Preface

When I was 14, I fell in love with the universe. The discovery occurred with a one-time view of Saturn through a telescope at a local “star party.” There was something so calm and comforting about gazing skyward at the twinkling dots spread across an inky black cosmos. Somewhere amid all the apparent serenity out in the universe things must be incredibly more complicated than they were for an earthbound kid. So in the cool spring of America’s Bicentennial year, I found a new habit: taking my family’s pair of 7×50 binoculars, grabbing a sleeping bag, and, with my dog in tow, wandering out from the edge of our neighborhood into a cornfield and lying down, taking long stares at the star fields and glows of the Milky Way above the southern Ohio landscape.

The sessions went from a few minutes at first, to hours after a couple of weeks. I might as well have been on the Moon; civilization was shielded, the neighborhood tucked away behind a low screen of trees, the sky fortunately dark, and my only companions Oscar the border collie, the occasional rustle of a squirrel or raccoon, and the deep beauty of the sky above. As Earth slowly rotated, I saw the universe’s whole show – as far as we can see it from our place in the cosmos. Gradually, I learned constellations, recognized bright stars as my friends, and squinted toward the positions of fuzzy objects I couldn’t quite make out – clusters of stars and glowing gas clouds in the Milky Way Galaxy – as seen through the old binoculars. Before I knew it, I had been taken into another world. I didn’t know exactly why I’d gone, except that this world was alluring for the mystery and the grandeur of the vastness of space.

The last Great Comet we explored was Comet Skjellerup-Maristany (C/1927 X1), which lit up Earth’s skies during the final weeks of 1927. If we fast-forward 30 years, we come to the first Great Comet of the modern era, Comet Arend-Roland (C/1956 R1). Belgian astronomers Sylvain Arend (1902–1992) and Georges Roland (1922–1991) discovered the comet on photographic plates made on November 8, 1956. At the time of discovery, Arend-Roland already glowed at 10th magnitude and had a short tail. But the astronomers delayed announcing the discovery until November 19, and thereafter other images of the comet turned up, as early as September 11, 1956.

Comet Arend-Roland brightened slowly. Its orbit made clear that the comet would pass closest to Earth on April 21, 1957, at a distance of 85.3 million km. This followed its closest passage to the Sun on April 8. By year’s end 1956 Arend-Roland was glowing at about 9th magnitude and sported a tail stretching just 8 arcminutes long. The first few weeks of 1957 saw the comet continue to brighten slowly, reaching magnitude 8.5 by late February. By February 27, the comet slipped too close to the Sun to observe, and astronomers had to wait until it reemerged in the morning sky.

A generation ago, capturing your own photographic portraits of the sky was a difficult and complex process. Cameras could be tricky, choosing the right film was an arduous process, and technology simply wasn’t what it is today – the process of guiding your telescope accurately as Earth rotated and the sky seemed to move could be such an exacting process that it drove people crazy. Now, producing high-quality pictures of the sky is still a somewhat tricky process, but it’s much easier than it used to be.

As with any sky targets, you can capture pictures of comets in a variety of ways. Astronomical objects are very faint compared with normal scenes of people at the beach, so long exposures are generally needed, regardless of the equipment setup or technique used.

Types of comet photography range from pretty simple to quite complex and of course demand increasing sophistication of equipment and knowledge. The easiest way to capture a comet is simply to mount a camera on a tripod, fitted with a wide-field lens, and shoot an unguided exposure that will capture the comet amid constellations. More ambitious is to mount your camera “piggyback” style on a guided platform or telescope, to capture a time exposure of a smaller field of view that tracks the sky to preserve small, round star images. The most sophisticated method is prime focus photography, in which the camera is attached to the telescope and the telescope is used as a giant lens. Because of the high resolution and small field of view, the tracking must be superb and the exposure calculated to capture the desired details in the comet. All three basic methods will be discussed in this chapter.

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.

Foreword

With the appearance of two bright comets in the year 2013, sky watchers around the world are preparing to train their telescopes on this pair of wonders in the night. The starlike central portions surrounded by soft haze, and followed by gossamer tails, appear rarely enough in our lives that they surely deserve our full attention and our passion.

David J. Eicher’s book brings the magical world of comets to life. It is not an arcane mathematical textbook but a celebration of these slowly wandering objects. It delves into Tycho’s Comet of 1577, the comet that led to the great Danish astronomer’s discovery that comets pass through the sky well beyond the orbit of the Moon. The book devotes considerable space to Halley, the most famous of comets and the first comet shown to be periodic.

Comets are famous not just for what they are, but also for what they can do. In 1994 Comet Shoemaker-Levy 9 performed the first major cosmic collision witnessed by humanity when it collided with Jupiter. My role in the discovery of that comet is a story that dates back to 1965, while I was a teenage camper at the Adirondack Science Camp near Lewis, New York. Our camp director asked that each participant display a project at the camp’s annual science fair. He also insisted these projects need not be completed by the date of the fair; he wanted us to stretch our limits, to conceive and conduct a project that could last a lifetime, and one that could also fail. “Failure,” he told us, “is the great teacher. If everything you do in science is a great success, then you probably haven’t learned anything.”

There are comets and then there are comets. I didn’t know it when I caught my first glimpse of Comet West back in the spring of 1976. As I gazed at the brilliant nucleus and twin tails of that magnificent visitor, I was looking at a Great Comet. Astronomers like to use the term “Great Comet” for exceptionally bright comets, but there is no official classification as such.

Nonetheless, both in the minds of amateur astronomers and in the historical record, it’s pretty clear when a comet is great. You know a Great Comet when you see one. It’s a brilliant, naked-eye spectacle that causes the most casual viewer, your neighbor Fred or your Aunt Martha, who has never really observed the sky, to look up at the glowing specter and say, “WOW!” – or maybe even something a little stronger.

A number of factors influence how bright and well developed a comet will become. First and foremost is how close the comet will be to the Sun at perihelion. The closer the comet is to the Sun, the brighter it will be, as it will warm and outgas significantly more as it is closer to the Sun. Because of the inverse square law, normal objects are only one-quarter as bright when they are twice as far away from the Sun.

One of history’s great lovers of comets is now remembered for just about anything but comets. French astronomer Charles Messier (1730–1817), who was born in Badonviller, Lorraine, and died in Paris, spent much of his career observing comets at a critical time in the history of observational astronomy. Messier was lured into astronomy by the excitement of seeing the Great Comet of 1744, with its multiple tails, and by observations of an annular eclipse of the Sun in 1748. And it was the return of Halley’s Comet in 1759 that played a critical role in pushing Messier forward into his studies and cataloging of comets and cometlike objects.

In 1750, only about 50 comets were well known since the beginning of time. Keeping track of comets and studying their motions were the critical assignments for anyone interested in the subject. Messier traveled from his hometown to Paris in 1751, seeking his fortune, and was hired by astronomer Joseph-Nicolas Delisle (1688–1768) as a draftsman and recorder of observations. Although Messier first set about copying a map of the Great Wall of China, he was also instructed in the use of astronomical instruments. Messier also took a position as clerk at the Marine Observatory in Paris.

The science of understanding comets has changed dramatically in the 60 years since Fred Whipple hypothesized the basic model of cometary nuclei and Jan Oort suggested the existence of the large cloud that most comets call home. Over the past 20 years some really interesting twists and turns in the road have confronted planetary scientists: The distinction between comets and asteroids has blurred significantly and is based in some ways on where these icy bodies happen to be at the present time rather than intrinsic differences in their makeup.

The past decade has also witnessed an explosion of science, conjecture, and speculation about the role comets have played in our own existence. Did comets provide substantial amounts of water to Earth, thereby making the development of life on our planet either possible or easier? And even more dramatically, astronomers have focused on the organic molecules discovered in comets for years now, wondering whether they may have deposited organics onto early Earth that may have led to the development of life. We’ve seen that comets and asteroids are destroyers of life when they cause large impacts on our planet. Were they also the promoters of life? Do we owe our very existence to them?

Comets have always been afforded a special place in the minds of human beings. In fact, from the earliest days of recorded history up through the 17th century, most people thought they were harbingers of doom or portents or some incredibly important events – perhaps good but usually bad. Thought to be specters from beyond, ghosts sent from heaven or hell and carrying wickedness or good, comets spent the entire history of human culture being misunderstood until just the last 300 years. And the real understanding of comets, as we’ve seen, has only developed over the past century. It’s difficult to blame the ancients for singling out comets, as so few things in the sky seem to change over short time intervals. To most, the sky, filled with stars, seems essentially static. Even the motions of the planets, the Moon, and the Sun are comfortable. But comets were outside the realm of the comfortable, and their uninvited, sudden appearances would shake the foundations of belief.

The Roman philosopher Lucius Annaeus Seneca, aka Seneca the Younger (ca. 4 b.c.–a.d. 65), summarized the early take on comets – as opposed to the always-visible parts of the heavens, the Sun, Moon, and so on – in one of his writings. “No man is so utterly dull and obtuse,” he penned,

The universe is a dynamic, active, often violent place. Stars, planets, and galaxies undergo rapid change, constantly worked on by their environments. Seeing Earth, the Sun, or the Milky Way as it was long ago is just not possible. Comets, however, give astronomers a precious storehouse of ancient information. They are relatively pristine, ancient blocks of ice that allow us to peer back in time.

Astronomers’ knowledge of comets is still developing, but the knowledge base is vastly larger than it was a generation ago. Strangely, planetary scientists have gleaned much of what they know about comets without knowing precise numbers on some of their most basic physical properties. All we know about the shapes, sizes, and albedos (how reflective of sunlight the comet is) of the nuclei of comets has been gained from the small number of close visits to comets by spacecraft. In this way, only about two dozen comets are well known.

When astronomers observe comets from Earth, they face challenges like making assumptions about the object’s albedo, which they can then use to calculate a presumed size. Recent measurements of a small number of comets such as 9P/Tempel 1 with the Spitzer Space Telescope, in 2005, suggested a low albedo of about 4 percent for that comet. The Deep Impact mission measured the comet at 7.6 by 4.9 km, and the inferred mass for Tempel 1 is about 75 trillion kg, on the basis of a density of 0.62 g/cm3 calculated from data from Deep Impact , the spacecraft that encountered the comet. (By comparison, water has a density of 1 g/cm3.)