With Comet ISON on the way, we are excited to bring you an excerpt from a brand new book on comets by David J. Eicher, editor of Astronomy Magazine. Make sure to pick up a copy of the book from Cambridge University Press.
COMETS!: Visitors from Deep Space, by David J. Eicher
Copyright © 2013 David J. Eicher. Reprinted with the permission of Cambridge University Press.
Chapter 1: Strange Lights in the Sky
When I was young I fancied becoming a doctor. The allure of medicine, of diagnosing diseases, of understanding the complexity of the human body – it all seemed endlessly fascinating. It offered a universe of ideas and challenges you could lose yourself in that could help person after person through challenges with illness and health. And then, in the midst of that momentum, when I was 14, in my little southwestern Ohio town, I went to a so-called star party.
Someone had set up a Criterion Dynascope 6-inch reflector, one of those telescopes with a long white tube and with the eyepiece fixed high at the upper end, and I peered in to take my first telescopic look at Saturn. That moment changed my life. Seeing the radiant light from Saturn’s bright orange globe, encircled by golden orange rings, incised by a black gap, made me gasp. The pinpoint of a little saturnian moon hovered nearby. Everything just stopped. I was transfixed by the vision of another world – live, in real time – right before my eyes.
It was early 1976, and the crisp winter air was not yet ready to surrender to spring. Infected with this new awareness of the universe around me, I needed to find out everything I could – to take many more looks through telescopes, or in my case, my dad’s pair of binoculars. Just as I was scrambling my first set of primitive equipment together, a friend called and gave me some promising news. “If you’re getting into astronomy, you’re in luck,” he blurted out. “There’s a bright comet that’s gonna be amazing soon, but you’ll have to get up early in the morning to see it!”
Along came another magic moment. Wandering out into the backyard, stepping across into the adjoining cornfield, and gazing up at the stars of Aquarius, I was thunderstruck at the sight. The icy cold air, dead silence of the early morning, and strange adventure of being out alone in a field before dawn added to the eerie, almost mystical sight that hovered over the planet. There, starkly visible in plain sight, like a shimmering sword hanging over Earth, was the bright glow of a comet with a fuzzy, starlike head and a long tail skirting upward and to the left. This was my first look at Comet West, the first look of many.
To someone who lived his whole life to that point on a “2-D planet,” like most of us beset by issues of daily life, this was a dose of sudden magic. Who knew that you could simply walk out and so easily see such a range of incredible sights in the universe, far away from Earth? And not only was Comet West a spectacular sight, bright enough to be stunning in its odd and unexpected appearance, but it showed me in just a day or two that objects in the heavens changed rapidly. The comet altered appearance when viewed through a telescope and changed position in the sky from night to night. I was catching on that there’s a whole lot more to this universe than I might have believed just a few days earlier.
Each day the comet rose in the early morning sky in the east, the tail peeking above the horizon first and then finally the head clearing the trees and moving up to complete the stunning portrait. Each morning it was fully visible in a dark sky before the creeping glow of dawn finally moved in and broke up the show. Here was a daily adventure, one that revealed the universe around us as a dynamic and unpredictable place. It demonstrated loudly that we inhabit just one little tiny spot in the cosmos, indeed even a small corner of our solar system. That late winter and early spring, Comet West became one of the Great Comets of the 20th century, peaking at magnitude –3, making it brighter than the planet Jupiter.
Strangely, you don’t have to go very far back into history to reach the point when great thinkers believed comets were local phenomena, emissions of gas or smoke hovering in Earth’s atmosphere. But then astronomers realized that if they were close, comets would be seen against slightly different star backgrounds from different places on Earth, and that didn’t happen. The realization came on that comets are distant objects – at least much more distant than the Moon – moving through the solar system in very strange ways compared to the regular orbits of the planets.
So what exactly are comets, anyway? The answer requires looking at the way planetary scientists believe the solar system formed, some 4.6 billion years ago. The solar system consists of our Sun, a medium-sized star, and its attendant planets and other assorted debris, the whole collection being one of perhaps 400 billion stars and attendant small bodies in the Milky Way Galaxy. (And the universe contains some 125 billion galaxies astronomers know of – it’s a rather large place.)
Scientists believe the solar system formed as a giant disk as gravity pulled material inward, eventually assembling enough mass to enable the Sun to “turn on” its nuclear fusion and begin life as a star. The so-called solar nebula, the disk formed and spun in rotation by gravity as the solar system coalesced, contained lots of material that didn’t make it into the Sun itself. Some of this material was eventually driven off by radiation pressure from the Sun’s intense energy, but some continued to join together by gravity, sticking little bits into larger bits, and building planets.
But Sun and planets alone do not make a solar system. Several distinct zones make up our star’s system. The innermost zone contains the terrestrial planets, including Earth. Next comes the asteroid belt, a region of rocky debris containing thousands of subplanet-sized bodies that together make up less mass than Earth’s Moon. Next come the giant planets, including Jupiter and Saturn. And the outer zone contains the comets, icy bodies of frozen gases and dust.
Most comets are far, far away and exist in several groups. Some comets are locked up with other debris in the so-called Kuiper Belt, a disk of icy bodies that extends from about 4.5 billion km out to 7.5 billion km from the Sun – in the region of Pluto and other dwarf planets. (The Sun itself spans a mere 1.4 million km.) Other, more remote comets exist in a huge shell surrounding the solar system called the Oort Cloud. Planetary scientists believe some 2 trillion comets may exist in the Oort Cloud, nearly all of which never make their way in toward the Sun (and into our skies). The Oort Cloud extends a staggering distance into deep space, perhaps as many as 1.5 light years from the Sun; that’s 40 percent of the way to the nearest star beyond our Sun. The comets themselves are typically just a few kilometers across. And some other zones and families of comets exist too.
In later chapters, we’ll explore the great complexity of comets, their origins, and where they live in detail, and you’ll see how incredibly rare a thing it is for a comet to move into the inner solar system and become terrifically bright. We’ll discover that asteroids and comets, not long ago believed to be two separate things, are now blurring the lines of their relationships. We’ll absorb the findings from spacecraft missions and ground-based telescopes that have studied both bright and faint comets and expanded our knowledge of these mysterious visitors. We’ll revisit how comets have affected human culture, how people have celebrated or dreaded them throughout history. And we’ll examine the best ways to observe and photograph comets from your own backyard, or whatever dark-sky sites you prefer.
The appearance of a bright comet in Earth’s skies is one of the most exciting astronomical events of all. In fact, no other type of astronomy-related happening comes close in getting new people interested in the night sky. Whenever a really bright comet appears, chatter rises, club memberships increase, attendance at star parties zooms, circulations of astronomy magazines climb, and new blood enters the hobby of astronomy.
It’s happened that way time after time since astronomy became an organized hobby, most recently with the Great Comets Ikeya-Seki (1965), West (1975/6), Halley (1985/6), Hyakutake (1996), and Hale-Bopp (1996/7). Now more than 15 years have passed since the last terrifically bright, well-placed comet has graced the skies of Northern Hemisphere viewers. But the time may have come.
My friend David Levy, one of history’s most successful comet hunters, has a favorite saying. “Comets are like cats,” he claims. “They have tails, and they do precisely what they want.” This underscores one of the great challenges with comets – their lack of predictability. The latest go-around occurred in September 2012, when astronomers discovered a potentially bright comet that could dazzle observers the world over in the fall of 2013.
On September 21, 2012, astronomers Vitali Nevski from Vitebsk, Belarus, and Artyom Novichonok of Kondopoga, Russia, captured images of a new fuzzy object in the sky. Their instrument of choice was the 16-inch Santel reflector at Kislovodsk Observatory in Russia, along with a program of automated asteroid detection called
CoLiTec. The telescope is one of 18 dedicated by the Russian Academy of Science to detection and tracking of faint objects in the sky, the network collectively termed the International Scientific Optical Network (ISON).
When the Russian astronomers alerted others that they suspected a comet, astronomers at the Mount Lemmon Survey in Tucson, part of the Catalina Sky Survey, and astronomers at the Pan-STARRS telescope in Hawaii checked earlier images and also found the object. The next night, more observations were made by Italian astronomers at the Remanzacco Observatory, using another network, this one called iTelescope. The Minor Planet Center in Cambridge, Massachusetts, clearinghouse for such astronomical discoveries, announced the new comet on September 24, 2012, three days after its discovery at a terrifically faint magnitude of 18.8.
As with all comets, following its discovery and verification by other astronomers, the new fuzzy object received a designation, C/2012 S1, and its popular name would not be the name of one of the discoverers but, following international agreements, the search network abbreviation. So C/2012 S1 (ISON), or informally, Comet ISON (Figure 1.1), was born. (Many such search facilities have uncovered multiple comets, however, so care needs to be used in throwing around the term Comet ISON or those of other networks or surveys.)
ISON is exciting to astronomers because of its great potential as a so-called sungrazer – a comet that will swoop in very close to the Sun and therefore brighten dramatically. At perihelion, its closest approach to the Sun, the comet will pass a mere 1.8 million km from our star’s glowing “surface.” When this happens, on November
28/29, 2013, the comet could be dramatically bright, a significant fraction as bright as the Full Moon. But that will take place in a daytime sky, when the comet is only 1.3° northeast of the Sun.
Fortunately, the comet should be dazzling in a nighttime sky as well – to be more precise, in the early morning sky in mid-November. The comet could then shine as bright as the planet Venus and may well become the brightest comet ever seen by anyone now alive.
Another reason ISON’s potential is exhilarating is that its orbit resembles that of another famous comet, C/1680 V1 (Kirch), which came to be called the Great Comet of 1680. Because the orbits are so similar, some astronomers have speculated may have originated from the same parent body. If this is so, ISON may present a historic show as well. The Great Comet of 1680 was one of the brightest comets of the 17th century and was plainly visible during the day. And its distance from Earth at closest approach was nearly the same as ISON’s will be. ISON will reach perigee, its closest passage of Earth, on December 26, 2013, some 63 million km from Earth.
At the turn of the New Year 2013, ISON glowed faintly at 16th magnitude as it floated among the stars of Gemini, near the bright twins Castor and Pollux. In addition to professionals at research observatories, amateur astronomers began to image the comet with a great sense of anticipation. Because of the orbital geometry of its path through the inner solar system, ISON will commence a big, semicircular, clockwise loop through the sky beginning in spring 2013, traversing Leo, Virgo, Scorpius, Hercules, and Ursa Minor by January 2014.
But the comet’s great brightness will be a long time coming. By midsummer 2013 ISON will brighten to be an intriguing telescopic fuzzball; it likely won’t be until early fall that ISON hits the range of being an impressive comet as viewed with binoculars. In October, the excitement will build as ISON’s magnitude rises above 10, and sometime close to Halloween, the comet will become a naked-eye object.
Comet fever should grip the astronomy world – and maybe pop culture too – when ISON slinks across southern Leo and into Virgo during the first week of November. By then, the comet will rise to 6th magnitude, and a few days later it will gain another magnitude and be visible with the eye alone from a suburban site. The comet then should increase by a magnitude every few days and will dazzle viewers who rise to see it in the early morning hours, perhaps 4 A.M. on into dawn. (That’s the time slot occupied by Comet West during those memorable first few weeks of 1976.)
If predictions pan out, by about November 25 the comet will have become impressively bright, shining at negative magnitudes, and situated in eastern Virgo, approaching the border with Scorpius. November 28 is the comet’s perihelion, its closest point to the Sun. ISON may then be as bright as Venus, or as much as 100 times brighter yet. If so, it will outshine everything in the sky save for the Sun and the Moon.
But remember that we’re talking about the comet’s total magnitude, its brightness if all of the light were compressed into a pointlike source. Because the comet is spread out over a large area, little areas of it will not appear as bright as Venus. But we’re still talking about a comet that could cast shadows – a remarkable event that’s unprecedented in our lifetimes.
At its brightest moment, ISON could shine at magnitude –9.5. By then it will be a daytime object a mere 1.3° from the Sun. This will make seeing the comet at its brightest difficult; trained observers who block out the disk of the Sun will be able to see it, but it will not be an easy observation when the comet is so close to the solar disk.
Comet ISON lies right in the head of Scorpius at perihelion and thereafter swings north toward Hercules. The first week of December should see it as a 1st-magnitude object with a sweeping tail, and Northern Hemisphere viewers will be well placed to see the comet as it slowly fades toward month’s end. By January 8, 2014, the comet will be a mere 2° from Polaris, the North Star, and will have dimmed to about 6th magnitude, reaching the naked eye limit once again.
Of course predicting comet magnitudes makes for a dangerous game. The comet’s orbit is well known, but assumptions about the comet’s composition, how solid it is, its reflectivity, and how volatile its gases and dust are make the brightness of ISON uncertain. Recent astronomical history knows one great story of a comet that everyone believed would certainly be dramatically bright, and in the end it fizzled. That is the story of Comet C/1973 E1, Kohoutek.
Shortly after its discovery, Comet Kohoutek was touted as the “comet of the century.” Among the prognosticators who believed Kohoutek would put on a spectacular show was Carl Sagan, not yet world famous as the creator of Cosmos, the book and television miniseries, but famous enough as a compelling scientist (and astronomy professor at Cornell) to appear on the Tonight Show alongside Johnny Carson.
Sagan predicted a sensational view of the comet as Kohoutek brightened late in 1973 and early in 1974. The comet, after all, promised a great deal to astronomers as they studied its orbit. It had been discovered on March 7, 1973, by Czech astronomer Luboš Kohoutek (1935–), who along with other astronomers found that the comet was a long-period object with a hyperbolic orbit that would carry it extremely close to the Sun. The date of perihelion was fixed as December 28, which would provide the world with an end-of-year, holiday spectacle.
Astronomers excitedly found the comet would pass close to Earth and quite close to the Sun, a mere 21 million km. The blowing of the horn about how fantastic Kohoutek would be ramped up expectations and created quite a flurry of attention in the popular media and culture, aside from Sagan’s regular pronouncements.
The comet’s effects ranged from the ridiculous to the sublime. David Berg, founder of the Children of God, predicted a doomsday event for January 1974. In December 1973, jazz musician Sun Ra put on a Comet Kohoutek show. The comic strip Peanuts featured the comet over a week-long span as Snoopy and Woodstock hid under a blanket from the mysterious light from the sky. The comet influenced musical works at the time or later by Pink Floyd, R.E.M., Journey, Kraftwerk, and Weather Report.
And the reasons for optimism were valid. The feeling was that, with such an orbit, Comet Kohoutek must be an Oort Cloud object, originating from far out in the solar system and therefore fresh, rich in volatile gas and dust that would stream off the comet as it warmed in the glow of sunlight like water vapor taking to the air on a foggy London morning. Astronomers believed the comet had never been to the inner solar system before and therefore was a good, solid object.
But as Kohoutek approached the inner solar system, it lagged significantly behind the predicted magnitudes. Comet Kohoutek seemingly fooled the experts on two counts: In hindsight, it may well have originated from the closer Kuiper Belt, not the distant Oort Cloud, and therefore could have had a relatively rocky composition with minimal volatile ices, gas, and dust. Moreover, rather than reflecting sunlight efficiently and developing significant tails spread across the sky, the comet partially disintegrated as it approached perihelion, prior to its closest approach to Earth. Thus, although it became a naked-eye comet, to many, Kohoutek was an outright dud.
The lesson is simple: No one can accurately predict a comet’s brightness beforehand, even knowing its orbit well, because of many small but potentially important unknown factors. ISON will be a great sight: no doubt. Only by early 2014 will we all know whether it was really the comet of our lifetimes, the century, several centuries, or just another pretty good comet. But the upside with this discovery is that it could “fizzle” compared to the predicted magnitudes and still be a remarkably, perhaps historically bright comet. That’s pretty encouraging. I urge you to follow the comet’s progress in Astronomy magazine and on the magazine’s Web site, www.Astronomy.com.
Well, whether it be the excitement over observing a bright comet or the anticipation of what might be to come, one question soon comes to mind: Just what exactly is a comet, anyway? The Greeks originated the word kometes, which translates to “long-haired,” referring to what early observers thought of as “hairy stars” because of their observed glowing tails. Although most people think of comets as a “streak” of light in the sky, a comet is really a tiny body floating along some kind of orbit in the solar system. Comets are clumps of frozen ices, gas, and dirty rock the diameter of a small town – they span an average of 5 kilometers or so across – and only when this frozen chunk approaches the inner solar system and heats up from the Sun’s warmth does it begin to outgas and produce a tail, becoming a spectacle in the sky.
Later chapters will describe the nature and physics of comets in detail, but for now, suffice it to say that its physical body is called the nucleus. The observational parts – coma, a hazy cloud of light surrounding the nucleus, and tails – arise from the solar heating as the comet approaches the Sun. The nucleus is really the physical
being of the comet, and planetary scientists believe cometary nuclei formed in the proto solar system some 4.6 billion years ago as icy outlying material that did not gravitationally clump together into larger objects.
As solar system bodies go, comets are tiny. You could stack 2,500 of them side by side across Earth’s equator. Yet they are plentiful – the deep recesses of the outer solar system may hold as many as 2 trillion comets. The bulk of a comet is frozen ices and gases; it is chiefly composed of water, carbon monoxide, carbon dioxide, formaldehyde, and methanol. Along with the frozen ices and gases are variable amounts of dust.
The unpredictable nature of comets as they approach the Sun and warm up results from their variable composition, the amounts of various gases and dust, the “freshness” of the comet – some have taken previous trips to the inner solar system – and the dynamics of the orbit. When the frozen block of comet approaches the Sun and warms, its ices begin to sublimate, transforming directly from a solid to a gas, and this creates the diffuse coma, which is surrounded by a large halo of hydrogen. The coma also contains dust grains liberated from being locked in the ice.
As the comet continues to warm, more gases escape and dust grains leap forth, and as the volume increases, the solar wind and radiation pressure from the Sun push these particles into a gas or plasma tail (typically bluish) and often a separate dust tail (white to yellowish in color), pointing away from the Sun. Comets are one major type of small body in the solar system. The other consists of rocky bodies without frozen gases that mostly live in separate places – the asteroids. Together, comets and asteroids make up the vast majority of the small bodies in the solar system. For hundreds of years, astronomers classified comets and asteroids as two completely different creatures – apples and oranges. As we’ll see later, however, at least in some cases, this distinction is becoming blurred as new discoveries are made.
One important feature that distinguishes comets is their peculiar orbital track around the Sun. They typically have large orbital eccentricities – orbits that differ markedly from circles – and high orbital inclinations, often tipping them at a strange angle compared with the orbits of the planets. Eccentricities are sometimes elliptical, sometimes parabolic, and sometimes even hyperbolic in the cases of comets that have been influenced by the giant planets and slung like pinballs in a crazy game of orbits. The angle of cometary orbits relative to the plane of the solar system is essentially without limits. Clearly, chaos in the early solar system was instrumental in setting up the paths of these celestial wanderers. Many are well behaved; others drop down at crazy angles like dive bombers; some even have so-called retrograde orbits, moving around the Sun in the opposite direction of Earth and the other planets.
As with all sciences, astronomy was for hundreds of years chiefly a game of classification. To help understand the orbits of comets – and where they might be coming from – planetary scientists created a dividing line for comets based on their orbital periods.
Long-period comets, those with periods of more than 200 years, are governed by highly elliptical orbits, and they reside at the outer limits of the solar system. These creatures spend their lives in a celestial deep freeze, a measurable fraction of the distance to the nearest star away, and move in only rarely and briefly to our part of the cosmos. In fact, an enormous shell of comets surrounds the solar system, and the distinguished Dutch astronomer Jan H. Oort (1900–1992) proposed in 1950 the existence of the source of these long-period comets, and the great sphere of cometary nuclei took on his name, the Oort Cloud.
An interesting class of long-period comets exists in the Kreutz Sungrazers. They are named for German astronomer Heinrich Kreutz (1854–1907), who demonstrated their relationships. This family of comets is characterized by orbits that carry the celestial visitors very close to the Sun, which sometimes makes them exceptionally bright. Sungrazers sometimes plow straight into the Sun or break apart as a result of the Sun’s gravitational influence.
By contrast, short-period comets have orbits of 200 years or less and are further divided into two distinct groups. These objects are much closer residents of the solar system. They comprise the Halley-type comets, which have periods of 20 to 200 years, and the Jupiter-family comets, with periods of less than 20 years. The Halley class feature orbits that are randomized, just as the long-period comets do, but the orbits of the Jupiter-family comets are inclined more closely to the ecliptic plane, as are those of the planets.
Of the 2 trillion comets that may exist in the Oort Cloud, astronomers have observed and cataloged about 4,200. Some 1,500 of these are Kreutz Sungrazers and 484 are short-period comets. In recent years astronomers have even detected comets in extrasolar planetary systems, the first in observations of the Beta Pictoris system in 1987. Of the 10 so-called exocomets discovered to date, all were detected around young stars, and they may help tighten the picture of how solar systems form.
This is a pretty strong indicator of how fast astronomy is moving as a science. Several hundred years ago many of the planet’s best thinkers believed that comets were atmospheric phenomena. Now we’re observing comets that are dozens of light-years away.
Come to think of it, that’s one of the things that struck me as a teenager, lying out in that field, gazing up at Comet West. Suddenly, after I learned a little about what comets are, it hit me. They hammer home the immensity of the cosmos. Yes, they are relatively nearby. But seeing them move from night to night – changing their place against the backdrop of the stars glistening behind them – is extremely powerful. I think it triggers something deep within the soul.
And that seems always to have been the case. The earliest records of cometary observations are from China and date from about the year 1000 B.C. Similar observations may have been made by inhabitants of the marshy land in southeastern Mesopotamia known as Chaldea. By about 550 B.C., Greek philosophers recorded comets as wandering planets. In his scheme of spherical shells making up the cosmos, Aristotle (384–322 B.C.) wrote in Meteorology (ca . 330 B.C .) that comets are residents of the lowest such sphere and called them “dry and warm” atmospheric exhalations.
Not only were comets viewed as local phenomena, but for centuries they were also taken as portents of doom, omens of some impending event, usually a disaster. Only with the writings of Thomas Aquinas (1225–1274) and Roger Bacon (ca. 1214–1294) did the notion that comets may not be lurking in Earth’s atmosphere begin to step forward. But further intellectual work on the subject would really have to wait until the world emerged from the gloomy deep freeze of the Middle Ages.
Real progress on understanding comets stepped up when Paolo dal Pozzo Toscanelli (1397–1482), an Italian mathematician and astronomer, observed what would come to be known as Halley’s Comet in 1456, along with a number of other comets during the previous and following decades. Observations made by Toscanelli and later by Danish nobleman and astronomer Tycho Brahe (1546–1601) began to define comets more precisely. Tycho’s observations of Comet C/1577 V1 in particular demonstrated the comet’s distance as being much farther away than the Moon.
In the late 17th century, German amateur astronomer Georg S. Dorffel (1643–1688) observed two bright comets in 1680 and 1681 and realized the two comets were one comet – C/1680 V1 – seen before and after perihelion, and that the comet had a parabolic orbit about the Sun. This provided ammunition for the great physicist Isaac Newton (1642–1726), who used his newfound theory of gravitation to demonstrate the comet moved in a giant elliptical orbit and passed only 230,000 km above the Sun’s surface.
And then the astronomer whose name would almost become synonymous with comets entered the stage. English astronomer Edmond Halley (1656–1742) calculated the orbits of a dozen well-known comets and found that one in particular, the Great Comet of 1682 (1P/1862 Q1), was periodic. Further, he postulated the comet’s period as 76 years (its period has varied between 76 and 79 years).
When in 1758 this comet was recovered as predicted by Halley, this time by German astronomer Johann G. Palitzsch (1723–1788), the first chapter of the story of cometary orbits fell together, neatly bundled. Newton’s theory of gravitation was validated to a distance way out beyond the planet Saturn, and Halley’s prediction rang true, solidifying the most celebrated name in comets, “Halley’s Comet.”
Astronomers continued to observe comets and refine their techniques for determining orbits over the 18th and early 19th centuries. To their puzzlement, scientists increasingly found that some comets had orbits like nice parabolas while others were contained in the inner solar system, closer to Earth than Jupiter. Astronomers believed that somehow the giant planet was gravitationally dominating the orbits of these comets, or perhaps even that Jupiter itself was ejecting the comets.
It was the French astronomer Pierre-Simon Laplace (1749–1827) who realized that a gravitational mechanism enabled Jupiter to capture these comets and influence their orbits, concentrating some of the short-period comets in one area. Thus, the Jupiter family of comets was born.
As telescopes improved, so did the opportunities for observing comets. The 1835 appearance of Halley’s Comet afforded multiple observers the opportunity to make detailed drawings of the comet’s structures, such as jets, streamers, and brighter and darker portions of the coma. German mathematician and astronomer Friedrich
Bessel (1784–1846) observed the comet and supposed he saw particles streaming out of the comet’s nucleus and being forced back into a tail that aimed away from the Sun. By this time, comets had become real – they were interacting with the solar system around them.
As the 19th century edged onward, astronomers discovered more bits of evidence that comets were important parts of the cosmos, not just meaningless debris. In 1866 and the year that followed, Italian astronomer Giovanni Schiaparelli (1835–1910) identified two annual meteor showers, the Perseids and the Leonids, with two comets – 109P/Swift-Tuttle and 55P/Tempel-Tuttle, respectively. The fact that these comets were losing particles that later intersected Earth’s orbit, shooting into our atmosphere and creating glowing streaks, linked comets with meteors.
Around this same time, astrophysics entered the study of comets when Italian astronomer Giovanni Donati (1826–1873) and English astronomer William Huggins (1824–1910) made the first spectroscopic observations of comets. They found the spectroscopic bands seen in several comets and in a gas flame were similar, and they detected a broad continuum indicating that the comets were reflecting sunlight.
The two great early tools in the arsenal of astrophysics, photography and spectroscopy, soon became the standard for comet research. The English photographer William Usherwood (1821–1915) took the first photograph of a comet when he recorded C/1858 L1 Donati, in 1858. Good spectra of cometary tails were made soon after the turn of the 20th century, and a big event in 1910, the next apparition of Halley’s Comet, gave astronomers the opportunity to produce some of the earliest papers on the physics of comets.
Leaps in understanding comets would have to wait. Not until the 1950s did several events take place that would push the understanding of comets forward. In 1950, American astronomer Fred Whipple (1906–2004) proposed the icy conglomerate model of a comet’s structure and composition (now universally and lovingly known as the “dirty snowball” model). The seeds of this idea went back to the late 1930s, but Whipple was the first to put them all together. Astronomers hadn’t yet understood how molecules were locked away in a comet’s frozen nucleus. Arguments over the source of the gas and dust from a comet had originated nearly a century earlier, and astronomers still couldn’t explain how a comet’s nucleus, when warmed, could produce such a vast coma. Moreover, they couldn’t understand how comets could be repeatedly warmed and refrozen over countless millennia and still survive their trips close to the Sun time and time again.
But Whipple overcame the uncertainties about a comet’s physical structure by building on the much earlier work of Laplace and Bessel. Whipple’s dirty snowball model proposed an icy nucleus that produced increasing quantities of gases by sublimation as the comet bathed in increasingly warm sunlight. The conglomerate part arose from the fact that the sublimation also released meteoric dust.
Whipple’s model struck a successful chord because it suddenly explained a whole spectrum of what astronomers had observed in comets for decades. It seemingly explained how comets could produce large amounts of gas. It explained how jets and other structures could be observed near the nuclei of comets. It explained nongravitational effects in comets that resulted from outflowing gas from the coma. It explained how sungrazing comets in the Kreutz group could survive close passages to the Sun. And it explained how comets produce meteor streams that in turn cause meteor showers in Earth’s sky. Small details still puzzled Whipple and others, but the model he proposed caught on and nearly all astronomers believed in it.
It seemed that by mid-20th century the understanding of comets was coming together nicely, because it was also at about the same time that Jan Oort proposed his model for the huge shell of comets that surrounded the solar system, far from the Sun. The Oort Cloud hypothesis was also a long time in coming. In the early 20th century Swedish-Danish astronomer Svante Elis Stromgren (1870–1947) showed that hyperbolic orbits observed in comets must be due to gravitational perturbations by Jupiter. This meant that although comets originated from distant locales, they were not coming from interstellar space.
In 1932 Estonian astronomer Ernst Opik (1893–1985) suggested that comets might be harbored in a distant cloud, which was stable somehow against the gravitational effects of passing stars. But it was Oort, in 1950, who actually studied the orbits of 19 comets and mathematically deduced the existence of the cloud on the basis of the semimajor axes – the half-lengths of the longest portions – of the comets’ orbits.
Among the amazing inferences of Oort’s work was that comets could remain in stable orbits to distances of about 200,000 astronomical units. (An astronomical unit is the distance between Earth and Sun, about 149.6 million km.) That incredible distance is some three-quarters of the way to the nearest star.
Some of these distant comets would, though, be gravitationally “kicked” inward by the influence of nearby stars. Over the 4.6-billion-year history of the solar system, Oort proposed, the orbital inclinations of these comets would have been totally randomized. And he further suggested that, in order to explain the number of new comets astronomers were discovering, the cloud of comets that would bear his name probably contains 200 billion comets. He also postulated that the total mass of the Oort Cloud would be about one-third the mass of Earth.
Along with his young student Maarten Schmidt (1929–) – who 13 years later would discover the first quasar – Oort studied the differences between “old” and “new” comets. The pair defined so-called new comets as fresh comets that were making their first appearance in the inner solar system, whereas old comets were returning for another trip around the Sun. New comets appeared to be richer in dust and brightened more slowly than older comets.
As research continued, support for the Oort Cloud model of comets only increased. The ability to produce refined, far more accurate orbits for comets increased throughout the 1970s, chiefly through the work of the English astronomer Brian Marsden (1937–2010) and Czech astronomer Zdenĕk Sekanina. Not only has evidence for the cloud strengthened, but astronomers now believe that comets are gravitationally knocked into the inner solar system from the Oort Cloud by the overall gravity of the galaxy as a whole more so than by passing stars.
Oort believed in a vastly distant cloud of comets. But he also toyed with the idea of a second, closer disk of comets that could explain the replenishment of the Oort Cloud and potentially be another source of distant comets. In 1981 American astronomer J. G. Hills proposed in detail the existence of this inner cloud, which could extend inside 20,000 astronomical units – one-tenth the way to the limit of the Oort Cloud.
Most planetary scientists came to believe that Oort Cloud comets formed in the region between Jupiter and Neptune and then migrated outward into the cloud. Astronomers now believe that as many as 2 trillion comets exist in the Oort Cloud. They also think that most surviving comets in the cloud formed in the region between Saturn and Uranus.
The science of comets seemed to be crystallizing quite nicely through the 1970s and early 1980s. And then the biggest buildup of hype, insanity, real science, and explosion of the amateur astronomy hobby our time has seen occurred. It all came riding along with the most recent appearance of Halley’s Comet, which would take place in 1985 and 1986.
In their quest to study the most famous comet in history, planetary scientists planned multiple spacecraft missions that would encounter the celestial visitor, as well as readying their battery of ground-based telescopes. The intense study of Comet Halley (formal cataloged name: 1/P Halley) was so voluminous and the scientific results so important, that the moment became a dividing line in cometary science. The two resulting eras were simply “before Halley” and “after Halley,” as if they were referring to the life of Jesus of Nazareth.
The appetizer in this great assault on a tiny, frozen block of ice occurred on September 11, 1985, when the International Cometary Explorer ( ICE ) spacecraft shot through the plasma tail of Comet 21/P Giacobini-Zinner. This comet has a nucleus some 2 km across and was discovered by French astronomer Michel Giacobini in 1900 and German astronomer Ernst Zinner in 1913. Although it wasn’t armed with a camera, ICE measured particles, waves, fields, and plasma in the comet’s tail en route to Halley. The probe passed within 8,000 km of Giacobini-Zinner’s nucleus and confirmed astronomers’ notions about the plasma tails of comets, measured ions in the tail, and measured a neutral electric current inside the tail.
The spacecraft missions to Halley heated up significantly in the spring of 1986, when five separate probes encountered the famous visitor. The first was the Soviet Vega 1 probe, which flew past Halley on March 6, 1986. As it approached, Vega 1 revealed two bright areas on the comet’s nucleus. These turned out to be jets of material emanating from the comet. The craft showed Halley’s nucleus was exceptionally dark and had a temperature of 300 to 400 K, warmer than expected. At closest approach, Vega 1 whizzed just 8,889 km from Halley’s nucleus as it snapped 500 images of the coma.
Second in line was the Japanese probe Suisei (“Comet”) which encountered Halley on March 8 at a distance of 150,000 km. Suisei imaged Halley from such a large distance because its mission was to capture ultraviolet images of the comet’s huge surrounding shell of hydrogen gas. The probe took up to 6 images per day of the comet and succeeded in its mission.
Third in the line of Halley probes was the Soviet Vega 2, which reached its closest approach on March 9. Sister craft of Vega 1, this probe (like Vega 1) encountered Venus first and then proceeded to the comet. As it approached Halley, Vega 2 commenced by snapping 100 images and then unleashed a science suite of studying the physical parameters of the nucleus, shape, temperatures, and surface properties. Altogether, the probe captured 700 images with better resolution than its sister craft.
The next Halley probe was the Japanese Sakigake (“Pathfinder”), Japan’s first interplanetary spacecraft. Encountering the comet on March 11, at the very great distance of 7 million km, the craft measured plasma wave spectra, solar wind ions, and interplanetary magnetic fields.
Finally, there was Giotto. The craft was named for Italian Renaissance painter Giotto di Bondone (1266/7–1337), who included Halley’s Comet as the “Star of Bethlehem” in his Adoration of the Magi, which he painted in 1304–1306. Engineered by the European Space Agency, Giotto flew past Halley on March 14 at a distance of
596 km and was struck by small cometary particles in the process.
Giotto provided the best images of Halley, revealing its nucleus to be a coal-black, peanut-shaped chunk of ice measuring 15 by 10 km. About 10 percent of the comet’s surface outgassed, producing the coma. The spacecraft’s analysis of Halley’s composition indicated the comet consists of 80 percent water ice, 10 percent carbon monoxide, 2.5 percent methane and ammonia, and a blend of hydrocarbons, iron, and sodium. Images revealed the comet was a miniature version of larger solar system bodies, showing small-scale features such as craters, ridges, and mountains. No outgassing was visible on the side of the comet aimed away from the Sun, only from the “daytime” side.
The first great round of spacecraft missions launched to a comet had revealed an enticing story of the basics of what makes up one of these icy bodies. Later flybys of other comets added a great deal to the story. On September 21, 2001, NASA’s Deep Space 1 probe whizzed past Comet 19/P Borrelly (Figure 1.2), an opportunity that produced much higher resolution images of a comet’s nucleus than Giotto’s images of Halley. The spacecraft was something of a systems test mission, and, ironically enough, several of its functions failed. Despite this, the images of Borrelly depicted an incredibly dark cometary nucleus with several areas of outgassing.
Three years later, NASA’s Stardust probe encountered Comet 81P/Wild 2 (Figure 1.3), a long-period comet that had been knocked into a short-period orbit by Jupiter. Here the mission was expanded into much more ambitious territory. Not only would Stardust study and image the comet from close range, but it also carried an aerogel collector and a return probe that would collect samples of the comet’s dust and return them to Earth. (The collector also scooped up interstellar dust particles.) High-resolution images of the comet were also a primary objective.
On January 2, 2004, Stardust sped past Wild 2 at the relatively low velocity of 6.1 kilometers per second. The spacecraft was moving so slowly that the comet overtook it as the two orbited the Sun. The closest approach was 237 km, a little more distant than had been planned, as mission controllers became concerned over dust particle collisions with the craft. Some two years later the sample return capsule separated from Stardust and reentered Earth’s atmosphere, plummeting downward at 12.9 kilometers per second before deploying a parachute and slamming into the Utah desert.
Scientists published their initial findings on the returned samples by the end of 2006. They had a great deal to choose from; a million specks of dust had been deposited onto the collector’s surface, and some 45 impacts from interstellar dust were found. Scientists announced they had identified a wide range of organic compounds in the sample, complex hydrocarbons, abundant silicates including pyroxene and olivine, some pure carbon, and methylamine and ethylamine.
Significantly, in 2011, researchers at the University of Arizona announced the discovery of iron and copper sulfide minerals in samples from Comet Wild 2, suggesting the formation of these minerals in liquid water. This was staggering, as previously no one had imagined that cometary nuclei could warm enough to melt a portion of their water ice. They also discovered some other secrets locked in the cometary grains.
Flybys have not been the only source of cometary drama in the solar system. In 2005 NASA launched the Deep Impact craft, a dual-purpose probe. Not only would Deep Impact approach Comet 9P/Tempel, a periodic Jupiter-family comet, but it would also strike right into the comet’s nucleus, blasting material upward and studying the plume of debris. The aim was to answer fundamental questions about the nature of the nucleus, its composition, and perhaps even its origin.
The dramatic impact of Deep Impact was slated for July 4, 2005, and an enormous media buildup accompanied the mission. On April 25 the spacecraft opened the action by taking its first image of Tempel (Figure 1.4) at a distance of 64 million km. Sixty-nine days before impact, the probe spotted the comet with its medium resolution imaging camera and began an aggressive program of photographing the comet. Cameras recorded two periods of outburst from Tempel, on June 14 and June 22, and a week after the last outburst, controllers began to orient the craft for its strike.
Deep Impact released the impactor portion of the craft from the main probe and positioned to take a front seat in the comet’s path – so the comet would in fact strike it. On the morning of July 4 – WHAM! The impact happened just as mission controllers expected it to, and images from the main probe showed a flash of light on impact. The impactor sent back pictures up until about 3 seconds before it struck. The energy released from the collision was equivalent to 5 tons of TNT, and the comet briefly lit up with a sixfold increase in brightness.
The science from Deep Impact surprised everyone involved. Scientists found the crater formed by the impact measured 100 meters across and 30 meters deep. The blast had liberated 5 million kg of water and 10 to 25 million kg of dust. The material consisted of much more dust and less water ice than scientists had thought they would find. The material was much finer grained than astronomers had guessed, consisting of particles akin to talcum powder rather than sand. Silicates and sodium were found in abundance but also clays and carbonates, suggesting the presence of liquid water for formation.
Astronomers likened the composition of the comet to a snow bank and suggested that as much as 75 percent of the space inside the comet was simply vacant. They concluded that Tempel 1 had formed in the icy outer solar system in the region of Saturn and Neptune.
Deep Impact was hardly finished, however. The craft received a second life in a dual-purpose mission that was dubbed EPOXI, short for Extrasolar Planet Observation and Deep Impact Extended Investigation – quite a mouthful. After several jugglings of a potential mission, the newly dubbed EPOXI swung past Comet 103P/Hartley (Figure 1.5), a small periodic comet of the Jupiter family, on November 4, 2010. Hartley thus became the fifth comet visited by a spacecraft and the smallest comet yet seen up close. The diameter of this potato-shaped block of ice is 1.2 to 1.6 km.
EPOXI ’s flyby showed that Comet Hartley was outgassing primarily carbon dioxide. The encounter distance of 700 km allowed the spacecraft’s cameras to capture impressive images. Scientists found the “waist” of the peanut-shaped nucleus had been redeposited onto the comet. It also revealed the comet orbits along one axis but spins across another. And it revealed the comet’s large ends contain relatively bright, blocklike objects as large as 16-story buildings.
Nor was Stardust through. Set for an extended mission and renamed Stardust-NExT (for New Exploration of Tempel 1), the orbiter that had previously collected particles would in 2007 redirect to Tempel 1. Stardust would now look at Tempel 1 to see what changes may have taken place to the comet since it was visited by Deep Impact. It would extend the mapping of Tempel 1, making it the most studied nucleus of a comet to date. And it would measure the mass and density of particles in the comet’s coma.
The encounter date for Stardust-NExT and Tempel 1 was set for February 15, 2011. As it flew past the comet at a distance of 181 km, the spacecraft recorded 72 images. They revealed far more than Deep Impact showed, and the analysis of these images continues. As of this writing, one more comet flyby mission is in the works – the European spacecraft Rosetta mission, launched in 2004, will encounter and launch a landing probe onto Comet 67P/Churyumov-Gerasimenko on November 10, 2014. This will introduce yet another new era in the history of exploring comets with spacecraft.
Along with ground-based observations, space missions to comets have given astronomers a window into the early history of the solar system. Comets represent relatively pristine material from the formation of our Sun’s family, and so studying them in detail sheds light on conditions dating back to the period when the Sun and its planets were forming. In subsequent chapters we’ll explore what astronomers know about comets from their various means of study in much greater detail.
Before moving on, however, you ought to know about a somewhat sticky subject – the nomenclature of comets. How comets are designated and named comes about through a straightforward process, but one that has changed conventions over time (Figure 1.6). The body charged with the authority to name celestial objects is the International Astronomical Union (IAU), a worldwide group of professional astronomers created in 1919 and consisting of about 9,900 members.
The IAU’s current comet naming guidelines were adopted in 2003. Many years ago, informal names were bandied about in all manner of ways. Not until the 20th century were comets systematically named for their discoverers. For decades, this system worked smoothly as typically one or two surnames neatly fit a given comet, as with Levy, Wilson-Harrington, or McNaught-Hartley.
Recently, however, the world has sprouted networks of search telescopes designed specifically for finding comets and asteroids against the stellar background. These wide-field charge-coupled device (CCD) surveys have turned up numerous comets and – aside from making the probability of a human discovering a comet even more challenging – have changed the way comets are named. Now there are, or are likely to be, numerous examples of LINEARs, ISONs, NEATs, PANSTARRS, and others, named for the search networks.
Following the suspected discovery of a comet, astronomers around the world know to alert the IAU’s Central Bureau for Astronomical Telegrams, at Harvard University in Cambridge, Massachusetts. The Central Bureau, or CBAT, was founded in Kiel, Germany, in the 1880s as the world’s first clearinghouse for astronomical observations and migrated to Copenhagen, Denmark during World War I and finally Cambridge in 1965. Three important astronomers have directed the CBAT since its move to the United States, and they are familiar to all astronomers – Owen Gingerich (1930–), director from 1965 to 1968; Brian Marsden, director from 1968 to 2000; and Daniel W. E. Green, director from 2000 to the present.
Not only does the CBAT alert other observatories so they can start observing a suspected comet right away (and look for observations they might have already captured), but it rides herd over the calculation of an orbit and other parameters such as positions, magnitudes, sizes, and other observational data. The CBAT issues these late-breaking observations on the famous IAU Circulars, both on printed card-sized alerts and in electronic form.
Despite the many changes over recent years, comets are still named, if possible, with the surnames of their discoverers. The CBAT prefers to limit the name to two independent discoverers, if appropriate, although in the past three or even more names have sometimes been used. Chronology rules – the person who found the comet first gets his or her name in the first spot. And of course search network names work the same way. In rare cases extremely bright sungrazing comets have become visible suddenly to a large number of people all at once, and in these cases the CBAT has adopted the phrase “Great Comet” or a similar designation rather than using discoverers’ names.
All these rules apply to comets in modern times. In the old days, comets were usually named after the year in which they were found. So we had the “Great Comet of 1680,” the “Great September Comet of 1882,” and so on. For a time, after the discovery that some comets were periodic, only the periodic comets were named for their discoverers. So we had Halley’s Comet, Encke’s Comet, and Biela’s Comet, while comets that appeared just once were still identified by the year of their appearance.
The naming of comets is one thing, the designations another. Before 1994, comets were given a provisional designation based on the year of their discovery and followed by a lowercase letter indicating the order of discovery within a year (a, b, c, etc.). So Comet West (originally 1975n) was the 14th comet discovered in 1975.
Additionally, once a comet was observed thoroughly, through its perihelion passage, it was given a permanent designation consisting of its perihelion year followed by a Roman numeral indicating the order of its perihelion passage for the year. So Comet West became 1976 VI, the sixth comet to reach perihelion in 1976.
Because of the cumbersome nature of this system, in 1994 the IAU changed the naming conventions. The system is now more systematic and also creates several categories of comets that help classify them, as the number of comets discovered steadily increases. Comets are designated with the discovery year followed by a letter indicating the half-month of discovery and a number indicating the order of discovery. So the first comet discovered in the first half of January 2014 would be designated 2014 A1.
Additionally, classifications of comet types have a set of prefixes attached. P/ indicates a periodic comet, a comet that has been observed at more than one perihelion and with a period of less than 30 years. C/ indicates a nonperiodic comet. X/ indicates a historical comet for which no reliable orbit could be calculated. D/ indicates a periodic comet that broke apart or has been lost. A/ indicates an object that was first identified as a comet but is actually an asteroid.
It takes a bit of getting used to, but the system of nomenclature in place over the past 20 years works well. Only a few mysterious flies are in the ointment; just a handful of bodies in the solar system are classified as both comets and asteroids and stand forth as examples of the increasing ambiguity between the small body types.
These include 2060 Chiron (95P/Chiron), 4015 Wilson-Harrington (107P/Wilson-Harrington), 7968 Elst-Pizarro (133P/Elst-Pizarro), 60558 Echeclus (174P/Echeclus), and 118401 LINEAR (176P/LINEAR). More to come on these later.
So I had come a long way from my first look at Comet West in the Ohio backyard, opening a doorway to the icy, distant past of the solar system. What I didn’t appreciate at first, however, was that I had opened a special door – one that led to a world of what astronomers like to call Great Comets.