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Boice D. C. Soderblom L. A. Britt D. T. Brown R. H. Sandel B. R. Yelle R. V. Buratti B. J. Hicks Nelson Rayman Oberst J. Thomas N. 《Earth, Moon, and Planets》2000,89(1-4):301-324
NASA's Deep Space 1 (DS1) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images(47–58 m pixel). A smooth, broad basin containing brighter regions and mesa-likestructures is present in the central part of the nucleus that seems to be the source ofjet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger thanabout 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about10% actively sublimating. Two types of dust features are seen: broad fans and highlycollimated “jets” in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller “jets” near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dustis rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far. 相似文献
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Jian-Yang Li Peter C. Thomas Max J. Mutchler Christopher T. Russell Marc D. Rayman 《Icarus》2011,211(1):528-534
We report an improved measurement of the rotational axis orientation of Asteroid (4) Vesta. By analyzing and combining all previous measurements using a limb-fitting technique from ground/HST data collected from 1983 to 2006, we derive a pole solution of (RA = 304.5°, Dec = 41.5°). Images of Vesta acquired with the Wide Field Camera 3 onboard the Hubble Space Telescope (HST) in February 2010 are combined with images from the Wide Field Planetary Camera 2 on HST obtained in 1994, 1996, and 2007 at similar spatial resolution and wavelengths to perform new measurements. Control point stereogrammetry returns a pole solution of (305.1°, 43.4°). An alternate method tracks surface features and fits their projected paths with ellipses to determine a great circle containing the pole for each HST observation. Combined, the four great circles yield a pole solution of (309.3°, 41.9°). These three solutions obtained with almost independent methods are within 3.5° of each other, suggesting a robust solution. Combining the results from all three techniques, we propose an improved value of the rotational axis of Vesta as RA = 305.8° ± 3.1°, Dec = 41.4° ± 1.5° (1-σ error). This new solution changes from (301°, 41°) reported by Thomas et al. (Thomas, P.C., Binzel, R.P., Gaffey, M.J., Zellner, B.H., Storrs, A.D., Wells, E. [1997a]. Icarus 128, 88-94) by 3.6°, and from (306°, 38°) reported by Drummond and Christou (Drummond, J.D., Christou, J. [2008]. Icarus 197, 480-496) by 3.4°. It changes the obliquity of Vesta by up to ∼3°, but increases the Sun-centered RA of Vesta at equinox by ∼8°, and postpones the date of equinox by ∼35 days. The change of the pole position is less than the resolution of all previous images of Vesta, and should not change the main science conclusions of previous research about Vesta. 相似文献
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