Study of Physical Characteristics of High Apogee Space Debris Yongna Mao, Jianfeng Wang, Xiaomeng Lu, Liang Ge, Xiaojun Jiang (National Astronomical Observatories, Beijing, 100012, China) Abstract Date acquisition and analysis methods of high apogee space debris (HASD) s optical characteristics are discussed in this paper. Key words: Space Debris; High Apogee; Time-series photometry; simultaneous multi-color photometry; Optical cross section 1. INTRODUCTION HASD (orbital altitude higher than 20,000km) are too distant for radar tracking and high resolution optical imaging. Non-imaging optical observation is most suitable for determining their physical characteristics, such as shape, size, AMR, attitude, and outer-layer material. The physical characteristics of MHASD can be obtained by using time series photometry, simultaneous multi-color photometry, spectroscopy, and photopolarimetry. Our study involves time-series photometry, simultaneous multi-color photometry, optical cross section, and low-resolution spectroscopy. Our research is based on observations carried out by telescopes at Xinglong Observatory, National astronomical observatories, Chinese academy of science (NAOC). Xinglong observatory is an optical observatory, and dedicated to astrophysics research. It is located in the Yan Mountain, about 165 kilometers northeast of Beijing, and about 900 meters above the sea level. There are 220-240 spectroscopic nights and 100-120 photometric nights per year. The average seeing is 1.5. 2. TIME-SERIES PHOTOMETRY For our space debris observations, we upgraded the astronomical optical telescope to be capable to automatically observe orbital debris. So when we observed space debris, the telescope works under three modes: the Survey mode, the telescope keeps stationary or tracks in the sidereal rate, used for searching un-cataloged debris. The Track mode, the telescope tracks the TLE orbit, used for precise astrometry and photometry. The Scan Mode, combination of survey mode and telescope pointing. Pointing the telescope in a specified sky zone sequence and the telescope runs in the survey mode in every zone for a few seconds, used for searching un-cataloged debris. For time-series photometry, Table 1 shows the parameters of the telescope system. The telescope located at Xinglong, with 500mm diameter. Small aperture telescopes typically perform better than larger ones in optical observations of space debris because of their higher tracking speed and larger field of view. But the limited light
gathering power is a challenge to high precision photometry. We adopt observations with the telescope locked on the targets during exposures to improve the signal to noise ratio (SNR). So exposure time must be considered carefully, increasing exposure time can improve the photometric accuracy, but results in long and overlapping background star trails. See the Figure 1, the same target with the exposure time of 0.5 seconds, 5 seconds, and 10 seconds. The middle panel is better obviously. Table 1 Parameters of the space debris telescope Parameter Value Telescope aperture 500mm Telescope focal length 4000mm CCD detector PI VersArray 1300 1340 Pixel size 20micron 20micron Image scale 1.0 /pixel FOV 22 23 Figure 1 The same target with different exposure time: 0.5 seconds, 5 seconds, and 10 seconds (from left to right) Data reduction was followed by standard IRAF procedures, including CCD reduction, bias frame subtraction, flat fielding, and cosmic ray spiking, and aperture photometry. The flux of GEO debris was calibrated by using Landolt standard stars under photometric nights, or using field stars of USNO star catalog during non-photometric conditions. During non-photometric conditions, the calibration procedures included detection of trailed field stars, and photometry of trailed field stars by isophotal method. For the figure 2, there are our preliminary results, between the upper panels and the lower panels, we can obtain the technical status; and between the left panels and the right panels, different attitude stabilizing modes are given. The other preliminary result is shape modeling. We established different diffuse reflection models, including ball shape, cylindrical shape and cubic shape. And we compared our observed curves to the models. The spin rate is an important result in our research. By the optical observations, we can obtain light curves of space debris. So we can analyze the spin rate of those targets, just by the spectral analysis.
Figure 2 The preliminary results of time-series photometry 3. SIMULTANEOUS MULTI-COLOR PHOTOMETRIC OBSERVATIONS Simultaneous multi-color photometry is another method for our space debris research. We introduce a 3-channel CCD photometer for simultaneous multi-color photometric observations. Table 2 shows the parameters of the instrument. Figure 3 is the optical layout of the 3-channel CCD photometer. It is composed of five components. Folding mirror is located between the telescope and the Philips prisms to keep the photometer in a compact shape. The Philips prism (Figure 4) is the heart of the 3-channel CCD photometer. It is a Philips type trichroic beam splitter prism, used to separate the three color beams. Three groups of correctors/reducers for the three channels, in order to obtain optimized optical quality. And in each channel, there is a filter slot and a CCD detector. Table 2 Parameters of the 3-channel CCD photometer
g channel r channel i channel FOV 18.8 18.8 18.2 17.6 9.2 9.2 Plate scale ( /pixel) 0.54 0.81 0.54 Efficiency 58.6% 64.0% 61.8% Limiting magnitudes 15.13 16.57 15.68 Differential photometric accuracy System transformation accuracy 0.5% 0.3% 0.9% 3.0% 2.0% 2.0% Figure 3 Optical layout of the 3-channel CCD photometer Figure 4 Structure of the Philips prism Similar to the time-series photometry, the data reduction of the simultaneous multi-color photometry was followed by standard IRAF procedures. But the calibration method is different slightly. We also calibrate the flux of GEO debris by using Sloan standard stars under photometric nights. But under the non-photometric conditions, we can use background stars from different field stars to fit the calibration parameters. Because of a number of methods for data reduction, e.g. image calibration, background fitting, targets detection and location, isophotal analysis, and flux calibration, we can ensure a photometric accuracy of better than 5%, even under ordinary, non-photometry conditions. Simultaneous multi-color photometric data on more than 30 high apogee space debris in SDSS s g, r and i bands were collected. Through the data analysis, we found color information could play a role in space debris classifications, as well as analyzing their properties of attitude and outer-layer materials. Figure 5 shows multi-color light curves of space debris, the blue one is the g channel, and little contribution to the total light curve-the red one.
Figure 5 Simultaneous multi-color light curve of space debris 4. OPTICAL CROSS SECTION Radar cross section is usually used to characterize the size of the space debris. But GEO debris is too distant for radar tracking and high resolution optical imaging. However, optical cross section is a better method to characterize the size of them. Our research of optical cross section (OCS) includes four steps: calibration of the influence of the observation can obtain high accuracy luminosity of space debris; the second and third steps are correction of the influence of revolution and rotation, including distance, phase angle, and attitude. By the ground test of material albedo, we can obtain optical cross section finally. Below is the Correction formula of distance and phase angle. R is the distance of between the space debris and observation station, and Φ is the phase angle between the Sun and the observation station. 2 R 180 φc FluxC = f ( R, φ) = Flux φ 2 ( R RC ) 180 Table 3 is our result of the sizes of space debris. Our study showed that optical cross section can characterize well with the real size of space debris with about 50% error. As one information of the physical characteristic, OCS can characterize space debris size better, because of the short wavelengths, and long detection distance. Table 3 Result of the sizes of space debris Size(m) Range of cross section(m 2 ) mid-value(m 2 ) OCS(m 2 ) Relative Error_OCS 2.16 2.46 3.15 5.314-10.312 7.81 11.74 50.3% 2.10 2.27 3.40 4.767-10.514 7.64 6.08 20.4% 2.80 4.90 3.80 10.640-23.129 16.88 8.11 52.0% 3.80 2.80 4.30 10.640-20.297 15.47 15.89 2.7% 3.40 3.50 5.80 11.900-28.301 20.10 4.94 75.4%
2.20 1.72 2.00 3.440-5.803 4.62 6.52 41.1% 3.10 2.50 6.10 7.750-24.293 16.02 17.31 8.1% Diameter 2.16, Length 6.59 3.664-14.234 8.95 6.69 25.3% Diameter 2.10, Length 4.10 3.464-8.610 6.04 8.36 38.4% 3.60 2.70 4.30 9.720-19.350 14.54 7.84 46.1% 7.30 3.62 3.62 13.104-37.372 25.24 57.13 126.3% 3.40 3.50 5.80 11.900-28.301 20.10 10.10 49.8% 2.20 1.72 2.00 3.440-5.803 4.62 11.08 139.8% 5. LOW-RESOLUTION SPECTROSCOPY For a new attempt, we introduce low-resolution spectroscopy for space debris research. We use the reflector, whose aperture is 2.16m, and a Cassgrain spectrograph. The resolution is about 1 nm. Compared with the photometric data, low-resolution spectroscopy would carry more information about the outer-layer materials of the debris. 6. CONCLUTION We present observations of HASD by using time-series photometry, simultaneous multi-color photometry, optical cross section and low-resolution spectroscopy, respectively, at Xinglong Observatory, NAOC. Our preliminary results indicate that optical observations provide useful information on space debris physical characteristics. REFERENCES [1] Thomas Schildknecht. Optical surveys for space debris. Astron Astrophys Rev(2007)14:41-111. [2] TANG Yi-jun, JIANG Xiao-jun, WEI Jian-yan. Review of optical observations of high apogee space debris. Journal of Astronautics, Vol.29 No.4. [3] TANG Yi-jun, JIANG Xiao-jun, WANG Jian-feng. 3-axis stabilization GEO satellites diffuse reflection optical. [4] Sanchez D J, Gregory S A. Photometric measurements of deep space satellites [J]. Proceedings of SPIE 2000, 4091:164-182 [5] Tamara E P, Stephen A G, Kim L. Electro-optical signatures comparisons of geosynchronous satellites[j]. Proceedings of SPIE 2002, 4847:332-336 [6] Mao Y N, Jiang X J, Lu X M. A 3-channel CCD photometer at Xinglong observatory. RAA, 13 (2013) 239-252