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排序方式: 共有423条查询结果,搜索用时 31 毫秒
31.
Stewart Nozette Paul Spudis Ben Bussey Robert Jensen Keith Raney Helene Winters Christopher L. Lichtenberg William Marinelli Jason Crusan Michele Gates Mark Robinson 《Space Science Reviews》2010,150(1-4):285-302
The Miniature Radio Frequency (Mini-RF) system is manifested on the Lunar Reconnaissance Orbiter (LRO) as a technology demonstration and an extended mission science instrument. Mini-RF represents a significant step forward in spaceborne RF technology and architecture. It combines synthetic aperture radar (SAR) at two wavelengths (S-band and X-band) and two resolutions (150 m and 30 m) with interferometric and communications functionality in one lightweight (16 kg) package. Previous radar observations (Earth-based, and one bistatic data set from Clementine) of the permanently shadowed regions of the lunar poles seem to indicate areas of high circular polarization ratio (CPR) consistent with volume scattering from volatile deposits (e.g. water ice) buried at shallow (0.1–1 m) depth, but only at unfavorable viewing geometries, and with inconclusive results. The LRO Mini-RF utilizes new wideband hybrid polarization architecture to measure the Stokes parameters of the reflected signal. These data will help to differentiate “true” volumetric ice reflections from “false” returns due to angular surface regolith. Additional lunar science investigations (e.g. pyroclastic deposit characterization) will also be attempted during the LRO extended mission. LRO’s lunar operations will be contemporaneous with India’s Chandrayaan-1, which carries the Forerunner Mini-SAR (S-band wavelength and 150-m resolution), and bistatic radar (S-Band) measurements may be possible. On orbit calibration, procedures for LRO Mini-RF have been validated using Chandrayaan 1 and ground-based facilities (Arecibo and Greenbank Radio Observatories). 相似文献
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Robert Shiurba Tatsuo Hirabayashi Shin Kiyokawa Akimasa Fukui Yuko Miyanaga Issey Kojima Makoto Asashima 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,23(12):2041
In most ectotherms, environmental temperature has differential effects on growth and differentiation. For example, amphibian size at maturity decreases with increasing temperature. To address how radiant heat in the form of far-infrared radiation (FIR) may affect development of the aquatic ectotherm Xenopus laevis, we continuously irradiated swimming larvae as they developed into young adults. Here we report evidence that FIR promotes growth of these organisms in an aqueous environment. 相似文献
34.
Robert G. Stone 《Space Science Reviews》1973,14(3-4):534-551
The remote sensing of low frequency nonthermal radio emission is the astronomy of field and particle phenomena. Observations conducted from space lead to information about the composition and dynamic processes occurring in planetary magnetospheres as well as within the interplanetary and interstellar medium. The potential of this technique is demonstrated by considering observations obtained from Earth orbit missions.This is one of the publications by the Science Advisory Group. 相似文献
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37.
Jie Jiang Robert H. Cameron Dieter Schmitt Manfred Schüssler 《Space Science Reviews》2013,176(1-4):289-298
To reproduce the weak magnetic field on the polar caps of the Sun observed during the declining phase of cycle 23 poses a challenge to surface flux transport models since this cycle has not been particularly weak. We use a well-calibrated model to evaluate the parameter changes required to obtain simulated polar fields and open flux that are consistent with the observations. We find that the low polar field of cycle 23 could be reproduced by an increase of the meridional flow by 55% in the last cycle. Alternatively, a decrease of the mean tilt angle of sunspot groups by 28% would also lead to a similarly low polar field, but cause a delay of the polar field reversals by 1.5 years in comparison to the observations. 相似文献
38.
Robert O. Pepin 《Space Science Reviews》2000,92(1-2):371-395
Xenon plays a crucial role in models of atmospheric evolution in which noble gases are fractionated from their initial compositions
to isotopically heavier distributions by early hydrodynamic escape of primordial planetary atmospheres. With the assumption
that nonradiogenic Xe isotope ratios in present-day atmospheres were generated in this way, backward modeling from these ratios
through the fractionating process can in principle identify likely parental Xe compositions and thus the probable sources
of noble gases in pre-escape atmospheres. Applied to Earth, this approach simultaneously establishes the presence of an atmospheric
Xe component due principally to fission of extinct 244Pu and identifies a composition called U-Xe as primordial Xe. Pu-Xe comprises 4.65±0.30% of atmospheric 136Xe, and 6.8±0.5% of the present abundance of 129Xe derives from decay of extinct 129I. U-Xe is identical to the measured composition of solar-wind Xe except for deficits of the two heaviest isotopes – an unexpected
difference since the modeling otherwise points to solar wind compositions for the lighter noble gases in the primordial terrestrial
atmosphere. Evidence for the presence of U-Xe is not restricted to the early Earth; modeling based on a purely meteoritic
data set defines a parental component in chondrites and achondrites with the same isotopic distribution. Results of experimental
efforts to measure this composition directly in meteorites are promising but not yet conclusive. U-Xe also appears as a possible
base component in interstellar silicon carbide, here with superimposed excesses of 134Xe and 136Xe six-fold larger than those in the solar wind. These compositional differences imply mixing of U-Xe with a nucleogenetic
heavy-isotope component whose relative abundance in the solar accretion disk and in pre-solar environments varied both spatially
and temporally.
In contrast to Earth, the U-Xe signature on Mars was apparently overwhelmed by local accretion of materials rich in either
chondritic Xe or solar-wind Xe. Data currently in hand from SNC meteorites on the composition of the present atmosphere are
insufficiently precise to constrain a modeling choice between these two candidates for primordial martian Xe. They likewise
do not permit definitive resolution of a 244Pu component in the atmosphere although its presence is allowed within current measurement uncertainties.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
39.
The Near-Earth Plasma Environment 总被引:1,自引:0,他引:1
Robert F. Pfaff Jr. 《Space Science Reviews》2012,168(1-4):23-112
An overview of the plasma environment near the earth is provided. We describe how the near-earth plasma is formed, including photo-ionization from solar photons and impact ionization at high latitudes from energetic particles. We review the fundamental characteristics of the earth’s plasma environment, with emphasis on the ionosphere and its interactions with the extended neutral atmosphere. Important processes that control ionospheric physics at low, middle, and high latitudes are discussed. The general dynamics and morphology of the ionized gas at mid- and low-latitudes are described including electrodynamic contributions from wind-driven dynamos, tides, and planetary-scale waves. The unique properties of the near-earth plasma and its associated currents at high latitudes are shown to depend on precipitating auroral charged particles and strong electric fields which map earthward from the magnetosphere. The upper atmosphere is shown to have profound effects on the transfer of energy and momentum between the high-latitude plasma and the neutral constituents. The article concludes with a discussion of how the near-earth plasma responds to magnetic storms associated with solar disturbances. 相似文献
40.
This paper describes a system of navigation by measurement of the Doppler shift in the radio transmissions from a near-Earth satellite, and simple equipment for making the measurements. It also shows that the current information which is needed by the navigator can be reduced to a number containing six decimal digits. Finally, it shows that the computations needed for the navigator to obtain a fix can be performed witlh the aid of tables that can be prepared at least 18 months in advance. Using these tables and a simple slide rule, but no other computational aids, a fix accurate to about 500 meters can be obtained in about 10 minutes. More accuracy can be obtained at the expense of time, or time can be saved at the expense of accuracy. 相似文献