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141.
D. Koschny V. Dhiri K. Wirth J. Zender R. Solaz R. Hoofs R. Laureijs T.-M Ho B. Davidsson G. Schwehm 《Space Science Reviews》2007,128(1-4):167-188
ESA’s Rosetta mission was launched in March 2004 and is on its way to comet 67P/Churyumov-Gerasimenko, where it is scheduled
to arrive in summer 2014. It comprises a payload of 12 scientific instruments and a Lander. All instruments are provided by
Principal Investigators, which are responsible for their operations.
As for most ESA science missions, the ground segment of the mission consists of a Mission Operations Centre (MOC) and a Science
Operations Centre (SOC). While the MOC is responsible for all spacecraft-related aspects and the final uplink of all command
timelines to the spacecraft, the scientific operations of the instruments and the collection of the data and ingestion into
the Planetary Science Archive are coordinated by the SOC. This paper focuses on the tasks of the SOC and in particular on
the methodology and constraints to convert the scientific goals of the Rosetta mission to operational timelines. 相似文献
142.
Maciel J.J. Slocum J.F. Smith J.K. Turtle J. 《Aerospace and Electronic Systems Magazine, IEEE》2007,22(11):17-20
Large apertures are of great benefit to applications that are prime powered limited as is found on aerostat and other airborne platforms. Electronically scanned array antennas are often proposed for these applications. However, increasing the aperture area with conventional array technology is met with prohibitive cost, weight, and prime power increases because of the dense spacing of phase shifters and/or active T/R modules. This discusses the recent development of RF MEMS (Microelectromechanical System) switch technology and the use of these switches in a Radanttrade lens configuration for arrays of approximately 10 m2 at X-band. A proof-of-concept 0.4 m2 MEMS Electronically Steerable Antenna (ESA) containing 25,000 MEMS switches has been successfully designed, fabricated, and tested. The 0.4 m2 MEMS ESA was then integrated with an AN/APG-67 radar system to form the MEMS Demonstration Radar System. The MEMS Demonstration Radar System successfully detected both airborne and ground moving targets during a series of extensive radar demonstrations. This is believed to be the first large scale employment of MEMS switches in a scanning antenna and radar system. The low-cost, lightweight, and low power technology demonstrated can enable weight and power constrained platforms with electronic steering. 相似文献
143.
A novel stochastic optimal control guidance law is presented for a missile with bounded acceleration. The nonlinear optimal guidance law (NOGL) is obtained by numerically solving the stochastic optimization problem. Since the certainty equivalence principle is not valid in the investigated problem, the resulting NOGL depends on the conditional probability density function of the estimated states. It is shown that the NOGL is also nonlinear in the estimated zero effort miss distance, and that the probability density function of the miss distance is non-Gaussian. The dependence of the new guidance law on the acceleration limit is investigated and it is shown that only for an extremely large acceleration limit does the proposed guidance law degenerate to the classical optimal linear one 相似文献
144.
Although the elemental composition in all parts of the solar photosphere appears to be the same this is clearly not the case
with the solar upper atmosphere (SUA). Spectroscopic studies show that in the corona elemental composition along solar equatorial
regions is usually different from polar regions; composition in quiet Sun regions is often different from coronal hole and
active region compositions and the transition region composition is frequently different from the coronal composition along
the same line of sight. In the following two issues are discussed. The first involves abundance ratios between the high-FIP
O and Ne and the low-FIP Mg and Fe that are important for meaningful comparisons between photospheric and SUA compositions
and the second involves a review of composition and time variability of SUA plasmas at heights of 1.0≤h≤1.5R
⊙. 相似文献
145.
The characteristics of the insulator wear process in the discharge chamber of a stationary plasma thruster (SPT) are analyzed. The results of studying the erosion resistance to ionic spraying of different ceramic compositions are presented. It is shown experimentally that the integral characteristics (the value of thrust and thrust efficiency) as well as the SPT lifetime can be improved in changing from the conventional hot-pressed boron silicate (BN + SiO2) ceramics to the BN + Si3N4 ceramic compositions. 相似文献
146.
介绍了NIST开展的微波散射参数(S参数)及功率测量业务。包括在这两项业务中的各自的测量内容,系统的校准技术,对系统及所用的标准件的描述和对于不同测量所考虑的测量不确定度。此外还介绍了在S参数测量及功率测量的未来工作方向。 相似文献
147.
Emergence of a Habitable Planet 总被引:2,自引:0,他引:2
Kevin Zahnle Nick Arndt Charles Cockell Alex Halliday Euan Nisbet Franck Selsis Norman H. Sleep 《Space Science Reviews》2007,129(1-3):35-78
We address the first several hundred million years of Earth’s history. The Moon-forming impact left Earth enveloped in a hot
silicate atmosphere that cooled and condensed over ∼1,000 yrs. As it cooled the Earth degassed its volatiles into the atmosphere.
It took another ∼2 Myrs for the magma ocean to freeze at the surface. The cooling rate was determined by atmospheric thermal
blanketing. Tidal heating by the new Moon was a major energy source to the magma ocean. After the mantle solidified geothermal
heat became climatologically insignificant, which allowed the steam atmosphere to condense, and left behind a ∼100 bar, ∼500 K
CO2 atmosphere. Thereafter cooling was governed by how quickly CO2 was removed from the atmosphere. If subduction were efficient this could have taken as little as 10 million years. In this
case the faint young Sun suggests that a lifeless Earth should have been cold and its oceans white with ice. But if carbonate
subduction were inefficient the CO2 would have mostly stayed in the atmosphere, which would have kept the surface near ∼500 K for many tens of millions of years.
Hydrous minerals are harder to subduct than carbonates and there is a good chance that the Hadean mantle was dry. Hadean heat
flow was locally high enough to ensure that any ice cover would have been thin (<5 m) in places. Moreover hundreds or thousands
of asteroid impacts would have been big enough to melt the ice triggering brief impact summers. We suggest that plate tectonics
as it works now was inadequate to handle typical Hadean heat flows of 0.2–0.5 W/m2. In its place we hypothesize a convecting mantle capped by a ∼100 km deep basaltic mush that was relatively permeable to
heat flow. Recycling and distillation of hydrous basalts produced granitic rocks very early, which is consistent with preserved
>4 Ga detrital zircons. If carbonates in oceanic crust subducted as quickly as they formed, Earth could have been habitable
as early as 10–20 Myrs after the Moon-forming impact. 相似文献
148.
D.R. Japaridze M.Sh. Gigolashvili V.J. Kukhianidze 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2007,40(12):1912-1916
The differential rotation of the patterns of the large-scale solar magnetic field during solar activity cycles 20 and 21 is investigated. Compact magnetic elements with the polarity of the general solar magnetic field have larger speed of rotation than the elements with the opposite polarity. The surface of the Sun was divided by 10°-zones. In all of them the average rotation rate of the magnetic elements with negative polarity is little higher than that of the magnetic elements with positive polarity, except for 50°-zone of the south hemisphere and at the 10° latitude of the north hemisphere.
The rates of differential rotation for large-scale magnetic elements with negative and positive polarities have similar behavior for both cycles of the solar activity.
The rotation rate varies at polarity reversal of the circumpolar magnetic fields. For the cycle No 20 in 1969–1970 the threefold reversal took place in the northern hemisphere and variations of rotation rate can be noticed for magnetic elements both with positive and negative polarity for each 10°-zone in the same hemisphere. 相似文献
149.
Martin A. Lee 《Space Science Reviews》2007,130(1-4):221-229
Gradual solar energetic particle (SEP) events are evidently accelerated by coronal/interplanetary shocks driven by coronal
mass ejections. This talk addresses the different factors which determine the composition of the accelerated ions. The first
factor is the set of available seed populations including the solar wind core and suprathermal tail, remnant impulsive events
from preceding solar flares, and remnant gradual events. The second factor is the fractionation of the seed ions by the injection
process, that is, what fraction of the ions are extracted by the shock to participate in diffusive shock acceleration. Injection
is a controversial topic since it depends on the detailed electromagnetic structure of the shock transition and the transport
of ions in these structured fields, both of which are not well understood or determined theoretically. The third factor is
fractionation during the acceleration process, due to the dependence of ion transport in the turbulent electromagnetic fields
adjacent to the shock on the mass/charge ratio. Of crucial importance in the last two factors is the magnetic obliquity of
the shock. The form of the proton-excited hydromagnetic wave spectrum is also important. Finally, more subtle effects on ion
composition arise from the superposition of ion contributions over the time history of the shock along the observer’s magnetic
flux tube, and the sequence of flux tubes sampled by the observer. 相似文献
150.
F. Capitanio A. Bazzano P. Ubertini G. De Cesare M. Del Santo A. Tarana A. Joinet 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2816-2819
On March 2003, IBIS, the γ-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be an HEAO-1 transient, namely H1743-322. The spectral and temporal evolutions of the source were observed by INTEGRAL in different periods. Also RXTE observed the source for the first time on 2003 March 29 during a PCA Galactic bulge scan. The source flux decayed below the RXTE PCA sensitivity limit in November 2003, then in April 2004 it was again detected by INTEGRAL. On July 3, 2004 the source was again detected by RXTE/PCA at a 2–10 keV intensity of 16 mCrab and on July 7, reached 69 mCrab. Recently, a new outburst was observed on August 2005. We briefly summarise here the behaviour of the source observed by INTEGRAL from March 2003 to August 2005. The new outbursts of the source and the analysis of all the data collected (now public) give a global view of the spectral and time behaviour of this X-ray transient. 相似文献