The dynamics of detumbling a randomly spinning spacecraft using externally mounted, movable telescoping appendages are studied both analytically and numerically. Two types of telescoping appendages are considered: (a) where an end mass is mounted at the end of an (assumed) massless boom; and (b) where the appendage is assumed to consist of a uniformly distributed homogeneous mass throughout its length. From an application of Lyapunov's second method, boom extension maneuvers can be determined to approach either of two desired final states: close to a zero inertial angular velocity state and a final spin rate about only one of the principal axes. Recovery dynamics are evaluated analytically for the case of symmetrical deployment. Numerical examination of other asymmetrical cases verifies the practicality of using movable appendages to recover a randomly tumbling spacecraft. 相似文献
Astronomical infrared spectra are used to confirm the existence of complex organic molecules produced by ultraviolet photoprocessing of interstellar grain mantles. This material is shown to be the major component of the interstellar grains between the sun and the galactic center and, by inference, constitutes more than 10 million solar masses — or close to one part in a thousand of the entire mass of the milky way galaxy. It may be demonstrated that the primitive chemistry of the earth's surface was dominated by these extraterrestrial molecules after aggregated into comets if the rate of comet impacts with the earth was comparable with that required to account for the extinction of species over the past 300 million years.
Ultraviolet irradiation of bacterial spores has been studied for the first time under simulated interstellar conditions. The inactivation time predicted for the less dense regions of space is at most several hundred years. Within molecukar clouds it is shown on theoretical and experimental grounds that this t the estimated cloud. However survival of spores during their initial exposure to the solar ultraviolet presents a problem for panspermia because it requires that in the process of ejection from the earth's surface they must be enclosed within a cocoon (or mantle) of ultraviolet absorbing material of 0.6 μm thickness. Thus, although panspermia can not be rejected on the basis of lack of interstellar survival there may remain insurmountable obstacles to its occuring because of the very special protective shield requirements during ejection from its planetary source. 相似文献
The “VIS-channel” (the channel is sensitive between about .4 and 1.1 μm wavelength) of the European geostationary satellite Meteosat-2 is calibrated by the method of “vicarious calibration by means of calculated radiances”. The calibration constant, which connects the 6-bit-counts of the VIS-channel of the Meteosat-2 with the corresponding “effective radiances” is determined to be cSAT = 2.3 W·m?2·sr?1/count with an accuracy of ± 10% (preliminary values). The calibration constant is valid for “gain 0” and the period until October 1981. The result means, that the VIS-channel of Meteosat-2 at the beginning of its lifetime is about 15% more sensitive than that of Meteosat-1 was at its end. 相似文献
The structure of a spherically symmetric stable dark `star' is discussed, at zero temperature, containing 1) a core of quarks in the deconfined phase and antileptons 2) a shell of hadrons in particular n, p, and – and leptons or antileptons and 3) a shell of hydrogen in the superfluid phase. If the superfluid hydrogen phase goes over into the electromagnetic plasma phase at densities well below one atom (10 fm)3, as is usually assumed, the hydrogen shell is insignificant for the mass and the radius of the `star'. These quantities are then determined approximatively: mass = 1.8 solar masses, radius = 9.2 km. 相似文献
The ionospheric HEATING facility near Tromsø, Norway, has recently been used to modulate the ionospheric temperature and thus conductivity at frequencies from 200 to 6000 Hz during times of moderate electrojet current. Analysis of the received VLF signals shows generation down to the lowest frequency (200 Hz). 相似文献
This paper presents the Sixth Catalogue of galactic Wolf-Rayet stars (Pop. I), a short history on the five earlier WR catalogues, improved spectral classification, finding charts, a discussion on related objects, and a review of the current status of Wolf-Rayet star research.The appendix presents a bibliography on most of the Wolf-Rayet literature published since 1867.Visiting astronomer, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory.Visiting astronomer, European Southern Observatory. 相似文献
The role of convection in the core helium flash is simulated by two-dimensional eddies interacting with the thermonuclear runaway. These eddies are followed by the explicit solution of the 2D conservation laws with a 2D finite difference hydrodynamics code. Thus, no phenomenological theory of convection such as the local mixing length theory is required.Our core helium flash is violent, producing a deflagration wave. This differs from the detonation wave (and subsequent disruption of the entire star) produced in previous spherically symmetric violent core helium flashes as the second dimension provides a degree of relief which allows the expansion wave to decouple itself from the burning front. Our results predict that a considerable amount of helium in the core will be burned before the horizontal branch is reached and that some envelope mass loss is likely. 相似文献
Space exploration is an emblematic domain of space activities where traditionally only established space powers have been active. However, new actors are demonstrating great interest in it, principally for international prestige reasons, with an increasing number making ambitious plans. Complementing national endeavours, international cooperation has become a central element of most countries' exploration strategy, since the costs of doing it alone are so great. Europe's development into a fully fledged actor in space exploration requires a shared assessment of the future challenges, threats and opportunities with which it will be confronted in order to derive the best options for cooperation to lead and anticipate rather than follow and endure change. 相似文献
The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) experiment is one of four instruments on NASA’s Thermosphere–Ionosphere–Energetics and Dynamics (TIMED) satellite. SABER measures broadband infrared limb emission and derives vertical profiles of kinetic temperature (Tk) from the lower stratosphere to approximately 120 km, and vertical profiles of carbon dioxide (CO2) volume mixing ratio (vmr) from approximately 70 km to 120 km. In this paper we report on SABER Tk/CO2 data in the mesosphere and lower thermosphere (MLT) region from the version 1.06 dataset. The continuous SABER measurements provide an excellent dataset to understand the evolution and mechanisms responsible for the global two-level structure of the mesopause altitude. SABER MLT Tk comparisons with ground-based sodium lidar and rocket falling sphere Tk measurements are generally in good agreement. However, SABER CO2 data differs significantly from TIME-GCM model simulations. Indirect CO2 validation through SABER-lidar MLT Tk comparisons and SABER-radiation transfer comparisons of nighttime 4.3 μm limb emission suggest the SABER-derived CO2 data is a better representation of the true atmospheric MLT CO2 abundance compared to model simulations of CO2 vmr. 相似文献
Two Precision Solar Photometric Telescopes (PSPT) designed and built at the U.S. National Solar Observatory (NSO) are in operation in Rome and Hawaii. A third PSPT is now in operation the NSO at Sunspot, NM. The PSPT system records full disk solar images at three wavelengths: K line at 393.3 nm and two continua at 409 nm and 607 nm throughout the observing day. We currently study properties of limb darkening, sunspots, and network in these images with particular emphasis on data taken in July and September 1998. During this period, the number of observations per month was high enough to show directional properties of the radiation field surrounding sunspots. We show examples of our PSPT images and describe our study of bright rings around sunspots. 相似文献