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Alberto Cellino 《Space Science Reviews》2000,92(1-2):397-412
The observational evidence of a gradient in composition of the solid matter in the Solar System as a function of heliocentric
distance, with more volatile elements increasingly abundant at larger heliocentric distances, is in general agreement with
commonly accepted ideas about condensation of solid material in the early protoplanetary disk. In this respect, the gradual
transition from regions mostly populated by S-type objects in the inner asteroid belt to the C-type dominated outer belt, and to the D-type dominated Trojan clouds has generally been interpreted as diagnostic of a zoning of the asteroid population in terms
of mineralogic composition and thermal history. The situation, however, is not completely clear, and several contradictory
facts still need an explanation. Another major problem concerns the origin of meteorites. Understanding the history of these
bodies is critically important, since most of our ideas about the composition of the original protosolar nebula and protoplanetary
matter come from meteoritic data. In this respect, very recent advances have been achieved in the knowledge of dynamical transport
mechanisms. This can have important consequences for the characterization of plausible meteorite parent bodies.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
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Pulsars are natural cosmic clocks. On long timescales they rival the precision of terrestrial atomic clocks. Using a technique called pulsar timing, the exact measurement of pulse arrival times allows a number of applications, ranging from testing theories of gravity to detecting gravitational waves. Also an external reference system suitable for autonomous space navigation can be defined by pulsars, using them as natural navigation beacons, not unlike the use of GPS satellites for navigation on Earth. By comparing pulse arrival times measured on-board a spacecraft with predicted pulse arrivals at a reference location (e.g. the solar system barycenter), the spacecraft position can be determined autonomously and with high accuracy everywhere in the solar system and beyond. We describe the unique properties of pulsars that suggest that such a navigation system will certainly have its application in future astronautics. We also describe the on-going experiments to use the clock-like nature of pulsars to “construct” a galactic-sized gravitational wave detector for low-frequency (\(f_{GW}\sim 10^{-9} \text{--} 10^{-7}\) Hz) gravitational waves. We present the current status and provide an outlook for the future. 相似文献