Understanding how comets work--what drives their activity--is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO(2), which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.
An investigation has been made of the source and magnitude of anisotropic material properties of cancellous bone in the proximal epiphysis of the human tibia. Results are reported for stiffness measurements made in three orthogonal directions on 21 cubes of cancellous bone before testing to failure along one of the three principal axes. The structure is approximately transversely isotropic. Strength and stiffness are linear with area fraction for loading along the isotropic axis. Strength is proportional to stiffness for all directions. A finite element model is proposed, based on experimental observations, which enables one to predict the elastic constants of cylindrically structured cancellous bone in the tibia from morphological measurements in the transverse plane.
We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its shortterm temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000) = 8 ± 4°, 54 ± 1°(obliquity = 48 ± 1°). This direction is likely changing, but the change is probably <6°on the sky over the $81.6 days of the DIXI encounter. The magnitudes of M and E at closest approach (JD 2455505.0831866 2011-11-04 13:59:47.310) are 30.0 ± 0.2 m 2 /s and (1.56 ± 0.02) Â 10 À3 m 2 /s 2 respectively. The period of rotation about the instantaneous spin vector, which points in the direction (RA, Dec; J2000) = 300 ± 3.2°, 67 ± 1.3°at the time of closest approach, was 14.1 ± 0.3 h. The instantaneous spin vector circulates around M, inclined at an average angle of 33.2 ± 1.3°, with an average period of 18.40 ± 0.13 h at the time of closest approach. The period of roll around the principal axis of minimum inertia (''long'' axis) at that time is 26.72 ± 0.06 h. The long axis is inclined to M by $81.2 ± 0.6°on average, slowly decreasing through encounter. We infer that there is a periodic nodding motion of the long axis with half the roll period, i.e., 13.36 ± 0.03 h, with amplitude of $1°again decreasing through encounter. The periodic variability in the circulation and roll rates during a cycle was at the 2% and 10-14% level respectively. During the encounter there was a secular lengthening of the circulation period of the long axis by 1.3 ± 0.2 min/d, in agreement with ground-based estimates, while the period of roll around the long axis changed by $À4.4 min/d at perihelion. M decreased at a rate of À0.038 (m 2 /s) per day in a roughly linear fashion. Assuming a bulk density between 230-300 kg/m 3 and a total volume for the nucleus of 8.09 Â 10 8 m 3 , the net torque acting on the nucleus was in the range 0.8-1.1 Â 10 5 kg m 2 /s 2. In order to bring the spacecraft photometric and imaging data into alignment on the direction of M, the directions of the intermediate and short principal axes of inertia had to be adjusted by 33°(on the sky) from the values indicated by the shape model with an assumed homogeneous interior. The adjusted direction of the intermediate axis is RA, Dec = 302°, À16.5°. The morning and evening terminators in the images are identified, and the variation of the insolation at three regions on the nucleus associated with active areas calculated. The plume of water vapor observed in the inner coma is found to be directed close to the direction of local gravity over the sub-solar region for a range of reasonable bulk densities. The plume does not follow the projected normal to the surface at the sub-solar point.
Mixtures of 1‐methyl‐3‐ethylimidazolium chloride (MEICl) and aluminum chloride are molten salts at room temperature and below. Acidic melts, which contain
AlCl3
mole fraction
false(Nfalse)
greater than 0.5, can be used as catholytes, with basic melts
false(N
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