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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

Comet Ephemerides for the EPOXI and Stardust-NExT Flybys

The recently successful flybys of Comet 103P/Hartley 2 by the Deep Impact spacecraft and Comet 9P/Tempel 1 by the Stardust spacecraft each presented different challenges from the perspective of comet ephemeris prediction and spacecraft targeting. Hartley 2 is a small, highly active comet, with nongravitational accelerations that proved very difficult to model, requiring some amount of "cometchasing" by the spacecraft navigators. In contrast, Tempel 1 is a far larger and less active comet. It showed very stable ephemeris behavior and at flyby was within 1-sigma of predictions issued more than a year prior to encounter. This happenstance was fortuitous because the Stardust spacecraft had very little fuel margin available for comet ephemeris errors.

non-gravitational accelerations↗
NASA NTRS1978

Comet Tempel 2: Orbit, ephemerides and error analysis

The dynamical behavior of comet Tempel 2 is investigated and the comet is found to be very well behaved and easily predictable. The nongravitational forces affecting the motion of this comet are the smallest of any comet that is affected by nongravitational forces. The sign and time history of these nongravitational forces imply (1) a direct rotation of the comet's nucleus and (2) the comet's ability to outgas has not changed substantially over its entire observational history. The well behaved dynamical motion of the comet, the well observed past apparitions, the small nongravitational forces and the excellent 1988 ground based observing conditions all contribute to relatively small position and velocity errors in 1988 -- the year of a proposed rendezvous space mission to this comet. To assist in planned ground based and earth orbital observations of this comet, ephemerides are given for the 1978-79, 1983-84 and 1988 apparitions.

Yeomans, D. K.↗
NASA NTRS1986

A program for the accurate generation of ephemerides for Halley's Comet

This is a discussion of an orbit determination computer program designed at the Jet Propulsion Laboratory to support the International Halley Watch and the European Space Agency's Giotto flight project to comet Halley. The program processes observational data received from the Astrometric Network of the International Halley Watch for the purpose of generating very accurate, up-to-date cometary ephemerides for Halley. An accurate comet ephemeris is essential if the Giotto spacecraft is to be navigated closely enough to the comet to satisfy its encounter objectives. Features discussed include the modelling of the nongravitational properties of Halley and the associated variational equations. The software has been implemented on several computers including the Honeywell and Siemens computers at the European Space Operations Center in Darmstadt, West Germany.

Ekelund, J. E.↗
NASA NTRS1988

Comet and asteroid dynamics

In order to provide observers with accurate ephemerides of comets and asteroids, up-to-date astrometric positions must be used to improve the existing orbits. For active comets, nongravitational forces must be taken into account; these forces are assumed due to the rocket-like effect of outgassing cometary ices and are used to characterize the volatility and rotation properties of icy cometary nuclei. In an effort to improve ephemeris accuracies, the benefits of a new nongravitational force model for comets as well as new radar data types are being investigated. The first successful attempts to improve the orbits of close Earth approaching asteroids using radar data have been completed for asteroids 1982XB and 1986JK. The radar Doppler measurements of 1982XB, made on Dec. 5-6, 1987, were represented to less than 0.1 Hz while the Doppler observations of 1986Jk, made on May 28 - June 1, 1986, were represented to within a few Hz for each of the 11 measurements. Last minute orbit updates for asteroid 324 Bamberga allowed a successful stellar occultation prediction to be made on Dec. 8, 1987. A new paradigm for the cometary nongravitational force model has been successfully tested on a few comets. This new model allows the water vaporization curve to peak on either side of perihelion, thus introducing a nongravitational force via an asymmetric radial force, rather than through a symmetric transverse effect that the old model requires.

Yeomans, Donald K.↗
NASA NTRS1982

Cometary ephemerides for spacecraft flyby missions

The determination of cometary ephemerides is considered with reference to proposed missions to Comet Halley. It is noted that the ability of spacecraft without onboard navigation capability to fly close to target comets is limited chiefly by the comet's ephemeris uncertainty. The situation is complicated by nongravitational forces acting on a comet's nucleus and the paucity of observers currently making astrometric observations of comets. The nongravitational forces affecting Comet Halley are consistent with the rocket effect of an outgassing water ice nucleus; the nucleus is apparently rotating in a direct sense about a stable spin axis. It is emphasized that accurate Comet Halley ephemerides for close spacecraft flybys will require continued efforts to refine the existing nongravitational force model. In addition, the flyby mission to Comet Halley will require a well-organized network of astrometric observers. For this purpose, an Astrometry Network is being set up within NASA's International Halley Watch program.

Yeomans, D. K.↗
NASA NTRS1984

The orbits of Comets Halley and Giocobini-Zinner

Ongoing activities to improve the orbits and ephemerides of comets Halley and Giacobini-Zinner are outlined. The nongravitational acceleration model of Marsden et al (1973) is used to represent the motion of these two comets over their observed intervals, and recent orbital updates are presented.

Yeomans, D. K.↗
NASA NTRS1984

Predictions of occultations of stars and radio sources by Comet Halley

Cataloged cometary ephemerides were compared with cataloged star and radio source positions to generate a comprehensive list of expected occultations by Halley's Comet. The comet path was integrated according of Yeoman's orbit no. 12 and account was taken of gravitational and nongravitational perturbations. A coma of 22,500 km was assumed for star occultation and a 675,000 km radius was projected for radio occultations, the latter to allow for effects of an ion tail. The ephemerides were projected back along the comet track and produced a good fit with observed fixes.

Bowell, E.↗
NASA NTRS1991

Comet and asteroid dynamics

In order to provide the ground-based observing community and NASA flight projects with accurate comet and asteroids ephemerides, improvements are being made to the existing dynamic models and new data type are being investigated. For active comets, non-gravitational forces must be taken into account; these forces are assumed due to the rocket-like thrusting of outgassing cometary ices.

Yeomans, Donald K.↗
NASA NTRS1986

Orbit determination for the Mariner Mark II Comet Rendezvous/Asteroid Flyby mission - The orbiting phase

The Comet Rendezvous/Asteroid Flyby (CRAF) mission is the first of the Mariner Mark II mission set, designed to explore the outer solar system. Major objectives of orbit determination will be determine the positions and masses of the comet and asteroid and the relative position of the spacecraft, which is important to accurate pointing of the scan platform on which the narrow angle camera and scientific instruments are positioned. Position prediction is also important, since continuous commuication with the spacecraft will not be possible. The small gravitational attractions and poorly known ephemerides of the comet and asteroid, and the small, slow spacecraft orbit about the comet, pose significant new problems for orbit determination. Results of simulations studying the effectiveness of key data types, the accuracies of estimates, and prediction capabilities, are presented.

Weeks, C. J.↗
NASA NTRS1981

Cometary ephemerides - needs and concerns

With the use of narrow field-of-view instrumentation on faint comets, the accuracy requirements upon computed ephemerides are increasing. It is not uncommon for instruments with a one arc minute field-of-view to be tracking a faint comet that is not visible without a substantial integration time. As with all ephemerides of solar syste objects, the computed motion and reduction of these observations, the computed motion of a comet is further depenent upon effects related to the comet's activity. Thus, the ephemeris of an active comet is corrupted by both observational errors and errors due to the comet's activity.

Yeomans, D. K.↗
NASA NTRS1986

IRAS observations of comets

The moderate spatial resolution and high sensitivity of the Infrared Astronomical Satellite (IRAS), which surveyed the celestial sphere during 1983 at wavelengths of 12, 25, 60, and 100 microns, were particularly well suited to detecting extended thermal emission from cometary dust. Sources with infrared color temperatures characteristic of solar system bodies, and at the ephemerides position of known comets were selected for analysis by the IRAS Asteroid Data Analysis System (ADAS). The data base is now available for use by researchers. This paper describes the development of the data base, details its entries, and presents a statistical analysis of its contents. The IRAS survey contains multiple observations of many periodic comets. A brief description and analysis is given of the observed infrared and derived physical properties for several comets of special interest.

Walker, R. G.↗
NASA NTRS1986

International Halley Watch: Discipline specialists for astronomy

The Astrometry Network of the International Halley Watch (IHW) has organized and coordinated more than 300 astrometric observers from 47 countries in an effort to provide for accurate positional information on comets Halley and Giacobini-Zinner. This information is being used to constantly update the existing orbits and ephemerides for the various flight projects to these two comets and also for the other ground-based observers within the IHW organization. The astrometric data, as well as the resultant orbits and ephemerides, are being distributed to all requesting observers and they will be cataloged within the general IHW archive system.

Yeomans, D. K.↗
NASA NTRS1998

Optical Studies of Active Comets

This grant was to support optical studies of comets close enough to the sun to be outgassing. The main focus of the observations was drawn to the two extraordinarily bright comets Hyakutake and Hale-Bopp, but other active comets were also studied in detail during the period of funding. Major findings (all fully published) under this grant include: (1) Combined optical and submillimeter observations of the comet/Centaur P/Schwassmann-Wachmann 1 were used to study the nature of mass loss from this object. The submillimeter observations show directly that the optically prominent dust coma is ejected by the sublimation of carbon monoxide. Simultaneous optical-submillimeter observations allowed us to test earlier determinations of the dust mass loss rate. (2) We modelled the rotation of cometary nuclei using time-resolved images of dust jets as the primary constraint. (3) We obtained broad-band optical images of several comets for which we subsequently attempted submillimeter observations, in order to test and update the cometary ephemerides. (4) Broad-band continuum images of a set of weakly active comets and, apparently, inactive asteroids were obtained in BVRI using the University of Hawaii 2.2-m telescope. These images were taken in support of a program to test the paradigm that many near-Earth asteroids might be dead or dormant comets. We measured coma vs. nucleus colors in active comets (finding that coma particle scattering is different from, and cannot be simply related to, nucleus color). We obtained spectroscopic observations of weakly active comets and other small bodies using the HIRES spectrograph on the Keck 10-m telescope. These observation place sensitive limits to outgassing from these bodies, aided by the high (40,000) spectral resolution of HIRES.

Jewitt, David↗
NASA NTRS1996

Helium and Neon in Comets

Two comets were observed with EUVE in late 1994. Both comet Mueller and comet Borrelly are short-period comets having well established orbital elements and accurate ephemerides. Spectra of 40 ksec were taken of each. No evidence for emission lines from either Helium or Neon was detected. We calculated limits on the production rates of these atoms (relative to solar) assuming a standard isotropic outflow model, with a gas streaming speed of 1 km/s. The 3-sigma (99.7% confidence) limits (1/100,000 for He, 0.8 for Ne) are based on a conservative estimate of the noise in the EUVE spectra. They are also weakly dependent on the precise pointing and tracking of the EUVE field of view relative to the comet during the integrations. These limits are consistent with ice formation temperatures T greater than or equal to 30 K, as judged from the gas trapping experiments of Bar-Nun. For comparison, the solar abundances of these elements are He/O = 110, Ne/O = 1/16. Neither limit was as constraining as we had initially hoped, mainly because comets Mueller and Borrelly were intrinsically less active than anticipated.

Jewitt, David↗
NASA NTRS1984

The astrometry network of the IHW and the P/Crommelin trial run results

In an effort to test the techniques and communication lines of the entire International Halley watch (IHW) organization, comet Crommelin was selected as a comet that would act as a trial run for the future observations of comets Halley and Giacobini-Zinner. Astrometry network members were asked to provide a few observations of comet Crommelin during a period within a few months on either side of the comet's perihelion passage. Special comet Crommelin star catalogs were distributed and each observer was asked to observe, reduce and transmit their data as quickly as possible. Some 372 astrometric observations from 42 different observatories were received at the orbit determination center. Orbit information and up-to-date ephemerides were computed and distributed by the same communication channels. The comet Crommelin trial run was successful in pointing out some areas that need to be improved to make the data more accurate and the communication channels more efficient.

Yeomans, D. K.↗
NASA NTRS1998

Radar Studies in the Solar System

We aid in study of the solar system by means of ground-based radar. We have concentrated on: (1) developing the ephemerides needed to acquire radar data at Arecibo Observatory and (2) analyzing the resultant data to: test fundamental laws of gravitation; determine the size , shape, topography, and spin vectors of the targets; and study the surface properties of these objects, through their scattering law and polarization characteristics. We are engaged in radar observations of asteroids and comets, both as systematically planned targets and as "targets of opportunity." In the course of the program, we have prepared ephemerides for about 80 asteroids and three comets, and the radar observations have been made or attempted at the Arecibo Observatory, in most cases successfully, and in some cases on more than one apparition. The results of these observations have included echo spectra for the targets and, in some cases, delay - Doppler images and measurements of the total round-trip delay to the targets. Perhaps the most dramatic of these results are the images obtained for asteroids (4179) Toutatis and 1989PB (Castalia), which were revealed to be double-lobed objects by the radar images. Besides these direct results, the radar observations have furnished information on the sizes and shapes of the targets through analysis of the Doppler width of the echoes as a function of time, and on the surface properties (such as composition, bulk density, and roughness) through analysis of the reflectivity and of the polarization state of the echoes. We have also refined the orbits of the observed asteroids as a result of the Doppler (and in some cases delay) measurements from the radar observations. Although the orbits of main-belt asteroids accessible to ground-based radar are quite well known from the available optical data, some near-Earth objects have been seen by radar very soon after their optical discovery (for example, 199OMF, just eight days after discovery). In such cases. the radar results ensure that the object in question can be anticipated and identified at the next apparition. We have also participated in radar studies of the terrestrial planets. The results of these studies have included both planetary topography profiles from the analysis of round-trip delays to points along the target Doppler equator and determinations of the target spin state. The latter is of special interest in the case of Venus, which is very close to, but not on, a multi-body spin-orbit resonance such that Venus rotates 12 times for every 8 Earth orbits and 13 Venus orbits. As a result, Venus presents nearly the same face toward Earth at each inferior conjunction. Our latest results confirm that the spin state of Venus is slightly off the resonance. The delay measurements from planetary 2 ranging have also been used in combination with other types of range data in testing general relativity with increasing accuracy. We have also been engaged in radar studies of planetary satellites. Using our ephemerides, Arecibo made radar observations of the Galilean satellites of Jupiter and of Mars' satellite Phobos during the favorable opposition seasons (1988-1992 for Jupiter and 1990 for Mars). An attempt was also made to observe Deimos, but without detecting an echo. In 1997, an attempt was made to observe Saturn's satellite Titan, using the newly upgraded Arecibo radar system for transmitting and the Goldstone radar for receiving, but no echo was detected. The study of satellites by radar is in many ways similar to that of asteroids. The results from these observations have included characterization of the surface properties from the reflectivity and polarization ratio, as well as (in the case of the large satellites of Jupiter) the variation of reflectivity with incidence angle.

Shaprio, Irwin I.↗