The extreme ultraviolet explorer

The Extreme Ultraviolet Explorer (EUVE) Mission is described. The purpose of this mission is to search the celestial sphere for astronomical sources of extreme ultraviolet (EUV) radiation (100–1000Å). The search will be accomplished with the use of three EUV telescopes, each sensitive to different bands within the EUV band. A fourth telescope will perform a high sensitivity search of a limited sample of the sky in a single EUV band. In six months, the entire sky will be scanned at a sensitivity level comparable to existing surveys in other more traditional astronomical bandpasses. A substantial number of EUV sources such as hot white dwarfs and stellar coronae are certain to be discovered given our current knowledge. More uncertain is what entirely new classes of objects will be discovered as EUV sources. A moderate resolution (～ 5Å) spectroscopy option is being considered which would cover the band from 80 to 600Å. This instrument would be capable of providing spectra of at least the 100 brightest EUV sources and would be utilized entirely on a Guest Investigator basis.     

Initial results from the extreme ultraviolet explorer

Data obtained during the first five months of calibration and science operation of the Extreme Ultraviolet Explorer (EUVE) are presented. Spectra of an extragalactic object were obtained; the object is detectable to wavelengths longer than 100 Å, demonstrating that extragalactic EUV astronomy is possible. Spectra of a hot white dwarf, and a late-type star in quiescence and flaring are shown as examples of the type of spectrographic data obtainable with EUVE. Other objects for which broad band photometric mode data have been obtained and analyzed include an RS CVn star and several late-type stars. The backgrounds in the EUVE detectors are quite low and the character of the diffuse astronomical EUV background has been investigated using these very low rates. Evidence is presented showing that, contrary to previously published reports, EUVE is about three times more sensitive than the English Wide Field Camera in the short wavelength bandpass covered by both instruments. Only limited information has been extracted from the longer bandpasses covered only by EUVE. Nonetheless, the brightest EUV source in the sky, a B star, has been discovered and is detected only in these longer bandpasses.     

The extreme ultraviolet explorer and the local interstellar medium

The Extreme Ultraviolet Explorer (EUVE) mission is described with emphasis on the overall capabilities of the instrumentation and the relevance of the mission to determining parameters of the local interstellar medium. The primary purpose of the mission is to search the celestial sphere for astronomical sources of extreme ultraviolet (EUV) radiation (100–1000Å). The search will be accomplished with the use of three EUV telescopes, sensitive to different segments of the EUV band. A fourth telescope will perform a high sensitivity search of a limited sample of the sky in the shortest wavelength bands. The all-sky survey will be carried out in the first six months of the mission. The second six months of the mission, conducted entirely by Guest Investigators selected by NASA, will be devoted to spectroscopic observations of selected EUV sources. The instrumentation is now well developed. The mirrors meet the requirements of the mission with the best mirror having a full-width half energy spread of 8 arc seconds and a surface finish of 20Å. Prototype thin film bandpass filters have been flown on the Space Shuttle and their performance optimized. Prototype detectors have been developed which have 600 by 600 pixel imaging capability and up to 80% quantum efficiency in parts of the EUV band. A newly invented, high efficiency, grazing incidence spectrometer using variable line space gratings is under development; prototype gratings have been fabricated which provide 75% of theoretical efficiency. An end-to-end (star to as-received flight data) model of the mission has been constructed. Hypothetical, but realistic, flux data from continuous astronomical sources absorbed by intervening material have been processed through this model and show the ability of the EUVE instrumentation to measure such parameters as the hydrogen to helium ratio of the local interstellar medium.     

Two years of INTEGRAL observation of the BHC IGR J17464-3213

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.     

Rocket observation of the ultraviolet spectrum of comet Halley

An ultraviolet sounding rocket telescope/spectrograph experiment observed Comet Halley on 26 February 1986, 17 days after perihelion. From the long-slit spectra, the production rates of O, C, and CO are calculated. The derived water production rate is a lower limit of 5.0 × 10²⁹ s⁻¹ and the volume mixing ratio of CO to H₂O is 21%. The predicted brightness distribution from a radial outflow model with H₂O and CO as parent molecules are in accordance with the measured spatial profiles of OI and CO emissions. The ratio of the production rates of CO to C is 2.7 which is consistent with the carbon source being the photodissociation of CO. However, the radial outflow model which best fits the CO data predicts significantly weaker CI emissions than was observed. A better fit to the carbon data is found when an inner coma source of C at a rate of 3% of the water production rate is included in the model.     

Two years of INTEGRAL result on X-ray binaries

INTEGRAL is the ESA lead International Gamma-Ray Astrophysical Laboratory, successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two γ-ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to obtain deep observations of the Galactic Centre (GCDE) and to exploit regular scan of the whole Galaxy Plane since the beginning of the mission. This paper will briefly review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS γ-ray Imager onboard INTEGRAL, and make a comparison with the previous scenario depicted by the BeppoSAX and RXTE results.     

Neutral winds in the polar thermosphere as measured from dynamics explorer

Remote sensing measurements of the meridional thermospheric neutral wind using the Fabry-Perot Interferometer on Dynamics Explorer have been combined within-situ measurements of the zonal component using the Wind and Temperature Spectrometer on the same spacecraft. The two data sets with appropriate spatial phasing and averaging determine the vector wind along the track of the polar orbiting spacecraft. A study of fifty-eight passes over the Southern (sunlit) pole has enabled the average Universal Time dependence of the wind field to be determined for essentially a single solar local time cut. The results show the presence of a “back-ground” wind field driven by solar EUV heating upon which is superposed a circulating wind field driven by high latitude momentum and energy sources.     

行星三体引力摄动对卫星探测器大气制动的影响

考虑行星引力在其卫星探测器大气制动过程中的显著摄动,建立了基于Milankovitch参数的平均轨道动力学模型并对土卫六探测器进行仿真。首先,将轨道参数转换为无奇异的Milankovitch参数,考虑探测卫星的大气阻力、扁率摄动以及行星引力摄动,建立了半解析轨道方程。其次,以土卫六探测器为对象,选择不同的土星初始方位角进行有大气和无大气情况下的数值仿真,并进行比较分析。结果表明,土星初始方位角的选择会引起土卫六大气制动轨道偏心率和近拱点高度在不同范围内震荡,极大地影响大气制动效果。      

Images of the Earth's aurora and geocorona from the dynamics explorer mission

Several results from analyses of auroral and geocoronal images from the Dynamics Explorer Mission are summarized. (1) The motion of the transpolar arc of a theta aurora is found to be correlated with the y-component of the interplanetary magnetic field. The arc motion is in the general direction of the y-component. (2) A sequence of global images of a small auroral substorm shows the initial development of intense luminosities in a relatively small spatial region, or ‘bright spot’, in the pre-midnight sector of the auroral oval and a subsequent appearance of an expanding area of lesser intensities at lower latitudes and contiguous to the midnight boundary of the bright spot. This evolution of auroral luminosities is interpreted in terms of acceleration of electrons in the boundary layer of the magnetotail plasma sheet to produce the bright spot and subsequent injection into, and eastward drift within, the plasma sheet to form the diffuse area of lesser intensities. (3) A series of images of the Earth's geocorona in scattered solar Ly α emissions is used to obtain a best-fit spherical model of atomic hydrogen densities in the Earth's exosphere. A Chamberlain model provides an adequate fit to radial distances of 4.5 R_E, beyond which an exponential fit is used. The geocoronal tail is detected as an asymmetric increase in scattered Ly α intensities in the anti-solar direction.     

Observation of the lunar topography by the laser altimeter LALT on board Japanese lunar explorer SELENE

The SELENE Laser Altimeter (LALT) is designed to map the Moon’s topography and will be launched in summer 2007. LALT incorporates Q-switched Cr doped Nd:YAG laser (1064 nm) with an output energy of 100 mJ and 1 Hz repetition frequency for about one year mission period. The laser pulse travels to the Moon’s surface and reflections from the surface are detected by a silicon avalanche photo-diode. The ranging distance is 50–150 km with about 5 m accuracy. Several corrections for accurate ranging data are investigated. The flight hardware has been qualified and passed all the integration tests. A principal goal of the LALT instrument is to obtain a much more detailed lunar topographic map which is superior in global coverage, measurement accuracy and number of data points to previous observations and models. The overall science objectives of LALT are (1) determination of lunar global figure, (2) internal structure and surface processes, (3) exploration of the lunar pole regions, and (4) reduction of lunar occultation data.     

探测器触地关机软着陆稳定性分析

深空探测器软着陆过程中发动机未能正常关闭将极大降低其软着陆稳定性,因此引入了触地关机着陆方案。针对触地关机方案,建立了腿式探测器动力学分析模型及发动机推力控制模型。考虑着陆环境与探测器着陆状态的不确定性,采用蒙特卡罗模拟分别对探测器在主发动机关闭失败时、触地关机着陆方案下、带有姿态控制的触地关机着陆模式下的软着陆稳定性进行了分析。采用描述性采样方法抽取样本点,基于均值估计相对误差建立模拟终止准则,在保证模拟精度的前提下提高蒙特卡罗模拟效率。计算并对比了3种着陆方式下探测器软着陆可靠度,触地关机方案可在主发动机关闭失败后将探测器稳定着陆可靠度提升11.6%,姿态控制的引入可进一步将安全着陆的可靠度提升9.7%。      

Gamma-ray burst localization with the international solar polar mission

The European Space Agency's Solar Polar spacecraft is scheduled for launch in 1986. A solar X-ray and cosmic gamma ray burst detector will be aboard. Although the solar polar mission will not provide the long baselines originally planned, due to the cancellation of the NASA spacecraft, it is shown that arrival time analysis between the remaining ESA spacecraft and other missions will nevertheless achieve extremely precise localizations.     

Wide field ultraviolet observations of Comet Halley with the FAUST Spacelab I instrument

The wide field (7.5°), arc minute imaging, and spectroscopic capabilities of the Far Ultraviolet FAUST telescope which will be flown on Spacelab I can provide valuable information on Comet Halley. The use of the FAUST instrument in obtaining images of the hydrogen coma at 1216 Å, and in obtaining objective grating spectroscopy from 1300–3300 Å of the comet and tail, are described. The FAUST images would provide large field of view data that are required for model calculations of gas production rates and the determination of scale lengths and lifetimes of ion species.     

Spectroscopy of the extreme ultraviolet background radiation

Observations in the far ultraviolet and soft x-ray bands suggest that the interstellar medium contains several components of high temperature gas and should be emitting in the extreme ultraviolet. Indeed diffuse radiation has been detected in the extreme ultraviolet with photometric instruments, but no spectral measurements exist below 520Å. We have designed a unique grazing incidence spectrometer to study the diffuse emission between 80 and 650Å with 10 to 20Å resolution. The instrument was launched on a Black Brant sounding rocket from White Sands Missile Range on April 22, 1986. Our preliminary analysis shows the expected geocoronal and interplanetary HeI 584Å emission, and possibly other features which may originate in the hot ionized interstellar gas. Flux limits to these possible emission lines are compatible with previous broad band measurements.     

Observations of ultraviolet spectra of H II regions and galaxies with IUE

The ultraviolet spectra, obtained with the International Ultraviolet Explorer, of a sample of H II regions and the nuclear regions of spiral and elliptical galaxies are described. The star formation rates in the nuclei of spiral galaxies are similar to the star formation rate in the solar neighbourhood. The data indicate that the current thinking on the synthesis of carbon and nitrogen in galaxies has to be revised and the K-corrections determined from the ultraviolet spectra of galaxies when compared with the photometry of distant galaxies suggests colour evolution of galaxies at z > 0.3.     

Variations of the ground-level ultraviolet radiation in East Siberia

Daily measurements of the ground-level ultraviolet radiation (UVR) for the years 1998–2000 were used to investigate the seasonal UVR variation in the East Siberian region. The study revealed the asymmetry of the seasonal UVR variation with respect to the angular altitudes of the Sun, caused by the seasonal variation of the total ozone content (TOC), and possibly by the presence of the snow cover. Characteristics of the occurrence of short-lasting increases in UVR are considered, which are associated with TOC anomalies and with high atmospheric transparency. Fluctuations of the ground-level UVR with periods of 2.8, 6–13 and 26 days are identified. A study is made of the dependence of the ground-level UVR on solar activity. Statistically significant correlation coefficients with solar activity indices are obtained.