|Naoteru Gouda (2011), Scholarpedia, 6(10):12021.||doi:10.4249/scholarpedia.12021||revision #193927 [link to/cite this article]|
JASMINE is an abbreviation of Japan Astrometry Satellite Mission for Infrared Exploration. Two satellites are planned as a series of JASMINE project (Gouda et al. 2010, Gouda et al. 2011). These are JASMINE (at the moment, the JASMINE mission is officially called "the Small-JASMINE" mission at ISAS/JAXA. But, hereafter, it is described as JASMINE through this paper) and Nano-JASMINE. Two JASMINE missions deal with the positions and proper motions of celestial objects. The JASMINE mission (officially the Small-JASMINE mission) was selected by ISAS/JAXA (Institute of Space and Astronautical Science/the Japan Aerospace Exploration Agency) in May 2019 as the unique candidate for the JAXA Competitive Middle-Class Science Missions No.3. JASMINE will observes towards a region of the Galactic nuclear bulge around the Galactic center and other small regions, which include interesting scientific targets, with accuracies of 25 \(\mu\!\)arcseconds in an infrared Hw-band (1.1\(\sim\)1.7 \(\mu\!\)m) for bright stars. The target launch date is mid-2020s. On the other hand, Nano-JASMINE uses a very small nano-satellite. Nano-JASMINE will operate in zw-band (0.6\(\sim \)1.0\(\mu\!\)m) to perform an all sky survey with an accuracy of 3 milliarcseconds for positions, parallaxes and proper motions.
Infrared Space Astrometry
There is much interstellar dust on the Galactic plane especially around the Galactic center. Light at visual wavelengths gets absorbed and/or scattered effectively by the interstellar dust. This effect gives rise to the decrease of the number of observable stars at visual wavelengths. Furthermore the decrease of photon numbers reduces the accuracies of positions of the observed stars. On the other hand, the effect of absorption by the interstellar dust is less effective for at infrared wavelengths. Consequently, infrared space astrometry has an advantage when observing towards the Galactic plane, especially towards the Galactic center, which is hidden by the interstellar dust when observing at visual wavelengths. Also the very interesting star formation regions, surrounded by much interstellar dust, are of great interest.
Overview of JASMINE
JASMINE was selected by ISAS/JAXA (Institute of Space and Astronautical Science/the Japan Aerospace Exploration Agency) in May 2019 as the unique candidate for the JAXA Competitive Middle-Class Science Missions No.3. At the moment, the JASMINE mission is officially called "the Small-JASMINE" mission at ISAS/JAXA. The target launch date is mid-2020s. The final data catalogue will be released to the public around FY 2030. JASMINE will determine positions and parallaxes accurate to < \(\sim\) 25 \(\mu\!\)arcseconds for ~12000 stars, including 7000 bulge stars (Hw<12.5mag) towards a region of the Galactic nuclear bulge around the Galactic center (Hw-band: 1.1~ 1.7 micron). Proper motion precisions of < \(\sim\) 25 \(\mu\!\)arcseconds/year are expected. Furthermore Small-JASMINE provides about 98000 stars, including 67000 bulge stars, with proper motion precisions of less than 125 \(\mu\!\)arcseconds/year for brighter than Hw~15 mag.The observation regions for the primary scientific objectives (key project) in the Galactic nuclear bulge are shown in the Fig.2. The regions include the following region 1 and region 2 as its minimum;
(1) A circle with a radius of 0.7 ° from the Galactic center (region 1)
(2) A rectangular area with longitude from -2 ° to 0.7 ° and latitude from 0 ° to+0.3 °. The survey will be done with a single beam telescope of which the diameter of the primary mirror is about 30 cm (Yano et al. 2011).
The basic designs of JASMINE and technical problems have been investigated mainly at the JASMINE Project office, National Astronomical Observatory of Japan (NAOJ) in collaboration with JAXA, Kyoto University and other institutes.
JASMINE has the following three primary scientific goals.
(1) To reveal the Milky Way's central core structure and formation history by measuring the distances and the motions of the stars located as far as 26 thousand light-years away with high-precision astrometry observations in the near-infrared band.
(2) To explore the formation history of the Milky Way related to the origin of human beings by revealing the evolution of the Galactic structures, which caused the radial migration of the Sun and other stars with their planetary systems.
(3) To find Earth-like habitable exoplanets, taking advantage of the time-series photometry capability required for the precision astrometry.
The project is unique in that unlike the optical space astrometry mission, “Gaia Project”, operated by the European Space Agency (ESA), the same astronomical object can be observed frequently (once per 100 minutes), and observation will be performed in the near-infrared band, in which the effect of absorption by dust is weak. This project will help to achieve revolutionary breakthroughs in astronomy and basic physics, including the formation history of the Galactic nuclear bulge (Galactic Center Archeology); Galacto-seismology; the supermassive black hole at the Galactic Center; the gravitational field in the Galactic Nuclear Bulge, the activity around the Galactic Center; formations of star clusters; the orbital elements of X-ray binary stars and the identification of the compact object in an X-ray binary; the physics of fixed stars; star formation; planetary systems; and gravitational lensing. The main scientific objectives in the Galactic nuclear bulge is schematically shown in Fig.3. Such data will allow for the compilation of a more meaningful catalog when combined with data from terrestrial observations of the line-of-sight velocities and chemical compositions of stars in the bulge (e.g., APOGEE (Majewski et al. 2010) and BRAVA (Howard et al. 2008)).
Due to satellite operations, there are periods when astrometric observations towards the Galactic center direction are not possible. In such a period, in order to utilize the unique features of the JASMINE satellite (its capability of highly frequent observations in the near-infrared region), we plan to observe a few specific astronomical objects in the Galaxy. A good example is transit observations utilizing the continuous photometric observations of JASMINE. It is possible to search for Earth-type planets that are expected to be in the habitable zones around M-type stars, which are low mass red stars belonging to the main sequence. JASMINE dominates the other missions for explorations of this type of exo-planet. Furthermore, JASMINE will be Japan’s first satellite mission for the exploration of exo-planets.
Preliminary specifications of the satellite system
The satellite system consists of a bus module and a mission module. Detailed design of the bus module is now under consideration by satellite companies. The mission module consists of a telescope, electronics box, X-band antenna and GPS unit. Large sun shield and telescope hood are installed to prevent telescope from stray light invasion and temperature change (Fig.4). The JASMINE satellite will be launched by a type of the solid rocket (Epsilon Launch Vehicle) provided by JAXA in Japan. The orbit of JASMINE is a sun-synchronized orbit with above the altitude of 550 km. The main target (the Galactic nuclear bulge) observation is planned during spring and autumn seasons(Fig.5) because JASMINE cannot observe the direction toward the Galactic center in summer and winter seasons due bad thermal conditions for the observation. During one orbit around the Earth, JASMINE can make observation during only about a half of an orbit period (~50 minutes) when the Galactic bulge is visible (Fig.6). The telemetry rate should be larger than 2 Mbps. The weight of the JASMINE satellite is about 400 kg.
A candidate optical system for a JASMINE telescope is a modified Korsch system with three mirrors and four folding flats to fit the focal length into the available volume (Fig.7). The telescope has a circular primary mirror with 30 cm diameter and 3.9 m focal length. A candidate for materials of the telescope is CREARCERAM whose Coefficient of Thermal Expansion (CTE) is very low at about 278K which is the operation temperature around the telescope. The telescope provides a flat image plane that contains a large format infrared detector (HgCdTe) with a field of view of 0.6°×0.6°. A total of 4k×4k pixels of the detector with 10 \(\mu\!\)m pixel are simultaneously read out. The readout noise should be less than 30e\(^-\ .\)
The JASMINE mission has the following critical technical issues to be resolved for the satisfactions of the system requirements.
(1) Structural-thermal stability of the telescope: within 10nm change of the stellar positions on the focal plane during 50 minutes
(2) Vibrational stability of the telescope: within around 340 mas during the exposure time of 7.1 seconds
(3) Fabrication and adjustment of the optical system: adjustment of the focallength:50μm resolution, 1.25mm range
(4) Decreasing straight light: within 5 photons/pixel/sec
(5) Thermal control: temperature at the detector:<180K, Temperature at the telescope :278K, within 0.1 degree temperature variation at the telescope during 50 minutes, and within 0.7 degree temperature variation at the detector during 50 minutes.
The necessary technologies including the critical issues just described above requires us to carry out the following developments.
(a) Super-super invar: suppressing changes of distortion within a half orbit period is a major requirement on the optical system. To achieve this requirement, we need both athermal optics and high thermal stability of the optics. For athermal optics, we adopted CREACERAM as a material of mirrors and developed super-super invar as a material of telescope structure. Super-super invar was developed with helps of material industries, and its thermal expansion is nearly zero. Our measurement of its coefficient of thermal expansion (CTE) is 0±5x10-8/K.
(b) Thermal design of the telescope: the telescope is installed inside the “Telescope Panel Box” which has controlled heating and which insulates the telescope from the outside thermal environment. By this approach, we can relax a temperature stability requirement to ±1degree at the box wall and we can achieve ±0.1degree at the telescope structure. We confirmed this approach by thermal vacuum test of a BBM (Bread Board Model) of the telescope and telescope panel box.
(c) Qualification of mechanical environment tolerance: vibration test of the BBM (Fig.8) was conducted to confirm that mechanical design of the telescope would be feasible, and the structure mathematical model would have enough accuracy. The test results show that the measured mechanical propertied were equivalent to the design results.
(d) Satellite system feasibility analysis: JASMINE team is now conducting a preliminary study of the satellite system, which includes both a bus module and the mission module with different satellite manufacturers.
(e) Detector subsystem (DSS): Japanese JASMINE team is collaborating with a US team.
JASMINE will not observe two fields of view simultaneously, but will observe only one field of view at a time because these missions do not perform an all-sky survey, but focus on the restricted regions. A whole survey region is composed by linking stars in an overlap region between two consecutively observed adjacent fields; in a very short time period stars in neighbouring fields do not move. This is called a “frames-link” method (Gouda et al. 2010) which is similar to the block adjustment method (Eichhorn 1960, Zacharias 1992, Yu 2004). This method can be used when the number of stars in each small-field is so large that a large-frame of the whole survey region can be made with the required accuracy. The star density in the Galactic bulge is sufficient to apply this method.
The detailed explanation of the frames-link method for JASMINE is shown below (Fig.9).
(i) First stage: imaging a small-frame and centroid determination
One field of view (one frame) will be observed during 7 seconds. This imaging is repeated 16 times at the same field of view. One set of stellar images gathered by these 20 frames is called a "small-frame". A stellar image on one frame is analyzed in a window of 9 × 9 pixels.
(ii) Second stage: linking small-frames
The telescope moves towards an adjacent field of view overlapping the previous small-frame (the overlap area is about half the frame size) in 30 seconds. In about 50 minutes the telescope takes stellar images over the whole survey region around the Galactic center, covering it by 16 small-frames. The 16 small-frames are linked together by many stars in the overlap regions. The whole region, linked together by the 16 small-frames is termed "a large-frame". Small-frames are linked by employing their common stars to create a large-frame. In an overlap region there are about 1000 common stars (brighter than 12.5 mag). This number is large enough to create a large-frame with the required positional accuracies of stars.
(iii) Final stage: combining all large-frames
The above procedure at the first and second stage is repeated during the whole mission life of about 3 years and finally about 8000 large-frames will be observed. Each large-frame covers the common survey area. Combining these 8000 large-frames allows to determine the astrometric parameters with the target accuracy; the large-frames will be combined to form a single global frame. We will determine the individual star's astrometric parameters (position, proper motion, and parallax) on this global frame. Here the origin and orientation, the absolute value of the size and distortion, of each large-frame should be calibrated by calibration stars measured by Gaia. The astrometric parameters and the nature of each large-frame mentioned just above will be determined applying a least-squares fit to traces of stellar motions on the global frame with information of calibration stars. The entire process is iterated until corrections to the parameters converge.
The JASMINE mission requires that the precisions of parallaxes for brighter stars (<12.5mag) must be equal to or less than 25μas. Repeated measurements of the stellar position improve the precision of the parallax by reducing the random errors. In JASMINE, 0.15M-0.6M measurements will be performed for each star. Single observation errors will be about 4mas. and so that the parallax precision is expected to be around 25μas by the multiple measurements. However, although statistical (random) errors will reduce according to square root of N raw, systematic errors should be removed by some processes. The estimation is important process for astrometry to reduce systematic errors. By modelling of systematic errors, self-calibration will be carried out. It is possible to model the systematic errors by the use of the fact that we can presume that relative stellar positions on the celestial sphere do not change in short periods. Hence, we can model the systematic noise by fitting of some mathematical functions by the constraints of the fact that the relative stellar positions do not change in short periods. Regarding long period, if effects of binaries, ex-planets, gravitational lens and/or hot spots can be neglected for a target star, we can assume that the trajectory of such a star has a definitive shape, that is, helical motion. This fact is used as a constraint for the calibration of the systematic noises in longer periods. In this way, we remove the systematic noises by the use of our measured data, or the self-calibration. In the actual procedure, we modify the modeling of the systematic noises in the following way.
We define modelling error and residual as follows.
Modelling error ≡ (best fitting function for the model of an error) ─ (true error)
Residual of estimation ≡ (estimated function by observations) ─ (best fitting function for the model)
Residuals have no correlation, and we can expect that the residual errors decrease as 1/√N with large N (the number of observations). On the other hand, the modelling error is a systematic error. It does not decrease even if many measurements are performed. If a modelling error is larger than the required precision, we can find that error because the precision approaches a constant value with increasing number of observations. We modify the model until the precision meets the required precision (Fig.10).
More detailed reduction procedure contains 3 parts.
As the 1st step, we estimate the shape of PSF from downlinked data to get PSF fitted center of stellar images. An estimated model of PSF shape is built from measured stellar images. This estimated PSF includes not only diffraction but all effects, i.e. non-uniformity of the response of inter and intra pixel(s), electronic response, coating of detector surface and filter, etc. We call it “effective PSF”, or simply “ePSF”. We do not need any information about the individual effects.
As the 2nd step, a large frame is made by combining unit frames (Fig.9). The process of combining the PSF-fitted centers of all 20 x 16 unit frames to the common coordinate system defined on the large frame is called the frame link method. This mapping from the unit-frame coordinate system to the large-frame requires us to take into account the calibration of two effects: (large scale, variable) “optical distortion” and (small-scale, constant) “image distortion”. We omit the detailed explanations here.
As the final step, we estimate parallaxes, proper motions, and positions at certain time by the use of the time series of the large-frames.
Overview of the Nano-JASMINE mission
Nano-JASMINE is a nano-size satellite of which the size and weight are (50cm)3 and about 35 kg, respectively (Hatsutori et al. 2011). Nano-JASMINE will operate in the zw-band (0.6 \(\sim\) 1.0 \(\mu\!\)m). The target accuracy of parallaxes is about 3 milliarcseconds at zw=7.5 mag (Kobayashi et al. 2011). Moreover high-accuracy proper motions (\( \sim 0.1\) milliarcseconds/year) can be obtained by combining the Nano-JASMINE catalogue with the Hipparcos catalogue, as the decrease in the error of the proper motions is proportional to the inverse of the epoch difference between the two catalogues which for Hipparcos and the Nano-JASMINE catalogues will exceed 30 years. Nano-JASMINE mission performs an all-sky survey. The satellite, while rotating, observes simultaneously two fields of view, separated by a large and fixed basic angle. Tracing great circles allows to achieve their intended resolution; fields of view can be linked based on this basic angle. This is the same as for the Hipparcos and Gaia missions. Nano-JASMINE will be put in a Sun-synchronized orbit with the altitude of about 800 km. Launch oppotunities for the Nano-JASMINE satellite are under consideration.
Design of the mission system
The mission part of the Nano-JASMINE satellite consists of a telescope, CCD, CCD controller, stellar image extractor, and mission control CPU (Kobayashi, et al. 2010). Following the principles of the Hipparcos and Gaia instruments, the Nano-Jasmine telescope has two apertures, separated by 99.5 degrees, and projecting through a beam combiner and a 5 cm diameter primary mirror on a single focal plane at a focal length of about 1.7 m. One CCD with 1k \(\times\)1k pixels is situated in the focal plane. Richey-Chretien type telescope is adopted to achieve the diffraction limited performance over the wide field of view (0.5\(^\circ \times\)0.5\(^\circ\)).The telescope and telescope structure are made entirely of aluminum alloy to reduce thermal distortion (Hatsutori, et al. 2011). The mirrors are coated with gold and the final wave front error is less than 1/14.
A 1k \(\times\)1k Fully Depleted CCD (FDCCD) developed by Hamamatsu Photonics Co. is put at the focal plane of the telescope (Kobayashi et al. 2010). At red wavelength regions, the response of the FDCCD is much better than that of a conventional CCD. A combination of the CCD response and the long-pass filter, which has cut-off wavelength of 0.6 microns, defines the zw-band filter. The CCD is controlled with the time delay and integration mode by the TDI board, which makes the charge transfer rate synchronize the satellite spin so that the CCD can detect as much as photons of stellar images without blur of the stellar images.
All of the components will be degraded by space radiation during the mission time of around 2 years. The charge transfer efficiency of the CCD will be reduced by the space radiation. Radiation experiments for the CCD suggest that the point spread function of stellar images will be changed by the space radiation and so the accuracy of the determination of stellar positions will be affected (Hatsutori, et al. 2011). A correction model for the determination of the centroids of stellar images should be used in the data analysis for Nano-JASMINE.
Design of the bus system and the orbit of Nano-JASMINE
The Nano-JASMINE satellite must achieve strict requirements during the mission (around 2 years) to obtain target accuracies of astrometry data (Sako et al. 2007, Inamori, et al. 2011). The attitude of the satellite should be controlled to an accuracy of 0.05° and furthermore the attitude should be stabilized to better than 740 milliarcseconds per 8.8 seconds accuracy during the observation. The attitude control of the Nano-JASMINE satellite is given as follows; Nano-JASMINE is equipped with many types of attitude sensors which have different levels of accuracy. These are a star tracker, fiber optical gyroscope, MEMS gyroscope, Sun sensor as well as coarse and fine magnetic sensors. The most accurate and final attitude sensor is to use outputs of stellar images obtained by the mission instruments. Blurred images of the stars will be analyzed and used for the attitude stabilization during the mission. The satellite spin axis should be aligned to the mission CCD's charge transfer direction and moreover the spin velocity of the satellite should be matched with the charge transfer velocity of the CCD to correct the blurred stellar images (Inamori et al. 2011). This fine attitude control is performed using small reaction wheels.
The temperature of the CCD must be less than -50°C to reduce thermal noise. In addition, the temperature stability of the telescope frame must be less than 0.1° per about 100 minutes to avoid the time variations of the telescope structure, especially to stabilize the angle of the beam combiner within 1 milli-arcseconds per about 100 minutes. The requirement for the temperature is achieved by the following strategy (Sako, et al. 2007); The key design point is the use of a Multi-Layered thermal Insulation (MLI). The temperature stability is achieved by covering the telescope with a thermal shield. The telescope and the shield are hold by adiabatic columns which do not conduct heat and can release heat stress. The CCD is cooled by emission from a radiation plate on the top of the satellite. Besides, the satellite attitude is kept so that the radiation plate is always towards non-Sun and non-Earth direction to maintain to maintain the same thermal environment.
The Nano-JASMINE satellite uses two FPGAs as an on-board computer. The on-board computer functions for the control of mission instruments, the attitude control, telemetry and the extraction of the stellar images from the raw data (Yamada et al. 2009).
The telemetry is operated in the S-band and the telemetry rate is 100 kbps for the high-speed downlink of scientific data, and 10 kbps for the uplink and low speed downlink of the house keeping data of the satellite. The high-speed downlink will be operated by using a radio telescope with 10 m aperture located at Mizusawa VLBI observatory belonged to National Astronomical Observatory of Japan. The uplink and the low speed down link will be operated normally by a radio antenna located the University of Tokyo for the nominal operations.
A SD-card with 2 GB capacities is used for the data storage on board.
The flight model (FM) of the Nano-JASMINE satellite has been accomplished. The development of the satellite has been done by National Astronomical Observatory of Japan and Kyoto University for the mission system , and the University of Tokyo for the bus system and ground stations.
The orbit of Nano-JASMINE is a sun-synchronized orbit whose altitude is about 800 km.
Observing strategy and data analysis
The observing strategy and a method used in the data analysis for Nano-JASMINE will be similar to what is planned for Gaia. Hence the use of Nano-JASMINE data is useful to check algorithms that are to be used in the Gaia data analysis. Furthermore, the algorithm of Gaia data analysis can be applied to Nano-JASMINE. There exists currently a very good collaboration on Nano-JASMINE data analysis with the Gaia data analysis team (Yamada et al. 2011).
Features of the Nano-JASMINE catalogue
Nano-JASMINE will provide 3 mill-accuracies of the positions and parallaxes of the stars brighter than zw=7.5 mag. The number of the stars brighter than zw=7.5 mag is estimated to be about 200,000 by using cataloged stars in the Guide Star Catalogue version 2.3, the Hipparcos and the Tycho catalogue. The capability of the telemetry for Nano-JASMINE guarantees that all stars brighter than zw=7.5 mag can be obtained on the ground. Although stars for darker than zw=7.5 mag. can be obtained on board, the number of these dark stars which can be obtained on the ground depends the actual capability of the telemetry which will be determined by the detailed orbit and attitude. If all the stars for brighter than zw=9 mag. can be obtained on the ground, the number of the stars is estimated to be around 520,000. The first version of the Nano-JASMINE catalogue will provide the proper motions of some stars with accuracies of \(\sim\)0.2 milliarcseconds using 0.5 \(\sim\) 1 year data obtained by Nano-JASMINE and the Hipparcos catalogue. It will be published in 2 years after the launch date. The new data of accurate proper motions will certainly provide interesting scientific results. The second version of the Nano-JASMINE catalogue will provide the positions, the parallaxes and the proper motions with better accuracies than those in the first version, for all stars obtained by Nano-JASMINE, and will be published in around 4 years after the launch date.
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