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1.
During a failed Eros orbit insertion maneuver on 20 December 1998, more than 28 kg of hydrazine were expended by attitude control jets on the NEAR Shoemaker spacecraft. Deposition of burn products on the outer optic of the multispectral imager, or MSI, resulted in a wavelength-dependent degradation of the system point-spread function (PSF). The scattered light is progressively worse in the shortest and longest wavelength filters, especially at 450 and 1050 nm. The degraded PSF was characterized using numerous images of Canopus acquired subsequent to the anomaly. There is no evidence for temporal change in the PSF since the burn abort incident. A fast Fourier transform-based image restoration method using the optimal filter recovers most of the spatial resolution of the original images while preserving radiometric accuracy for the 550- to 1000-nm images. This procedure has enabled nearly unimpeded monochrome imaging of asteroid morphology and select 5-color measurements at a scale of ∼5 pixels.  相似文献   

2.
High-resolution imaging acquired with the Near Earth Asteroid Rendezvous Shoemaker (NEAR Shoemaker) spacecraft is used to elucidate the spectral properties and spatial distribution of color units on Asteroid 433 Eros. Previous workers mapped four distinct types of color units on the surface (bright streaks, dark soils, ponded materials, average regolith). These units exhibit albedo and color boundaries but there is no evidence to indicate they represent distinct rock types. Rather the units are thought to show evidence of complex regolith transport and sorting processes. Here we report the results of a comprehensive study of all viable color MultiSpectral Imager (MSI) data to identify and characterize the distribution and nature of color units across the whole asteroid. Due to a spacecraft upset that resulted in contamination of the MSI optics, color images are affected with a scattered light problem that hampers interpretation of subtle color contrasts, even after a rigorous remediation. To constrain interpretations of the MSI color data we characterize this residual scattered light and demonstrate how complete correction would affect derived color ratios. Results of our comprehensive study are consistent with previous mapping—confirming that bright streaks, average regolith and dark soils fall on a mixing line, consistent with space weathering effects. We find that the ponded deposits do not fall on this putative mixing line. The color and reflectance of the ponded deposits are consistent with some combination of compositional, grain size and maturity variations from the average regolith. Additionally we show that spectral separation of the four units on ratio plots would only increase with full removal of residual scattered light, especially for features that are small in terms of pixels. Global analysis of the Eros color units illustrates complex regolith processes and grain sorting that may hold clues to understanding space weathering processes and the link between asteroids and meteorites.  相似文献   

3.
As part of the NEAR Radio Science investigation, a global solution that includes both spherical and ellipsoidal harmonic gravity fields of Eros, Eros pole and rotation rate, Eros ephemeris, and landmark positions from the optical data was generated. This solution uses the entire one-year in orbit collection of X-band radiometric tracking (Doppler and range) from the Deep Space Network and landmark tracking observations generated from the NEAR spacecraft images of Eros. When compared to a constant density shape model, the gravity field shows a nearly homogeneous Eros. The Eros landmark solutions are in good agreement with the Eros shape model, and they reduce the center-of-mass and center-of-figure offset in the z direction to 13 m. Most of the NEAR spacecraft orbits are determined in all directions to an accuracy of several meters. The solution for the ephemeris of Eros constrains the mass of Vesta to 18.2±0.4 km3/s2 and reduces the uncertainty in the Earth-Moon mass ratio.  相似文献   

4.
Abstract— From April 24 to May 14, 2000, the Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission's near infrared spectrometer (NIS) obtained its highest resolution data of 433 Eros. High signal‐to‐noise ratio NIS reflectance spectra cover a wavelength range of 800–2400 nm, with footprint sizes from 213 times 427 m to 394 times 788 m. This paper describes improvement in instrument calibration by remediation of internally scattered light; derivation of a “pseudo channel” for NIS at 754 nm using Multispectral Imager (MSI) Eros approach maps at 951 and 754 nm; synthesis of a 3127‐spectrum high‐resolution data set with the improved calibration and expanded wavelength coverage; and investigation of global and localized spectral variation with respect to mineralogy, composition, and space weathering of Eros, comparing the findings with previous analyses. Scattered light removal reduces the “red” slope of Eros spectra, though not to the level seen by telescopic observations. The pseudo channel completes sampling of Eros' 1 micron (Band I) absorption feature, enabling direct comparison of NIS data with other asteroid and meteorite spectra without additional scaling or correction. Following scattered light removal and wavelength range extension, the spectral parameters of average Eros plot well inside the S(IV) field of Gaffey et al. (1993) and are consistent with the L6 chondrite meteorite fields of Gaffey and Gilbert (1998). Although Eros shows no evidence of mineralogical heterogeneity, modest spectral variations correlate with morphologically and geographically distinct areas of the asteroid. Eros bright‐to‐dark spectral ratios are largely consistent with laboratory “space weathering” experiment results and modeling of space weathering effects. Eros brightness variation unaccompanied by significant spectral variation departs from “lunar‐type”—where band depths, slopes, and albedoes all correlate—and “Ida‐type”—where significant spectral variation is unaccompanied by corresponding brightness variation. The brightest areas on Eros—steep crater walls—have lesser spectral slope and deeper Band I, consistent with exposure of “fresher,” less space weathered materials. Bright crater slope materials have opx/(opx + olv) of 0.24–0.29 and may be more representative of the subsurface mineralogy than “average” Eros, which is probably affected by space weathering. The floors of the large craters Psyche and Himeros have lower albedo and contain the most degraded or altered looking materials. NIS spectra retain a “red” spectral slope at greater than 2 microns. The recalibrated and expanded NIS spectra show better agreements with mixing models based on space weathering of chondritic mixtures.  相似文献   

5.
The NASA Solar Dynamics Observatory (SDO), scheduled for launch in early 2010, incorporates a suite of instruments including the Extreme Ultraviolet Variability Experiment (EVE). EVE has multiple instruments including the Multiple Extreme ultraviolet Grating Spectrographs (MEGS) A, B, and P instruments, the Solar Aspect Monitor (SAM), and the Extreme ultraviolet SpectroPhotometer (ESP). The radiometric calibration of EVE, necessary to convert the instrument counts to physical units, was performed at the National Institute of Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF III) located in Gaithersburg, Maryland. This paper presents the results and derived accuracy of this radiometric calibration for the MEGS A, B, P, and SAM instruments, while the calibration of the ESP instrument is addressed by Didkovsky et?al. (Solar Phys., 2010, doi: 10.1007/s11207-009-9485-8 ). In addition, solar measurements that were taken on 14 April 2008, during the NASA 36.240 sounding-rocket flight, are shown for the prototype EVE instruments.  相似文献   

6.
From February 13 to May 13, 2000, the near-infrared spectrometer (NIS) instrument on the Near Earth Asteroid Rendezvous (NEAR) spacecraft obtained more than 200,000 spatially resolved 800- to 2500-nm reflectance spectra of the S-type asteroid 433 Eros. An important subset of the spectra was obtained during a unique opportunity on February 13 and 14, when the NEAR spacecraft flew directly through the 0° phase angle point between Eros and the Sun just prior to the orbital insertion maneuver. This low phase flyby (LPF) dataset consists of ∼2000 spectra of the northern hemisphere of Eros, obtained from 1° to 47° phase angle and at spatial resolutions of between 6×12 km to 1.25×2.50 km per spectrum. The spectra were calibrated to radiance factor (I/F, where I=observed radiance and πF=solar input radiance) and then photometrically corrected to normal albedo. The average northern hemisphere spectrum of Eros is similar to the asteroid's unresolved telescopic spectrum and exhibits absorption features near 1000 nm (Band I) and 2000 nm (Band II) consistent with an orthopyroxene to orthopyroxene+olivine (opx+ol) mixing ratio of approximately 0.38±0.08. The ensemble of NIS LPF spectra falls primarily within the S(IV) to upper S(III) fields of the Gaffey et al. (1993) S-asteroid classification scheme and exhibits Band I and Band II properties similar to those of ordinary chondrite meteorites. While some small spatially coherent spectral variations have been detected, neither the opx/opx+ol) mixing ratio nor other spectral parameters vary spatially by more than ∼1σ across the entire northern hemisphere of the asteroid, suggesting a remarkable homogeneity of the composition and mineralogy of the uppermost regolith. Spectral mixture modeling suggests that the presence of glass and/or a reddening agent like nanophase iron, likely formed from exposure of the regolith to the space environment, is a component of the surface of Eros. Reddening and darkening components could also explain the dissimilarity in overall spectral slope and albedo between Eros and other S(IV) asteroids and ordinary chondrite meteorites. The largest (but still weak) spectral variations across the surface are seen in the depths of Band I and Band II, which are greatest in and around the largest craters and at the 0° longitude “nose” of the asteroid, and in the Band II/Band I area ratio between the large impact craters Psyche and Himeros. These subtle NIS spectral variations are usually associated with albedo and surface slope variations seen in NEAR imaging and topographic data and appear to be related to downslope movement of regolith materials.  相似文献   

7.
The Solar–Stellar Irradiance Comparison Experiment {II (SOLSTICE {II), aboard the Solar Radiation and Climate Experiment (SORCE) spacecraft, consists of a pair of identical scanning grating monochromators, which have the capability to observe both solar spectral irradiance and stellar spectral irradiance using a single optical system. The SOLSTICE science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5%, to determine its variability with a long-term relative accuracy of 0.5% per year during a 5-year nominal mission, and to determine the ratio of solar irradiance to that of an ensemble of bright B and A stars to an accuracy of 2%. Those objectives are met by calibrating instrument radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Maryland. During orbital operations irradiance measurements from an ensemble of bright, stable, main-sequence B and A stars are used to track instrument sensitivity. SORCE was launched on 25 January 2003. After spacecraft and instrument check out, SOLSTICE {II first observed a series of three stars to establish an on-orbit performance baseline. Since 6 March 2003, both instruments have been making daily measurements of both the Sun and stars. This paper describes the pre-flight and in-flight calibration and characterization measurements that are required to achieve the SOLSTICE science objectives and compares early SOLSTICE{II measurements of both solar and stellar irradiance with those obtained by SOLSTICE {I on the Upper Atmosphere Research Satellite.  相似文献   

8.
Determination of Shape, Gravity, and Rotational State of Asteroid 433 Eros   总被引：5，自引：0，他引：5  
Prior to the Near Earth Asteroid Rendezvous (NEAR) mission, little was known about Eros except for its orbit, spin rate, and pole orientation, which could be determined from ground-based telescope observations. Radar bounce data provided a rough estimate of the shape of Eros. On December 23, 1998, after an engine misfire, the NEAR-Shoemaker spacecraft flew by Eros on a high-velocity trajectory that provided a brief glimpse of Eros and allowed for an estimate of the asteroid's pole, prime meridian, and mass. This new information, when combined with the ground-based observations, provided good a priori estimates for processing data in the orbit phase.After a one-year delay, NEAR orbit operations began when the spacecraft was successfully inserted into a 320×360 km orbit about Eros on February 14, 2000. Since that time, the NEAR spacecraft was in many different types of orbits where radiometric tracking data, optical images, and NEAR laser rangefinder (NLR) data allowed a determination of the shape, gravity, and rotational state of Eros. The NLR data, collected predominantly from the 50-km orbit, together with landmark tracking from the optical data, have been processed to determine a 24th degree and order shape model. Radiometric tracking data and optical landmark data were used in a separate orbit determination process. As part of this latter process, the spherical harmonic gravity field of Eros was primarily determined from the 10 days in the 35-km orbit. Estimates for the gravity field of Eros were made as high as degree and order 15, but the coefficients are determined relative to their uncertainty only up to degree and order 10. The differences between the measured gravity field and one determined from a constant density shape model are detected relative to their uncertainty only to degree and order 6. The offset between the center of figure and the center of mass is only about 30 m, indicating that Eros has a very uniform density (1% variation) on a large scale (35 km). Variations to degree and order 6 (about 6 km) may be partly explained by the existence of a 100-m, regolith or by small internal density variations. The best estimates for the J2000 right ascension and declination of the pole of Eros are α=11.3692±0.003° and δ=17.2273±0.006°. The rotation rate of Eros is 1639.38922±0.00015°/day, which gives a rotation period of 5.27025547 h. No wobble greater than 0.02° has been detected. Solar gravity gradient torques would introduce a wobble of at most 0.001°.  相似文献   

9.
COR1 is an internally occulted Lyot coronagraph, part of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument suite aboard the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft. Because the front objective lens is subjected to a full solar flux, the images are dominated by instrumental scattered light which has to be removed to uncover the underlying K corona data. We describe a procedure for removing the instrumental background from COR1 images. F coronal emission is subtracted at the same time. The resulting images are compared with simultaneous data from the Mauna Loa Solar Observatory Mk4 coronagraph. We find that the background subtraction technique is successful in coronal streamers, while the baseline emission in coronal holes (i.e. between plumes) is suppressed. This is an expected behavior of the background subtraction technique. The COR1 radiometric calibration is found to be either 10 – 15% lower, or 5 – 10% higher than that of the Mk4, depending on what value is used for the Mk4 plate scale, while an earlier study found the COR1 radiometric response to be ∼ 20% higher than that of the Large Angle Spectroscopic Coronagraph (LASCO) C2 telescope. Thus, the COR1 calibration is solidly within the range of other operating coronagraphs. The background levels in both COR1 telescopes have been quite steady in time, with the exception of a single contamination event on 30 January 2009. Barring too many additional events of this kind, there is every reason to believe that both COR1 telescopes will maintain usable levels of scattered light for the remainder of the STEREO mission.  相似文献   

10.
The Helioseismic and Magnetic Imager (HMI) instrument is a major component of NASA's Solar Dynamics Observatory (SDO) spacecraft. Since commencement of full regular science operations on 1 May 2010, HMI has operated with remarkable continuity, e.g. during the more than five years of the SDO prime mission that ended 30 September 2015, HMI collected 98.4% of all possible 45-second velocity maps; minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology. HMI velocity, intensity, and magnetic-field measurements are used in numerous investigations, so understanding the quality of the data is important. This article describes the calibration measurements used to track the performance of the HMI instrument, and it details trends in important instrument parameters during the prime mission. Regular calibration sequences provide information used to improve and update the calibration of HMI data. The set-point temperature of the instrument front window and optical bench is adjusted regularly to maintain instrument focus, and changes in the temperature-control scheme have been made to improve stability in the observable quantities. The exposure time has been changed to compensate for a 20% decrease in instrument throughput. Measurements of the performance of the shutter and tuning mechanisms show that they are aging as expected and continue to perform according to specification. Parameters of the tunable optical-filter elements are regularly adjusted to account for drifts in the central wavelength. Frequent measurements of changing CCD-camera characteristics, such as gain and flat field, are used to calibrate the observations. Infrequent expected events such as eclipses, transits, and spacecraft off-points interrupt regular instrument operations and provide the opportunity to perform additional calibration. Onboard instrument anomalies are rare and seem to occur quite uniformly in time. The instrument continues to perform very well.  相似文献   

11.
The main characteristics of the PENGUIN-M instrument are given. The instrument has been operating aboard the CORONAS-PHOTON spacecraft (SC) launched into orbit on January 30, 2009. The instrument includes the PENGUIN-MD detector unit (PMD) and the PENGUIN-ME electronic unit (PMD). The purpose of the experiment is to measure the degree of linear polarization of X-ray radiation from solar flares in the energy range of 20–150 keV and to obtain energy spectra of X-ray radiation from solar flares in the energy range of 2–500 keV. The paper describes the instrument, calibration procedure, and in-flight adjustment, and contains the first results of measurements.  相似文献   

12.
We report on radiometric and reflected light observations of 433 Eros at high time resolution, high accuracy, and broad spectral coverage. We use a thermal inertia model to estimate the thermal inertia, albedo, and size of Eros. We find an albedo of 0.125 ± 0.025 with axes of 39.3 ± 2.0 × 16.1 ± 0.8 km. Our estimate of the albedo is about 30% lower than previous estimates.  相似文献   

13.
David Morrison 《Icarus》1976,28(1):125-132
Radiometry of Eros at 10 and 20 μm demonstrates that the thermal conductivity of the upper centimeter of the surface is approximately as low as that of the Moon, suggesting that the asteroid has a regolith of highly porous rocky material. When combined with photoelectric photometry, these infrared measurements yield an effective diameter of Eros at maximum light of D0 = 22 ± 2 km and a geometric albedo of pv = 0.18 ± 0.03.  相似文献   

14.
The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft reached a separation angle of 180° on 6 February 2011. This provided a unique opportunity to test the intercalibration between the Sun–Earth Connection Coronal and Heliospheric Investigation (SECCHI) telescopes on both spacecraft for areas above the limb. So long as the corona is optically thin, at 180° separation each spacecraft sees the same corona from opposite directions. Thus, the data should appear as mirror images of each other. We report here on the results of the comparison of the images taken by the inner coronagraph (COR1) on the STEREO-Ahead and -Behind spacecraft in the hours when the separation was close to 180°. We find that the intensity values seen by the two telescopes agree with each other to a high degree of accuracy. This validates both the radiometric intercalibration between the COR1 telescopes, and the method used to remove instrumental background from the images. The relative error between COR1-A and COR1-B is found to be less than 10−9 B/B ⊙ over most of the field-of-view, growing to a few ×10−9 B/B ⊙ for the brighter pixels near the edge of the occulter. The primary source of error is the background determination. We also report on the analysis of star observations which show that the absolute radiometric calibration of either COR1 telescope has not changed significantly since launch.  相似文献   

15.
Photometric Stability of the Lunar Surface   总被引：1，自引：0，他引：1  
Hugh H. Kieffer 《Icarus》1997,130(2):323-327
The rate at which cratering events currently occur on the Moon is considered in light of their influence on the use of the Moon as a radiometric standard. The radiometric effect of small impact events is determined empirically from the study of Clementine images. Events that would change the integral brightness of the moon by 1% are expected once per 1.4 Gyr. Events that cause a 1% shift in one pixel for low Earth-orbiting instruments with a 1-km nadir field of view are expected approximately once each 43 Myr. Events discernible at 1% radiometric resolution with a 5 arc-sec telescope resolution correspond to crater diameters of approximately 210 m and are expected once every 200 years. These rates are uncertain by a factor of two. For a fixed illumination and observation geometry, the Moon can be considered photometrically stable to 1 × 10−8per annum for irradiance, and 1 × 10−7per annum for radiance at a resolution common for spacecraft imaging instruments, exceeding reasonable instrument goals by six orders of magnitude.  相似文献   

16.
The abundance of Fe relative to H is obtained by using resonantly scattered intensities of λ 284 of Fe XV that were measured with OSO-7 and resonantly scattered intensities of Lα of H I that were obtained by Gabriel (1971). Because of possible differences in electron densities along lines of sight for these non-simultaneous measurements and in relative calibrations, results are rather uncertain but still indicate that the average Fe abundance relative to H in the corona appears to be at least as large as a recent photospheric abundance. Some limitations in using this method for obtaining abundances are examined for future experiments with simultaneous measurements and well calibrated detectors.  相似文献   

17.
The Solar–Stellar Irradiance Comparison Experiment II (SOLSTICE II) is one of four experiments launched aboard the Solar Radiation and Climate Experiment (SORCE) on 25 January, 2003. Its principal science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5% and to determine solar variability with a relative accuracy of 0.5% per year during a 5-year long nominal mission. SOLSTICE II meets these objectives using a pair of identical scanning grating monochromators that can measure both solar and stellar irradiance. Instrument radiometric responsivity was calibrated to ∼3% absolute accuracy before launch using the Synchrotron Ultraviolet Radiation Facility (SURF) at the National Institute for Standards and Technology (NIST) in Gaithersburg, MD. During orbital operations, SOLSTICE II has been making daily measurements of both the Sun and an ensemble of bright, stable, main-sequence B and A stars. The stellar measurements allow the tracking of changes in instrument responsivity with a relative accuracy of 0.5% per year over the life of the mission. SOLSTICE II is an evolution of the SOLSTICE i instrument that is currently operating on the Upper Atmosphere Research Satellite (UARS). This paper reviews the basic SOLSTICE concept and describes the design, operating modes, and early performance of the SOLSTICE II instrument.  相似文献   

18.
We describe the imaging quality of the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) as measured during the ground calibration of the instrument. We describe the calibration techniques and report our results for the final configuration of HMI. We present the distortion, modulation transfer function, stray light, image shifts introduced by moving parts of the instrument, best focus, field curvature, and the relative alignment of the two cameras. We investigate the gain and linearity of the cameras, and present the measured flat field.  相似文献   

19.
Abstract— In late January 2001 the NEAR—Shoemaker spacecraft performed low‐altitude passes over the surface of 433 Eros. Coordinated observations of the asteroid surface were obtained at submeter resolution by the NEAR laser rangefinder and the multispectral imager. This paper presents three independent, coordinated observations of a 90 m pond adjacent to a granular debris flow, including the highest resolution altimetric measurements of ponded deposits on Eros. The ponded deposits appear to have been emplaced by fluid‐like motion of dry asteroidal regolith. A simple model of seismic agitation from impacts is developed to account for pond formation on Eros. The model predicts that ponds should form readily on Eros but not on the Moon, where ponds are not observed. The model also suggests that the absence of observable ponds in the largest craters of Eros, as well as on Phobos and Deimos, may be related to regolith depth.  相似文献   

20.
The results from a set of 12 solar corona radial velocity measurements in the 400-440 nm spectral band during the total solar eclipse of July 11, 1991 are reported. The measurements show that the orbital motion of the F-corona material near the sun in the ecliptic plane is consistent with Keplerian motion and predominantly, but not exclusively, prograde, as is usually assumed. This work demonstrates a method of using the measured radial velocities to sort out the relative amounts of K-corona, near-earth F-corona, near-solar F-corona, and scattered light in each measurement for each observation point W and E of the sun between 2.5Ro(solar radii) and 5Ro along the celestial equator and at three points north of the sun. The near-solar F-corona component is quite weak, contributing only 7-14% of the total signal in each case. The stronger diffraction component from near-earth F-corona is estimated to have been produced by particles with radii of about 11μ. In contrast, the scattered light component appears as strong zero-velocity features dominating all the measurements. The measurements W and E of the sun and near the ecliptic plane also show evidence of a red-shift velocity of at least 330 km s−1, suggestive of a high-speed dust outflow from the sun.  相似文献   

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