Sky & Telescope March 1987 [antikvár]

FOCAL POINTCometary Studies Come of AgeAbout a century ago, Mark Twain . noted with characteristic wit his amusement at the vast range of speculation that can result from a very meager piece of scientific data. Comets have always yielded abundant data but relatively little understanding.Were he alive today. Twain would surely have a wry view of the way our understanding of comets developed, or failed to develop, between Halley's passages in 1910 and 1986. And how could he fail to note with sarcasm that, even as the comet rounded the Sun last February, astronomers were still processing Halley data from 1910?While modern comet studies began with Whipple's snowball model and Oort's concept of a distant cometary reservoir, both in 1950, httle progress was made for the next 20 years. Most of what we know about comets has come in the last decade. We are, in fact, in the midst of an explosion of understanding about comets.I think four key factors have put us in this happy and historically enviable position. First, there have been important advances in theoretical tools and computational techniques. This has allowed scientists to study by statistical methods such phenomena as the ejection of molecules and dust grains from the elusive nucleus. Similarly, we have gained fresh insights on the Oort cloud and the way its comets are transferred to the inner solar system.Key to understanding the dust in a comet's coma has been research on the optical properties of materials that may make up the nucleus. Theoretical studies by computer have opened possibilities for determining the isotopic composition of comets from observations of molecular spectra, which reveal very complex environments of radiation that excite the molecules.Second, as infrared astronomy blossomed in the mid-1960's, new cometary phenomena came into view. We now know some details about how dust reflects sunlight and radiates heat, and we've observed the infrared signatures of numerous molecules. The growth of technology for making observations in the infrared, together with laboratory analysis of microscopic bits of cometary dust captured high in Earth's atmosphere, are great credits to modern science.Third, we now recognize the relation-ship of comets to other small bodies in the solar system, specifically asteroids, planetary satellites, and meteorites. The complete picture is still unclear, but we now have an appreciation of how some comets have become asteroids after the loss of their volatiles, and how some asteroids are ejected from the main belt into orbits that intersect those of the inner planets. Some meteorites can now be traced back to specific kinds of asteroids in their temporary Earth-crossing orbits, then to the asteroid belt, and perhaps in some cases to the Oort cloud reservoir. Similarly, we now begin to understand where comets formed in the condensing solar nebula.The fourth great fount of knowledge lies in the booty returned by the flotilla of spacecraft sent to encounter Comets Halley and Giacobini-Zinner, and from a worldwide network of astronomers at their telescopes. With sharpened toolsand insight, we have begun to synthesize our wealth of information into a new perspective about comets in general, and these two in particular. I am convinced that an understanding worthy of the expended effort will emerge.Were the spacecraft missions to Comet Halley necessary? The question seems to make no more sense than it does to ask if it was necessary for people to sail across the seas or take to the air. Succeeding generations will look back on these times and consider the spacecraft missions to comets as a visionary, essential, and perfectly natural step in our first efforts to explore the solar system.While a new view of the cometary nucleus was emerging from ground-based work, a controversy at least as large as a comet's coma would have continued to enshroud the problem for many years. Giotto and Vega have saved us a lot of valuable time by clarifying several basic properties of the comet's nucleus. They also gave us on-the-spot measurements of the dust and gas composition in the coma that probably could not have been made any other way.Equally vital toward understanding cometary composition and behavior is the proposed Comet Rendezvous Asteroid Flyby mission. Although NASA has just announced its preliminary payload, the mission has not yet gained congressional approval. Looking farther ahead, the crowning achievement in cometary studies will be the successful return to Earth of samples extracted from a comet's nucleus. The missions to Comet Halley were an essential step toward this goal.Although NASA may at the moment be searching for direction, planetary scientists have a clear vision of the necessary next steps in the exploration of the solar system. Having recognized the importance of comets in understanding the conditions in which the solar system formed, additional missions to these frozen little worlds will play a crucial role in that exploration.DALE P. CRUIKSHANKThe author, who tniliales a new Sky & Telescope feature this month, has been a key player in infrared studies of the solar system. Cruikshank shuttles between the telescopes atop Mauna Kea and the University of Hawaii's Institute for Astronomy in Honolulu.Focal Point invites contributions from readers who wish to comment on contemporary issues in astronomy and space science.