Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan
The asteroid 6489 Golevka (= 1991 JX) approached Earth to the
geocentric distance of 0.034 AU on June 9, 1995. The asteroid is an
Apollo object and has an earth-crossing orbit with perihelion and
aphelion distances of 1.0098 and 4.0328 AU, respectively (Fig.1).
Figure 1. Orbits of the asteroid 6489 Golevka and planets and their positions on June 9, 1995.
The asteroid was a very good target of international bi-static radar experiment since it had a high value of declination when it approached earth. Taking this opportunity, an international asteroid radar experiment was organized as a collaboration between Japan, Russia, and the United States. In the bi-static radar experiment, a high power radio signal was transmitted from 70m antenna at Goldstone, and the radar echo reflected at the surface of the asteroid was received by the 34m antenna at Kashima along with other antennae at Evpatoria in Ukraine, and at Goldstone. The radar echo was successfully detected and the asteroid became the first solar system object observed by means of radar from Japan. The name of the asteroid was proposed to the Minor Planet Center of the International Astronomical Union after the radar experiment to honor the success by taking two or three letters of three ground stations (GOLdstone-EVpatoria-KAshima = GOLEVKA) and it was officially approved in January, 1996 (Minor Planet Circular 26245, 1996).
In this report, the results of the Golevka radar observations and the future prospects of asteroid radar experiments will be discussed. The full detail of the Golevka experiment can be found in the papers by Koyama et al. (1996) and by Zaitsev et al. (1996).
At Kashima, the received signal was down-converted to about
10 kHz and sampled at 48 kHz sampling rate after a
low-pass-filter. The low-pass-filter is built-in in a DAT
digital data recorder unit, which has a cut frequency at 20 kHz.
The digitized data were recorded on a DAT tape. Both Right-Hand-
Circular-Polarization and Left-Hand-Circular-Polarization data
were recorded. A DAT tape can record two data channels for two
hours. The configuration of the observing system is shown in
Figure 3. Block diagram of the observation system set-up used at Kashima.
Observations were made for two hours on June 15, 1995, about a week after the closest approach of the asteroid to the Earth. The geocentric distance to the asteroid at the time of observations was about 0.048 AU. After the observations, the data recorded on a DAT tape were transferred to a UNIX workstation through a GP-IB interface. Power spectrum of the signal was calculated from 5 seconds of data duration at frequency resolution of 0.2 Hz, and then integrated for the period that the power of received radar echo was stable, which is a span of 54 minutes, by compensating the Doppler-shift (Fig.4). The radar echo is apparent in the RHCP power spectrum. The echo signal spectrum has a broad frequency width in spite the transmitted signal was a pure CW signal because of the rotation of the asteroid.
Figure 4. Power spectrum of the received signal integrated for 54 minutes with frequency resolution of 0.2 Hz.
The maximum dimension of the asteroid perpendicular to its apparent rotation axis D and its apparent rotation angular velocity omega can be related to Doppler frequency width W as,
Koyama, Y., M. Yoshikawa, J. Nakajima, M. Sekido, T. Iwata, A. M. Nakamura, H. Hirabayashi, T. Okada, M. Abe, T. Nishibori, T. Nakamura, T. Fuse, S. J. Ostro, D. K. Yeomans, D. Choate, R. A. Cormier, R. Winkler, R. F. Jurgens, J. D. Giorgini, K. D. Rosema, D. L. Mitchell, M. A. Slade, and A. L. Zaitsev, "Radar Observations of an Asteroid 6489 Golevka," Publ. Astron. Soc. Jpn.( submitted) 1996
Minor Planet Circular 26245, Minor Planet Center, International Astronomical Union, 1996
Mottola S., Erikson A., Harris A. W., Hahn G., Neukum G., Buie M. W., Sears W. D., Tholen D. J., et al. 1995, Bull. Amer. Astron. Soc. 27, 1055
Ostro S. J., Cambell D. B., Chandler J. F., Hine A. A., Hudson R. S., Rosema K. D., and Shapiro I. I. 1991, Science 252, 1399
Ostro, S. J., "Planetary radar astronomy," Rev. Modern Phys., 65, pp.1235-1279, 1993
Ostro S. J., Jurgens R. F., Rosema K. D., Hudson R. S., Giorgini J. D., Winkler R., Yeomans D. K., Choate D., et al. 1996, Icarus 121, 44
Zaitsev, A. L., S. J. Ostro, Y. Koyama, D. K. Yeomans, S. P. Ignatov,
M. Yoshikawa, D. Choate, A. G. Petrenko, R. A. Cormier, O. K. Margorin,
R. Winkler, V. V. Mardyshkin, R. F. Jurgens, O. N. Rghiga, J. D. Giorgini,
V. A. Shubin, M. A. Slade, A. P. Krivtsov, Y. F. Koluka, A. M. Nakamura,
A. L. Gavrik, D. V. Ivanov, F. S. Peshin,
"Intercontinental Bistatic Radar Observations of 6489 Golevka (1991 JX),"
Planetary and Space Sci. ( submitted) 1996