Engineering topics
Yeomans, D. K.
Publications and source records attributed to Yeomans, D. K..
Determination of Eros Physical Parameters for Near Earth Asteroid Rendezvous Orbit Phase Navigation
Navigation of the orbit phase of the Near Earth steroid Rendezvous (NEAR) mission will re,quire determination of certain physical parameters describing the size, shape, gravity field, attitude and inertial properties of Eros. Prior to launch, little was known about Eros except for its orbit which could be determined with high precision from ground based telescope observations. Radar bounce and light curve data provided a rough estimate of Eros shape and a fairly good estimate of the pole, prime meridian and spin rate. However, the determination of the NEAR spacecraft orbit requires a high precision model of Eros's physical parameters and the ground based data provides only marginal a priori information. Eros is the principal source of perturbations of the spacecraft's trajectory and the principal source of data for determining the orbit. The initial orbit determination strategy is therefore concerned with developing a precise model of Eros. The original plan for Eros orbital operations was to execute a series of rendezvous burns beginning on December 20,1998 and insert into a close Eros orbit in January 1999. As a result of an unplanned termination of the rendezvous burn on December 20, 1998, the NEAR spacecraft continued on its high velocity approach trajectory and passed within 3900 km of Eros on December 23, 1998. The planned rendezvous burn was delayed until January 3, 1999 which resulted in the spacecraft being placed on a trajectory that slowly returns to Eros with a subsequent delay of close Eros orbital operations until February 2001. The flyby of Eros provided a brief glimpse and allowed for a crude estimate of the pole, prime meridian and mass of Eros. More importantly for navigation, orbit determination software was executed in the landmark tracking mode to determine the spacecraft orbit and a preliminary shape and landmark data base has been obtained. The flyby also provided an opportunity to test orbit determination operational procedures that will be used in February of 2001. The initial attitude and spin rate of Eros, as well as estimates of reference landmark locations, are obtained from images of the asteroid. These initial estimates are used as a priori values for a more precise refinement of these parameters by the orbit determination software which combines optical measurements with Doppler tracking data to obtain solutions for the required parameters. As the spacecraft is maneuvered; closer to the asteroid, estimates of spacecraft state, asteroid attitude, solar pressure, landmark locations and Eros physical parameters including mass, moments of inertia and gravity harmonics are determined with increasing precision. The determination of the elements of the inertia tensor of the asteroid is critical to spacecraft orbit determination and prediction of the asteroid attitude. The moments of inertia about the principal axes are also of scientific interest since they provide some insight into the internal mass distribution. Determination of the principal axes moments of inertia will depend on observing free precession in the asteroid's attitude dynamics. Gravity harmonics are in themselves of interest to science. When compared with the asteroid shape, some insight may be obtained into Eros' internal structure. The location of the center of mass derived from the first degree harmonic coefficients give a direct indication of overall mass distribution. The second degree harmonic coefficients relate to the radial distribution of mass. Higher degree harmonics may be compared with surface features to gain additional insight into mass distribution. In this paper, estimates of Eros physical parameters obtained from the December 23,1998 flyby will be presented. This new knowledge will be applied to simplification of Eros orbital operations in February of 2001. The resulting revision to the orbit determination strategy will also be discussed.
Ancient Chinese Observations and Modern Cometary Models
Ancient astronomical observations, primarily by Chinese, represent the only data source for discerning the long-term behavior of comets. These sky watchers produced astrological forecasts for their emperors. The comets Halley, Swift-Tuttle, and Tempel-Tuttle have been observed for 2000 years. Records of the Leonid meteor showers, starting from A.D.902, are used to guide predictions for the 1998-1999 reoccurrence.
Tidal disruption and the appearance of periodic comet Shoemaker-Levy 9
A unified model is formulated to interpret quantitatively the observed characteristics of the nuclear train, the two dust trails, and the tail region of P/Shoemaker-Levy 9 in terms of a collisionally modified rotation velocity distribution of the comet's debris. Since there is no evidence for the comet's out-gassing, the model does not assume activity. The discruption of the parent comet was due primarily to tidal stesses during its extremely close approach to Juptier in July 1992. The original nucleus is found to have been most probably approximately 5 km in radius or, equivalently, approximately 10(exp 17) g in mass. The dynamical separation of the debris occurred apparently approximately 2.2 hr after the perijove passage, even though the actual fragmentation of the original mass is likely to have begun before closest approach. Physical breakp was accompanied by ubiquitous low-velocity collisions among the particulates, resulting in a rearrangement of the initial rotaional velocities into a rapidly 'thermalized' distribution, characterized by a long tail of relatively high velocities for the debris that populates the dust trails far from the nuclear train. Compelling evidence is presented for secondary fragmentation events, indicative of the comet's continuing disintegration. Secondary fragmentation may be caused by rotational bursting of the massive fragments that had been cracked but unbroken during the tidal disruption. Impact conditions and possible interactions of the comet's debris with the jovian system during the encounter in July 1994 are described.
The overlapping plates method applied to CCD observations of 243 Ida
The overlapping plates method has been applied to crossing-point Charge Coupled Device (CCD) observations of minor planet 243 Ida to produce absolute position measurements precise to better than 0.1 sec and differential position measurements precise to better than 0.06 sec. Although these observations numbered only 17 out of the 520 that produced the final ground-based Ida ephemeris for the Galileo spacecraft flyby, their inclusion decreased Ida's downtrack error from 78 to 60 km and its out-of-plane error from 58 to 44 km.
Navigation for Rendezvous and Orbit Missions to Small Solar-System Bodies
All previous spacecraft encounters with small solar-system bodies, such as asteroids and comets, have been flybys (e.g. Galileo's flybys of the asteroids Gaspra and Ida). Several future projects plan to build on the flyby experience and progress to the next level with rendezvous and orbit missions to small bodies. This presents several new issues and challenges for navigation which have never been considered before. This paper addresses these challenges by characterizing the different phases of a small body rendezvous and by describing the navigation requirements and goals of each phase. Prior to the encounter with the small body, improvements to its ephemeris and initial estimates of its physical parameters, e.g. size, shape, mass, rotation rate, rotation pole, and possibly outgassing, are made as accurately as ground-based measurements allow. This characterization can take place over years...
Navigation for Low-Cost Missions to Small Solar-System Bodies
A variety of low-cost space missions planned by NASA for flight in the late 1990's and early 2000's will involve rendevous with, and orbits about, small solar-system bodies such as asteroids and comets.
Master: A Discovery-Class Orbiter For The Detailed Study of Large Mainbelt Asteroids
MASTER (Mainbelt Asteroid Exploration/Rendezvous) focuses on the comprehensive global study of important large mainbelt asteroids. A three instument, four-investigation payload can be placed into orbit around 4 vesta within the cost/complexity envelope of the Discovery program using a Delta-II launch vehicle.
Navigation for Low-Cost Missions to Small Solar-System Bodies
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The Impact of Comet Showmaker-Levy 9 with Jupiter
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Predicting Close Earth Approaches of Asteroids and Comets
The motions of all known Earth approaching asteroids and comets with reasonably secure orbits have been numerically integrated forward in time to A.D. 2200. Special care was taken to use the best available initial conditions including orbits based upon radar data.
The Plate Overlap Technique Applied to CCD Observations of 243 Ida
The plate overlap technique has been applied to crossing-point CCD observations of minor planet 243 Ida to produce position measurements accurate to better than 0.
Targeting an asteroid - The Galileo spacecraft's encounter with 951 Gaspra
On 1991 October 29, the Galileo spacecraft provided the first in situ observations of an asteroid. To a significant degree, the successful encounter with asteroid 951 Gaspra was made possible by the high accuracy of the ground-based Gaspra ephemeris and the refinement to this initial ephemeris provided by the use of optical navigation images taken on board the spacecraft itself. The success of these celestial navigation efforts was dramatically demonstrated when Gaspra appeared in the central frame of the large mosaic of CCD images-less than one asteroid diameter from Gaspra's expected location in the camera's field of view. The corrections to Gaspra's ground-based ephemeris, based upon the optical on-board navigation images, were less than 80 km. The ground-based observation program that went into this ephemeris development is discussed and the accuracy of the ephemeris is used to validate the assumptions that went into the error analysis studies.
Europa, Ganymede, and Callisto - New radar results from Arecibo and Goldstone
Observations of the icy Galilean satellites, performed during 1987-1991 with the Goldstone 3.5 cm system and the Arecibo 13 cm system, show significant enhancements in the knowledge of the satellite's radar properties. The most prominent radar features are tentatively identified with Galileo Regio and the Valhalla basin. Estimates of echo Doppler frequencies show Callisto to be lagging its ephemeris by 200 +/- 50 km.
Asteroid and comet orbits using radar data
For the 30 asteroids and 4 comets for which radar astrometric data were given by Ostro (1991), orbits have been computed using both the radar and the existing optical measurements. The techniques required to process radar data in orbit determination solutions are outlined, and future radar observation opportunities for asteroids and comets are identified. For asteroids and comets that have only short intervals of optical astrometric data, the additional use of only a few radar observations allows a far more accurate extrapolation of their future motions. The use of radar data can often ensure an object's successful recovery at future earth returns and greatly assist efforts in monitoring the motions of the rapidly growing population of known near-earth objects, including their future close-earth approaches.
Asteroid radar astrometry
Measurements of time delay and Doppler frequency are reported for asteroid-radar echoes obtained at Arecibo and Goldstone during 1980-1990. Radar astrometry is presented for 23 near-earth asteroids and three mainbelt asteroids. These measurements, which are orthogonal to optical, angular-position measurements, and typically have a fractional precision between 10 to the -5th and 10 to the -8th, permit significant improvement in estimates of orbits and hence in the accuracy of prediction ephemerides. Estimates are also reported of radar cross-section and circular polarization ratio for all asteroids observed astrometrically during 1980-1990.
A comet among the near-earth asteroids?
Using available optical and radar observations, orbital solutions have been computed for 12 near-earth asteroids identified as possible extinct comets. For 10 of these objects, there is no evidence for the nongravitational effects that might indicate residual outgassing activity. For 1862 Apollo and 1566 Icarus, the inclusion of cometlike nongravitational effects in their equations of motion significantly reduces their RMS residuals and flattens their systematic residual trends. For Icarus in particular, the determined radial nongravitational parameter retains the same value when different datasets are employed and the predictive capability of the orbits is improved. While these results do not prove that 1566 Icarus is a comet masquerading as an asteroid, future apparitions of this object should be monitored for possible cometlike activity.