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Radar Observations and Physical Modeling of Asteroid 6489 Golevka R. S. Hudson School of Electrical Engineering and Computer Science, Washington State University, Pullman, Washington 99164-2752 E-mail:
[email protected] S. R. D. D.
J. Ostro, R. F. Jurgens, K. D. Rosema*, J. D. Giorgini, R. Winkler, Rose, D. Choate, R. A. Cormier, C. R. Franck, R. Frye, D. Howard, Kelley, R. Littlefair, M. A. Slade, L. A. M. Benner, M. L. Thomas**, L. Mitchell***, P. W. Chodas, and D. K. Yeomans Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109-8099
D. J. Scheeres Department of Aerospace Engineering, The University of Michigan, 3048 FXB, Ann Arbor, MI 48109-2140 P. Palmer Astronomy and Astrophysics Center University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 A. Zaitsev Institute of Radioengineering and Electronics Vedensky Square 1, 141120 Friazino, Russia Y. Koyama Kashima Space Research Center / Communications Research Laboratory 893-1 Hirai, Kashima-Machi, Ibaraki 314, Japan A. Nakamura Graduate School of Sci. & Tech., Kobe Univ. 1-1 Rokkodai-cho, Nada-ku, Kobe, 657-8501, Japan A. W. Harris DLR Institute of Space Sensor Technology and Planetary Exploration Rutherfordstrasse 2, 12489 Berlin, Germany M. N. Meshkov Special Design Office, Moscow Energetic Institute, Krasnokazarmennaia 14, 111250 Moscow, Russia. -----------------------------------------------------*current address: 5209 21st Ave NE, Seattle WA 98105 **current address: Scriptics Corp., 2593 Coast Ave., Mountain View, CA 94043 ***current address: Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450
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JPL E-mail mailstop ----------------------------------------------------------------------------Hudson 509 335-0922 335-3818
[email protected] Ostro 818 354-3173 354-9476 300-233
[email protected] Jurgens 818 354-4974 354-6825 238-420
[email protected] Rosema 818 393-2629 354-9476 300-233
[email protected] Giorgini 818 393-3107 393-1159 301-150
[email protected] [email protected] Winkler 760 255-8259 255-8515 DSCC-61 Rose 760 255-8259 255-8515 DSCC-61
[email protected] Choate 760 255-8259 255-8515 DSCC-74
[email protected] [email protected] Cormier 760 255-8358 255-8554 DSCC-74 Franck 818 354-6842 354-6825 238-420
[email protected] Frye 818 354-6874 354-6825 238-420
[email protected] Howard 818 354-8753 354-8153 125-177
[email protected] Kelley 760 255-8358 255-8554 DSCC-74
[email protected] [email protected] Littlefair 760 255-8355 255-8354 DSCC-74 Slade 818 354-2765 354-6825 238-420
[email protected] Benner 818 354-7412 354-9476 300-233
[email protected] Thomas 650 210-0136 210-0101
[email protected] Mitchell 510 643-1561
[email protected] Chodas 818 354-7795 393-1159 301-150
[email protected] Yeomans 818 354-2127 393-1159 301-150
[email protected] Scheeres 734 615-3282 763-0578
[email protected] Palmer 773 702-7972 702-8212
[email protected] Zaitsev +7-095-526-9047
[email protected] Koyama +81-299-847143 +81-299-847159
[email protected] Nakamura +81-78-803-6483 +81-78-803-5349
[email protected] Harris
[email protected] Meshkov +7-095-362-1029 +7-095-362-5576
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55 pages, 12 tables, 14 figure Keywords: ASTEROIDS, RADAR
editorial correspondence and proofs should be directed to:
R. S. Hudson School of Electrical Engineering and Computer Science, Washington State University, Pullman, Washington 99164-2752 E-mail:
[email protected] Phone: (509) 335-0999 Fax: (509) 335-3818
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We report 8510-MHz (3.5-cm) radar observations of the Earth-crossing asteroid (ECA) 6489 Golevka (1991 JX) obtained between June 3-15, 1995 at Goldstone, the Very Large Array, and the Evpatoria (Ukraine) and Kashima (Japan) radio antennas. Onedimensional Doppler spectra are used to estimate the object's convex hull, refine the ephemeris, and yield four possible pole directions. Three-dimensional modeling using twodimensional delay-Doppler images and published lightcurves unambiguously defines the pole and reveals an extraordinarily angular shape with flat sides, sharp edges and corners, and peculiar concavities. The equivalent diameter of the object is 530±30 m, with moments of inertia about the (long, intermediate, short) axes proportional to (1.00,1.38,1.39) ±0.1. The asteroid’s pole direction is λ = 202 ± 5° , β = −45 ± 5° , and its sidereal period is P = 6.0289 ± 0.0001hr .
The asteroid's circular polarization ratio, SC/OC = 0.23±0.02, is lower than the average for radar-detected near-Earth asteroids and reveals only a modest degree of nearsurface roughness at scales near the 3.5-cm wavelength. However the approximately Lambertian radar scattering law implies considerable surface roughness at larger scales. The asteroid’s radar scattering law is modeled as ρ cos n θ , with ρ = 0.25 ± 50% and n = 1.7 ± 0.7 giving an equivalent spherical albedo of 0.18 ± 50% . This value is in the middle
of the distribution of albedos of S-class asteroid’s previous imaged by radar. The Hapke parameters describing the object's optical scattering properties are w = 0.173 ± 0.006 , h = 0.024 ± 0.012 , B0 = 1.026 ± 0.448 , and θ = 19.5 ± 4.4° . Both the optical and radar
scattering properties are consistent with those of a typical S class asteroid. Goldstone-VLA plane-of-sky images do not resolve the asteroid but do provide astrometry with uncertainties less than 0.1 arcsec. Integration of an orbit based on all available radar and optical astrometry shows that Golevka has an insignificant probability of collision with any planet during at least the next nine centuries. We investigate Golevka’s dynamical environment, assuming uniform density. Some areas of the surface are characterized by large enough slopes that we expect they are exposed, solid, monolithic rock.
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INTRODUCTION Golevka was discovered in May 1991 at Palomar by E. F. Helin (Marsden 1991), three weeks before passing 0.036 AU from Earth. It was detected in June of that year at Arecibo and Goldstone (Ostro et al. 1991a) at a time when the available delay-Doppler data-acquisition capabilities were being improved. The observations included some of the first asteroid timedelay measurements with sub-microsecond resolution. In March 1995 Golevka was recovered at Siding Spring Observatory (Williams 1995) 3600 arcsec from the position predicted by an optical-only orbit, but only 5 arcsec from the position predicted from an orbit that includes the radar data from 1991. Golevka's orbit is close to the 3:1 mean-motion resonance with Jupiter and has a 3.995-year period. The 1995 approach to 0.034 AU on June 9 provided an excellent opportunity for ground-based investigations. Mottola et al. (1997, hereafter M+97) conducted an extensive international campaign of optical photometry and infrared radiometry, obtaining estimates of the asteroid's sidereal spin period (6.0264 ±0.002 hr), pole direction (β=35±10º, λ=347±10º), and Hapke parameters. They used radiometric observations to estimate the asteroid's approximate dimensions as 0.35 x 0.25 x 0.25 km. They concluded that it has a high visual geometric albedo (~0.6) marking it as an unusual object and tentatively assigned a V classification. At one point, the nominal design of a Clementine II mission included a Golevka encounter in June 1999 (Hope et al. 1997). Here we report observations that reveal this object as distinctly different from other asteroids imaged by radar or by spacecraft. The object’s unique characteristics are not those suggested by M+97. However we confirm those authors’ estimate of the asteroid’s elongation.
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OVERVIEW OF OBSERVATIONS We observed Golevka with the Goldstone X-band (8510-MHz, 3.52-cm) on June 3, 4, and 6–15, 1995 using a variety of radar configurations aimed at characterizing the object, refining its orbit, and establishing the technical feasibility of novel radar experiments. (Arecibo was being upgraded at the time and was unavailable.) Goldstone's 70-m antenna, DSS-14, was used for all transmissions. On June 14 we conducted radar aperture-synthesis observations, with Goldstone transmitting and the VLA receiving. The resultant images yield plane-of-sky positions with uncertainties of a few hundredths of an arcsec. On June 13–15 we carried out the first intercontinental radar astronomy experiment. These bistatic observations consisted of cw transmissions from Goldstone and reception at the Evpatoria (Ukraine) 70-m antenna on each of those dates (Zaitsev et al. 1997) and reception at the Kashima (Japan) 34-m antenna on June 15 (Koyama et al. 1995). We also attempted bistatic observations with reception at the Weilheim (Germany) 30-m antenna, but those were not successful. Throughout the bistatic experiments, we received echoes at DSS-13, the 34-m Goldstone antenna about 22 km from DSS-14. Monostatic observations with DSS-14 used three different configurations: a cw (Doppleronly) setup with 0.5Hz spectral resolution, a “low-resolution” delay-Doppler imaging setup with 0.25µs x 1.0Hz pixels, and a “high-resolution” imaging setup of 0.125µs x 0.5Hz that placed more than 100 pixels on the asteroid. We completed several hundred transmit/receive cycles (runs) each setup. The asteroid's ~6.0-hour synodic rotation period was evident from comparison of the delay-Doppler image sequences from consecutive days. Poleward motion of the subradar latitude was apparent from contraction of the echo bandwidth after closest approach. Most of the
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Goldstone tracks were more than six hours long, permitting very thorough orientational coverage of the asteroid. Echo spectra resulting from cw observations can be thought of as one-dimensional images, or brightness scans, across the target through a slit parallel to the asteroid's apparent spin vector. The bandwidth of a target's instantaneous echo power spectrum is proportional to the breadth, measured normal to the line of sight (LOS), of the target's pole-on silhouette. Hence, measurements of echo edge frequencies as functions of rotation phase can be used to estimate the shape of the silhouette's convex envelope (hull) as well as the frequency of hypothetical echoes from the asteroid's center of mass (Ostro et al. 1988). Delay-Doppler images are obtained by using a modulated waveform to resolve echoes in time-delay as well as Doppler frequency. Constant-Doppler planes are parallel to both the LOS and the target's apparent spin vector; constant-delay planes are normal to the LOS. These two orthogonal sets of planes cut the target into rectangular cells in a manner analogous to the way one cuts a potato to make french fries, and each image pixel shows the sum of echo power in one cell. Beyond the leading edge, each cell can capture echoes from surface regions on either side of the apparent equator (the so-called north-south ambiguity), so the surface-to-image mapping is potentially two-to-one or even many-to-one, very unlike the one-to-one mapping of normal optical imaging. Therefore, modeling is usually required to resolve the ambiguity and to allow accurate interpretation of images (Hudson 1993). Accurate reconstruction of the target's shape is possible if the data sample non-equatorial views of the target, because then each point on the surface produces echoes with a unique delay-Doppler trajectory and inversion of the data yields a unique solution. The ideal situation combines complete rotational phase coverage with enough latitude excursion for the radar to see the entire surface. For Golevka, the geometry turned out to
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be advantageous and echo strength was high, of the same order as the predictions listed in Table I. In all our monostatic observations, DSS-14 transmitted a signal toward the asteroid for a duration near the echo's roundtrip time delay (RTT). Then we changed to a receiving configuration and recorded echoes for a duration that was shorter than RTT by about 15 seconds, the time needed to move the antenna's sub-reflector and change the focus from the transmit horn to the receive horn. Each of these transmit-receive cycles, or runs, produced between 20 and 30 seconds of data. Each delay-Doppler observation used a repetitive, binary-phase-coded cw waveform (e.g., Ostro 1993 and references therein) with a 127-element code and one of four time resolutions, or “bauds” (∆t = 11, 2, 0.25, or 0.125 µs; 1 µs provides 150 m of range resolution). The most important steps in real-time processing of digitized samples of the received signal's voltage were decoding via cross-correlation with the transmitted code, sorting of voltage samples by range bin, and spectral analysis with a 64-point Fast Fourier Transform (FFT). This procedure produced 64 x 127 arrays of echo power. Each array's unaliased frequency window equals 1/(PR x NCOH), where the code repetition period, PR = 127∆t, defines the waveform's time-delay window and NCOH is the number of PR-long time series of voltage samples that were coherently summed prior to Fourier analysis. Table II lists the key characteristics of our setups. (For a block diagram of the Goldstone radar system see Fig. 1 of Ostro et al. 1996.) Our cw observations used a frequency-switching technique identical to that described most recently by Ostro et al. (1992). During the monostatic cw runs, the transmitter was switched between two frequencies 250 Hz apart, and the echoes collected were sorted into two corresponding groups of power spectra, which then were summed.
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Each sum had echo at a different frequency, so either sum contained a sample of the echo-free noise background at the frequency of the other sum's echoes. Such a "frequency-hopped" data set permits reliable background removal and data normalization. During the VLA and international experiments, the transmitter frequency was held constant. Signals received at DSS-13 were downconverted to 50 MHz and passed over a fiber optic cable to DSS-14, where they were processed. To facilitate removal of the noise background contributed at DSS-14 (from the 50-MHz intermediate frequency down to baseband), we switched the frequency of the local oscillator at the first mixing stage between two frequencies 250 Hz apart. The pass bands of the low-noise amplifiers at DSS-13 were stable and very clean, and therefore this approach sufficed to produce reliable spectra. Our Goldstone-VLA observations used techniques described by de Pater et al. (1994). For the G-VLA portion of this work DSS-14 illuminated the object with a CW signal whose frequency was continually adjusted to compensate for Doppler shifts so that the return would arrive at the VLA at 8510 MHz. The VLA recorded the opposite-sense circular polarization return. At the time of the observations, the VLA was being reconfigured from the DnA-array to the A-array. On June 14, only 12 of the 24 antennas that provided useful data were in A-array locations. However, the longest baselines (36 km) were present so that full angular resolution could be obtained for a point-like source. (The VLA's finest frequency resolution, 384 Hz, is much too coarse to resolve Golevka echoes in frequency.) The observations were carried out while tracking the position interpolated from JPL orbit solution 21. Approximately every 10 minutes, the phase of the array was calibrated by observing the astrometric calibration source 2203+317 (positional uncertainty:
