The Target Bodies for the CONTOUR Mission: Comets 2P/Encke and 73P/Schwassmann-Wachmann 3-C Draft of an article to be published in Space Science Reviews February 2002 Donald K. Yeomans 301-150 Jet Propulsion Laboratory Pasadena, CA 91109 Tel. (818) 354-2127 FAX (818) 393-1159 donald.k.yeomans@jpl.nasa.gov Michael J.S. Belton Belton Space Exploration Initiatives, LLC 430 South Randolph Way Tucson, AZ 85716 Tel. (520) 795-6220 FAX (520) 795-6220 belton@azstarnet.com Comets 2P/Encke, and 73P/Schwassmann-Wachmann 3 are among the most diverse and active short period comets. Comet Encke is likely to be a very evolved comet in a stable orbit while comet Schwassmann-Wachmann 3 underwent multiple splits in late 1995 and may now reveal some young and fresh interior surfaces. To date, comet Encke has been observed at 58 returns to perihelion (1786, 1795, 1805, 1818-19, 1822-1941, 1947-2000) while Schwassmann-Wachmann 3 has been observed at five returns (1930, 1979, 1989-90, 1994-96, 2000-01). The present paper will cover the history, orbital evolution, observing conditions, and physical characteristics for these two CONTOUR comets. A detailed discussion of the dust and gas environments for these comets is presented by Feldman and Cochran (2002). COMET 2P/ENCKE: INTRODUCTION AND HISTORY The following brief historical introduction to the work on comet Encke's orbit is a summary of more complete histories by Yeomans (1991) and Sekanina (1991a). These two sources also provide the references for the following brief historical introduction. Comet 2P/Encke (hereafter comet Encke) has the peculiar distinction of being discovered four separate times with none of the discoverers being Johann Encke. The first discovery of the comet was on 1786 Jan. 17 by Pierre Mechain at Paris. Because of its faintness, only two nights of observations by Mechain, Charles Messier, and Jean-Dominque Cassini were available and these did not allow an orbit determination. Nearly ten years later on 1795 Nov. 7, comet Encke was again discovered when Caroline Herschel at Slough England discovered her sixth comet. The comet had made a close Earth approach (0.256 AU) on Nov. 9, 1795 and observations continued until November 29 when the comet was lost due to its solar proximity (low solar elongation angle). These observations permitted only a rough orbit to be determined. Another ten years passed before the comet was again discovered, this time on 1805 Oct. 20 by Jean Louis Pons at Marseille France with independent discoveries in Frankfurt (Johann Huth) the next night and on October 22 by Alexis Bouvard. The final discovery of the comet took place on 1818 Nov. 26 when Pons discovered it yet again. Baron Von Zach conjectured that the comets of 1818-19 and 1805 were the same and Heinrich Olbers suspected that the comets discovered in 1818, 1795 and 1786 were all the same object. In 1819, Johann Encke, a student of Carl Gauss, confirmed Olber's conjecture by integrating the comet's motion under the perturbative influences of all known planets except Uranus. Encke also integrated the motion forward in time to successfully predict the next perihelion passage time as 1822 May 24, the second successful comet return prediction (2P/Encke). As a result of Encke's work that concluded that the comets discovered in 1786, 1795, 1805, and 1818 were one and the same object, the comet became known as Encke's comet, although Encke himself always modestly referred to the comet as that of Pons. Interplanetary Resisting Medium Encke noted that observed time of perihelion passage in 1822 was a few hours earlier than his prediction and in 1823, he suggested an interplanetary resisting medium to explain this discrepancy. This resisting force, which was supposed proportional to the medium's density and to the square of the comet's orbital velocity, was envisioned as either an extension of the solar atmosphere or the debris from cometary or planetary atmospheres left in space. Using his model of the resisting medium to reduce the comet's orbital period by about 2.5 hours per return (0.16 m/s), Encke successfully predicted his comet's returns between 1825 and 1858. After studying some of the observations of comet Halley in 1835, Friedrich Bessel noted some sunward emanations that resembled a burning rocket. Bessel argued that a resisting medium was not evident in the motion of the planets and he suggested that the orbital periods of comets could be shortened by the radial, rocket-like thrusting of the comet itself if this effect was asymmetric with respect to perihelion. As an example, Bessel calculated how much comet Halley's orbital period would be shortened if its sunward emanations took place before perihelion and lasted 23 days beyond Oct. 2, 1835. If the daily mass loss from the nucleus was assumed to be 1/1000 of the total the result was a shortening of the comet's period by 1,107 days. While a considerable overestimate (Halley's orbit is actually lengthened by 4 days per period), Bessel's rocket effect anticipated the currently accepted explanation for these effects advanced by Fred Whipple in 1950. In addition, Bessel clearly considered that the nuclei of comets were solid. However, contemporary views clearly favored Encke's resisting medium. In 1878, Friedrich Emil von Asten published a study of Encke's motion over the 1818 - 1875 interval trying to test whether Encke's resisting medium or Bessel's rocket-like thrusting was correct. From the computed decrease in the orbital eccentricity, he concluded that Encke's hypothesis was preferred and that since the so-called nongravitational effects were not evident in comets with larger perihelion distances, he suggested that the resisting medium did not extend much beyond the orbit of Mercury. While Encke's motion over the 1818 - 1868 interval was consistent with the resisting medium hypothesis, the return in 1871 was not the usual 2.5 hours early. Von Asten speculated that the apparent disappearance of the nongravitational effects in 1871 was due to a collision of an asteroid with the comet in June 1869. Jons Oskar Backlund, a Swedish astronomer who became director of Pulkovo Observatory in 1895, received the RAS-London gold medal for his work on comet Encke's motion in 1909, yet he had only limited success in explaining this comet's motion. Backlund rejected von Asten's asteroid collision hypothesis and concluded that the comet's orbital period decreased in a uniform fashion over the 1818 - 1858 interval but subsequently, this decrease became less apparent. Backlund's explanation allowed the density of the resisting medium, which took the form of a meteoric ring near the comet's perihelion, to decrease with time. Since a resisting medium could only decrease a comet's orbital period, the final blow to the resisting medium occurred with the discovery that some comets showed a secular increase, rather than a decrease, in their orbital periods. In 1933, Michael Kamienski showed this to be true for the motion of periodic 14P/Wolf and seven years later, Albert Recht demonstrated the same type of behavior for periodic comet 6P/d'Arrest. COMET ENCKE'S ORBITAL CHARACTERISTICS AND NONGRAVITATIONAL ACCELERATIONS Among the short-periodic comets, comet Encke has the shortest orbital period and its perihelion distance of 0.33 AU is the second smallest. The perihelion distance for comet 96P/Machholz 1 is smaller at 0.12 AU. Because it can get no closer to Jupiter than 0.9 AU, the size and orientation of comet Encke's orbit are very stable with time. Table 1 presents the current osculating orbital elements for comets Encke and Schwassmann-Wachmann 3. The nongravitational parameters (A1, A2) represent the radial and transverse rocket-like accelerations acting upon the comet's nucleus as a result of its outgasing at 1 AU from the sun (Marsden et al., 1973). Interactive (two-body) orbital movies are available for comets 2P/Encke and 73P/Schwassmann-Wachmann 3 at: http://neo.jpl.nasa.gov Table 1. Osculating orbital elements (J2000) for comets 2P/Encke and 73P/Schwassmann-Wachmann 3 (Fragment C). Both orbital element sets were computed using planetary ephemeris DE405 and included asteroid perturbations from Ceres, Pallas, and Vesta. Encke Schwassmann-Wachmann 3 JPL Reference Solution: K004/12 K012/12 No. Observations: 273 356 Observation Arc: 1989/06/01-2001/08/28 1994/12/28 - 2001/11/20 Epoch 2003 Dec 27.0 2006 May 25.0 Eccentricity 0.8473391 0.6932314 Perih. dist. (AU) 0.3384615 0.9391412 Perihelion passage 2003 Dec 29.876312 2006 Jun 7.169859 Arg. of Perih. (deg.) 186.49870 198.80518 Long. Asc. Node (deg.) 334.58746 69.89584 Inclination (deg.) 11.76962 11.39612 A1 {AU/(day2)} 6.96 (4.42) x 10-11 9.877 (0.086) x 10-9 A2 {AU/(day2)} -1.232 (0.017) x 10-11 0.682 (0.026) x 10-9 A (AU) 2.2176466 3.0613988 P (Years) 3.302 5.356 Comet Encke's Nongravitational Effects The nongravitational accelerations experienced by comet Encke were used by Whipple (1950) to introduce and test his icy conglomerate model for a cometary nucleus. Vaporizing nucleus ices would expel gas preferentially toward the sun thus introducing a radial acceleration away from the sun. The nucleus rotation, either prograde or retrograde, would be expected to introduce a transverse acceleration of either sign as a result of the time delay between cometary noon and the time when the outgasing reached a maximum. Those using the nongravitational acceleration model introduced by Marsden et al. (1973) normally solve for only the radial (A1) and transverse (A2) nongravitational acceleration parameters. While there is a third nongravitational acceleration parameter (A3) that is normal to the comet's orbit plane, it is not often used since this component rarely improves the orbital solutions. The change of the comet Encke's radial and transverse nongravitational parameters with time are displayed in Figures 3a and 3b. It is clear that the transverse parameter (A2) varies in a sinusoidal fashion and that it is currently very close to zero. It seems likely that for this comet, as well as for other active comets, the nongravitational accelerations are not primarily due to a time delay between cometary noon and the time of maximum outgasing but rather due to a radial acceleration that can act asymmetrically with respect to perihelion. This idea, originally put forward by Bessel in the early nineteenth century, was adapted for general computational use by Yeomans and Chodas (1989). For a comet (e.g. d'Arrest) that outgases preferentially post-perihelion, orbital energy is introduced (A2 > 0) and the comet's semi-major axis increases slightly with time. For comets that outgas preferentially before perihelion, their semi-major axes decrease slightly with time and A2 < 0. In the orbit given for comet Encke in Table 1, the mean value of A2 over the 1989 - 2000 time interval is slightly negative. However, from Figure 1, we note that values for A2 have been becoming less negative since 1820 - probably because the comet's propensity to outgas preferentially before perihelion has been decreasing to a point where, currently, the comet's outgasing is nearly symmetric with respect to perihelion. Kamel (1991) has analyzed the light curves of comet Encke over its recorded history and finds that the comet's brightness has not faded noticeably in the period 1832-1987 but the time of maximum brightness has shifted from about 19 days pre-perihelion in the middle of the nineteenth century to a few days after perihelion currently. Kamel also noted that the shifting time of peak brightness correlates well with the decrease in the nongravitational effects upon the comet's orbit. COMET ENCKE'S ORBITAL EVOLUTION Marsden and Sekanina (1974) numerically integrated the motion of comet Encke back to 1570 using constant values for the transverse nongravitational parameter (A2) while Sitarski (1988) used a value of A2 that varied as a sinusoidal function of time to integrate the comet's motion back to 1201. In the last eight centuries, the perihelion and aphelion distances have changed by less than 0.01 and 0.04 AU respectively. The orbital line of apsides rotates very slowly in an easterly direction (~0.9 degree/century). Whipple (1940) noted that the Taurid meteors are associated with comet Encke and Whipple and Hamid (1952) pointed out that some of these meteoroid particles left the parent some 4700 years ago while the comet was on an orbit that was similar to its present day trajectory. While the orbit of comet Encke is remarkably stable in both its size and orientation over long periods of time and it has been observed as a naked eye object several times since its discovery in 1786, there are apparently no records of the comet in the ancient Chinese annals (Hasegawa, 1979; Whipple and Hamid, 1972). Thus the comet may have been inactive, or dormant, during some of the centuries prior to its discovery in 1786. Using estimates of the comet's gas and dust production rates, Reach et al. (2000) estimate that the physical lifetime of comet Encke is about 3000 - 10,000 years assuming that the nucleus bulk density is about one gram per cubic centimeter. FUTURE VIEWING OPPORTUNITIES Because its orbit size and orientation are very stable, general conclusions can be drawn as to when the comet is easily observable. When the comet reaches perihelion between mid-September and the end of January, the comet is observable from the northern hemisphere before perihelion while if the comet reaches perihelion between late April and early August, the comet is observable after perihelion from the southern hemisphere. Perihelia returns in other times of the year result in rather unfavorable observation scenarios. The closest Earth approaches to within 0.2 to 0.3 AU are possible when the comet reaches perihelion in December or the second half of May (Sekanina, 1991a). For example, in 2003 comet Encke reaches perihelion on Dec. 29 allowing pre-perihelion viewing from the northern hemisphere and an approach to Earth of 0.26 AU on November 17, 2006. An orbital diagram of comet Encke and an illustration of its observing geometry at various apparitions are given in Figures 4 and 5. PHYSICAL CHARACTERISTICS OF COMET ENCKE'S NUCLEUS Beginning with its discovery in 1786, comet Encke has had a long observational history. Spectral observations have been made from ground-based telescopes as well as from instruments on board the OGO-5, IUE, EUV, HST, ISO, SOHO and PVO spacecraft. This comet has been extensively observed in wavelength regions as diverse as radio, radar, infrared, visible, ultraviolet, and X-ray. This set of comprehensive observations has allowed modeling efforts to estimate an OH production rate approaching 1029 molecules per second near perihelion and about 3 x 1027 molecules/sec at the CONTOUR encounter time (Sekanina, 1991b). A'Hearn et al. (1983) noted that the production rate of OH drops by a factor of 3 at 0.75 AU pre-perihelion and that all measured production rates are lower post-perihelion than at similar pre-perihelion heliocentric distances. However, A'Hearn and Schleicher (1988) note that at least between heliocentric distances 0.75 and 1.0 AU, the OH production rate is nearly symmetric with respect to perihelion but the radial outflow velocity, as measured via the Greenstein effect in IUE spectra taken in November 1980 and August 1987, show pre-perihelion outflow velocities some five times larger than corresponding post-perihelion outflow velocities (i.e., 1.6 vs. 0.3 km/s). Thus the comet's nongravitational effects would also be expected to be asymmetric with respect to perihelion with the result (noted above) that the comet's orbit has been losing a slight bit of orbital energy for nearly the last two centuries. The comet shows signs of activity at all positions around its orbit and, even at aphelion, variability attributable to coma particles has been seen. Meech et al (2001), from a study of observations from six aphelion passages, find that the total magnitude can vary by up to 2 magnitudes. However, no extended coma has been seen at aphelion. Although a lack of a continuous spectrum indicates a very low density of micron-sized dust, the meteor showers associated with comet Encke as well as the dust trails detected in Encke's orbital path by the Infrared Space Observatory suggest that larger dust particles are being emitted (Reach et al., 2000). The dust trail following the parent comet has a central core that is 2 x 104 km wide composed of particles about 5 cm in size and Reach et al. (2000) estimate that the water and dust lost per orbit are 2.1 x 1012 grams and 2-6 x 1013 grams respectively. The infrared observations of comet Encke by Gehrz et al. (1989) suggest relatively large dust grains (5 - 10 microns) while the work of Lisse et al. (2002) suggests the presence of even larger dust grains (> 20 microns). Lisse et al. (2002) also suggest a nucleus albedo of 0.05 (0.02) and a dust to gas ratio of 2.3. From their analysis for ground-based mid-infrared and optical observations along with ISO and Hubble Space Telescope observations, Fernandez et al. (2000) suggest that the nucleus is elongated (at least one aspect ratio is 2.6), its radius is 2.4 km, and the nucleus rotation period is likely to be 15.2 hours, a result that agrees with an earlier light curve analysis by Luu and Jewitt (1990). They also note that the geometric albedo of the nucleus is 0.05 (0.02) and the phase coefficient is 0.06 magnitudes/degree, making comet Encke one of the most phase darkened objects in the solar system. Together with comets Halley and Borrelly, comet Encke is one of only three cometary nuclei for which the spin axis (more correctly the direction of the total angular momentum vector) is roughly known. Sekanina (1988, 1991b) used observations of fan structures in the coma and synthetic computer-generated images of Encke's coma morphology to model the activity of comet Encke. Based upon the hypothesis that the fan structure is symmetric about the nucleus spin axis, Sekanina suggests that the nucleus currently has its spin axis nearly in its orbit plane (R.A. and Dec., 1950.0 of 205 and +2 degrees) and that an active area is present near both polar regions at a latitude of +55 degrees and -75 degrees. Festou and Barale (2000) reached similar conclusions on the direction of the spin axis, but based upon the morphology of coma gas outflow. Some confirmation of these results comes from the thermal observations of the shape of the inner coma and the near nucleus dust trail observed by Reach et al. (2000) using ISO. They find that the spin axis must be nearly parallel to the orbital plane to provide an explanation of what is seen. Finally Samarasinha (2002) has shown in computer simulations of the evolution of the rotational angular momentum vector of an elongated nucleus under typical jet outflows, that over time the vector slowly migrates toward the orbital plane and circulates around the general direction of perihelion. According to Sekanina, the spin axis location has been rather constant for more than a half century. The sizes of the northern and southern active areas are estimated to be respectively 0.4 and 0.6 km2 with depth erosion rates of about 4.5 and 6.5 meters per orbit. The active area near the northern pole is found to dominate the comet's activity on the inbound leg of the orbit until several days before perihelion and again beginning some two months post-perihelion while the active region near the southern pole dominates the activity at the other times near and after perihelion. Water production rates derived from the ultraviolet observations of comet Encke near perihelion in 2000 by the SOHO spacecraft were in excellent agreement with Sekanina's (1991b) activity model (Raymond et al., 2002). Sekanina attributes the persistent pre-perihelion fan-like coma to a collimated stream of material arising from a deep vent that sinks below the surface by several hundred meters. The suggested pre-perihelion CONTOUR spacecraft encounter time may offer a view of a very evolved (possibly quite bizarre) cometary landscape. Determining the rotation state of comet Encke has become a bit of a puzzle in that various periods (i.e., 22.4, 15.1, 11.1, 8.7 hours) have been suggested by the five different observational studies that have been reported. Some of these periods appear to be harmonically related ( e.g., 11.1 ~ 22.4/2, and 3*[15.1/2] ~ 22.4). We may have a situation similar to what existed for comet Halley where ground-based observations were seemingly in conflict until spacecraft observations showed the nucleus was in an excited or complex rotation state (Belton, 1990). If this is the case for comet Encke, then it becomes necessary to modify Sekanina's hypothesis on the nature of coma fans to one in which the fan structure is symmetric about the total angular momentum vector rather that the spin axis in order to explain its stability as determined by the orientation of the outgasing fans for nearly 100 years. Meech et al. (2001) have pointed out that determinations of Encke's rotation period could be confused because the comet is active near aphelion, sometimes appearing as many as two magnitude brighter intrinsically. If the rotation period of comet Encke is fully relaxed (i.e., unexcited), then one would expect the nucleus to be rotating about its shortest axis with no significant precessional motion of the spin axis. This would be called rotation in a short axis mode (SAM). However, Belton (2001) suggests, based upon an axially symmetric model for the nucleus, that the comet may be rotating in a long axis mode (LAM). This latter excited state has two rotation periods, rather like a football thrown with an imperfect spiral. The first period of 8.7 hours is then attributable to the rotation of an active area about the longest axis and the second period of 15.2 hours is due to the free precession of the long axis which circulates about the angular momentum vector. In the Belton model, the angular momentum vector is inclined by 52 degrees with respect to the long axis of the nucleus and the net spin period is about 6 hours if the axial ratio is 2.6:1 as suggested by Fernandez et al. (2000). Table 2. Physical characteristics for comet 2P/Encke Nucleus Rotation Aspect Geometric Pole Direction Radius (km) Period (hrs) Ratio Albedo RA/Dec. deg. (1950) Reference 0.5-3.8 Kamoun et al. (1982) 22.4 (0.08) >2 Jewitt & Meech (1987) < 4.4 Campins (1988) 2.5 - 6.4 Gehrz et al. (1989) 2.2 - 4.9 15.08 (0.08) >1.8 Luu & Jewitt (1990) 205/+2 Sekanina (1991b) 198/0 Festou & Barale (2000) 2.4 (0.3) 15.2 (0.03) >2.6 0.047 (0.023) Fernandez et al. (2000) 1-3 Reach et al. (2000) 11.08 (0.01) Meech et al. (2001) a=5.8 15.1 0.05 198 (10) Belton (2001) b=c=2.6 8.7 2.2 0 (10) COMET 73P/SCHWASSMANN-WACHMANN 3 Comet 73P/Schwassmann-Wachmann 3 (hereafter SW3) was discovered on 1930 May 2 by Arnold Schwassmann and Arno Arthur Wachmann at the Bergedorf observatory near Hamburg Germany (see figure 6). The comet was first recognized as a short period object toward the end of May 1930 when the English orbit computer A.C.D. Crommelin fit an orbit to the observations made on May 2, 12 and 22, 1930. The comet passed within 0.062 AU of the Earth on May 31 and was last seen during this initial apparition on 1930 August 24. The intrinsic faintness of the comet, a less than secure orbit, and unfavorable observing conditions for the 1935-36 perihelion return prevented the comet from being recovered for the next eight returns to perihelion and it was not re-discovered until 1979 when the observing conditions were the most favorable since the 1930 discovery apparition. A comet was discovered on 1979 August 13 by J. Johnson and M. Buhagiar at Perth Observatory in Australia and M.P. Candy noted the new comet's orbital similarity to the long-lost comet SW3. SW3 was then re-discovered in 1979, although the actual time of perihelion passage was 34 days later than predicted. The comet was missed at its next return to perihelion in 1985-86 but it has been observed during its last three apparitions in 1989-90, 1994-96, and 2000-01. The current orbital elements are given in Table 1. An orbital diagram and one showing its observing conditions at various apparitions are given in figures 9 and 10. The pre-discovery history of SW3's motion suggests that this comet may be a relatively recent addition to the Earth's neighborhood. A Jupiter close approach on November 30, 1882 to 0.23 AU dropped the comet's perihelion distance down from 1.70 AU to 1.34 AU and a subsequent Jupiter close approach to 0.22 AU on September 22, 1894 dropped in further to 1.07 AU which is near its present value. Although integrations of the comet's motion back prior to these Jupiter close approaches would be uncertain, it seems likely that SW3 has only recently entered the region near the Earth's orbit. Sekanina (1989) studied the coma fans associated with comet SW3 and noted that the fan's axis was pointing in the general direction of, but not exactly at, the sun. He concluded that this type of feature (also noted for comet Encke) is characteristic of one or more active regions located near the sunlit rotation pole of the nucleus. The continuous nature of the fan-like structure requires that the nucleus spin axis be pointed continuously toward the sun and hence the spin pole must be near the comet's orbital plane. However the motion of the nucleus fan-axis could not be fitted to a model where the spin axis is fixed in space. The active area is located within 19 degrees of the sunlit pole but the pole itself is rapidly processing (peak rate of ~1.4 degrees/day) so that no single pole direction is appropriate. Sekanina estimates that the total mass loss per orbit from an active area of 0.8 km2 is 1.7 x 1012 grams, assuming a bulk density of 0.3 g/cm3. This would imply that the current active area would be rapidly eroding by about 7 meters per orbit. Using a magnitude estimate given by F. Baldet (Meudon) for a starlike feature in the elongated nuclear condensation of comet SW3 near an Earth close approach on May 30, 1930, Sekanina estimated an equivalent nucleus diameter of 2.0 km assuming an albedo of 0.04 and a phase coefficient of 0.033 magnitude/degree. During this comet's return to perihelion in 1995, observations made at the Nancay radio telescope in Meudon France indicated that the OH gas production rate had a dramatic eight-fold increase to 2.2 x 1029 molecules per second during the period September 11-13, 1995 (Crovisier et al. 1995; Sekanina et al., 1996). By September 17, the comet's had moved away from the sun's glare enough that visual observations were possible. The comet was then apparent magnitude 8.3, nearly 100 times brighter than expected. Perihelion was on September 22 and the comet maintained its brightness until the beginning of October when the comet brightened to magnitude 6. The comet then began to fade in brightness but flared again to magnitude 6.3 on October 22. The comet became more diffuse during December 1995 and January 1996 with rapid fading of the comet in January noted. By February 1996, the apparent magnitude was about 14. Later observations of the comet after solar conjunction were made by T.B. Spahr and C.W. Hergenrother on September 20-21, 1996 when only fragment C was visible at magnitude 22-23. The multiplicity of the nucleus of SW3 was first noted during a December 12-14, 1995 observing run at ESO La Silla (Boehnhardt and Kaufl, 1995). Sekanina et al. (1996) suggested that the fragment B probably was the first to separate from the parent comet (eastern most fragment C) in late October, several seeks after the flaring activity. The separation of the short-lived fragment A occurred some two weeks later. A fragment D and others were reported but none of these fragments remained visible long enough to be clearly identified. From the observations of fragments C, B, and a new fragment E in late November 2000, Sekanina (2000) concluded that fragment B separated from main fragment C on October 11, 1995 (+/-4 days) and that the new fragment E actually separated from fragment C in mid-December 1995 and remained unobserved until December 2000. Hence, toward the end of November 2000, fragment B was still observable but some 2.5 - 3 magnitudes fainter then the main nucleus C and fragment E was also apparent being about 1.5 to 2 magnitudes fainter than C (Figures 6 and 7). The fact that the nucleus of SW3 split into several pieces for no obvious reason suggests that this cometary nucleus is extremely fragile and that freshly cleaved surfaces may reveal the interior structure of this comet when the CONTOUR spacecraft flies by on June 19, 2006. The exposure of fresh ices on the comet might also explain the comet's apparent brightening (2-3 magnitudes) in the 2000 observations when compared with those made before the comet split in 1995. Based upon observations taken before the comet split in 1995, Boehnhardt et al. (1999) estimated that the radius of the nucleus was less than 1.1 km (assuming an albedo of 0.04). These authors conclude that the surface of comet SW3 is likely crusted over to a large degree with only a few percent of its surface area active. Of particular interest is the Earth close approach to within 0.08 AU on May 12, 2006, one of only six known cometary close Earth approaches to within 0.1 AU in the twenty first century. This close approach will allow ground-based optical and radar measurements to refine the pre-encounter ephemeris accuracies to such an extent that an extremely close flyby to within a few tens of kilometers could be attempted. This close approach will also allow an extensive ground-based observing campaign (including radar measurements) to complement the in situ spacecraft observations. A summary of the physical characteristics for both CONTOUR targets is collected in Table 3. While these values will be useful for planning purposes, with the upcoming in situ observations of the CONTOUR spacecraft, they will be replace with definitive values in the coming months. Table 3: Accepted Physical Characteristics for the CONTOUR target bodies 2P/Encke 73P/SW3-C Radius (km) 2.4 (0.3) 1.0 Aspect ratio >2.6 Rotation Period (hrs) 15.2 (0.01) Geometric Albedo 0.047 (0.023) 0.04 (assumed) Pole Direction (B1950) 198 (10) & 0 (10) precessing pole RA/Dec. (degrees) References: A'Hearn, M.F., Millis, R.L., and Thompson, D.T. (1983). The disappearance of OH from comet P/Encke, Icarus, vol. 55, pp. 250-258. A'Hearn, M.F. and Schleicher, D.G. (1988). Comet P/Encke's nongravitational force. Astrophysical Journal, vol. 331, pp. L47 - L51. Belton, M.J.S. (1990). Rationalization of comet Halley's Periods. Icarus, vol. 86, pp. 30-51. Belton, M.J.S. (2001). The excited rotation state of 2P/Encke. Bull. AAS, vol. 32, p. 1062. Boehnhardt, H. and H.U. Kaufl (1995). IAU Circular 6274 dated 1995 Dec. 13. Boehnhardt, H., N. Rainer, K. Birkle, and G. Schwehm (1999). The nucleus of comets 26P/ Grigg-Skjellerup and 73P/Schwassmann-Wachmann 3. Astronomy and Astrophysics, vol. 341, pp. 912-917. Campins, H. (1988). An anomalous dust production in periodic comet Encke. Icarus, vol. 73, pp. 508 - 515. Crovisier, J., N. Biver, D. Brockelee-Morvan, P. Colom, E. Gerard, L. Jorda and H. Rauer (1995). IAU Circular 6227 dated September 13, 1995. Feldman, P. and A. Cochran (2002). This volume. Fernandez, Y.R., C.M. Lisse, S.B. Peschke, H.W. Weaver, M.F. A'Hearn, P.P. Lamy, T.A. Livengood, T. Kostiuk (2000). Physical properties of the nucleus of comet 2P/Encke. Icarus, vol. 147, pp. 145-160. Festou, M.C. and Barale, O. (2000). The asymmetric coma of comets. I. Asymmetric outgassing from the nucleus of comet 2P/Encke. Astronomical Journal, vol. 119, pp. 3119-3132. Gehrz, R.D., E.P. Ney, J. Piscitelli, E. Rosenthal, A.T. Tokunaga (1989). Infrared photometry and spectroscopy of comet P/Encke 1987. Icarus, vol. 80, pp. 280-288. Hasegawa, I. (1979). Orbits of Ancient and Medieval comets. Publications, Astronomical Society of Japan, vol. 31, p.257-270. Jewitt, D. and K. Meech (1987). CCD Photometry of comet P/Encke. Astronomical Journal, vol. 93, pp. 1542-1548. Kamel, L. (1991). The evolution of P/Encke's light curve: No secular fading, a vanishing perihelion asymmetry. Icarus, vol. 93, pp. 226-245. Kamoun, P.G., D.B. Campbell, S.J. Ostro, G.H. Pettengill, I.I. Shapiro (1982). Comet Encke: Radar detection of nucleus, Science, vol. 216, pp. 293-295. Lisse, C.M., S.B. Peschke, M.F. A'Hearn, Y.R. Fernandez, E. Grun, H.U. Kaufl, T. Kostiuk, D.J. Lien, T.A. Livengood, D.J. Osip (2002). Icarus, submitted. Luu, J. and Jewitt, D. (1990). The nucleus of comet P/Encle. Icarus, v. 86, p. 69 - 81. Marsden, B.G., Sekanina, Z. and Yeomans, D.K. (1973). Comets and Nongravitational Forces. V., Astronomical Journal, vol. 78, pp. 211-225. Marsden, B.G. and Sekanina, Z. (1974). Comets and nongravitational forces. VI. Periodic comet Encke 1786-1971. Astronomical Journal, vol. 79, pp. 413419. Meech, K.J., Y. Fernandez, J. Pittichova (2001). Aphelion activity of 2P/Encke. Bulletin of the American Astronomical Society, vol. 33, No. 3, p. 1075. Raymond, J.C., M. Uzzo, T.-K. Ko, S. Mancuso, R,. Wu, L. Gardner, J.L. Kohl, B. Marsden, P.L. Smith (2002). Far UV observations of comet 2P/Encke at perihelion. Astrophysical Journal, vol. 564, pp. 1054 - 1060. Reach, W.T., M.V. Sykes, D. Lien, J.K. Davies (2000). The formation of Encke Meteoroids and dust trail. Icarus, vol. 148, p. 80-94. Samarasinha, N.H. (2002). Cometary Spin States, Their Evolution, and the Implications, In Conference Series of the Astronomical Society of the Pacific: IAU Colloquium 168, In Press. Sekanina, Z. (1988). Outgassing asymmetry of periodic comet Encke I. Apparitions 1924-1984. Astronomical Journal, vol. 95, pp. 911 - 924. Sekanina, Z. (1989). Nuclei of two Earth-grazing comets of fan-shaped appearance. Astronomical Journal, vol. 98, pp. 2322-2345. Sekanina, Z. (1991a). Encke, The Comet. Journal, Royal Astronomical Society of Canada, v. 85, pp. 324 - 376. Sekanina, Z. (1991b). Cometary activity, discrete outgassing areas, and dust-jet formation. In Comets in the Post-Halley Era (R.J. Newburn, Jr. ed.), Kluwer, Dordrecht, p. 769-823. Sekanina, Z. (2000). IAU Circular 7541 dated 2000 December 13.. Sekanina, Z., Boehnhardt, H.U. Kaufl, K. Birkle (1996). Relationship between outbursts and nuclear splitting of comet 73P/Schwassmann-Wachmann 3. Cometary Science Team preprint series No. 163, Jet Propuslsion Laboratory. Sitarski, G. (1988). Long-Term motion of comet Encke. Acta Astronomica, vol. 38, pp. 269-282. Whipple (1940). Photographic meteor studies. III. The Taurid Shower. Proceedings American Philosophical Society, vol. 83, pp. 711-745. Whipple, F.L. (1950). A Comet Model. I. The Acceleration of Comet Encke. Astrophysical Journal, vol. 111, pp. 375-394. Whipple, F.L. and Hamid, S.E. (1952). On the origin of the Taurid meteor streams. Helwan Observatory Bulletin No. 41, pp. 1-30. Whipple, F.L. and Hamid, S.E. (1972). A search for Encke's comet in ancient Chinese records: A progress report. In, The motion, evolution of orbits and origins of comets (G.A. Chebotarev, E.I. Kazimichak-Polonskaya, and B.G. Marsden, eds.), pp. 152-154. Yeomans, D.K. (1991). Comets: A Chronological History of Observation, Science, Myth, and Folklore. John Wiley and Sons. Yeomans, D.K. and Chodas, P.W. (1989). An Asymmetric Outgassing Model for Cometary Nongravitational accelerations, Astronomical Journal, vol. 98, pp.1083-1093. Figure 1. Although comet Encke was actually first discovered by the French comet observer, Pierre Mechain on January 17, 1786, it was the German astronomer, Johann Franz Encke (1791 - 1865) seen here who in 1819 showed that the comets seen in 1818-19, 1805, 1795 and 1786 were the same comet returning to perihelion at intervals of 3.3 years. This comet was then the second comet, after Halley, shown to be periodic. In honor of Encke's work, the comet was given his name thereafter. Figure 2. This image was taken by James V. Scotti (Spacewatch) on 1994 January 5. He used a 0.9-m Spacewatch telescope. The nuclear region is visible and the comet displays a large coma and an ion tail that extends upwards with a slight tilt to the right. Copyright (c) 1994 by James V. Scotti Figure 3: The radial sun-comet (A1) and transverse (A2) nongravitational parameters are plotted as a function of time for comet Encke. Figure 4: An ecliptic plane projection of comet Encke's orbit showing the comet and planetary positions (Mercury through Jupiter) at the CONTOUR encounter date of Nov. 12, 2003. Figure 5: Comet Encke viewing conditions for apparitions between 1990 and 2013. This figure is drawn in a rotating reference frame so that, for a given apparition, the Earth and Sun positions remain fixed and only the comet's apparent motion is depicted. The small open circles on the comet's apparent path represent its position at 30 day intervals before (-) or after (+) perihelion. For example, the view of comet Encke at the 2000 perihelion, as seen from the Earth (near 3 o'clock), was poor since the comet, sun and Earth were then aligned at superior conjunction. Figure 6. At the Bergedorf observatory near Hamburg Germany, Friedrich Karl Arnold Schwassmann (1870-1964) here on the right and Arno Arthur Wachmann (1902-1990) on the left, discovered their third periodic comet on May 2, 1930. This discovery was made in the course of their routine search for minor planets. Figure 7: Comet 73P/Schwassman-Wachmann-3 taken by J. Scotti on Dec. 27, 1996 at the Spacewatch 0.91 m telescope. The elongated nucleus shows clear signs of nucleus splitting. Image by James V. Scotti for the Spacewatch Project, Lunar and Planetary Laboratory, the University of Arizona. Figure 8: 73P/SW3 taken by Ken-ichi Kadota (Ageo, Saitama, Japan) on November 28, 2000 using a 0.18 meter f/5.5 reflector. The main nucleus C, and fragments B and E are evident. Copyright(c) 2000 Ken-ichi Kadota Figure 9: Orbital diagram for comet Schwassmann-Wachmann 3 with the comet and planetary positions (Mercury through Jupiter) shown for the CONTOUR encounter date of June 19, 2006. Figure 10: Comet Schwassmann-Wachmann 3 viewing conditions for apparitions between 1990 and 2006. This figure is drawn in a rotating reference frame so that, for a given apparition, the Earth and Sun positions remain fixed and only the comet's apparent motion is depicted. The small open circles on the comet's apparent path represent its position at 30 day intervals before (-) or after (+) perihelion. The view of the comet during the 2006 apparition is about as good as it can get since the comet spends most of its time near opposition.