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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. 39 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- 3 keV Kilo electron volt = 10 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– −9 nT Nanotesla (10 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 4 4 (10 –3 × 10 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). 18 pc Parsec: 1 pc = 3.086 × 10 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. −3 of solar wind at 1 AU solar distance is 5 cm , average The study of comets has been closely coupled with sci- −1 speed at earth is 400 km s , 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 3/2 p = a 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 √ 2 L = µa(1 − e ) (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 spacecraft at comet Giacobini–Zinner in September 1985. In the foregoing equation, µ = GM⊙, where G is the gravitational constant and M⊙ the solar mass. The orbit of a comet is prograde (or direct) if its inclination Because of the retrograde orbital motion of comet Hal- ◦ ◦ (i ) < 90 , and retrograde (or indirect) if i > 90 . ley, the encounter speeds during flyby observations were Because of perturbations from the passing stars, plane- −1 −1 very high (68 km s for Giotto and 79 km s for Vega 1). tary gravitational scattering, and/or jet forces from surface High-speed dust impact at close approaches to the comet outgassing, the orbital elements can change as a func- nucleus became a hazard for the spacecraft. This prob- tion of time. The orbital elements determined at a certain lem caused a temporary loss of radio link of Giotto to the time (t0) are called osculating elements for this particu- earth when the probe was at a distance of about 1000 km lar epoch. To infer the future or past orbital elements of from the cometary nucleus. As a result, no images were a comet, orbital integrations taking into account all rele- obtained beyond that point even though the spacecraft was vant perturbation effects must be carried out from t = t0 targeted to approach the comet as close as 600 km on the sunward side. Several other instruments were also dam- aged. In spite of these calculated risks, the comet missions as a whole achieved major scientific successes beyond ex- pectations. In other arenas of cometary research, there has been very interesting progress as well. The new concept of an 3 4 inner Oort cloud between 10 AU and 10 AU is one of them. In the present work, we shall make use of these many new results to attempt to build a concise picture of modern cometary physics. Because of the page limit, it is impos- sible to go into all details. For further information, the reader should consult the appropriate references as listed. II. ORBITAL DYNAMICS FIGURE 3 Description of a cometary orbit with i denoting the The Keplerian orbit of a comet can be characterized by inclination; ω is the argument of perihelion; is the longitude of several orbital elements, namely, the semimajor axis (a), the ascending node; and γ is the equinox. P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 Cometary Physics 343 to the time interval of interest. Because of limitations of stellar molecular clouds, and the galactic tidal forces (see computing machine time, computational accuracies, and Section IX), a continuous influx of new comets from the observational uncertainties, such calculations usually do Oort cloud to the inner solar system can be maintained. not cover a time period much more than a few thousand In addition to the classical Oort cloud, made detectable years if very accurate orbital positions are required. To via the injection of a constant flux of new comets, the 3 investigate long-term evolutions, statistical methods are presence of a massive inner Oort cloud between 10 AU 3 often used such that the orbital behavior of a sample of and 5 × 10 AU has recently been postulated. Only the comets over a time span of millions to billions of years very infrequent passages of stars would scatter comets in can be described. this region into the inner solar system. Such events could Certain invariants in celestial mechanics are useful give rise to the so-called comet showers lasting about 2–3 in identifying the orbital characteristics of small bodies million years at irregular intervals of 20–30 million years. (comets and asteroids) in the solar system. In the restricted For a comet in the Oort reservoir, with radial distance 4 three-body problem with the perturbing planet (Jupiter in r > 10 AU from the sun, its angular momentum can be this case) moving in a circular orbit, the Tisserand invari- expressed in terms of the orbital velocity (Vc) and the angle ant is defined as (θ) between the comet–sun radius vector and the velocity [ ( )] 1/2 vector (see Fig. 7), that is, 1 q TJ = + 2 2q 1 − cos i (6) 2 1/2 a 2a r Vc sin θ = [µa(1 − e )] (7) where a and q are in units of Jupiter’s semimajor axis, aJ . which yields [cf. Eq. (4)] Most of the short period comets have TJ < 3. 2µq 2 2 V θ ≈ (8) Comets are generally classified into three orbital types c 2 r according to their periods: namely, the long-period comets for small θ . Assuming that the velocity vectors of comets with p > 200 years; the intermediate-period comets with in the Oort region are sufficiently randomized by stellar P between 20 and 200 years; and finally, the short-period perturbations, we can express the distribution function comets with P < 20 years. A compilation of the orbital sin θ θdθ data of the observed comets shows that there are in total fθ (θ)dθ = dθ ≈ (9) 644 long-period comets 25 intermediate-period comets, 2 2 and 88 short-period comets. Although such classification In combination with Eq. (8) we find that the perihelion is somewhat arbitrary in the divisions of the orbital peri- distance of new comets should be ods, it makes very clear distinctions in the inclination dis- µdq tribution. The inclinations of the short-period comets are fq (q)dq = 2 2 (10) 2V r ◦ c mostly less than 30 , and those of the long-period comets have a relatively isotropic distribution. As discussed be- In other words, fq is independent of q. The popula- −4 tion of long-period comets with 1/a ≈ 10 AU, however, low, one active research topic at the present moment is is a mixture of new comets and the evolved ones with whether these different inclination distributions are also one or more perihelion passages through the solar sys- indicative of different dynamic origins of these comet tem. Because of planetary perturbations, the perihelion populations. distribution of the evolved comets would be significantly Note that among the long-period comets, there is a modified. Numerical simulations show that the frequency group of sun-grazers, called the Kreutz family, that might distribution of perihelion distance of long-period comets have originated from the breakup of a single large comet. should be highly depleted inside the orbit of Jupiter. It A number of them have perihelion distances inside the becomes constant only for q ≳ 30 AU where the planetary sun. The SOLWIND and Solar Maximum Mission satel- perturbation effects are no longer important. lites detected 13 such comets, while the SOHO space solar Note that the velocity of a long-period comet near its observatory detected more than 200 of the Kreutz family −1 aphelion in the Oort region is of the order of 200 m s . comets (see Fig. 4). Figure 5 shows the 1/a distribution for the new, long- On the other hand, the average speed (V∗) of a passing star −1 −4 −1 relative to the solar system is about 30 km s . The effect period comets in units of 10 AU . As first discussed by −4 −1 of stellar perturbation can therefore be treated using the Oort (1950), the peak at 1/a < 10 AU suggests that impulse approximation as follows. With closest approach the new comets mostly come from an interstellar reservoir in the form of a spherical shell surrounding the sun. The distances given as Dc and D⊙, respectively, the velocity in- 4 crements received by the comet and the sun can be given as inner radius of this so-called Oort cloud is at about 10 4 AU and that of the outer radius at about 3–5 × 10 AU. 2GM∗ Dc V = (11) c Because of the perturbing effects of passing stars, inter- 2 V∗ D P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 344 Cometary Physics FIGURE 4 The appearance of a sun-grazing comet observed by the SOHO coronograph instrument. Photo origin: ESA. and ing to numerical calculations, the typical energy change ( E) per perihelion passage is a function of the perihe- 2GM∗ D ⊙ V = . (12) ⊙ 2 lion distance and orbital inclination. For a comet with i V∗ D ∗ ◦ ◦ −4 −1 between 0 and 30 , E > 10 AU if q < 15 AU, and for The overall effect in the velocity change of the comet i between 150◦ and 180◦, E > 10−4 AU−1 only if q < 5 relative to the sun is then AU. This difference is due to the larger relative veloci- ties of retrograde comets during planetary encounters and δV c = Vc − V ⊙ (13) resulting smaller orbital perturbations. The excess of ret- 9 Over the age of the solar system (4.5 ×10 years), the rograde orbits in the frequency distribution of the incli- root-mean-squared velocity increment of a comet as nations of long-period comets may be a consequence of a result of stellar perturbations can be estimated to be such a selective effect in planetary encounters. −1 VRMS ≈ 100 m s . As this value is comparable to the The orbital transformation of long-period comets into 4 orbital velocity of a comet in circular motion at 9 × 10 short-period comets is one of the possible outcomes of AU, the outer boundary of the Oort cloud may be set at a sequence of random walk processes. The capture effi- this distance as a result of stellar perturbations. ciency depends on the inclinations and other orbital el- As mentioned before, the binding energy of a new comet ements of the comets in question. For example, a “cap- −4 −1 is of the order of 10 AU . During its passages through ture” zone exists for long-period comets with 4 AU < ◦ the inner solar system, a comet will be subject to gravi- q < 6 AU and i < 9 . Another possible scenario is that tational perturbations by the planets. The most important the majority of short-period comets are not supplied by perturbing planet is Jupiter, followed by Saturn. Accord- long-period comets with isotropic inclination distribution P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 Cometary Physics 345 FIGURE 5 Distributions of short-period comets (solid circles) and asteroids (open circles) plotted in a diagram of semimajor axis (a) vs eccentricity (e). A indicates the transjovian region, B is the Jupiter family of weak cometary activity, C is the Jupiter family of strong cometary activity, D is the minor planet region, and E is the apollo region. The thick dashed curve denotes the critical value of TJ = 3 (with cos i = 1) separating the cometary region (B + C) with TJ < 3 from the asteroidal region (D and E) with TJ > 3. [From Kresak, L. (1985). In “Dynamics of Comets: Their Origin and Evolution” (Carusi, A., and Valsecchi, G. B., eds.), IAU coll. 83, Reidel, Dordrecht, pp. 279–302.] P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 346 Cometary Physics [i.e., fi (i ) α sin i], but rather by a comet belt of low inclinations, located just outside the orbit of Neptune. The dynamical influence of Jupiter can be recognized in the aphelion distribution of short-period comets that tend to cluster near the semimajor axis of Jupiter. Furthermore, the distribution of the longitudes of perihelion (ω) for such a Jupiter family has a mimimum near the perihelion of longitude (ωJ) of Jupiter. The final fate of the orbital evolution of short-period comets would be determined by perturbation into escape orbit via close encounter with Jupiter or other planets, direct collision with a planet, or catastrophic fragmentation by hypervelocity impact with interplanetary stray bodies or crashing into the sun (see Fig. 4). The fragmentation process would lead to the formation of a meteor stream composed of small dust particles. Meteor streams could also be produced by partial fragmentation, surface crater- ing, and, of course, outgassing activities. For example, the short-period comet P/Encke is associated with the meteor stream S. Taurids, and P/Giacobini–Zinner is associated with the October Draconids. The Geminids stream is connected with the Apollo object 3200 Phaethon. Since Apollo objects are basically defined as Small bodies in Earth-crossing orbits (and Amor objects are bodies in Mars-crossing orbits), suggestions FIGURE 6 Frequency distribution of the original reciprocal semi- −3 −1 have been made that 3200 Phaethon may in fact be a de- major axes of long-period comets with (1/a)orig< 10 AU . [From Kresak, L. (1987), in “The Evolution of the Small Bodies funct cometary nucleus. There are several possible clues of the Solar System,” (Fulchignoni, M., and Kresak, L., eds.) Proc. supporting the hypothesis that short-period comets could of the Intern. School of Physics “Enrico Fermi,” Course 98, Societa be a source for the Apollo–Amor objects. First, several Italiana di Fisica, Bologna-North-Holland publ. co., pp. 10–32.] of these Apollo–Amor objects are in relatively high in- ◦ clinations (>30 ), which are very unusual for the asteroidal population. Second, three Apollo–Amor objects have aphelia beyond Jupiter’s orbit. Finally, the Apollo object 2201 Oljato has a surface UV reflectance very different from that of the asteroids. Short-period comets like P/Arend–Regaux and P/Neujmin II are almost inactive; therefore, they might have reached the turning point of becoming Apollo–Amor objects. Recent dynamical calculations show that, in addition to the injection from the main-belt asteroidal population via chaotic motion near the 3:1 Jovian commensurability and the ν6 secular reso- nance in the inner boundary of the asteroid belt, a significant fraction of the Apollo–Amor objects (≈2400 in total) could indeed come from short-period comets. III. GENERAL MORPHOLOGY Despite its brilliance in the night sky, a comet has a solid nucleus of only a few kilometers in diameter. The bright- ness comes from the dust, gas, and ions emitted from the FIGURE 7 The encounter geometry of a passing star with the nucleus. For example, during its 1986 passage near the 4 solar system. earth’s orbit, comet Halley lost on the order of 3.1 × 10 kg P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 Cometary Physics 347 of volatile ice per second and about an equal amount in and small nonvolatile dust particles. The expansion of the neu- 1275.6 tral gas (mostly water and carbon monoxide) from the cen- log p(mm Hg) = − + 0.00683T + 8.307 T tral nucleus would permit the formation of a large coma for T < 138 K (17) visible in optical, ultraviolet, and infrared wave-lengths. The optical emission in a cometary coma is largely from The latent heat of vaporization of water ice is the excitation of the minor constituents, such as CN and C2, by the solar radiation. These radicals are the daugh- L(T ) = 12420 − 4.8T (18) ter products from photodissociation of parent molecules −1 with L(T ) in cal mole . For CO2 ice we have such as HCN, C2H4, C2H6, and other more complicated 2 molecules. The dirty snowball model of the cometary nu- L(T ) = 12160 + 0.5 T − 0.033 T (19) cleus first proposed by Whipple (1950) describes the nu- Finally, the sublimation rate is related to the equilibrium cleus as a mixture of frozen ice and nonvolatile grains. The vapor pressure by the following equation: main components of the volatile ice are H2O, CO2, CO, H2CO, CH4, and NH3, followed by minor species such as ( )1/2 m ˙ HCN, C2H4, C2H6, CS2, and others. m Z(T ) = p(T ) (20) 2π K T ˙ A. Surface Sublimation Figure 8 shows the variations of Z(T ) as a function of 1 the solar distance r with ⟨cos θ cos φ⟩ = , Av = 0.02, and 4 An icy nucleus will begin to evaporate significantly at εIR = 0.4. For CO2 ice, a significant level of surface subli- perihelion approach to the sun once its surface tempera- mation starts at r ≈ 10 AU; strong outgassing would occur ture exceeds a certain value. Since the gas pressure in the at r ≈ 2–3 AU for H2O ice. cometary coma is much smaller than the critical pressure at the triple point, direct sublimation from the solid phase into vapor will occur. Under steady-state conditions, the energy equation in its simplest form can be written as −τ −2 F⊙e (1 − Av)r cos θ cos φ L(T ) 4 ˙ = εIRσ T + Z − K (T )∇T |s (14) NA where F⊙ is the solar flux at 1 AU, τ is the optical depth of the dust coma, Av is the surface albedo, r is the helio- centric distance in AU, φ and θ are the local hour angle and latitude, εIR is the infrared emissivity, σ is the Stefan– Boltzmann constant, T is the surface temperature, L(T ) is the latent heat of sublimation, NA is the Avogadro num- ˙ −2 −1 −1 ber, Z (in units of molecules cm s str ) is the gas production rate, and K (T ) is the thermal conductivity at the surface. Another important equation is the Clapeyron–Clausius equation. For water we have −2445.5646 log p(mm Hg) = + 8.2312 log T T − 0.01677006 T + 1.20514 −5 2 × 10 T − 6.757169 (15) with the equilibrium vapor pressure p in units of millime- FIGURE 8 The variations of the sublimation rates of H2O and ters of mercury. And for CO2 we have CO2 ices as a function of the solar distance. The surface albedo Av is taken to be 0.02 and the infrared emissivity εIR is assumed −1367.3 to be 0.4. Two extreme cases of the orientation of the spin axis log p(mm Hg) = + 9.9082 for T > 138 K (isothermal rapid rotator and pole-on rotator) are shown. The sur- T (16) face sputtering rate via solar-wind protons is also given. P1: ZCK Final Encyclopedia of Physical Science and Technology EN003E-124 June 13, 2001 22:11 348 Cometary Physics B. Gas Comas and Ion Tails wind protons will lead to the formation of an ion tail flow- + ing in the antisolar direction. In optical wavelengths, CO As the parent molecules (H2O, CO, CO2, CH4, H2CO, + and H2O are the most visible among the ions. The gen- etc.) move away from the nucleus, they will be either dis- eral features of the interaction of the cometary coma with sociated or ionized by solar ultraviolet radiation. The pho- the solar radiation and interplanetary plasma are sketched 5 todissociation life-time for water molecules is about 10 s in Fig. 10. at 1 AU solar distance. With an expansion speed (Vn) of −1 around 1 km s , most of the water molecules will be C. Dust Coma and Mantle dissociated into hydrogen atoms and hydroxide (OH) at 5 a cometocentric distance of a few 10 km. The OH will The gaseous drag from the expanding outflow is able to also be dissociated subsequently. A similar process occurs carry dust particles away from the nuclear surface. There for other parent molecules. As a result, at a cometocentric is, however, an upper limit on the particle size to be ejected 6 distance of a few 10 km, the extended neutral coma will in this manner. The critical particle size is reached when be made up only of atomic species (H, C, O, N, S, . . . ). the gravitational attraction of the nucleus outweighs the ˚ In Lyman alpha emission at 1216 A, the atomic hydrogen gaseous drag. The stronger the surface gas sublimation cloud of a comet can become the most prominent structure rate, the larger will be the critical size (see Section VI). in the solar system (Fig. 9). The ionization of the cometary For comet Halley, its out-gassing rate at 1 AU solar dis- neutrals by solar radiation and charge exchange with solar tance would enable the emission of solid particles up to a FIGURE 9 The SWAN experiment on the SOHO spacecraft observed a huge atomic hydrogen cloud surrounding Comet Hale–Bopp. The hydrogen cloud is 70 times the size of the Sun. Photo origin: ESA/NASA.