The Impact of Comet Showmaker-Levy 9 with Jupiter
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comet Shoemaker-Levy 9 asteroid galileo Jupiter↗
Engineering topics
Chodas, P. W.
Publications and source records attributed to Chodas, P. W..
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.
Comets vaporizing ices Earth asteroid 2340 Hathor↗
Venus gravity and topography: 60th degree and order model
We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.
Konopliv, A. S.↗
Estimating the Impact Probability of a Minor Planet with the Earth
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Asteroid Comets Impact Probability↗
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.
Yeomans, D. K.↗
The rotation period, direction of the north pole, and geodetic control network of Venus
Three related activities that use Magellan data to derive improved estimates of the rotation period and direction of the spin axis of Venus are discussed. These are the computation of the Magellan geodetic control network, the use of measurement landmarks identified in overlapping image strips to improve the spacecraft ephemeris, and the use of common points identified on both the Venera 15/16 and the Magellan images. Since seven years separate the acquisition of the Magellan and Venera images, it should be possible to compute an accurate rotation period and possibly the spin vector of Venus.
Davies, M. E.↗
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.
Yeomans, D. K.↗
The spin vector of Venus determined from Magellan data
A control network of the north polar region of Venus has been established by selecting and measuring control points on full-resolution radar strips. The measurements were incorporated into a least-squares adjustment program that improved initial estimates of the coordinates of the control points, pole direction, and rotation rate of Venus. The current dataset contains 4206 measurements of 606 points on 619 radar strips. The accuracy of the determination is driven by spacecraft ephemeris errors. An accurate estimate of the rotation period of Venus was obtained by applying an ephemeris improvement technique. The second cycle closure orbits improved ephemeris solutions for 40 orbits (376-384, 520-528, 588-592, 658-668, 1002-1010, 1408-1412, 1746-1764, and 2166-2170) are included and fixed in the geodetic control computations, thus trying the network to the J2000 coordinate system.
Davies, M. E.↗
Galileo asteroid encounter navigation
The Galileo spacecraft will be targeted to encounter one or more asteroids during its cruise to Jupiter. Accurate navigation will maximize science return from these asteroid flyby opportunities. Navigation errors for these encounters are dominated by uncertainties in the asteroid ephemeris, which is obtained from fits to ground-based observations. As the spacecraft approaches, on-board optical navigation dramatically improves knowledge of the spacecraft-relative asteroid position normal to the line of sight, while correlations in the asteroid ephemeris provide moderate improvement along the approach direction. The remaining uncertainty in encounter time can be further reduced only by improving the ground-based asteroid ephemeris. Uncertainties perpendicular to the line of sight can be reduced by improving the timing of optical navigation images and their placement with respect to the star background. At the closest approach to the asteroid Gaspra, the one-sigma errors in knowledge of the spacecraft position are less than 10 km in position and 25 seconds in encounter time.
Murrow, D. W.↗
An asymmetric outgassing model for cometary nongravitational accelerations
The standard nongravitation force model has been modified by allowing the water-vaproization curve to peak a certain number of days (DT) either before or after perihelion, and the modification is shown to often improve the data fit. The best fit to the astrometric data is generally achieved for a value of DT corresponding to the offset in the comet's visual light curve. The asymmetric model, which more accurately mimics the comet's outgassing history, suggests only weakly that the rotation direction of Comet Halley is direct, and it does not provide evidence that the rotation axis of Comet Kopff passed through its orbit plane in the early twentieth century. Approximate lag angles of 4 deg for Comet Halley and 10 deg for Comet d'Arrest are found using the asymmetric model, and it is noted that these values are lower than those obtained with the standard model.
Yeomans, D. K.↗
Radar observations of asteroid 1986 JK
The asteroid 1986 JK was observed with a 3.5 cm-wavelength radar in May and June, 1986, at less than 0.029 AU; its radar echo power circular polarization ratio indicates single backscattering from smooth surface elements. A working model constructed for the asteroid in light of these radar data postulates a 1-2 km object whose shape has little elongation and some polar flattening. Orbital and physical characteristics are rather cometlike. The radar astrometric data obtained are noted to be extremely powerful for orbit-improvement, so that a search ephemeris whose uncertainty is an order-of-magnitude smaller than that based on relevant optical data alone can be prepared by combining optical and radar data.
Ostro, S. J.↗
Radar astrometry of near-earth asteroids
An uncertainty analysis is conducted for four asteroids with different optical and radar observation histories in order to ascertain the degree to which near-earth asteroid ephemerides can be improved by means of radar observations. For any given set of optical and radar observations and their associated errors, the angular and earth-asteroid distance uncertainties are calculated as functions of time through 2001. The radar data furnished only a small absolute improvement in such cases as that of 1627 Ivar, where a long history of optical astrometric data exists, but dramatic reductions in ephemeris uncertainties are obtainable for asteroids having only short optical data histories.
Yeomans, D. K.↗
ISEE-3/ICE navigation analysis
Efforts to assure communications continuity and navigational accuracy for the ISEE-3 (now called the International Cometary Explorer - ICE) spacecraft as it heads toward a passage through the Giacobini-Zimmer (G-Z) comet tail are reviewed. Sufficient energy was gained with a series of lunar swingbys to take the rendezvous attempt possible. Communications will be by an S-band link though the upgraded Deep Space Network. The G-Z encounter is scheduled for September 1983. A 512 bps data rate will be supported by 64-3 m dishes at various global sites. The tracking will continue for 6 mos as the spacecraft moves into an orbit that parallels that of Halley's Comet. Ground-based recovery of the G-Z location in the spring of 1984 provided data for calculating the ephemerides required for configuring a major course correction for ICE 100 days before rendezvous.
Efron, L.↗
Comet Halley ephemeris uncertainties in 1985-1986
For the planned flyby missions to Comet Halley in March 1986, the comet's ephemeris uncertainties completely dominate the spacecraft-comet miss distance. In an effort to determine realistic Comet Halley ephemeris uncertainties, a statistical covariance analysis was conducted using the actual data in 1909-1910-1911 and simulated data in 1984-1985-1986. In 1985-1986, Comet Halley's ephemeris uncertainties are very sensitive to the comet's orbital position, the optical data noise, data schedule, and whether or not the old data is included in the orbital solutions. The comet's ephemeris uncertainties in March 1986 are relatively insensitive to reasonable center of light/center of mass offsets and also to possible radar data taken in late November 1985. Accurate Space Telescope observations made in early March 1986 might significantly improve upon the comet's position uncertainties for the various intercepting spacecraft.
Yeomans, D. K.↗