Stephen M. Slivan
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology
The purpose of this application is to generate a minor planet ephemeris file in the "observing catalog" format used at the Magellan telescopes, that is of accuracy sufficient to acquire the object field with the telescope.
MEPHEM uses a 2-body calculation for the minor planet position with respect to the Sun, for which the orbit is modeled as constant over time with no gravitational perturbations. The orbit elements accurately represent the real orbit at only one particular epoch in time, so calculations of minor planet positions closer in time to the epoch will generally be more accurate than those calculated for times farther from the epoch, but neither the magnitude nor the direction of the error can be expressed as a simple function of time. Since MEPHEM is designed specifically to calculate short-term ephemerides for use during an observing run, aiding observers by formatting the output for use by the Magellan TCS, using the 2-body model with orbit elements for an appropriate epoch should yield positions of sufficient accuracy to acquire the object field with the telescope.
The MEPHEM server will normally retrieve updated minor planets orbit elements from the Minor Planet Center daily.
For accurate positions over a longer span of time than is supported by MEPHEM a perturbed ephemeris is needed instead; two resources to calculate perturbed ephemerides (but not in Magellan TCS format) are:
The following two plots show the differences in right ascension (red) and in declination (green) between the 2-body positions of (4) Vesta calculated by MEPHEM and perturbed positions calculated using JPL HORIZONS. On each plot the calculation at the elements epoch 2004 Jul 14 is marked by a black circle, where the positions coincide (differences are 0) as expected. For Vesta the orbit is well-established, and the cumulative effect of gravitational perturbations on the orbit is easily seen in the nonzero differences as the MEPHEM positions are calculated farther from the elements' epoch date. The Earth oppositions of Vesta which bracket the epoch occur on 2003 Mar 31 and 2004 Sep 17.
The next two plots are the same as those above except that the object is Earth-approaching (4179) Toutatis. The 2004 Sep 29 encounter with Earth introduces an abrupt and marked error in the 2-body calculations.
If an ephemeris calculation is requested for a time that's outside the span of Earth orbit data in the file, an approximate Earth position will be used based on "low precision formulae for the Sun's coordinates" (AA 2004 edition, page C24) instead, the RA and Dec rates won't be output, and a warning message will logged for the user.
1
2 Pallas
(3)
(4) Vesta
954 Li
3360 1981 VA
Note that "954 Li" isn't misinterpreted as a provisional
designation because 954 isn't greater than 1800.
When requesting an ephemeris for an unnumbered object, specify the target by its (new-style) provisional designation. No distinction is made between upper-case and lower-case letter characters. Here are some sample valid input strings for unnumbered objects (the year must be greater than 1800):
1982 YA
2000 SG344
2000 SG 344
An interactive converter for permanent, provisional, and temporary minor planet designations ( http://cfa-www.harvard.edu/iau/MPDes.html) is maintained by the IAU Minor Planet Center.
A variety of warning messages can appear if there's some problem using the Earth orbit elements file when precise Earth positions are calculated. Any of the "using approx Earth calc" messages mean that the precise calculation was not possible, and an approximate Earth position has been used based on "low precision formulae for the Sun's coordinates" (AA 2004 edition, page C24) instead.
MINOR PLANET CENTER ORBIT DATABASE (MPCORB)
This file contains published orbital elements for all numbered and unnumbered
multi-opposition minor planets for which it is possible to make reasonable
predictions. It also includes published elements for recent one-opposition
minor planets and is intended to be complete through the last issued Daily
Orbit Update MPEC. As such it is intended to be of interest primarily
to astrometric observers.
Software programs may include this datafile amongst their datasets, as
long as this header is included (it is acceptable if it is contained
in a file separate from the actual data) and that proper attribution
to the Minor Planet Center is given. Credit to the individual orbit
computers is implicit by the inclusion of a reference and the name of
the orbit computer on each orbit record. Information on how to obtain
updated copies of the datafile must also be included.
The work of the individual astrometric observers, without whom none of
the work of the Minor Planet Center would be possible, is gratefully
acknowledged.
New versions of this file, updated on a daily basis, will be available at:
ftp://cfa-ftp.harvard.edu/pub/MPCORB/MPCORB.DAT
CGIC License Terms Basic License CGIC, copyright 1996, 1997, 1998, 1999, 2000, 2001, 2002 by Thomas Boutell and Boutell.Com, Inc.. Permission is granted to use CGIC in any application, commercial or noncommercial, at no cost. HOWEVER, this copyright paragraph must appear on a "credits" page accessible in the public online and offline documentation of the program. Modified versions of the CGIC library should not be distributed without the attachment of a clear statement regarding the author of the modifications, and this notice may in no case be removed. Modifications may also be submitted to the author for inclusion in the main CGIC distribution.
| Last modified 2006 Mar 08 |
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