JET PROPULSION LABORATORY INTEROFFICE MEMORANDUM 312.F - 01 - xxx August 24, 1999 TO: Distribution FROM: D.K. Yeomans, M.S. Keesey, and R.N. Wimberly SUBJECT: Comet Encke orbit and uncertainty analysis update REFERENCES 1. To: Distribution. JPL IOM 312.F-99-080, dated Aug. 24, 1999 2. Marsden, B.G., Sekanina, Z. and Yeomans, D.K. (1971). Astron. J., vol. Orbit and uncertainty analysis information is updated for the CONTOUR target comet 2P/Encke. INTRODUCTION This memo is a continuation of the information given in Reference 1. The orbit of 2P/Encke has been updated and denoted K004-6 as it represents the 6th solution using 4 apparitions of observations. This solution included the perturbations of all the planets as well as the minor planets Ceres, Pallas, and Vesta. This solution used 251 observations over the 1989 - 2000 interval and included the affects of the so-called nongravititational accelerations used to represent the rocket-like outgasing of the comet near perihelion (Reference 2). Radial and transverse nongravitational parameters (A1, A2) are included in the solution set along with the six orbital elements. ORBIT UPDATE FOR COMET 2P/ENCKE: Planetary Ephemeris: DE405 Small Body Ephemeris: SB-CPV405-01 No. Observations: 251 Observation Arc: 1989 Jun 1 - 2000 Aug 25 ---- Residual Summary ---- RA Dec Total Mean .011 .059 .060 RMS, unweighted .874 .809 .842 ---- Corrected Elements (J2000): Solution K004/6 Epoch 2451880.50000 = 2000 Dec 2.00000 Post-Fit Std.Dev. Eccentricity 0.846907846 .000000100 Perih. distnace 0.339532730 .000000254 au Perih. Passage 2451797.1668288 .0000651886 d 2000 Sep 9.66683 Longitude of Asc. Node 334.5947869 .0000614 deg Argument of Perihelion 186.4887588 .0000783 deg Inclination 11.7558953 .0000127 deg A1 2.58044E-12 6.0E-11 A2 -1.23191E-11 1.6946E-13 A 2.217832334 0.000000299 au P 1206.4004573 .00024406 d 3.30289 yr UPDATE FOR COMET 2P/ENCKE UNCERTAINTY ANALYSIS The following uncertainty analysis uses all of the observations included in the above orbit solution plus 20 simulated observations over the interval from 2003 August 5 - November 11. An observations noise of one arc second was used throughout and the radial nongravitational parameter (A1) was assumed to be uncertain at the 100% level. The transverse nongravitational parameter (A2) was included in the solution set along with the 6 orbital elements at the epoch of osculation. The uncertainty analysis output is given in terms of the so-called RTN and Plane-of-Sky coordinate systems. The heliocentric RTN coordinate system is defined by a Sun-comet unit vector (R), a unit vector normal to R and also normal to the comet's orbit plane (N) and by a transverse unit vector (T) that completes the right handed, orthogonal system such that T = N x R. In the plane-of-sky coordinate system, the RA and DEC coordinates lie in the plane of sky as viewed from Earth with RA pointing in the direction of increasing right ascension and DEC pointing in the direction of increasing declination. The R vector is directed in the range direction, from the Earth to the comet. Table 1. Summary of CONTOUR Comet Encke Ephemeris Uncertainties (baseline values, 1-sigma) at various times including the CONTOUR spacecraft encounter time on Nov. 12, 2003. The given values represent 1-sigma estimates (in km) of the uncertainty ellipsoid axes in the RTN coordinate system and in each case, observations upto the given time have been processed in the orbit determination process. T(2003) RR RT RN July 30.5 288 188 101 Aug. 4.5 295 191 99 Ground-based astrometric observations begin Aug. 9.5 295 190 96 Aug. 14.5 296 188 94 Earth Swingby Aug. 19.5 295 186 92 Aug. 24.5 294 184 89 Aug. 29.5 292 181 87 Sep. 3.5 289 177 84 Sep. 8.5 285 174 82 Sep. 13.5 279 169 79 Sep. 18.5 273 164 76 Sep. 23.5 265 159 73 Sep. 28.5 256 152 70 Oct. 3.5 246 146 67 Oct. 8.5 236 139 64 Oct. 13.5 225 131 61 Oct. 18.5 214 124 57 Oct. 23.5 203 117 54 Oct. 28.5 192 111 50 Nov. 2.5 180 105 47 Nov. 7.5 166 99 44 Nov. 12.5 149 93 41 CONTOUR Close Approach Nov. 17.5 156 101 37 Nov. 22.5 163 112 34 ________________________________________________________ Distribution: W.E. Bollman D.V. Byrnes L.A. Cangahuala A.B. Chamberlin P.W. Chodas J.D. Giorgini C.A. Halsell R.J. Haw C.E. Helfrich G.R. Hintz R.A. Jacobson M.S. Keesey G.W. Null W.M. Owen, Jr. S.P. Synnott B.G. Williams