MSFC Stream Model Preliminary Results: Modeling Recent Leonid and Perseid Encounters

The cometary meteoroid ejection model of Jones and Brown [Physics, Chemistry, and Dynamics of Interplanetary Dust, ASP Conference Series 104 (1996b) 137] was used to simulate ejection from comets 55P/Tempel-Tuttle during the last 12 revolutions, and the last 9 apparitions of 109P/Swift-Tuttle. Using cometary ephemerides generated by the Jet Propulsion Laboratory’s (JPL) HORIZONS Solar System Data and Ephemeris Computation Service, two independent ejection schemes were simulated. In the first case, ejection was simulated in 1 h time steps along the comet’s orbit while it was within 2.5 AU of the Sun. In the second case, ejection was simulated to occur at the hour the comet reached perihelion. A 4th order variable step-size Runge–Kutta integrator was then used to integrate meteoroid position and velocity forward in time, accounting for the effects of radiation pressure, Poynting–Robertson drag, and the gravitational forces of the planets, which were computed using JPL’s DE406 planetary ephemerides. An impact parameter (IP) was computed for each particle approaching the Earth to create a flux profile, and the results compared to observations of the 1998 and 1999 Leonid showers, and the 1993 and 2004 Perseids.     

Is the D/H Ratio in the Comet Coma Equal to the D/H Ratio in the Comet Nucleus?

We present a simple, semianalytic model of the vaporization of H₂O and HDO ice from a comet nucleus. We use this model to show that the flux of HDO relative to H₂O can be much higher, at times, than would be expected from the D/H ratio in the nuclear ice itself. This effect varies with position in the comet's orbit. It is negligible sufficiently near the Sun but could lead to erroneous interpretations of the primordial D/H ratio in cometary ice if measurements are made in other parts of the cometary orbit.     

Polarimetry Of Comet Hale-Bopp: Linear And Circular Polarization, Stellar Occultation

Comet Hale-Bopp was observed with the 2.6-m and 1.25-m telescopes of the Crimean Astrophysical Observatory on March 9 and 11, 1997. We determined the linear and circular polarization in the coma. For the dust coma, the linear polarization varied from 7.8% to 12.4%. The degree of circular polarization was always negative and did not exceed 0.3% with an accuracy of ±0.04% on average. The passage of a bright star through the cometary coma was monitored with polarimetry and photometry. The wavelength dependence and spatial variations of optical thickness of dust are obtained from the stellar occultation. This revised version was published online in July 2006 with corrections to the Cover Date.     

无锡梅村坠冰的研究

１９８４年１１月１７日，从天空飞来一个冰块，坠落于锡山市（原无锡县）梅村乡陈三房村的麦田里（“梅村坠冰”）。现场和有关部门调查排除了人为成因，气象成因以及来自飞机的可能性。坠冰样品的实验分析研究的初步结果表明，它跟地球物质有差别，与地外物质相似，这有利于该坠冰来自无外的看法，或许可能来自慧星。坠洋的深入研究对于揭示慧星性质，探讨水以及有关地质，气象，天文问题可能有重大意义。     

1997年3月4日海尔-波普彗星的小规模分裂

１　引言彗核分裂是彗星的最重要特征之一．至１９８２年止文献记录共观测到２２个彗核分裂,３３个次核［１］．１９８６年哈雷彗星回归时观测到彗核的分裂［２］．１９９６年紫金山天文台２００ｍｍ赤道望远镜和青岛观象台同时观测到百武彗星的分裂［３］．海尔波普彗星从１９９５年８月到１０月曾爆发５次［４］．１９９６年３月至１２月不断有喷流射出,彗核活动日趋剧烈［５］．１９９７年３月４日我们观测到彗核的小规模分裂及喷流特征,３月３日我们观测到海尔波普彗星近核照片上有喷流,这可能是３月４日彗核小块分裂的前奏．３…     

Predictions for the Aurigid Outburst of 2007 September 1

The September 2007 encounter of Earth with the 1-revolution dust trail of comet C/1911 N1 (Kiess) is the most highly anticipated dust trail crossing of a known long period comet in the next 50 years. The encounter was modeled to predict the expected peak time, duration, and peak rate of the resulting outburst of Aurigid shower meteors. The Aurigids will radiate with a speed of 67 km/s from a radiant at R.A. = 92°, Decl. = +39° (J2000) in the constellation Auriga. The expected peak time is 11:36 ± 20 min UT, 2007 September 1, and the shower is expected to peak at Zenith Hourly Rate = 200/h during a 10-min interval, being above half this value during 25 min. The meteor outburst will be visible by the naked eye from locations in Mexico, the Western provinces of Canada, and the Western United States, including Hawaii and Alaska. A concerted observing campaign is being organized. Added in proof: first impression of the shower. Prepared as a contribution to the conference proceedings of “Meteoroids 2007”, to be published in the journal “Earth, Moon, and Planets”.     

Near-infrared observations of Comet C/2004 Q2 (Machholz)

Our observations of Comet C/2004 Q2 (Machholz) in the range from 1.2 to 4.8 µm indicate that the material outflowed from the cometary surface in the form of fragments that separated into gas and dust under sublimation on time scales of the order of days. The albedo of these fragments in the range under study was wavelength independent, while the dust was heated by the Sun to an equilibrium temperature at a cross section Qλ inversely proportional to the wavelength.     

Dust Activity in Comet 67P/Churyumov–Gerasimenko from February 20 to April 20, 2003

Broadband imaging of Comet 67P/Churyumov–Gerasimenko has provided more data on the characterisation of the target of the ESA Rosetta Mission. The comet monitoring between r _h=2.37 and r _h=2.78 AU postperihelion shows a prominent dust coma which extends up to ≈ 25,000 km from the nucleus, and a long dust structure in approximately anti-tail direction, reaching at least 230,000 km, identified as a neck-line structure. The non-isotropic dust emission is detected from the structures in the inner coma, and it is reflected on the slope of linear fits of surface brightness profiles vs. cometocentric projected distance in log–log representation as m ≈ 0.83−0.941. Besides the long dust spike at position angle of 295°, the morphological study of the dust coma confirms the presence of two structures at position angles of 95 and 195° where the overabundance of dust can be as high as 50% at ρ ≤ 30,000 km. The A f ρ parameter derived from our R broadband data is 26.0 and 29.8 cm at r _h=2.37 and 2.48 AU postperihelion. The dust reflectivity S′(λ), a measurement of the dust colour, is 0.061±0.019, a rather neutral colour.     

Nitrogen Sulfide In Comets Hyakutake (C/1996 B2) And Hale–Bopp (C/1995 O1)

The chemistry of both nitrogen and sulfur presents interesting problems in comets.In this paper, we use a model of cometary comae with gas-phase chemical kineticsand gas dynamics to predict molecular abundances in the inner coma region for twoof the brightest comets in the past 20 years, Hyakutake (C/1996 B2) and Hale–Bopp(C/1995 O1). In this progress report we concentrate on the gas-phase chemistry of thenitrogen sulfide (NS) radical at a heliocentric distance of 1 AU to study the abundanceof NS using a detailed photo and chemical reaction network with over 100 species andabout 1000 reactions. The results are compared with recent observations of CometHale–Bopp and reveal that conventional gas-phase reactions schemes do not produceNS in sufficient quantities to explain the observations. We plan to continue therefinement of the model to improve agreement with observational constraints.