COMETS

• COMETS ARE MOUNTAIN-SIZED, DIRTY SNOWBALLS THAT FORMED AT THE DAWN OF THE SOLAR SYSTEM. OCCASIONALLY ONE APPROACHES THE SUN, CHANGES RADICALLY IN SIZE AND APPEARANCE, AND BECOMES BRIGHT ENOUGH TO BE SEEN.

An estimated 1 trillion comets exist in the freezing outer reaches of the solar system. Unchanged since the planets formed, each is a lump of snow, ice, and rocky dust: a cometary nucleus. These icy bodies are too small to be seen from Earth, but if one ventures into the planetary part of the solar system, it can develop a spectacular glowing halo and tails, making it bright enough to be detected. Many comets are found using telescopes, often accidentally by asteroid hunters, but many also pass unnoticed. Some revisit us regularly, with return periods ranging from a few years to hundreds. Others are unexpected and may not pass our way again for thousands or millions of years—or ever. Newly discovered comets take the name of the discoverer. The greatest number of discoveries, over 2,500, has been made by the SOHO spacecraft. Close to the Sun When a comet gets closer to the Sun than the asteroid belt, solar heating causes its nucleus to lose mass. This material forms a coma— a huge cloud of gas and dust around the nucleus—and two tails that continuously disperse into space. Each time the nucleus passes the Sun, a layer of surface material about 3 ft (1 m) deep is used up in a fresh coma and tails. A comet such as Halley, which orbits the Sun every 76 or so years, will eventually run out of material and vanish.

Comets in the inner solar system By the end of 2013, astronomers had detected about 5,000 comets passing through the planetary part of the solar system. Around 500 are short-period comets, such as Comet Tempel

1. First recorded in 1867, Tempel 1 returned in 1873 and 1879 but then did not reappear until 1967, due to a change in its orbit. Comet Halley is an intermediate-period comet first recorded in 240 BCE and seen 30 times since. The long-period Comet Hyakutake appeared brightly in Earth’s sky in 1996. It previously visited the Sun 17,000 years before, but on its 1996 orbit, gravitational interaction with the giant planets disturbed its orbit so much that it will not return for 70,000 years. Short-period comets h

• COMET ORBITS MOST COMETS EXIST WITHIN THE OORT CLOUD, FAR BEYOND THE PLANETS AND OUR VISION. WE KNOW THEY EXIST BECAUSE OCCASIONALLY ONE OF THESE BODIES IS DIVERTED INTO THE INNER SOLAR SYSTEM AND FORMS A COMA AND TAILS.

Cometary orbits, unlike planetary ones, are highly elliptical (oval), so a comet’s distance from the Sun varies greatly over time. Comets that leave the Oort cloud and travel toward the Sun are classed according to how long one orbit takes. Short-period comets hug the plane of the planets and have periods of less than 20 years. Intermediate-period comets pass close to the Sun every 20–200 years and their orbits have a wide range of inclinations. Long-period comets, which are also randomly inclined, have periods ranging from 200 years to tens of million of years. Some travel so far from the Sun that they may fly halfway to nearby stars. Cometary orbits are affected by the gravitational fields of the planets. Short-period comets have become trapped in the inner solar system by Jupiter’s gravity, and Jupiter can easily flip a comet from a short orbit back to a longer one. Some long-period comets are ejected from the solar system altogether and sail off into the galaxy. Others are pulled closer to the Sun, giving astronomers a chance of detecting them.

• MISSIONS TO COMETS IN THE PAST 30 YEARS OUR KNOWLEDGE OF COMETS HAS IMPROVED ENORMOUSLY, THANKS TO A SMALL NUMBER OF SPACECRAFT THAT HAVE SAILED THROUGH THE GLOWING COMAS AROUND COMETS TO VISIT THEIR ICY NUCLEI.

Hidden in the glare of their brilliant comas, and too small to be viewed with telescopes, comet nuclei can be seen clearly only by spacecraft. The first craft to return detailed images of a comet nucleus was Giotto, which launched in 1985 and passed within 375 miles (600 km) of Halley’s Comet less than a year later. Its images confirmed the theory that comets are made of dirt and snow. More ambitious missions followed, including NASA’s Stardust, which scooped a sample of dust from Comet Wild 2 and brought it back to Earth, and ESA’s Rosetta, the first craft designed to land on a comet nucleus.

Notable Comets Comets have inspired and frightened humanity since the dawn of history. Some of the more notable are discussed here.

Comet Arend-Roland

Comet Arend-Roland (C/1956 R1) was a long-period comet remarkable for its anomalous second tail, which projects toward rather than away from the Sun. It was one of the brightest naked-eye com- ets of the 20th century. It was discovered photographically on the night of Nov. 8–9, 1956, by S. Arend and P. Roland at the Royal Observatory, Uccle, Belg. Its perihelion passage (i.e., its closest approach to the Sun) occurred on April 20, 1957. Because it was discovered months before perihelion, lengthy obser- vations could be carried out. The anomalous tail appeared for a few nights late in April, changing direction from night to night and appearing as a sharp spike aimed at the Sun. This was an effect of perspective, of viewing edgewise a fan of debris from the comet, scattered ahead of it along its orbit. Biela’s Comet Biela’s Comet was a short-period comet discovered (1826) by and named for the Austrian astronomer Wilhelm, Baron von Biela (1782–1856). It was identified by Biela as a periodic comet that returned every 6.6 years. Biela’s Comet underwent remarkable transformations; it was observed in 1846 to break in two, and in 1852 the fragments returned as twin comets that were never seen thereafter. In 1872 and 1885, however, when Earth crossed the path of the com- et’s known orbit, bright meteor showers (known as Andromedids, or Bielids) were observed, lending strength to astrono- mers’ deduction that all meteor showers are composed of fragments of disinte- grated comets. The official designation is 3D/Biela, where the letter D stands for “dead.” Chiron Chiron was once thought to be the most distant known asteroid. It is now believed to have the composition of a comet nucleus—i.e., a mixture of water ice, fro- zen gases, and dust. Chiron was discovered in 1977 by the American astronomer Charles Kowal and classified as an asteroid with the number.2060. It is about 200 km (125 miles) in diameter and travels in an unstable, eccentric orbit between the orbits of Saturn and Uranus with a period of about 50.7 years. In 1989 two other Americans, Karen J. Meech and Michael Belton, detected a fuzzy luminous cloud around Chiron. Such a cloud, termed a coma and being a distinguishing feature of comets, consists of dust and entraining gases expelled from the cometary nucleus when sunlight vaporizes its ices. On the basis of this discovery, Chiron was reclassified as a comet. Subsequently, additional asteroid-size icy bodies in orbits similar to that of Chiron were discovered and given the class name Centaur objects.

Encke’s Comet

The faint Encke’s Comet has the shortest orbital period (about 3.3 years) of any known; it was also only the second comet (after Halley’s) to have its period estab- lished. The comet was first observed in 1786 by Pierre Méchain. Johann Franz Encke in 1819 calculated that sightings of apparently different comets in 1786, 1795, 1805, and 1818 were in fact appearances of the same comet, whose short orbital period he was able to deduce. The comet was named in his honour. Encke also found the comet’s period to be decreas- ing by about 2 1/2 hours in each revolution and showed that this behaviour could not be explained by gravitational perturba- tions (slight changes in an orbit) caused by planets.

Comet Hyakutake

Comet Hyakutake (C/1996 B2) was a long-period comet that, because of its relatively close passage to Earth, was observed as one the brightest comets of the 20th century. It was discovered on Jan. 30, 1996, by the Japanese ama- teur astronomer Hyakutake Yuji using large binoculars. Visible to the naked eye in late February of that year, it became spectacular in March, devel- oping a long blue plasma tail and a white dust tail that was much shorter but wider. It finally became five or six times as bright as a first-magnitude star when it passed Earth at a mere 0.1 AU (15 million km [9.3 million miles]) on March 24–25. It faded away in early April and reached perihelion (closest distance to the Sun) at 0.23 AU from the Sun on May Comet Ikeya-Seki The long-period Comet Ikeya-Seki is one of a group of Sun-grazing comets having similar orbits, including the Great Comet of 1882. Comet Ikeya- Seki was discovered Sept. 18, 1965, by two Japanese amateur astronomers, Ikeya Kaoru and Seki Tsutomu . Moving in a ret- rograde orbit, the comet made its closest approach to the Sun on Oct. 21, 1965, at a distance less than a solar radius from the surface. The comet was then bright enough to be seen with the naked eye in daylight. Like the similarly spectacular Great Comet of 1882, it was fragmented by tides induced by its proximity to the Sun; Ikeya-Seki gave astronomers their fi rst chance since 1882 to study a comet in such conditions. It is assumed that the group of Sun-grazing comets to which Ikeya-Seki belongs represents the rem- nants of a single, larger comet that also was fragmented by solar tides at some time in the past.

COMET MOREHOUSE

Comet Morehouse was a very bright comet in a retrograde, quasi-parabolic orbit, remarkable for variations in the form and structure of its tail. It was named after Daniel Walter Morehouse, a U.S. astronomer, and was observed from September 1908 to May 1909. On several occasions the tail appeared to break into fragments and to be com- pletely separated from the head. Also, the tail became visible at twice Earth’s distance from the Sun (2 AU), whereas most comets start to produce a visible tail only at about 1.5 AU from the Sun. These characteristics suggest that Morehouse was a “new” comet, coming straight from the Oort cloud. Comet Schwassmann- Wachmann 1 In 1927, the short-period Comet Schwassmann-Wachmann 1 was discovered photographically by the German astronomers Friedrich Carl Arnold Schwassmann and Arthur Arno Wachmann. It has the most nearly circu- lar orbit of any comet known (eccentricity 0.131) and remains always between the orbits of Jupiter and Saturn, having an orbital period of 16.4 years. It is also remarkable for outbursts in its bright- ness, which sometimes increases by several magnitudes in a matter of hours. These outbursts are determined to be the result of the transient development of a coma (faint atmosphere) of gas and dust, but, because this event occurs at random along the comet’s orbit, it cannot be explained by variations in solar heating of the comet’s nucleus. Rather, it is thought to be caused by heat-evolving chemical reactions occurring inside the nucleus or from the buildup and occa- sional release of internal gas as solar radiation gradually evaporates volatile materials below the crust of the nucleus.