched down on the surface of the comet on November 12, 2014, and transmitted data until its battery died. (Philae carried a solar-powered second battery, but it landed in an area with little Sun and could not charge the battery.) The Rosetta mission ended in 2016 when the spacecraft was last contacted.

Applications

The study of comets requires detailed knowledge of the composition of the outer regions of the solar system and the space between the last planet and one hundred thousand AU outward from the Sun. Comet studies also seek to understand complex gravitational interactions between bodies separated by wide distances and even gravitational interactions between tiny comets and the entire galaxy. Astronomers who study comets want to learn more about their makeup, their behavior when approaching the Sun, and the makeup and evolution of the early solar system.

New comets approaching the Sun for the first time have been held in deep freeze within the Oort Cloud and are thought to be composed of primordial material of the newly forming solar system. They have been tied up in the Oort Cloud for billions of years at temperatures barely above absolute zero. As they approach the Sun, their internal gases begin to stream away. Detailed study of an approaching comet’s outgassing can inform planetary scientists about the composition of the early solar system. Comets and their approach have also hinted at the existence of the elusive brown dwarfs, thought to be one of the most common bodies of interstellar space. Because they are so dim, they are all but invisible from Earth. On the other hand, because brown dwarfs are also thought to be plentiful, the study of comets and their orbits may give the first real clues to the former’s reality and abundance. The first serious studies of brown dwarfs came from observations made by the Spitzer Space Telescope, the final member of NASA’s Great Observatory program. Spitzer detected brown dwarfs from their infrared emissions.

In the early 1980s, galactic tidal action was merely speculation. Since then, careful study of comet orbits and their approaches to the inner solar system has favorably supported the theory of galactic tides. In the close approach of Halley’s comet by robotic spacecraft in 1986, a wealth of information was recovered on the shape, behavior, and composition of comets. The existence of the Oort Cloud and the concept of gravitational interactions by passing objects in space have led to the theory of periodic comet showers. Such comet showers, separated by tens of millions of years, may be responsible for certain mass extinctions on Earth. These extinctions might result from a shower of comets from within the Oort Cloud that are sent on their close approach to the Sun by the close passage of a star or brown dwarf to or through the Oort Cloud.

Samples of Comet Wild 2 were treated to many of the contamination safeguards used with the Apollolunar rocks. Analyses of Stardust’s captured comet material revealed some surprises: Tracks in the aerogel suggested solid materials larger than interstellar dust grains. crystals and other mineral crystals were found that required more than just mild heating, as would have been the case if the comet was largely composed of interstellar dust grains. This suggests that the theory of comet formation may need alteration. Inclusions of the components vanadium nitride, titanium, molybdenum, osmium, ruthenium, and tungsten were found, which would have required high heating. Samples also contained organic materials that were more primitive than were found in asteroidal material, compounds, such as polycyclic aromatic hydrocarbons.

The collision of Deep Impact's copper payload was equivalent to five tons of TNT. Comet Tempel 1 increased in brightness sixfold due to the event. As much as between ten and twenty-five million kilograms of comet material was ejected as a crater formed after the impact. Tempel 1 material was much finer than expected, more akin to talcum powder than a sandy grain. Data ruled out a loose aggregate or highly porous model of the comet’s structure. Indeed, rather unlike a dirty snowball as proposed by Whipple, Comet Tempel 1 was more like an icy dirt ball. Seen in the ejected materials, in addition to volatiles, were clays, carbonates, sodium, and crystalline silicates. After this encounter, the flyby portion of the Deep Impact spacecraft was redirected to a planned encounter with Comet Hartley 2 in 2010. Deep Impact got close enough to take readings on the gas emanating from the comet.

The Rosetta mission and the successful landing of the Philae lander marked a historical first in space exploration and provided scientists with new information that reshaped researchers' understanding of comets, including data on a comet's surface properties and interior that could not be gleaned from just measurements. Scientists hope to apply this information to future missions.

Context

Humankind has always looked to the heavens in awe, wonder, and sometimes in fear. No astronomical phenomenon except a total solar eclipse has historically evoked as much fear as comets. However, when the specter of fear is removed, they emerge as stunning objects in the sky. It was once believed that if Earth passed through the tail of a comet, its inhabitants would die; this theory has been discredited. Comets are messengers from a time long past. Most are chunks of dirty ice that have been locked away in the Oort Cloud for billions of years.

Comets have been used to judge vast distances, evaluate the early composition of the solar system, and even test the idea that the gravity of the entire galaxy can make a difference to the smallest of objects in space. Comets have been used as yardsticks to evaluate what may be the most common type of star in the galaxy, the brown dwarf—which, ironically, is difficult to observe, even in the infrared. Comets have been called dirty snowballs. Halley’s comet was so black that it was the darkest object ever seen in space. Comets have been called dirty snowballs. Halley’s comet was so black that it was the darkest object ever seen in space. However, planetary scientists have witnessed some of the most spectacular light shows from these dirty ice specks. Ultimately, comets may reveal some of the most fundamental secrets of the solar system and planets. From these tiny messengers, planetary scientists may unlock and examine pristine elements from the solar system's formation.

Debris from comets provides the material Earth passes through during annual meteor showers. For example, the Orionid meteor shower is leftover material from Halley’s comet, the Leonids meteor shower is associated with Comet Tempel-Tuttle, and the Perseid meteor shower is material from Comet Swift-Tuttle.

Historically, comets have come full circle from being seen as omens to be feared to celestial objects evoking a sense of wonder and to again being objects that should be feared if they come too close and perhaps even impact Earth. Comets represent a more troubling threat than asteroids, as comets are usually discovered only when they go past the orbit of Jupiter. As such, there is insufficient time to mount any mitigating effort if a new comet is determined to make a close pass or impact the Earth, as they have done in the past: Many believe that most of Earth’s water came from comets encountering the early Earth. The Siberian Tunguska event of 1908, itself a curiosity that has been explained by some (without any legitimate supporting evidence) as a nuclear explosion or even the impact of an unidentified flying object, is now believed to have been the result of a comet or asteroid impact, most likely an air burst explosion of the body, and draws attention to the potential for devastation that an impacting comet represents.

A week-long comet collision was observed in 1994 when nearly two dozen pieces of the shattered Comet Shoemaker-Levy 9 collided with Jupiter’s upper atmosphere. These pieces created temporary craters on the planet's surface, many of which were the size of Earth, indicating that a tremendous amount of energy was involved in this series of collisions. The Hubble Space Telescope and the Galileo spacecraft recorded the impacts. Several years later, scientists announced that Herschel, an infrared space telescope, had detected a substantial increase in water on the planet's southern hemisphere. Through subsequent analysis of the infrared imaging provided by Herschel, scientists determined that the water was concentrated in the areas where Shoemaker-Levy 9 fragments made contact with the planet and that the comet was responsible for delivering the water to Jupiter's southern hemisphere.

In 2011, the Spitzer Space Telescope detected indications of comets raining down in a different solar system than our own, yet the effect of the bombardment scarred our Moon and produced large amounts of dust.

It was believed since roughly 1950 that gravitational disruption of the Sun’s Oort Cloud by a close passage of another star was responsible for swarms of comets heading into the inner solar system, resulting in the bombardment of the planets. However, in late 2008, Hans Rickman of Sweden’s Uppsala Astronomical Observatory reported in Celestial Mechanics and Dynamical Astronomy the results of an updated computer simulation of the Oort Cloud investigated by his research group. If correct, their model indicates that sporadic stellar encounters, while crucial in generating fresh comets that head toward the inner solar system, are not the only mechanism for sending comets toward the planets. This model accounted for galactic gravitational tidal influences on the Oort Cloud and found that the threat from comets may be more constant than previously believed. If correct, the model reinforces the need to monitor the skies for incoming comets that might be headed our way.

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