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71.
C. M. Lisse M. F. A’Hearn T. L. Farnham O. Groussin K. J. Meech U. Fink D. G. Schleicher 《Space Science Reviews》2005,117(1-2):161-192
As comet 9P/Tempel 1 approaches the Sun in 2004–2005, a temporary atmosphere, or “coma,” will form, composed of molecules
and dust expelled from the nucleus as its component icy volatiles sublimate. Driven mainly by water ice sublimation at surface
temperatures T > 200 K, this coma is a gravitationally unbound atmosphere in free adiabatic expansion. Near the nucleus (≤ 102 km), it is in collisional equilibrium, at larger distances (≥104 km) it is in free molecular flow. Ultimately the coma components are swept into the comet’s plasma and dust tails or simply
dissipate into interplanetary space. Clues to the nature of the cometary nucleus are contained in the chemistry and physics
of the coma, as well as with its variability with time, orbital position, and heliocentric distance.
The DI instrument payload includes CCD cameras with broadband filters covering the optical spectrum, allowing for sensitive
measurement of dust in the comet’s coma, and a number of narrowband filters for studying the spatial distribution of several
gas species. DI also carries the first near-infrared spectrometer to a comet flyby since the VEGA mission to Halley in 1986.
This spectrograph will allow detection of gas emission lines from the coma in unprecedented detail. Here we discuss the current
state of understanding of the 9P/Tempel 1 coma, our expectations for the measurements DI will obtain, and the predicted hazards
that the coma presents for the spacecraft.
An erratum to this article is available at . 相似文献
72.
Deep Impact Mission Design 总被引:1,自引:0,他引:1
William H. Blume 《Space Science Reviews》2005,117(1-2):23-42
The Deep Impact mission is designed to provide the first opportunity to probe below the surface of a comet nucleus by a high-speed
impact. This requires finding a suitable comet with launch and encounter conditions that allow a meaningful scientific experiment.
The overall design requires the consideration of many factors ranging from environmental characteristics of the comet (nucleus
size, dust levels, etc.), to launch dates fitting within the NASA Discovery program opportunities, to launch vehicle capability
for a large impactor, to the observational conditions for the two approaching spacecraft and for telescopes on Earth. 相似文献
73.
Since its discovery in 1867, periodic comet 9P/Tempel 1 has been observed at 10 returns to perihelion, including all its returns
since 1967. The observations for the seven apparitions beginning in 1967 have been fit with an orbit that includes only radial
and transverse nongravitational accelerations that model the rocket-like thrusting introduced by the outgassing of the cometary
nucleus. The successful nongravitational acceleration model did not assume any change in the comet’s ability to outgas from
one apparition to the next and the outgassing was assumed to reach a maximum at perihelion. The success of this model over
the 1967–2003 interval suggests that the comet’s spin axis is currently stable. Rough calculations suggest that the collision
of the impactor released by the Deep Impact spacecraft will not provide a noticeable perturbation on the comet’s orbit nor
will any new vent that is opened as a result of the impact provide a noticeable change in the comet’s nongravitational acceleration
history. The observing geometries prior to, and during, the impact will allow extensive Earth based observations to complement
the in situ observations from the impactor and flyby spacecraft. 相似文献
74.
卢焰%贾新朝%陈兆生%姚草根%周奠华 《宇航材料工艺》2007,37(3):65-66
为了提高1Cr21Ni5Ti双相不锈钢的屈服强度,对其进行了铌微合金化,通过化学分析、金相观察、拉伸试验等,对微合金化前后的性能进行了对比研究。结果表明,经过Nb微合金化处理的1Cr21Ni5Ti屈服强度σ0.2显著提高,达到了技术条件的规定。 相似文献
75.
ZHAO Li-xiao LIU Fu-shun XU Hui-bin 《中国航空学报》2006,19(B12):224-227
The microstructures, the phase transformation characteristics, and the mechanical properties of the Ti50Ni47Fe3 alloy in as-forged and as-cross-rolled states were investigated. It is found that, after cross-rolling, the phase transformation temperature (Ms) of the alloy decreases drastically and the grains get refined. Moreover, its yield strength and fracture strength after cross-rolling hit 540 MPa and 687 MPa respectively, up by about 200 MPa over those in as-forged state. 相似文献
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设计了一种计算机实时金相定量分析系统(简称:计算机分析系统),该系统采用先进的图像处理技术实现定量金相测量.利用该系统对Cr12型钢进行了定量金相分析,完成对模具失效形式的分析,并提出了解决模具寿命短的锻造方法.在保证锻件质量前提下,对锻后热处理工艺的主要特性和某些现象的可行性作了探讨性研究.提出了掌握Cr12型钢热加工关键技术是保证模具质量,提高模具寿命的主要途径. 相似文献