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Cometary Physics P1: ZCK Final Qu: 00, 00, 00, 00 Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 Cometary Physics W.-H. Ip Institutes of Astronomy and Space Science, National Central University, Taiwan I. Introduction II. Orbital Dynamics III. General Morphology IV. Cometary Spectra V. Cometary Nuclei VI. Dust VII. Atmosphere VIII. Plasma IX. Origin X. Prospects GLOSSARY Wachmann 3, and d’Arrest with the spacecraft to be launched in 2002. a Semimajor axis of an orbit. DE (disconnection event) The major disruption of a AU Astronomical unit = 1.496 × 1013 cm, which is the cometary ion tail by the apparent ejection of large semimajor axis of the earth’s orbit around the sun. plasma condensation or separation of the main ion tail Blackbody An idealized perfectly absorbing body that from the comet head. absorbs radiation of all wavelengths incident on it. Deep Impact An American space mission to comet Bow shock A discontinuity defining the interface of the 9P/Tempel 1 for active Impact experiments. The sched- transition of a supersonic flow to a subsonic flow as it uled launch date is in 2004. encounters an obstacle. Deep Space 1 An American Spacecraft with ion propul- Carbonaceous chondrites A special class of meteorites sion to flyby comet Borrelly in 2002. containing up of a few percent carbon. eV Electron volt = 1.6 × 10−12 ergs. Contact surface A surface separating two flows of dif- Giotto A spacecraft from the European Space Agency for ferent chemical and/or flow properties. flyby observations of comet Halley in March 1986. Comet showers The infrequent injection of a large num- HST Hubble Space Telescope, an optical telescope ber of new comets into the inner solar system by passing launched into geocentric orbit by NASA in 1990. stars at close encounters with the solar system. ICE International Cometary Explorer, an interplanetary CONTOUR An American space mission to perform spacecraft (ISEE 3), which was redirected to intercep- flyby observations of comets Encke, Schwassmann- tion of comet Giacobini/Zinner in September 1985. 339 P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 340 Cometary Physics IHW International Halley Watch, an international pro- will be in 2004 and the sample will be parachuted back gram to coordinate the ground-based observations of to the earth in 2006. comet Halley. Suisei A Japanese spacecraft for in-situ plasma measure- IMS Ion mass spectrometer that can make mass- ments at comet Halley in 1986. separating measurements of ions. VEGA Two Soviet spacecraft dedicated to close flyby ISO Infrared Space Observatory, an infrared telescope observations of comet Halley in 1986 after performing launched into geocentric orbit by ESA in 1993. a series of measurements at Venus swingbys. Jet force The repulsive force on a cometary nucleus due to anisotropic gas emission. The jet force was in- voked to explain the nongravitational effects observed COMETS are known to be the Rosetta Stone to deci- in cometary orbital evolutions. pher the origin of the solar system. The chemical com- keV Kilo electron volt = 103 eV. position of the volatile ice, the mineralogical properties KT boundary The Cretaceous–Tertiary boundary sepa- and isotopic abundances of the dust grains, and finally the rating two distinct geological structures about 65 mil- physical structure of the cometary nuclei all carry funda- lion years ago. This boundary is characterized by a thin mental information on the condensation and agglomera- layer of enhanced iridium deposition that might have tion of small icy planetesimals in the solar nebula. The come from an impact event of a comet or an asteroid process of comet–solar-wind interaction is characterized of kilometer size. by very complex and interesting plasma effects of spe- Mass extinction Episodes of large-scale disappearances cial importance to solar system plasma physics; also, the of life forms on earth. Statistical correlations suggest a different aspects of momentum exchange, energy trans- periodicity of about 26 million years. fer, and mass addition in a plasma flow are closely linked Meteoroid A small solid particle orbiting around the sun to large-scale outflows in interstellar space. For example, in the vicinity of the Earth. the generation of massive outflows from the T Tauri stars NMS Neutral mass spectrometer, which can make mass- and other young stellar objects and their interaction with separating measurements of neutral gas. the surrounding medium are reminiscent of the comet– nT Nanotesla (10−9 T) units of magnetic field strength. solar-wind interaction. The formation of small structures Oort cloud The reservoir of new comets at large distances called cometary knots in the Helix planetary nebula is (104–3 × 104 AU) from the sun. reminiscent of the comet–solar wind interaction (Fig. 1). PAH Polycyclic aromatic hydrocarbons, which might be The cometary plasma environment therefore could be con- representative of the very small grains in interstellar sidered a laboratory simulating many basic astronomical space. processes. POM Formaldehyde polymers of the form of (H2CO)n, with n = 10–100. The chainlike polymers could be ter- minated and stabilized by the addition of monovalent I. INTRODUCTION atoms or ions (i.e., HO–CH2–O–..CH2–CN). pc Parsec: 1 pc = 3.086 × 1018 cm. The historical context of cometary research is an interest- Rosetta A European mission designed to collect surface ing document of progress in science. In Oriental records, material from the short-period comet P/Wirtanen with a comets were usually called guest stars or visiting stars to lander to be launched from a mother spaceship orbiting describe their transient appearances in the celestial sphere. around the cometary nucleus. The spacecraft will be One of the earliest systematic records of comets can be launched in 2003. found in the Chinese astronomical history of 467 B.C. The Sakigake A Japanese spacecraft for remote-sensing mea- historian Kou King-ting noted in 635 B.C. that cometary surements at comet Halley in 1986. tails point away from the sun. In Europe, study can be Solar nebula The primordial disk of gas and con- traced back to Aristotle’s postulation that comets should densed matter befor the formation of the planetary be associated with transient phenomena in the earth’s at- system. mosphere. As astronomy started to blossom in the Middle Solar wind A radial outflow of ionized gas (mostly pro- Ages, Tycho Brahe, Kepler, and Newton all paid special tons) from the solar corona. The mean number density attention to the dynamical nature and origin of comets. of solar wind at 1 AU solar distance is 5 cm−3, average The study of comets has been closely coupled with sci- speed at earth is 400 km s−1, and the mean electron entific and technical progress in the past few centuries. temperature is 20,000 K. The most important example is comet Halley, which was Stardust An American sample return mission to flyby deduced to be in elliptic orbit around the sun with a pe- comet Wild 2 for dust collection. The comet encounter riod of 75.5 years by Edmond Halley in 1682. His famous P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 Cometary Physics 341 FIGURE 1 Structures of the so-called cometary knots observed at the Helix Nebula (NGC 7293). They are formed by interstellar cloudlets being blown off by intense ultraviolet radiation from the central massive stars. Photo origin: NASA. prediction that this comet should return in 1758 has be- investigations of the nucleus rotation, coma activities, and come a milestone in astronomy. Three returns later, in dynamics of the plasma tail all require comparisons of the March 1986, comet Halley was visited by a flotilla of spacecraft measurements with the observational results spacecraft taking invaluable data revealing the true nature gathered by the IHW and other similar programs. of the most primitive bodies in the solar system. In to- Before the Halley encounters, a first look at the tal, six spacecraft from the former Soviet Union, Japan, cometary gas environment was obtained by the NASA Europe, and the United States participated in this interna- spacecraft International Cometary Explorer (ICE), at tional effort (see Fig. 2). In addition, an extensive network comet Giacobini–Zinner on September 11, 1985. Even for ground-based comet observations was organized by though it did not have a scientific payload as com- NASA to coordinate the study of comet Halley. This pro- prehensive as the payloads carried by the Giotto and gram, called International Halley Watch (IHW), proved Vega probes, many exciting new observations pertinent to be successful in maintaining a high level of cometary to comet–solar-wind interaction were obtained. Because research in all areas. The long-time coverages provided ICE went through the ion tail of comet Giacobini–Zinner, by ground-based observations are complementary to the the experimental data are particularly useful in addressing snapshots produced by spacecraft measurements. In fact, questions about the structures of cometary plasma tails. P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 342 Cometary Physics the inclination (i), the eccentricity (e), the argument of perihelion (ω) and the longitude of the ascending node () with respect to the vernal equinox (γ ). Their mutual relations are shown in Fig. 3. The orbital period is given by p = a3/2 years (1) where a is in units of AU. Thus, for comet Halley, a = 17.9 AU and P = 76 years. The closest distance to the sun, the perihelion, is given by q = a(1 − e) (2) and the largest distance from the sun, the aphelion, is given by Q = a(1 + e) (3) The total specific angular momentum is expressed as L = µa(1 − e2) (4) whereas the component perpendicular to the ecliptic plane is 2 FIGURE 2 A schematic view of the encounter geometries of sev- Lz = µa(1 − e ) · cos i (5) eral spacecraft at comet Halley in March 1986 and the ICE space- craft at comet Giacobini–Zinner in September 1985.
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