Icarus 148, 37–51 (2000) doi:10.1006/icar.2000.6483, available online at http://www.idealibrary.com on Radar Observations and Physical Model 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. J. Ostro, R. F. Jurgens, K. D. Rosema,1 J. D. Giorgini, R. Winkler, R. Rose, D. Choate, R. A. Cormier, C. R. Franck, R. Frye, D. Howard, D. Kelley, R. Littlefair, M. A. Slade, L. A. M. Benner, M. L. Thomas,2 D. L. Mitchell,3 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, Michigan 48109-2140 P. Palmer Astronomy and Astrophysics Center, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637 A. Zaitsev Institute of Radioengineering and Electronics, Vedensky Square 1, 141120 Friazino, Russia Y. Koyama Communications Research Laboratory, Kashima Space Research Center, 893-1 Hirai, Kashima-Machi, Ibaraki 314, Japan A. Nakamura Graduate School of Science and Technology, Kobe University, 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 and M. N. Meshkov Special Design Office, Moscow Energetic Institute, Krasnokazarmennaia 14, 111250 Moscow, Russia Received February 21, 2000; revised April 20, 2000 convex hull, refine the ephemeris, and yield four possible pole direc- We report 8510-MHz (3.5-cm) radar observations of the Earth- tions. Three-dimensional modeling using two-dimensional delay- crossing asteroid (ECA) 6489 Golevka (1991 JX) obtained between Doppler images and published lightcurves unambiguously defines June 3 and June 15, 1995, at Goldstone, the Very Large Array the pole and reveals an extraordinarily angular shape with flat sides, and the Evpatoria (Ukraine) and Kashima (Japan) radio antennas. sharp edges and corners, and peculiar concavities. The equivalent One-dimensional Doppler spectra are used to estimate the object’s 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) = = 1 Current address: 5209 21st Ave. NE, Seattle WA 98105. 0.1. The asteroid’s pole direction is 202 5 , 45 5 , = 2 Current address: Scriptics Corp., 2593 Coast Avenue, Mountain View, CA and its sidereal period is P 6.0289 0.0001 h. 94043. The asteroid’s circular polarization ratio, SC/OC = 0.23 0.02, 3 Current address: Space Sciences Laboratory, University of California, is lower than the average for radar-detected near-Earth asteroids Berkeley, CA 94720-7450. and reveals only a modest degree of near-surface roughness at 37 0019-1035/00 $35.00 Copyright c 2000 by Academic Press All rights of reproduction in any form reserved. 38 HUDSON ET AL. scales near the 3.5-cm wavelength. However, the approximately The 1995 approach to 0.034 AU on June 9 provided an excel- Lambertian radar scattering law implies considerable surface lent opportunity for groundbased investigations. Mottola et al. roughness at larger scales. The asteroid’s radar scattering law is (1997, hereafter M+97) conducted an extensive international n modeled as cos , with = 0.25 0.12 and n = 1.7 0.7 giving campaign of optical photometry and infrared radiometry, ob- an equivalent spherical albedo of 0.18 0.09. This value is in the taining estimates of the asteroid’s sidereal spin period (6.0264 middle of the distribution of albedos of S-class asteroid’s previously 0.002 h), pole direction ( = 35 10,=347 10), and imaged by radar.The Hapke parameters describing the object’s opti- = = Hapke parameters. They used radiometric observations to es- cal scattering properties are w 0.173 0.006, h 0.024 0.012, . . B = 1.03 0.45, g =0.34 0.02, and ¯ = 20 5. Both the op- timate the asteroid’s approximate dimensions as 0 35 0 25 0 . tical and the radar scattering properties are consistent with those 0 25 km. They concluded that it has a high visual geometric of a typical S-class asteroid. albedo (0.6) marking it as an unusual object and tentatively Goldstone-VLA plane-of-sky images do not resolve the asteroid assigned a V classification. but do provide astrometry with uncertainties less than 0.1 arcsec. Here we report radar observations conducted on June 3, 4, Integration of an orbit based on all available radar and optical as- and 6–15, 1995 (Table I). Those observations and subsequent trometry shows that Golevka has an insignificant probability of modeling reveal Golevka to be an unusually shaped object with collision with any planet during at least the next nine centuries. surface properties fairly typical of an S-class asteroid. We investigate Golevka’s dynamical environment, assuming uni- form density. Some areas of the surface are characterized by large enough slopes that we expect that they are exposed, solid, mono- OVERVIEW OF OBSERVATIONS lithic rock. c 2000 Academic Press Key Words: asteroids; radar. Weobserved Golevka with the Goldstone X-band (8510-MHz, 3.52-cm) system using a variety of radar configurations aimed at characterizing the object, refining its orbit, and establishing the INTRODUCTION technical feasibility of novel radar experiments. Goldstone’s 70- m antenna, DSS-14, was used for all transmissions. On June 14 Golevka was discovered in May 1991 at Palomar by E. F.Helin we conducted radar aperture-synthesis observations, with Gold- (Marsden 1991), 3 weeks before passing 0.036 AU from Earth. stone transmitting and the VLA receiving. The resultant images It was detected in June of that year at Arecibo and Goldstone yield plane-of-sky positions with uncertainties of a few hun- (Ostro et al. 1991). In March 1995 Golevka was recovered at dredths of an arcsec. Siding Spring Observatory (Williams 1995) 3600 arcsec from On June 13–15 we carried out the first intercontinental radar the position predicted by an optical-only orbit, but only 5 arcsec astronomy experiments. These bistatic observations consisted of from the position predicted from an orbit that includes the radar cw transmissions from Goldstone and reception at the Evpatoria data from 1991. Golevka’s orbit is close to the 3 : 1 mean-motion (Ukraine) 70-m antenna on each of those dates (Zaitsev et al. resonance with Jupiter and has a 3.995-year period. 1997) and reception at the Kashima (Japan) 34-m antenna on TABLE I Experiment Overview Duration SNR Start–Stop DOY 1995 Date (hhmm–hhmm) RA Dec s RTT Date Run Setups 154 June 3 0900–1200 3.0 235 17 45 340 20 cw rng 155 June 4 0930–1220 2.8 240 20 42 480 27 cw 156 June 5 157 June 6 1015–1320 3.1 251 28 37 800 41 cw low 158 June 7 0750–1400 6.2 258 31 36 1000 50 cw G-V rng low 159 June 8 0750–1330 5.3 267 35 35 1200 55 Low high 160 June 9 0830–1515 6.7 277 38 34 1300 61 Low high 161 June 10 0830–1620 7.8 287 40 34 1300 61 Low high 162 June 11 0915–1645 7.5 299 40 36 1200 55 Low high 163 June 12 0910–1710 8.0 308 40 38 940 45 cw G-V low 164 June 13 0555–1717 11.3 317 39 41 690 35 cw G-E low high 165 June 14 0625–1755 11.5 324 38 44 570 29 cw G-E low high 166 June 15 0645–1755 11.2 331 37 47 390 21 cw G-E,K low high Note. DOY is day of year, RA is right ascension, Dec is declination, and RTT is echo roundtrip time delay (equal numerically to the approximate distance in 10–3 AU). Predicted values of the echo’s signal-to-rms-noise ratio (SNR) per date and the maximum SNR per run were based on conservative assumptions about the target and the radar system. Setups (see Table II) are abbreviated cw (continuous wave), rng (coarse-resolution ranging), low (low-resolution imaging), and high (high-resolution imaging). G-V, G-E, and G-K indicate observations that used cw transmissions from Goldstone (DSS-14) and reception of echoes at the VLA, Evpatoria, or Kashima (see text); in those experiments, we also received echoes at DSS-13. GOLEVKA RADAR OBSERVATIONS AND MODEL 39 June 15 (Koyama et al. 1995). We also attempted bistatic ob- from the DnA-array to the A-array. On June 14, only 12 of the servations with reception at the Weilheim (Germany) 30-m an- 24 antennas that provided useful data were in A-array locations. tenna, but those were not successful. Throughout the bistatic ex- However, the longest baselines (36 km) were present so that full periments, we received echoes at DSS-13, the 34-m Goldstone angular resolution could be obtained for a point-like source. (The antenna about 22 km from DSS-14. VLA’s finest frequency resolution, 384 Hz, is much too coarse Monostatic observations with DSS-14 used three different for resolving Golevka echoes in frequency.) The VLA received configurations: a cw (Doppler-only) setup with 0.5-Hz spec- echoes while tracking the position interpolated from JPL orbit tral resolution, a “low-resolution” delay-Doppler imaging setup solution 21. Approximately every 10 min, the phase of the array with 0.25-s 1.0-Hz pixels, and a “high-resolution” imaging was calibrated by observing the astrometric calibration source setup of 0.125 s 0.5 Hz that placed more than 100 pixels 2203 + 317 (positional uncertainty, <0.00200).