Numerical simulation of the LCROSS impact experiment

This study presents the results of the numerical modeling of the Lunar Crater Observation and Sensing Satellite (LCROSS) space experiment, which is scheduled for 2009 by NASA. It is demonstrated that a spacecraft with a mass of 2 tons impacting the Moon at a velocity of 2.5 km/s creates an ejecta plume with a size of more than 100 km and a mass exceeding 100 tons. The detailed characteristics of the ejecta are given and their relation to the impactor structure is investigated.     

Scouring the surface: Ejecta dynamics and the LCROSS impact event

The Lunar CRater Observation and Sensing Satellite mission (LCROSS) impacted the moon in a permanently shadowed region of Cabeus crater on October 9th 2009, excavating material rich in water ice and volatiles. The thermal and spatial evolution of LCROSS ejecta is essential to interpretation of regolith properties and sources of released volatiles. The unique conditions of the impact, however, made analysis of the data based on canonical ejecta models impossible. Here we present the results of a series of impact experiments performed at the NASA Ames Vertical Gun Range designed to explore the LCROSS event using both high-speed cameras and LCROSS flight backup instruments. The LCROSS impact created a two-component ejecta plume: the usual inverted lampshade “low-angle” curtain, and a high speed, high-angle component. These separate components excavated to different depths in the regolith. Extrapolations from experiments match the visible data and the light curves in the spectrometers. The hollow geometry of the Centaur led to the formation of the high-angle plume, as was evident in the LCROSS visible and infrared measurements of the ejecta. Subsequent ballistic return of the sunlight-warmed ejecta curtain could scour the surface out to many crater radii, possibly liberating loosely bonded surface volatiles (e.g., H₂). Thermal imaging reveals a complex, heterogeneous distribution of heated material after crater formation that is present but unresolved in LCROSS data. This material could potentially serve as an additional source of energy for volatile release.     

A ground-based observation of the LCROSS impact events using the Subaru Telescope

The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was an impact exploration searching for a volatile deposit in a permanently shadowed region (PSR) by excavating near-surface material. We conducted infrared spectral and imaging observations of the LCROSS impacts from 15 min before the first collision through 2 min after the second collision using the Subaru Telescope in order to measure ejecta dust and water. Such a ground-based observation is important because the viewing geometry and wavelength coverage are very different from the LCROSS spacecraft. We used the Echelle spectrograph with spectral resolution λ/Δλ ∼ 10,000 to observe the non-resonant H₂O rotational emission lines near 2.9 μm and the slit viewer with a K′ filter for imaging observation of ejecta plumes. Pre-impact calculations using a homogeneous projectile predicted that 2000 kg of ejecta and 10 kg of H₂O were excavated and thrown into the analyzed area immediately above the slit within the field of view (FOV) of the K′ imager and the FOV of spectrometer slit, respectively. However, no unambiguous emission line of H₂O or dust was detected. The estimated upper limits of the amount of dust and H₂O from the main Centaur impact were 800 kg and 40 kg for the 3σ of noise in the analyzed area within the imager FOV and in the slit FOV, respectively. If we take 1σ as detection limit, the upper limits are 300 kg and 14 kg, respectively. Although the upper limit for water mass is comparable to a prediction by a standard theoretical prediction, that for dust mass is significantly smaller than that predicted by a standard impact theory. This discrepancy in ejecta dust mass between a theoretical prediction and our observation result suggests that the cratering process induced by the LCROSS impacts may have been substantially different from the standard cratering theory, possibly because of its hollow projectile structure.     

How Good are the Predictions for Oscillation Frequencies

We have used available intermediate degree p-mode frequencies for solar cycle 23 to check the validity of previously derived empirical relations for frequency shifts (Jain et al., 2000). We find that the calculated and observed frequency shifts during the rising phase of cycle 23 are in good agreement. The observed frequency shift from minimum to maximum of this cycle as calculated from MDI frequency data sets is 251±7 nHz and from GONG data is 238±11 nHz. These values are in close agreement with the empirically predicted value of 271±22 nHz.     

The Soft X-ray Telescope for the SOLAR-A mission

The Soft X-ray Telescope (SXT) of the SOLAR-A mission is designed to produce X-ray movies of flares with excellent angular and time resolution as well as full-disk X-ray images for general studies. A selection of thin metal filters provide a measure of temperature discrimination and aid in obtaining the wide dynamic range required for solar observing. The co-aligned SXT aspect telescope will yield optical images for aspect reference, white-light flare and sunspot studies, and, possibly, helioseismology. This paper describes the capabilities and characteristics of the SXT for scientific observing.After the launch the name of SOLAR-A has been changed to YOHKOH.     

Predictions for the Aurigid Outburst of 2007 September 1

The September 2007 encounter of Earth with the 1-revolution dust trail of comet C/1911 N1 (Kiess) is the most highly anticipated dust trail crossing of a known long period comet in the next 50 years. The encounter was modeled to predict the expected peak time, duration, and peak rate of the resulting outburst of Aurigid shower meteors. The Aurigids will radiate with a speed of 67 km/s from a radiant at R.A. = 92°, Decl. = +39° (J2000) in the constellation Auriga. The expected peak time is 11:36 ± 20 min UT, 2007 September 1, and the shower is expected to peak at Zenith Hourly Rate = 200/h during a 10-min interval, being above half this value during 25 min. The meteor outburst will be visible by the naked eye from locations in Mexico, the Western provinces of Canada, and the Western United States, including Hawaii and Alaska. A concerted observing campaign is being organized. Added in proof: first impression of the shower. Prepared as a contribution to the conference proceedings of “Meteoroids 2007”, to be published in the journal “Earth, Moon, and Planets”.     

Predictions for eclipses of Nereid by Neptune

Neptune will eclipse its satellite Nereid (Neptune II) on 2007 April 27 from 00 to 06 h UT and on 2008 April 21 from 12 to 17 h UT, with uncertainties of about 3 h; and a third eclipse may occur on 2009 April 17. These events offer unique opportunities for astrometric and geophysical measurement.     

A DUAL mission for nuclear astrophysics

DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an improvement in sensitivity over previous technologies by at least an order of magnitude. The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic (LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is situated on a deployable structure at a distance of 30?m from the satellite. This configuration not only permits to maintain the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and, above all it provides ASCI with a continuous all-sky exposure.     

Orbit injection error analysis for the Lageos III mission

An analysis is presented of the orbital injection errors for the Lageos III satellite mission. Several methods are introduced for the solution of the Inverse Problem in the Theory of Errors. The novelty of the present approach consists in the use of the full geopotential covariance matrix in the error propagation equations. The GEM-T1 covariance matrix is used. It is found that by properly accounting for the correlation among the even zonal harmonic coefficients the acceptable error bounds increase by an order of magnitude with respect to the case when only the variances are used. The most stringent constraint, even when using the full covariance, is on inclination, whose nominal value must be realized within approximately 0.1° for the recovery of the Lense-Thirring precession to be successful at the 3% level (accounting only for injection errors). The associated tolerance in the semimajor axis is about 30 km while that in eccentricity is approximately 0.2. However, if the errors in semimajor axis and eccentricity can be kept to the routinely achievable levels respectively of 10 km and 0.004, then the tolerance in inclination can be relaxed to 0.2°.     

EIT: Extreme-ultraviolet Imaging Telescope for the SOHO mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 ), Fe XII (195 ), Fe XV (284 ), and He II (304 ) to provide sensitive temperature diagnostics in the range from 6 × 10⁴ K to 3 × 10⁶ K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.     

Searching for exosolar planets with the COROT space mission

The space mission COROT (to be launched in 2002) will continuously monitor the flux of a number of stars during 150 days periods with a very high photometric accuracy. One of its objectives is the detection of extra-solar planets by looking for their transits in front of the disk of several tens thousand stars. COROT accommodates a 25 cm telescope with low straylight, and 4 2048×2048 CCDs, 2 of which monitoring 5000 to 12000 stars simultaneously up to m_v= 16.5. The stability and noise performances should make easy the detection of Jupiter-like planets and possible the detection of Earth-like planets with radius 1.5R_⊕. Under study is a dispersive system that will allow to retrieve some chromatic information : this could be essential to discriminate actual transit events against stellar fluctuations that would mimic a transit and to identify properly the events occurring in binary stars. The mission, the instrument and the results of simulations are presented, together with a discussion on the number of expected events : the multiple ones with a short orbital period signature, or the single or double events identified by their (a)chromatic signature. The later ones may lead to the discovery of planets in the habitable zone.     

FRB Event Rate Predictions for the Ooty Wide Field Array

We developed a generic formalism to estimate the event rate and the redshift distribution of Fast Radio Bursts (FRBs) in our previous publication (Bera et al. 2016), considering FRBs are of an extragalactic origin. In this paper, we present (a) the predicted pulse widths of FRBs by considering two different scattering models, (b) the minimum total energy required to detect events, (c) the redshift distribution and (d) the detection rates of FRBs for the Ooty Wide Field Array (OWFA). The energy spectrum of FRBs is modelled as a power law with an exponent ?α and our analysis spans a range ?3≤α≤5. We find that OWFA will be capable of detecting FRBs with α≥0. The redshift distribution and the event rates of FRBs are estimated by assuming two different energy distribution functions; a Delta function and a Schechter luminosity function with an exponent ?2≤γ≤2. We consider an empirical scattering model based on pulsar observations (model I) as well as a theoretical model (model II) expected for the intergalactic medium. The redshift distributions peak at a particular redshift z _p for a fixed value of α, which lie in the range 0.3≤z _p ≤1 for the scattering model I and remain flat and extend up to high redshifts (z?5) for the scattering model II.     

Plasma diagnostics and simulation for the SMART-1 mission

SMART-1 is a technology demonstrator for using primary electric propulsion on interplanetary spacecraft. Hence, studying of the interaction of the plasma emitted by the thruster with the environment and the spacecraft is one of the top priorities during the mission. Two experiments (Electronic propulsion diagnostic package and Spacecraft potential, electron and dust experiment) are available to measure the electron densities and temperatures as well as wave electric fields during the operation of the electric propulsion thruster. Additionally, a retarding potential analyser, a quartz microbalance and a solar-cell sample will analyse data from slow charge-exchange ions which are a potential contamination source. ESTEC is developing a 3D particle-in-cell model in order to study the spacecraft/environment interactions on SMART-1 and interpret the measurements. In the present paper, we will review the contamination effects associated with electric propulsion and how the plasma sensors cover them. We further present preliminary results from the numerical simulation and show how the flight data will be used to validate the modelling code. A successful validation of the simulation will support future interplanetary and commercial missions featuring electric propulsion to reduce the risk of contamination and interference with on board instruments.     

利用IGS星历预报GPS卫星轨道

在动力学轨道拟合以及轨道积分的基础上,提出了基于IGS精密星历的GPS卫星轨道预报方法。该方法首先利用已知的IGS精密星历作为虚拟观测值,采用动力学方法拟合出GPS卫星的初始轨道和动力学参数,然后再通过积分来预报GPS卫星的轨道。主要讨论了基于不同弧段的IGS星历时,该方法对GPS卫星轨道的拟合和预报情况。研究结果显示:对于6 d弧段以内的IGS精密星历,其拟合轨道与IGS精密星历差值的三维RMS值均优于4 cm,随着拟合弧段的增加,拟合残差变大;当利用2～6 d弧段的IGS星历来预报GPS轨道时,大部分卫星第1天、第7天和第30天的三维预报精度可优于0.1 m、3 m和100 m。其中,2d弧段的IGS星历对GPS卫星第1天和第7天的预报结果最好,5 d弧段的IGS星历对GPS卫星第30天的预报结果最好。     

Planetary protection principles used for Phobos-Grunt mission

The article presents an analysis of the Phobos-Grunt mission, a classification of its phases in terms of planetary protection, and the main principles of activities management and definition of actions for fulfilling the planetary-protection requirements developed by Committee on Space Research.     

Mineralogical characterization of potential targets for the ASTEX mission scenario

In-situ investigation of asteroids is the next logical step in understanding their exact surface mineralogy, petrology, elemental abundances, particle size distribution, internal structure, and collisional evolution. Near-Earth asteroids (NEAs) provide us with ample opportunities for in-situ scientific exploration with lower Δv requirements and subsequently lower costs than their main belt counterparts. The ASTEX mission concept aims at surface characterization of two compositionally diverse NEAs, one with primitive and the other with a strong thermally evolved surface mineralogy. Here we present the first results of our ground-based characterization of potential ASTEX mission targets using the SpeX instrument on the NASA IRTF. Of the four potential targets we characterized, two (1991 JW and 1998 PA) have compositions similar to ordinary chondrite mineralogy. The other two targets (1994 CC and 1999 TA10) are thermally evolved objects with igneous formation histories. While 1994 CC is a triplet system and thus very challenging to orbit the V-type NEA, 1999 TA10 is the most interesting scientific ASTEX target identified so far.     

A graph based methodology for mission design

A space mission design methodology is presented, where initial and final orbits are connected through segments of periodic orbits. After a discretization of the solution space, the problem of mission design is transformed into an equivalent combinatorial optimization problem. Specifically, a graph is constructed that represents periodic orbits connected by the execution of impulsive maneuvers. A low computational complexity algorithm for this transformation is introduced. An efficient combinatorial optimization algorithm that solves the shortest path problem is described. Subject to the initial discretization of the solution space, an optimal sequence of coastal arcs is determined for a low total Delta-V mission. Finally, the proposed methodology is applied to the design of a hypothetical Saturn?CTitan system mission.