Radar Observations of Asteroid 1620 Geographos Ostro, Steven J.; Jurgens, Raymond F.; Rosema, Keith D. ...
Icarus (New York, N.Y. 1962),
05/1996, Letnik:
121, Številka:
1
Journal Article
Recenzirano
Goldstone radar observations of Geographos from August 28 through September 2, 1994 yield over 400 delay-Doppler images whose linear spatial resolutions range from ∼75 to ∼151 m, and 138 pairs of ...dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (S. J. Ostro, K. D. Rosema, R. S. Hudson, R. F. Jurgens, J. D. Giorgini, R. Winkler, D. K. Yeomans, D. Choate, R. Rose, M. A. Slade, S. D. Howard, and D. L. Mitchell 1995,Nature375, 474–477). Here we present individual images and co-registered sums over ∼30° of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features “inside” the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g cm−3. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on August 28 and to about 100 m on August 31, an improvement of two orders of magnitude over pre-observation ephemerides.
Radar Observations of Asteroid 7822 (1991 CS) Benner, Lance A.M.; Ostro, Steven J.; Rosema, Keith D. ...
Icarus (New York, N.Y. 1962),
02/1999, Letnik:
137, Številka:
2
Journal Article
Recenzirano
We report Doppler-only (cw) radar observations of near-Earth asteroid 1991 CS obtained at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm) on 1996 August 26, 27, 28, and 29. Weighted, ...optimally filtered sums of cw echoes achieve signal-to-noise ratios in excess of 300 per day and thoroughly cover the asteroid in rotation phase (synodic period=2.39 h, obtained photometrically by P. Pravecet al.1998,Icarus136, 124–153). A weighted sum of all cw spectra gives an OC radar cross section of 0.24±0.08 km2and a circular polarization ratio of 0.28±0.001. Our observations place up to 50 0.98-Hz resolution cells on 1991 CS at echo powers greater than 2 standard deviations of the noise. Variations of ∼10 Hz in the echo's 2-σ bandwidth are evident on each day and are consistent with the rotation period. Inversion of echo edge frequencies yields convex hulls of the pole-on silhouette for each day. The hulls have a mean elongation and rms dispersion of 1.18±0.02 and place a lower bound on the maximum pole-on dimension of 1.3 km/cosδ, where δ is the angle between the radar line-of-sight and the asteroid's apparent equator. The hulls suggest that 1991 CS has the least elongated pole-on silhouette of any of the nine near-Earth asteroids for which similar shape information from radar observations is available. If we assume that the projected area of 1991 CS is the same as that of a sphere with a diameter of 1.1 km, equal to the lower bound on the minimum breadth of the pole-on silhouette on August 28 and 29, then 1991 CS's radar cross section and absolute magnitude of 17.5 correspond to upper limits on the radar and optical geometric albedos of 0.25 and 0.14 that are consistent with its S-class taxonomy. The circular polarization ratio and radar albedo do not vary substantially as a function of rotation phase, suggesting that the near-surface of 1991 CS is relatively uniform. Among the 20 S-class asteroid radar detections previously reported, the circular polarization ratio of 1991 CS is greater than 11, comparable to 4, and less than 5.
Regulated electrolyte transport across polarized epithelia is central to organism homeostasis. Two major epithelial ion permeation pathways have been identified: the transcellular route through ...cells, mediated by channels, transporters, and exchangers, and the paracellular pathway between them, gated by the tight junction. While physiologic studies in a variety of epithelia have demonstrated the importance of both pathways to maintaining electrolyte homeostasis, significantly greater progress has been made in identifying and characterizing mediators of transcellular conductance. This is due to a fundamental difference in the nature of transcellular and paracellular permeability determinants. The channels, transporters, and exchangers responsible for transcellular permeability can be studied and characterized because each functions as a discrete unit, with the same structure, permeability, and regulation as other copies of the same protein. The paracellular pathway, however, is gated by the tight junction which is composed of a heterogeneous population of proteins whose coordinated function determines overall permeability. While physiologic studies have demonstrated the ion selectivity and permeability of many epithelia, the complexity of the tight junction structure has largely prevented identification of individual protein determinants of paracellular permeability. Genetic studies of two renal disorders of electrolyte homeostasis: recessive renal hypomagnesemia with hypercalciuria and nephrocalcinosis (HHN) and pseudohypoaldosteronism type II (PHAII), allowed a priori identification of paracellular permeability determinants. In the case of HHN, mutations in paracellin-1, a claudin family tight junction protein, result in renal magnesium wasting and hypercalciuria due to a defect in paracellular magnesium and calcium reabsorption in the kidney. PHAII, a disorder characterized by hypertension and hyperkalemia in the presence of otherwise normal renal function, is caused by mutations in the WNK1 and WNK4 serine/threonine kinases. The identification of these genes raised a number of questions about their role and localization in the kidney. Specifically, it was critical to examine the nephron-segment and subcellular localization of each. Additionally, an in vitro expression system for PCLN-1 provided a method to determine the effects of PCLN-1 expression on paracellular permeability. These studies provide the first evidence that there are specific molecular determinants of paracellular permeability and examine their relevance to human disorders of electrolyte homeostasis.
Unlike the major planets and main belt asteroids, some near-Earth objects (NEOs) make their closest approaches to Earth at high northern declinations and therefore are visible simultaneously from ...North America, Europe and/or Asia, providing occasional opportunities for intercontinental radar experiments. The first celestial target of intercontinental radar was the Earth-crossing asteroid 6489 Golevka (1991 JX), which in June 1995 passed 0.034AU from Earth at a declination of ∼40deg. High power (∼0.5MW), continuous-wave signals at 3.5cm wavelength were transmitted toward the asteroid from the 70m antenna at the JPL/NASA Goldstone Deep Space Communication Complex (DSCC) in California on 13, 14 and 15 June 1995 during several hours on each date. This illumination of the asteroid created an artificial radio source for astronomers anywhere on the asteroid-facing side of Earth. Five astronomical groups tried to detect the radar echoes and two succeeded. Detections were obtained on each of the three days by the 70m antenna at Evpatoria DSCC in Crimea and on June 15 by the 34m antenna at Kashima Space Research Center in Japan. The Goldstone 34m antenna monitored echoes throughout all the observations. From the results of the Goldstone-Evpatoria experiment it can be inferred that the asteroid is about 0.5km across, is not very elongated, possesses considerable surface irregularity and is very reflective presumably due to a large near-surface bulk density. This first intercontinental radar astronomy experiment can be considered as an initial step toward a global radar network for routine NEO investigations.
Goldstone radar observations of Geographos from August 28 through September 2, 1994 yield over 400 delay-Doppler images whose linear spatial resolutions range from approximately 75 - 151 m, and 138 ...pairs of dual-polarization (OC, SC) spectra with one-dimensional resolution of 103 m. Each data type provides thorough rotational coverage. The images contain an intrinsic north/south ambiguity, but the equatorial view allows accurate determination of the shape of the radar-facing part of the asteroid's pole-on silhouette at any rotation phase. Sums of co-registered images that cover nearly a full rotation have defined the extremely elongated shape of that silhouette (S. J. Ostro, K. D. Rosema, R. S. Hudson, R. F. Jurgens, J. D. Giorgini, R. Winkler, D. K. Yeomans, D. Choate, R. Rose, M. A. Slade, S. D. Howard, and D. L. Mitchell 1995, Nature 375, 474-477). Here we present individual images and co-registered sums over approximately 30 deg of rotation phase that show the silhouette's structural characteristics in finer detail and also reveal numerous contrast features "inside" the silhouette. Those features include several candidate craters as well as indications of other sorts of large-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may be related to the pattern of ejecta removal and deposition caused by the asteroid's gravity field. The asteroid's surface is homogeneous and displays only modest roughness at centimeter-to-meter scales. Our estimates of radar cross section and the currently available constraints on the asteroid's dimensions are consistent with a near-surface bulk density between 2 and 3 g/cc. The delay-Doppler trajectory of Geographos' center of mass has been determined to about 200 m on August 28 and to about 100 m on August 31, an improvement of two orders of magnitude over pre-observation ephemerides.
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