Current Orbit Research Articles

Astrometric-spectroscopy binary star orbits. IV - Beta Coronae Borealis

view Abstract Citations (23) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Astrometric-Spectroscopic Binary Star Orbits. IV. Beta Coronae Borealis Kamper, Karl W. ; McAlister, Harold A. ; Hartkopf, William I. Abstract From a combination of speckle interferometric and visual measurements with recent radial velocities, an improved current orbit was derived for beta Coronae Borealis with a period of 10.55 yr, an eccentricity of 0.538, periastron at 1980.472, and a semimajor axis of 0.2032 arcsec and have combined this with a long Allegheny parallax series to obtain a new parallax estimate of + 0.026 + or - 0.004 arcsec and a system mass of 4.3 + or - 2.0 solar masses, for which the error is due almost entirely to the uncertainty in the parallax. With this parallax, the system absolute magnitude becomes M(v) = + 0.7 and the absolute magnitude of the F0p primary becomes + 0.9 + or - 0.3. The orbital elements differ significantly from those derived from the Lick radial velocities of a half century ago, which also indicated the presence of a third body in the system. The recent velocities do not show this variation. The Lick radial-velocity evidence for a third body in the system was reexamined and it was found that its existence is statistically likely. It is possible that it cannot now be detected because perturbations have brought its orbit plane presently into the plane of the sky. Publication: The Astronomical Journal Pub Date: July 1990 DOI: 10.1086/115510 Bibcode: 1990AJ....100..239K Keywords: Astrometry; Astronomical Spectroscopy; Binary Stars; Stellar Orbits; Radial Velocity; Speckle Interferometry; Stellar Magnitude; Stellar Parallax; Astrophysics; STARS: PECULIAR A; STARS: BINARIES full text sources ADS | data products SIMBAD (1) Related Materials (3) Part 1: 1987AJ.....93..683K Part 2: 1987AJ.....94.1302K Part 3: 1989AJ.....98..686K

Circulation from a joint gravity field solution determination of the general ocean

With the development of satellite altimetry, it is possible to infer the geostrophic velocity of the surface ocean currents, if the geoid and the position of the satellite are known accurately. Errors in current geoid models and orbit computations, both due primarily to errors in the Earth's gravity field model, have limited the use of altimeter data for this purpose. The objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi‐stationary sea surface topography, ζ, and the model for the gravity field. Satellite tracking data from twelve satellites were used along with Seasat altimeter data for the solution. The estimated model of ζ compares well at long wavelengths with the hydrographic model of ζ. Covariance analysis indicates that the geoid is separable from ζ up to degree 9, at which point geoid error is comparable to the signal of ζ.

A new method of initial orbit determination

We present a new method of initial orbit determination for angles-only data. The technique is applicable with only three data sets but the formalism can incorporate arbitrarily large amounts of data. The algorithm rests on the fact that the orbital plane is usually very well determined, as a consequence of the central nature of the gravitational force, and a theorem of W. R. Hamilton's from 1846. His result states that the velocity vector describes a circle in the orbital plane in a particular manner. Tests on a variety of deep-space artificial satellites show that the method has promise. Unfortunately it does not work very well on the stressing cases of current practical application-high eccentricity orbits. As no other method does either, the opportunities still exist for a significant advance in this area of celestial mechanics.

The obliquity of Pluto

Pluto's obliquity (the angle between its spin axis and orbit normal) varies between ∼102 and ∼126° over a period of about 3 million years. These oscillations are nearly sinusoidal and quite stable, leading to only modest changes in the insolation regime. Thus, Pluto's rotation has been slightly retrograde ever since its current orbit and rotation rate were established.

Flow visualization reveals causes of Shuttle nonlinear aerodynamics

Selected oil-flow photographs, which show many of the complicated flow phenomena that have dominated the aerodynamic characteristics of the various Space Shuttle configurations, are presented and analyzed. The analysis covers the entire spectrum of Shuttle configurations from the early straight wing, fully recoverable orbiter and booster to the current double delta planform orbiter. For all the cases analyzed, the oil-flow photographs reveal that flow separation is the fundamental cause of the nonlinear aerodynamic characteristics. The success of these simple techniques in revealing the causes at the Shuttle nonlinear aerodynamics underscores their usefulness for other aircraft configurations dominated by flow separation and/or flow interference effects.

Observed magnetic substorm signatures at synchronous altitude

Magnetic field data from the geostationary satellites GOES 2 and GOES 3 are examined to study the development of substorm activity in the near‐earth nightside magnetosphere (around a radial distance of 6.6 RE). Substorm events are those in which a well‐defined single onset is seen at low latitudes on the ground. The field configuration change from more taillike to more dipolelike starts initially in a longitudinally localized region in association with the ground onset, and it develops westward and eastward, even when the simultaneous onset of the low‐latitude positive bay is recorded in a wide longitudinal region on the ground. It is also found that the variation caused by the field‐aligned current starts at the ground onset and reaches a peak at the time of the field change. The present results are consistent with the view that a substorm is associated with a disruption and subsequent conversion of the cross‐tail current to the field‐aligned current connected with the polar ionosphere. It is indicated that the cross‐tail current near synchronous orbit and its disruption are important in producing the field configuration change at synchronous orbit.

Density models for the upper atmosphere

Modeling the effects of atmospheric drag is one of the more important problems associated with the determination of the orbit of a near-earth satellite. Errors in the drag model can lead to significant errors in the determination and prediction of the satellite motion. The uncertainty in the drag acceleration can be attributed to three separate effects: (a) errors in the atmospheric density model, (b) errors in the ballistic coefficient, and (c) errors in the satellite relative velocity. In a number of contemporary satellite missions, the requirements for performing the orbit determination and predictions in near real-time has placed an emphasis on density model computation time as well as the model accuracy. In this investigation, a comparison is made of three contemporary atmospheric density models which are candidates for meeting the current orbit computation requirements. The models considered are the analytic Jacchia-Roberts model, the modified Harris-Priester model, and the USSR Cosmos satellite derived density model. The computational characteristics of each of the models are compared and a modification to the modified Harris-Priester model is proposed which improves its ability to represent the diurnal variation in the atmospheric density.

Nongravitational perturbations of long-period comets

The model of Marsden et al. (1973) is used to investigate the effect of nongravitational forces resulting from water ice sublimation on the orbits of long-period comets. The orbits of hypothetical comets with perihelion distances of 0.005 to 4 AU are integrated numerically along initially parabolic trajectories through one perihelion passage for nongravitational forces of 10 millionths to 200 millionths the solar attraction at 1 AU and lag angles of 0, 5, and 10 deg. The results indicate that the nongravitational perturbations are possibly of equal or greater importance than planetary perturbations for comets with small perihelia and that nongravitational forces could have brought sun-grazing comets to their current orbits in two or three returns. It is concluded that nongravitational forces are likely to be found only for those long-period comets that have large nongravitational accelerations to begin with and are bright enough to be discovered early and tracked over a very long arc of their orbits.

Gas drag in primordial circumplanetary envelopes: A mechanism for satellite capture

Some natural satellites may have been captured due to the gas drag they experienced in passing through primordial circumplanetary nebulas. This paper models such an encounter and derives the testable parameters from the known properties of current solar system objects and Bodenheimer's (1977, Icarus 31) model of the earliest phases of Jupiter's evolution. We propose that the clusters of prograde and retrograde irregular satellites of Jupiter originated when two parent bodies were decelerated and fragmented as they passed through an extended primordial Jovian nebula. Fragmentation occured because the gas dynamic pressure exceeded the parent bodies' strengths. These events must have occurred only shortly before the primordial nebula experienced hydrodynamical collapse so that subsequently the fragments underwent only limited orbital evolution. Because self-gravity exceeded the relative drag force, the fragments initially remained together, only to be dispersed at a later time by a collision with a stray body. Predictions of this hypothesis, such as orbital distance of the irregular satellites and size of the parent bodies, are found to be consistent with the observed properties of Jupiter's irregular satellites. In addition nebular drag at a later time may have caused the inner three Galilean satellites to undergo a modest amount of orbital evolution, accounting for their present orbital resonance. Gas drag capture of Saturn's Phoebe and Iapetus and Neptune's Nereid and Triton may also be possible. Reasonable differences in properties could explain why these satellites, in contrast to the Jovian ones, did not fracture upon capture. The current irregular satellites represent only a tiny fraction of the bodies captured by primordial nebulas. The dominant fraction would have spiraled into the center of the nebula as a result of continued gas drag and thus offer one source for the heavy element cores of the outer planets. If one is willing to postulate the presence of a massive gaseous nebula around primordial Mars, then gas drag capture could account for the origin of the Martian moons. We hypothesize that a single parent body was captured in a region of the nebula where the gas velocity approached the Keplerian value, that it fragmented upon deceleration into at least two bodies, Phobos and Deimos, and that continued nebular drag led to the low eccentricity and inclination that characterize the satellites' current orbits. Following the dissipation of this nebula, the more massive Phobos tidally evolved to its current position.

Secular changes in Jovian eccentricity: Effect on the size of capture orbits

The model of dynamic capture proposed by Bailey based on the elliptic restricted three-body problem is examined in the light of eccentricity variations in the orbits of the primaries. It is found that good agreement between theory and observations for the case of the outer satellites of Jupiter is obtained when the current orbital eccentricity of Jupiter is used, but this agreement does not follow from most past conditions. The current orbits do fall within the predicted limits. The implication for a purely capture origin for these satellites is discussed.

Orbiter Abort Guidance

Abort is defined as a premature or abnormal termination of a mission as a result of either a system or human failure. The abort guidance during the ascent phase, among all flight phases of the shuttle vehicle, is most critical; because it involves guiding a heavily loaded shuttle vehicle that is passing through severe environmental conditions. This paper describes a predictive guidance algorithm for the abort mode initiated during the orbiter ascent flight phase. Contrary to the concept of the minimization1 of fuel consumption during a nominal ascent flight phase, the Space Shuttle orbiter is required to deplete its fuel tank in order to be separated from the vehicle during the abort mode. The guidance objective during the abort mode is to attain a proper terminal energy state in relation to the landing site, consistent with the aerodynamic heating and loading constraints. In the absence of a well-defined payoff function for optimization during this mode, the abort guidance algorithm presented in this paper is formulated to minimize aerodynamic forces. Minimization of aerodynamic forces was considered because of the possibility that the failure that caused the initiation of the abort mode may have weakened the orbiter structure. Immediately after the abort mode initiation, consistent with the philosophy of the minimization of the aerodynamic forces, the orbiter is forced to fly a lifting trajectory to attain higher altitude, thereby reaching a low dynamic pressure environment. The predictive guidance algorithm uses the knowledge of the current orbiter state and the desired terminal state in relation to the landing site, and determines the time sequence of the powered maneuver and the orientation of the thrust vector throughout the abort mode. For example, the magnitude of the down-range component of the acceleration, the time at which to begin to return toward the desired landing site, and the radial acceleration necessary to attain the desired terminal altitude (while remaining in the low dynamic pressure environment) are computed. The key formulations in the predictive guidance algorithm are analytical expressions for the terminal state that the vehicle will reach for a given sequence of thrust orientations, and the associated Jacobian matrix. Therefore, these have been carefully developed in the paper. In the terminal phase of the abort mode, the predictive guidance reduces to a velocity to be gained scheme which has the desirable effect of reducing the deviations in the end-state prediction that results from variations and uncertainties in the vehicle characteristics. Detailed results for representative abort cases and estimates of the computer requirements are also included in the paper.

Orbital evolution of Lost City meteorite

The orbit of the Lost City meteorite is investigated to determine the influence of secular and encounter perturbations on the orbital evolution. Secular perturbations are negligible in the interval of 300–400 years and encounter perturbations are highly unlikely; therefore it is valid to interpret the short-lived cosmic radiogenic isotopes as having formed in the current orbit. During a 40,000-year numerical integration, the maximum inclination was 16°, the minimum 9.7°; the minimum eccentricity was 0.414, the maximum 0.447. Over a 500,000-year interval, it was found that the very long period secular terms were effective in preventing earth encounters for a substantial fraction of the time, suggesting that the hypothesis that the meteor was a surviving remnant of the early solar system ought not be completely dismissed.

ORBITAL CELLULITIS DUE TO SINUS INFECTION

Medical Journal of AustraliaVolume 1, Issue 6 p. 267-267 Abstracts From Current Medical Literature ORBITAL CELLULITIS DUE TO SINUS INFECTION First published: February 1938 https://doi.org/10.5694/j.1326-5377.1938.tb110211.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume1, Issue6February 1938Pages 267-267 RelatedInformation