Comet Halley

Halley’s Comet is the brightest, most famous of the known periodic comets. Definitive records of sightings go back more than two thousand years. The comet travels around the Sun roughly once every seventy-six years in a highly eccentric retrograde orbit inclined 20° to the ecliptic plane. Its orbital period has enabled many observers to see Halley’s Comet twice during their lifetimes.

Overview

For many years, the idea that comets were “dirty snowballs” has generally been accepted by astronomers. First proposed by Fred L. Whipple in 1950, this was one of several different ideas about the makeup of comets. The most popular idea was that they were “flying sandbanks,” or collections of interstellar dust and gas accreted as the Sun and planets periodically passed through vast clouds of interstellar matter in their journey through the galaxy. The Sun’s gravity drew in the material that eventually collected to form individual bodies. This idea was popular during the first half of the twentieth century and championed by British astronomers R. A. Lyttleton and Fred Hoyle. Since the middle of the nineteenth century, meteor streams have been associated with comets, and supporters of the “flying sandbank” model of cometary nuclei suggested that the particles within meteor streams arose from material escaping from comets as they moved through the solar system.

It is widely believed that cometary nuclei are composed of material that condensed from the solar Nebula simultaneously, as did the Sun and its planets. The European Space Agency (ESA) and other Spacecraft that intercepted and studied Halley’s Comet in March 1986 detected copious amounts of carbon, nitrogen, and oxygen. The materials given off by the comet signify that these objects were formed in the solar system's outer regions, where the extremely low temperatures necessary for them to solidify prevailed. Giotto revealed that the Nucleus of Halley’s comet is a tiny, irregularly shaped chunk of ice coated by a layer of very dark material measuring some fifteen kilometers long by eight kilometers wide. This layer is thought to be composed of carbon-rich compounds and has a very low albedo, reflecting merely 4 percent of incident light. This low Reflectivity makes the nucleus of Halley’s comet one of the darkest objects known. However, various bright spots were seen on the nucleus. A hill-type feature was found near the terminator, along with one resembling a crater near a line of vents. The vents seen on the nucleus appear to be fairly long-lived. Dust jets detected by the Russian Vega 1 and Vega 2 probes appear to have emanated from these vents, two identified by Giotto. Possibly, some of the larger vents have survived successive Perihelion passages.

Gas and dust that cause all the cometary activity seen (including the coma and tail) emanate from the nucleus via localized vents or fissures in the outer dust layer. These vents cover approximately 10 percent of the total surface area of the nucleus. They become active when exposed to the Sun and cease to expel material when plunged into darkness as the nucleus rotates. The force of these jets of material escaping from the nucleus plays a vital role in the comet’s motion around the Sun, affecting its orbital speed. Halley’s comet was several days late in reaching perihelion during the last apparition in 1986, a result of the jetlike effects of the matter being expelled as a consequence of Newton’s second law of motion. The late arrival of Halley’s comet was one of the factors examined by Swedish astronomer Hans Rickman, who attempted to calculate the mass of the nucleus from the amount of ejected material. Linking the ejection rate to the delay in perihelion, he judged the volume of the nucleus to be between 50 and 130 cubic kilometers. However, measurements obtained through spacecraft imagery revealed a volume closer to 500 cubic kilometers. The only conclusion was that the nucleus is markedly porous and far less dense than first anticipated, with an average density of no more than a quarter that of ice. This porosity meshes with the belief that comets formed in the outer regions of the solar nebula, where material coming together would remain loosely bound rather than compact.

The fact that the nucleus of Halley’s comet rotates is not in doubt. What remains unresolved is the period of rotation. Using photographs of the comet taken during its apparition in 1910, astronomers calculated the rotation period to be 2.2 days around an axis that was fairly well aligned with the poles of the comet’s orbit around the Sun. Results obtained by the Giotto, Vega, and Japanese Suisei probes appeared to support this value. However, ground-based observations carried out during 1986 indicated a rotation period of 7.4 days. This value was supported by other ground-based observations and results from the American Pioneer Venus orbiter, which examined Halley’s comet when it neared perihelion. Controversy ensued over these differing values, although a possible explanation has been suggested. The nucleus of Halley’s comet could display both periods of rotation: spinning around its axis and the Precession of the axis of rotation. The combination of rotation and precession is still contested by some astronomers, to some extent, because of the porosity of the nucleus. Any precessional properties would quickly disappear unless the nucleus were reasonably rigid.

Comets give off copious amounts of gas and dust that spread out in tails across large space areas. Investigation of this material can reveal much about the composition of cometary interiors. Many of the investigations carried out by the European, Japanese, and Soviet space probes were directed toward a survey of the material ejected by Halley’s comet. These investigations were supplemented by observations from ground-based astronomers and the American Pioneer Venus, International Cometary Explorer (ICE), and International Ultraviolet Explorer (IUE) spacecraft. As with the surface of Halley’s nucleus, the dust thrown off by the comet was very dark and may have emanated from the surface rather than the interior. Giotto and Vega carried out analyses of the dust. They found a mixture of different materials, including the lighter elements oxygen, hydrogen, nitrogen, and carbon, and the heavier elements—silicon, iron, and magnesium. The amount of carbon found during the investigations coincides quite well with the observed abundance of this material elsewhere in the galaxy, indicating comets are composed of interstellar material.

More than three-quarters of the gas ejected from the nucleus was found to be water vapor, which also appears to constitute more than 80 percent of the nucleus. The production rate varied during the interval these space probes examined the comet. Vega 2 found approximately sixteen tons of water coming from the nucleus during its flyby, while Vega 1 detected double that rate. These significant changes are reflected in Comet Halley’s brightness, sometimes varied by a factor of two or three from night to night. The velocity of the ejected vapor was found to be between 0.8 and 1.4 kilometers per second. This was the first time water had been positively identified in a comet, although cometary nuclei were widely thought to consist of dust and water ice. Carbon monoxide and carbon dioxide were also detected, although methane was not found. This is strange in that either any methane that existed in the comet may have been altered chemically during the period since the formation of the comet, or methane was lacking in the cloud of material from which the comet formed. If there is methane in Halley’s nucleus, it must constitute a very tiny percentage of the total makeup.

Processes involved in releasing gas from the nucleus may have played a prominent role in the evolution of its surface. It has been suggested that, as a comet approaches the Sun after spending its time in temperatures of approximately forty kelvins in the outer regions of the Sun’s influence, the Sun's warming effects can cause the ice within the nucleus to expand. This would result in heat generation and the release of trapped gas. Some of this gas may collect in pockets, which eventually explode, producing craterlike features similar to that imaged by Giotto.

Methods of Study

Halley’s comet is unusual (though not unique) because it was named for the astronomer who first calculated its orbital path rather than the person who discovered it. Edmond Halley observed a bright comet in 1682, and the impression of this sighting stayed with him and eventually expanded into a more profound interest in comets. In 1705, Halley began a study of several bright comets seen between 1337 and 1698. Using methods developed by Sir Isaac Newton, he worked on the orbital motions of some twenty-four comets seen during this period. His results showed many similarities between the orbits of the comets observed in 1531 and 1607 and the bright comet he had seen in 1682. The intervals between the sightings were also roughly identical at around seventy-six years. This led Halley to predict that these sightings were of the same comet and that it would reappear in 1758.

Halley died in 1743, although astronomers began a search for the returning comet as the date forecast by Halley drew near. French astronomer and mathematician Alexis-Claude Clairaut, with the help of Joseph-Jérome de Lalande and Madame Nicole Lepaute, attempted to calculate its orbital path in more detail. Considering the gravitational effects of Jupiter and Saturn, they calculated that the comet would reach perihelion on April 13, 1759. They published ephemerides (detailed star maps and charts) to help astronomers with their search. Many famous astronomers joined in, although the amateur astronomer Johann Georg Palitzsh from Dresden first spotted the comet on Christmas Day 1758. The reappearance was quickly confirmed, and the comet was named for Halley in honor of his accurate prediction of its return. Once a number of observations had been obtained, a revised orbit was calculated. It was found that Clairaut’s calculated perihelion date was in error by thirty-two days. Scientists were at a loss to explain this error, although they did not know about the existence of the two giant planets Uranus and Neptune, which were not to be discovered until 1781 and 1846, respectively.

Since the 1758 appearance, Halley’s comet was seen in 1835, 1910, and 1985-1986. Times of previous visits of the comet have been calculated by considering the gravitational effects of other Solar system bodies and plotting the comet’s orbital course backward through time. Dates estimated for previous apparitions have been substantiated by checking against ancient astronomical records, primarily those of Chinese astronomers. The first definite appearance of Halley’s comet took place in 240 BCE, although the 12 BCE appearance is the first about which detailed information is available. The most famous return was that of 1066, interpreted as a bad omen by the Saxons and, in particular, by Harold, the last of the Saxon kings. William of Normandy, who viewed the apparition as a good sign, invaded England, following which Harold died at the Battle of Hastings in October of that year. The Bayeux Tapestry depicts the comet suspended above Harold, who is seen tottering on his throne as his courtiers look on in awe and terror.

The 1531 appearance is important because it was one of two apparitions studied by Halley (the other being that of 1607) before his two deductions: that these historical sightings were of the same object and that the comet is a regular visitor to this region of the solar system. A comprehensive set of observations of the 1531 appearance was made by astronomer Peter Apian, who published his results in 1540. The 1607 appearance was observed and recorded by many astronomers, including Johannes Kepler. This was the last apparition of Halley’s comet before the introduction of the telescope.

Astronomers could plot its orbit with even greater accuracy after the comet’s reappearance in 1758 and the discovery of Uranus in 1781. Long before its scheduled return in 1835, many attempts were made to calculate the expected date of perihelion passage. The consensus was that Halley’s comet would pass closest to the Sun in November 1835. The search for the returning comet started as early as December 1834, almost a year before it was due to sweep through the inner solar system. However, the first sighting was not made until August 6, 1835, by Father Dumouchel and Francisco di Vico at the Collegio Romano Observatory. Confirmation came via Friedrich Georg Wilhelm von Struve, who saw the comet on August 21. Perihelion occurred on November 16.

Prominent among the astronomers who studied the comet during the 1835 apparition was Sir John Frederick Herschel, who was then based at a temporary observatory near Cape Town, South Africa. He was in the process of completing the sky survey started by his father, Sir William Herschel. He moved to South Africa to survey the southern stars that were visually inaccessible from England. John Herschel first attempted to locate the comet in late January 1835, although he saw it on October 28. The 1835 apparition was remarkable in that much activity was seen to occur in the comet. Before its temporary disappearance in the Sun’s rays, many changes were observed in the tail as it rounded the Sun. These disturbances continued after its reappearance. The tail was seen to vary noticeably in length. The head also altered in appearance, at times appearing almost as a point of light, while at others taking on a nebulous form. It was noticed that the coma expanded while undergoing a reduction in brightness, eventually becoming so dim that it merged into the surrounding darkness. Herschel’s final observation of Halley’s comet in mid-May 1836 was the last any astronomer made until the 1910 return. All data scientists have about the 1835 apparition are in the form of sketches and visual descriptions. Photography had not yet impacted astronomy, although its appearance in 1910, through the use of the camera, provided the most comprehensive and detailed study of Halley’s comet up to that time.

The third predicted return in 1910 was awaited eagerly by astronomers worldwide. From 1835 to 1910, visits were littered with numerous bright comets, such as the Great Comet of 1843, Donati’s Comet of 1858, and the Great September Comet of 1882. The latter is particularly significant in that it was the subject of the first successful attempt to photograph a comet. Sir David Gill obtained a good image in South Africa. Observation of Comet Morehouse in 1908 demonstrated that a series of photographs was an ideal means of monitoring cometary structural changes. (Comet Morehouse itself underwent many prominent changes that, coupled with the fact that Halley’s comet had suffered in a similar fashion three-quarters of a century before, whetted the appetites of astronomers who were gearing up for the forthcoming apparition.) The prolonged period of cometary activity following its last visit had allowed astronomers to perfect their observing techniques and paved the way for observations of the return of Halley’s comet.

The comet had passed Aphelion in 1872, after which it once more began its long journey toward the inner solar system. Astrophysics professor Max Wolf at Heidelberg, Germany, was the first astronomer to detect the returning visitor. A photographic plate was exposed on September 11 to 12 night and recorded the comet close to its expected position. It did not become visible to the naked eye until well into 1910. Before this, another bright comet made an unexpected appearance. The Great Daylight Comet was first spotted by diamond miners in Transvaal, South Africa, in the early morning sky on January 13, 1910. The discovery was confirmed four days later, and news of this spectacular discovery was distributed to the world’s observatories. Unlike Halley’s comet, which was to appear later that year, the Great Daylight Comet became a brilliant evening object for observers in the Northern Hemisphere. Its tail attained a maximum length of 30§ or more by the end of January. The comet became so bright that it was visible to the naked eye even in broad daylight (hence its name).

The Great Daylight Comet was widely mistaken for Halley’s Comet by many people expecting its return at about this time, although Halley’s Comet did not put on as grand a show. Bad weather and the fact that a full moon occurred at what should have been the best time for observation meant that astronomers north of the equator were disappointed. Yet, even working against these odds, they did obtain many useful photographs and were able to study the comet spectroscopically. The best results, however, were obtained from observatories in the Southern Hemisphere, notably at Santiago in Chile. From mid-April to mid-May 1910, Halley’s comet was in the same area of the morning sky as Venus, the two objects together forming a marvelous visual spectacle in the Constellation of Pisces. Much activity was noted in the comet's nucleus and tail. Sequences of photographs showed marked changes in the head, including material ejected from the nucleus and halos expanding out from the nucleus. The tail also underwent violent changes, with material being seen to condense in various regions. On April 21, the day following perihelion, the previously smooth northern edge of the tail became irregular and distorted. Material seemed to be thrown out in various directions, and parts of the tail seemed to be ejected into space, an event visible on photographs obtained at the time. For some days following perihelion, a jet of material from the nucleus seemed to be refueling the northern section of the tail. Once this activity ceased, the tail’s southern area increased in brightness. A few weeks after perihelion, the two types of cometary tail appeared as a straight and distinct gas tail contrasting with the fainter, more diffuse, and curved dust tail. Halley’s comet passed between the Sun and Earth on May 18, although despite many attempted observations, no trace of the nucleus could be seen as the comet transited the solar disk. This proved the nucleus must be tiny and the gas around it very tenuous. During this time, it was thought that the Earth may pass through the tail, although there is no evidence that this occurred. The pronounced curve of the tail seems to have taken it away from the Earth, preventing a passage of the planet through it. The closest approach of the comet to Earth was on May 20, when the distance between the two bodies was twenty-one million kilometers. For a time afterward, the comet became a prominent evening object for American observers, and many valuable results were obtained by astronomers at Lick Observatory and Mount Wilson Observatory in California. Many changes in the comet’s structure occurred, and many spectroscopic observations were taken. These showed the presence of many different molecules in the comet and helped astronomers understand its chemical constitution more clearly.

As the comet started on its journey back to the solar system's outer regions, it grew steadily fainter. It was last seen beyond the orbit of Jupiter in a photograph taken on June 15, 1915, on its way toward aphelion in 1948. The next return would be accompanied by an unprecedented campaign by astronomers and space scientists to expand their understanding of comets in general and Halley’s Comet in particular.

The return of 1985-1986 provided astronomers with their best chance of exploring a comet. Unlike other bright comets, many of which appear suddenly, the orbital path of Halley’s comet is known with great precision and accuracy. Therefore, it was possible to plan missions by robotic space probes to rendezvous with the comet during its last return. For a comet rendezvous mission, the comet's position at the time of interception must be known well in advance, as was the case with Halley’s comet. In all, five space probes were sent to examine the comet.

Two of these were the Soviet Vega probes, launched in December 1984, to release balloons into the Venusian atmosphere. Along their way, the probes encountered Halley’s comet on March 6 and March 9, 1986, at distances of 8,890 and 8,030 kilometers, respectively. Among the equipment they carried were cameras, infrared spectrometers, and dust-impact detectors.

The two Japanese probes carried out their investigations from greater distances. Sakigake, launched in January 1985, flew by the comet on March 11, 1986, at a distance of 6.9 million kilometers. Its primary purpose was to investigate the interaction between the Solar wind and the comet at a considerable distance from the comet. One of the main aims of Suisei, launched in August 1985, was to investigate the growth and decay of the hydrogen corona. Suisei flew past the comet on March 8, 1986, at 151,000 kilometers.

The most ambitious and successful probe dispatched to Halley’s Comet was the European Giotto, named in honor of the Italian painter Giotto di Bondone and launched on July 2, 1985. Giotto was cylindrical, with a length of 2.85 meters and a diameter of 1.86 meters. Its payload included numerous dust-impact detectors, a camera for imaging the nucleus and inner coma of Halley’s comet, and a Photopolarimeter for measuring the brightness of the coma. Giotto flew within 610 kilometers of the nucleus on March 14, 1986, at a speed of more than sixty-five kilometers per second. Data collected by Giotto were immediately transmitted back to Earth via a special High-gain antenna mounted on the end of the space probe facing away from the comet. The sixty-four-meter antenna at the Parkes ground station in Australia received information back on Earth. At the opposite end, Giotto was equipped with a special shield to protect it from impacts by dust particles during its passage through the comet’s halo.

Exploration of Halley’s comet by space probes was a truly international effort, the images and measurements obtained by the Soviet Vega craft helping scientists to target Giotto precisely. From Earth, the nucleus of a comet is hidden from view by the material surrounding it. Not until the Vega images were received was its position established and the subsequent trajectory of Giotto determined. During the close encounter, all instruments performed well, although disaster struck immediately before the closest approach to the nucleus. A dust particle weighing merely one gram impacted Giotto. This temporarily knocked the spacecraft and its antenna out of alignment with Earth, and for thirty tense minutes, contact was lost. The problem was rectified, and contact was reestablished. After the encounter, it was found that approximately half of the scientific equipment had suffered damage, although scientists could redirect the craft and put it on a course back to Earth. Tests carried out by the European Space Agency in 1989 paved the way for the reactivation of Giotto, which set up a pass within 22,000 kilometers of the Earth and placed Giotto in a new orbit that allowed it to intercept another comet. On July 10, 1992, Giotto flew close to Comet Grigg-Skjellerup at a point just twelve days in advance of the comet’s closest passage to the Sun, a time when its activity was approaching maximum.

Context

Although the study of Halley’s Comet has taught scientists much about comets, there remains much to learn about these ghostly visitors. Halley’s Comet provides a chance to investigate the solar system's origins. Cometary explorations by space probes could include rendezvous missions during which a probe would position itself close to a cometary nucleus for a prolonged period and perhaps send a lander to the nucleus's surface. The National Aeronautics and Space Administration (NASA) examined the possibilities of such a mission at the time of Halley’s 1986 visit. Known as Comet Rendezvous and Asteroid Flyby (CRAF), this mission would have enabled scientists to explore asteroids and comets closely. Unfortunately, budget cuts led to the cancellation of CRAF. Sample return missions, by which scientists can examine first-hand material plucked from the heart of a comet, also remain a possibility. Some astronomers and scientists hope for a mission that will carry a human crew to Halley’s comet during its next apparition in 2061.

More realistically, in the meantime, NASA was able to launch its Deep Space 1 probe and demonstrate the capability of an ion propulsion system to drive a spacecraft to effect rendezvous with an asteroid and a comet. On September 22, 2001, Deep Space 1 flew within 2,200 kilometers of Comet Borrelly, performing measurements and taking high-resolution images. The Deep Impact mission slammed a copper impactor into Comet Tempel 1 on July 4, 2005, to expel surface material and excavate a crater on the comet’s nucleus. The flyby portion of the Deep Impact spacecraft observed the collision of its impactor and analyzed material thrown up from the formation of an Impact crater on the cometary nucleus. Then, in January 2006, the Stardust mission returned samples to Earth released from Comet Wild 2; those samples were collected 240 kilometers from the comet’s nucleus. ESA launched the Rosetta spacecraft in 2004 and set it on a trajectory toward an encounter with the comet 67P/Churyumov-Gerasimenko in May 2014. Rosetta was designed to orbit the comet and later release a small lander named Philae to touch down on the comet’s nucleus and perform in situ analyses of surface materials.

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