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991.
Cometary nuclei consist of ices intermixed with dust grains and are thought to be the least modified solar system bodies remaining
from the time of planetary formation. Flyby missions to Comet P/Halley in 1986 showed that cometary dust is extremely rich
in organics (∼50% by mass). However, this proportion appears to be variable among different comets. In comparison with the
CI-chondritic abundances, the volatile elements H, C, and N are enriched in cometary dust indicating that cometary solid material
is more primitive than CI-chondrites. Relative to dust in dense molecular clouds, bulk cometary dust preserves the abundances
of C and N, but exhibits depletions in O and H. In most cases, the carbonaceous component of cometary particles can be characterized
as a multi-component mixture of carbon phases and organic compounds. Cluster analysis identified a few basic types of compounds,
such as elemental carbon, hydrocarbons, polymers of carbon suboxide and of cyanopolyynes. In smaller amounts, polymers of
formaldehyde, of hydrogen cyanide and various unsaturated nitriles also are present. These compositionally simple types, probably,
are essential "building blocks", which in various combinations give rise to the variety of involatile cometary organics.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
992.
The Martian surface is exposed to both UVC radiation (<280 nm) and higher doses of UVB (280-315 nm) compared to the surface of the Earth. Terrestrial organisms have not evolved to cope with such high levels of UVC and UVB and thus any attempts to introduce organisms to Mars, particularly in closed-loop life support systems that use ambient sunlight, must address this problem. Here we examine the UV radiation environment of Mars with respect to biological systems. Action spectra and UV surface fluxes are used to estimate the UV stress that both DNA and chloroplasts would experience. From this vantage point it is possible to consider appropriate measures to address the problem of the Martian UV environment for future long term human exploration and settlement strategies. Some prospects for improving the UV tolerance of organisms are also discussed. Existing artificial ecosystems such as Biosphere 2 can provide some insights into design strategies pertinent to high UV environments. Some prospects for improving the UV tolerance of organisms are also discussed. The data also have implications for the establishment of closed-loop ecosystems using natural sunlight on the lunar surface and elsewhere in the Solar System. 相似文献
993.
J Z Kiss R E Edelmann 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1999,24(6):755-762
In order to help resolve some of the controversy associated with ground-based research that has supported the starch-statolith theory of gravity perception in plants, we performed spaceflight experiments with Arabidopsis in Biorack during the January 1997 and May 1997 missions of the Space Shuttle. Seedlings of wild-type (WT) Arabidopsis, two reduced-starch strains, and a starchless mutant were grown in microgravity and then were given either a 30, 60, or 90 minute gravity stimulus on a centrifuge. By the 90 min 1-g stimulus, the WT exhibited the greatest magnitude of curvature and the starchless mutant exhibited the smallest curvature while the two reduced starch mutants had an intermediate magnitude of curvature. In addition, space-grown plants had two structural features that distinguished them from the controls: a greater number of root hairs and an anomalous hypocotyl hook structure. However, the morphological changes observed in the flight seedlings are likely to be due to the effects of ethylene present in the spacecraft. (Additional ground-based studies demonstrated that this level of ethylene did not significantly affect gravitropism nor did it affect the relative gravitropic sensitivity among the four strains.) Nevertheless, this experiment on gravitropism was performed the "right way" in that brief gravitational stimuli were provided, and the seedlings were allowed to express the response without further gravity stimuli. Our spaceflight results support previous ground-based studies of these and other mutants since increasing amounts of starch correlated positively with increasing sensitivity to gravity. 相似文献
994.
Thomas W. Becker 《Space Policy》1993,9(1)
The subject of space education is attracting increasing attention, but there are diverging views as to how it should be approached, as can be seen from the following two reports of Education Remote Sensing '92, a conference held in Cardiff, Wales, 28–30 June 1992. 相似文献
995.
C. Done 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):211-219
The X-ray properties of Active Galactic Nuclei (AGN) support recent theories which unify the Seyfert 1 nuclei with the Seyfert 2's and radio-quiet quasars. In these objects the underlying spectrum is strongly distorted by the effects of reflection from the accretion flow and by absorption in partially ionised material. These obscure any intrinsic changes in the spectrum, making it difficult (though not impossible) to constrain the nature of the emission process. Conversely, there is no evidence for either of these spectral distortions in the radio-loud AGN, supporting the hypothesis that the X-rays are dominated by beamed emission from the relativistic jet. 相似文献
996.
T. J. -L. Courvoisier A. Orr P. Bü hler A. Zchnder R. Henneck F. Stauffacher J. Biakhowski N. Schlumpf W. Schoeps A. Mchedlishvili R. Sunyaev V. Arefev A. Yascovich G. Babalyan M. Pavlinsky J. P. Delaboudini re T. Carone O. Siegmund J. Warren D. Leahy N. Salaschenko J. Platonov 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):299-302
EUVITA is a set of 8 extreme UV normal incidence imaging telescopes, each of them sensitive in a narrow band (λ/Δλ = 15 to 80), centered at wavelengths between 50 and 175 Å. Each telescope has an effective area of a few cm2; a field of view of 1.2° and a spatial resolution of 10 arcsec.
EUVITA will be flown on the Russian mission SPECTRUM X-G. This satellite will be launched in a highly eccentric orbit with a period of 4 days, allowing long, uninterrupted observations (e.g. 105 seconds). EUVITA's narrow spectral bands allow the measurement of source parameters such as temperature or power law index as well as interstellar absorption, and will resolve groups of strong lines emitted by optically thin hot plasmas. 相似文献
997.
C. De Jager 《Space Science Reviews》1993,66(1-4):7-19
The paper gives a summary of the situation mid-1993 of theory and observations regarding massive stars. I describe: stellar mass loss and its implications, pre-main-sequence evolution, the main sequence, problems of atmospheric instability, Luminous Blue Supergiants, Yellow Hypergiants, Wolf-Rayet stars and supernovae. 相似文献
998.
999.
A. Claret J. Ballet A. Goldwurm F. Lebrun J. P. Roques L. Bouchet M. C. Schmitz-Fraysse P. Mandrou R. Sunyaev E. Churazov M. Gilfanov N. Khavenson A. Dyachkov B. Novikov R. Kremnev V. Kovtunenko 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):735-738
We report the first detection of two soft gamma ray transients GX 354-0 and Nova Persei (GRO J0422+32) by the coded-mask telescope SIGMA. Only preliminary results are presented with regard to Nova Persei while special emphasis is given to the data on GX 354-0 which has been monitored during the 1992 February–April survey of the Galactic Center. GX 354-0 underwent two flares of about two week duration and its average spectrum is well described by a thermal Bremsstrahlung model or a broken power law whereas the Nova Persei spectrum seems to be similar to the one observed for GRS 1124-684 in Musca. 相似文献
1000.
R. A. Kimble A. F. Davidsen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1993,13(12):33-42
The Hopkins Ultraviolet Telescope (HUT) was flown aboard the space shuttle Columbia as part of the Astro-1 mission during December 1990. During the nine-day flight, HUT carried out 3 Å resolution spectrophotometry of a wide variety of astronomical objects, including a number of stellar targets, in the 912–1860 Å range of the far ultraviolet. A few nearby stars were observed in the 415–912 Å range of the extreme ultraviolet as well. For nearly all of these targets, the spectra obtained by HUT are the first ever obtained in the spectroscopically rich region between Lyman (1216 Å) and the Lyman limit (912 Å). Here, we present highlights of the results obtained by HUT in a variety of areas of stellar astronomy. 相似文献