Hairy Stars

In the not too distant past the appearance of comets in the sky was a reason for great terror and fear. With their bright and stellar like nuclei and long fanned tails, some called them "hairy stars." Comets were believed to be harbingers of doom and destruction. Even as recently as 1910, thousands of people reacted in terror to the news that the tail of Halley's Comet (properly referenced 1P/Halley) would sweep across Earth possibly poisoning the atmosphere of Earth with deadly gases. Salesman pushing gas masks made a fortune during the spring of 1910. But the comet passed with not a single noxious odor. Today comets are viewed with wonder and awe when one of the beautiful and rare bright ones flies past Earth.

The British astronomer Edmund Halley was among the first to begin to demystify comets. In the year 1705 after careful study of the orbits of bright comets that appeared in 1531,1607 and 1682 he realized that they followed paths that were virtually identical. Halley postulated from this that they must be the same object and boldly predicted that the comet would return in 1758. Halley died in 1742, but on Christmas night 1758 a German farmer and amateur astronomer named Johann Georg Palitzsch recovered the comet. Astronomers in recognition of Halley's achievement named the comet for him26. Astronomers of the time then backtracked through history and identified twenty-three previous appearances of the comet proving that it had been viewed by people at every return dating all the way back to the year 240 BC when Chinese observers wrote of a "broom star" and reported its motion from the eastern sky to the northern sky. Recreations of the 240 BC apparition bear out this observation. The ancient Babylonians saw the comet at its return of 164 BC. Sadly the comet's most observed return, its most recent was also the poorest in recorded history. 1P/Halley brightened to only magnitude +2.6 in March 1986 and its tail spanned only about 15 degrees in the early morning sky that year.

Though the return of 1986 was poor, amateur astronomers will get a treat at Halley's next return in 2061. A close fly-by of Jupiter in September 2060 will raise the comet's perihelion slightly and shorten the comet's orbital period by 1.3 years. The comet will then pass within .48 AU of Earth and only .05 AU from Venus (that's only about eight million kilometers!). Halley actually came closer to Earth in 1986, but did so while far from the Sun. The pass of 2061 will occur with the comet much closer to the Sun.

So what determines whether or not a comet will become bright in our skies? The first thing that one must always remember about predicting cometary brightness is that comets are notoriously unpredictable. Several factors must be considered when astronomers try to tell us whether or not a comet will become spectacular. First is whether or not the comet will pass close to the Sun. As a comet plummets into the inner solar system, the Sun heats up volatiles within the solid cometary nucleus, which burst through its icy covering throwing ice, dust and gas into space. These begin to reflect the Sun's light in an enormous area around the nucleus, which is called the coma or "head" of the comet. As the comet nears the Sun, the solar wind begins to push the dust and gas downwind from the comet forming a tail. The tail can have two distinct components. A whitish-yellow dust tail is the most prominent feature, but careful examination will also reveal the presence of a bluish gas tail. Both tails are obvious in the picture of Comet 1995 O1 Hale-Bopp. The gas tail tends to stream directly downwind from the Sun while the dust tail may fan out along the comet's direction of motion. This spreading of the tails is also evident in the image at the opening of this chapter. If the comet does not pass close to the Sun, then it will not be excited to an adequate degree or may not begin to discharge matter at all.

26 By tradition, new comets are named for their discoverers, rather than by them. A new comet is also given a designation number which begins with their year of discovery, a letter which denotes what two-week segment of the year they were discovered in, followed by a number issued in chronological order of discovery. Comets with known orbits are designated differently. They are given a sequential "P/" numbers such as 1P/Halley or 2P/Encke.

The comet must also pass close to Earth. Most comets are extremely small with a nucleus that measures less than one kilometer across. In order for such a tiny object to become bright, it must pass close enough to Earth for us to get a good look at it. If it does not come close to Earth, then it will appear many magnitudes fainter. A good example is that of Comet 2P/Encke, which has the shortest period of all of the known returning comets. Encke returns each 3.3 years. At two of its apparitions it passes fairly close to Earth and reaches minimal naked-eye brightness, but at each third apparition it is far from Earth and never brightens above magnitude +10.

Size also matters. The bigger a comet is, the more surface area it has available to blast dust and gas into space. It will also have more reservoirs of such material to exhaust into the void. So larger comets tend to have the opportunity to become brighter. But size is not the only key feature of the comet itself that determines how bright it might become. Astronomers now know that comets that are passing the inner solar system for the first time are often covered in a dark material that prevents them from outgassing at a substantial rate.

This was the unfortunate situation in 1974 when astronomers prematurely predicted that the massive Comet 1973 E1 Kohoutek would become one of the most brilliant in history. But as Kohoutek neared the Sun in 1974, its virgin dark coating prevented it from reaching the predicted brilliance and many never saw it, dealing the entire astronomical profession an embarrassing blow. Kohoutek did actually become relatively bright, reaching magnitude +3.0 in the morning sky. As the comet reached perihelion close to the Sun, viewers on Earth lost sight of it but astronauts on Skylab could see it against a dark sky and reported that it might have become as bright as magnitude -3.0! Earth-based viewers got a brief look at the comet at zero magnitude in early January 1974 but the comet rapidly faded as it moved away from the Sun, disappearing completely from naked-eye sight by the end of the month. Astronomers were understandably gun shy when in 1975, Comet 1975 V1 West was discovered and showed promise of becoming brilliant in the morning sky before dawn after passing perihelion less than ten million miles from the Sun on February 25,1976. Unlike Kohoutek, Comet West delivered, becoming bright enough to be viewed in daylight for several days shortly after perihelion and sporting as many as five distinct tails. Comet West is considered one of the premiere comets of the twentieth century.

So we have now learned that comets can become bright in some combination of four ways. They can pass very close to the Sun, they can pass very close to Earth, they can be enormously large or they can simply be enormously energetic. Comet 1P/Halley is fairly large (about seven kilometers along its longest axis), passes well within the orbit of Venus, usually passes within 0.5 AU of Earth at the same time and is a prodigious producer of dust and gas. So Halley combines all four elements. In 1994, amateur astronomers Alan Hale and Thomas Bopp independently found a new comet of about magnitude +10. That in itself is not surprising, but what was shocking was where the comet was when it was found. Comet 1995 O1 Hale-Bopp was still well beyond the orbit of Saturn! When it was found it was still nearly two years away from perihelion! The geometric circumstances of the coming apparition were to be very poor. Hale-Bopp would not come within 1 AU of the Sun, nor would it come within 1 AU of Earth. But Hale-Bopp was enormous. Estimates of its nucleus placed it at over 70 kilometers in size. The comet brightened to magnitude -1 and displayed a beautiful fanned 20 degree long tail. Hale-Bopp is an example of a comet that is intrinsically bright. It did not matter how far away it was, Hale-Bopp was going to put on a magnificent show. And boy did it ever! Hale-Bopp was visible to the unaided eye under dark skies for more than fifteen months and astronomers are still tracking the outgassing nucleus nine years later! Had Earth encountered the comet just five months later, it would have become brighter than Venus!

While the astronomical world was abuzz over the approach of Comet Hale-Bopp, the dedicated Japanese amateur Yuji Hyakutake was scanning the skies in January 1996 when he happened upon a faint fuzz ball in the sky. Comet 1996 B2 Hyakutake was the amateur's second comet in less than a year after more than twenty years of searching. The first comet was a nondescript tenth magnitude object that never developed, remaining well outside the orbit of Mars. But Hyaku-take's second comet would prove to be one of the grandest in history in its own right, upstaging the approaching Hale-Bopp. It has been twenty years since the grand display of Comet West and now we would be treated to two cometary spectacles in less than a year. Roaring out of the southern morning sky, Hyakutake brightened rapidly and reached zero magnitude as it passed almost directly over Earth's north pole at a distance of less than 13 million kilometers heading inbound towards the Sun. As it did so, some observers reported an ion tail of astonishing length. No account claimed less than 70 degrees and some observers claimed the tail stretched across more than 100 degrees of sky! Hyakutake was not a big comet, barely a kilometer across nor was it intrinsically bright, but it came very close to us. Hyakutake went on to curve within 5 million miles of the Sun before heading out of the inner solar system and putting on a less grand show for southern hemisphere observers.

Comet Hyakutake became brilliant because it passed near Earth, Hale-Bopp never came anywhere near Earth but became bright simply because of its brute size. There are many variables that contribute to cometary brightness, including whether or not they have been here before. Comet Kohoutek was a first-time visitor, but careful calculations show that Comet Hale-Bopp had been through the inner solar system before. That is, fifty thousand years before! Comet Halley is neither as big as Hale-Bopp, nor does it come as close to Earth as Hyakutake did. But a good combination of size, proximity and other factors combine to make it the Old Faithful of comets.

Though we were certainly spoiled by the appearance of two bright comets within a twelve-month span in 1996 and 1997, these are very rare events. The last comet to exceed magnitude zero before Hyakutake was Comet West twenty years prior and West was the first since Comet Ikeya-Seki blazed close to the Sun in 1965 becoming the brightest comet in recorded history. Ikeya-Seki was found a few weeks before perihelion and after solar conjunction blazed brighter than the full moon. Most comets remain extremely faint and are usually a challenge for even the largest amateur telescopes.

Since the mid-1980s comets have been prime targets for spacecraft exploration. An international armada greeted Comet Halley at its return in 1986. Five dedicated missions flew to Halley including the Russian probes Vega 1 and Vega 2, which studied the comet at medium range. Long-range investigations were carried out by the Japanese probes Suisei and Sakigake. The most prized observations were made by Europe's Giotto probe, which flew only 300 miles from Halley's nucleus and actually survived to the surprise of its controllers. Giotto returned the first images of the nucleus of a comet. Images returned showed a dark, potato-shaped rock spewing jets of gas and dust into space around it. Budgetary issues caused the United States to sit out Comet Halley but it since has launched several more missions to other comets since. These include the Deep Space 1 flyby of Comet 19P/Borrelly and the Stardust mission to obtain samples of material from the coma of Comet 81P/Wild 2. The European Rosetta mission is enroute to reach Comet 67P/Churyumov-Gerasimenko and enter orbit around it in the year 2011.

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