Radar Observations of Near Earth Asteroids

Yasuhiro Koyama (koyama(AT)nict.go.jp)

Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan

1. Introduction

Planetary radar technique has revealed its unique capability to investigate geometry and surface properties of various solar system bodies (Ostro, 1993). The radar technique has advantages over optical techniques in its high spatial resolution and ability to construct three dimensional images. Recently, deep space spacecraft can make more detailed investigations, but the planetary radar is still playing an important role since observation opportunities are far more frequent than space missions.

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).

2. Radar Experiment of the 6489 Golevka

In the bi-static radar experiment, high power radio signal was transmitted at 470 kW towards the asteroid 6489 Golevka from a 70m antenna at Goldstone, which is one of the worldwide deep space telecommunication facilities operated by Jet Propulsion Laboratory (JPL). The signals reflected from the surface of the asteroid were then received by the 34m antenna at Kashima (Fig.2). Since our objective was to detect the radar echo from an asteroid for the first time in Japan and to ensure the feasibility of conducting international asteroid radar experiments, a CW wave form was transmitted in Left-Hand-Circular-Polarization without any modulation which makes time delay measurements possible.

Figure 2. Principle of bi-static radar observations towards Near Earth Asteroids.

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 Fig.3.

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 ${\cal P}(\Delta f)$ 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 ${\cal P}(\Delta f)$ 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,

where f0 is the frequency of the central frequency of the received echo signal, c is the velocity of light, and theta is the angle between the apparent rotation axis and the direction towards the receiving station seen from the asteroid (Fig.5).

Figure 5. Geometric relation between the rotation axis of the asteroid and the observation site.

From Fig.4, by taking the threshold of signal existence to be ${\cal P}(\Delta f)$ = 2.96 from a confidence limit of 99%, the lower edge of the radar echo signal is between -1.8 Hz and -1.7 Hz, whereas upper edge is between 1.5 Hz and 1.6 Hz. Thus the frequency width of the radar echo can be estimated as W = 3.3 +/- 0.2 Hz. If we employ 6.02 hours as the rotation period of the asteroid (Mottola et al., 1995, Hudson and Ostro, 1995), D sin(theta) = 0.40 +/- 0.02 km. This value places a lower bound of about 0.4 km on the asteroid's maximum pole-on breadth. The total power of a radar echo signal is proportional to an effective cross section, rhorA, of a target asteroid where rhorA is a radar albedo and A is a cross section of the asteroid. The relation can be expressed as

Here, lambda is the wavelength of the radar signal, Ptx is the transmitting signal power, Gtx is the gain of the transmitting antenna, Rtx is the distance of the asteroid from the transmitting station, Prec is the received signal power, Grec is the gain of the receiving antenna, and Rrec is the distance of the asteroid from the receiving station. On the other hand, the standard deviation for power spectrum of noise, N, can be evaluated by equation,

where kB is Boltzmann's constant, Tsys is the system noise temperature of the receiving station, B is the frequency resolution of the power spectrum, and t is the integration time. The normalized total power spectrum of the received data obtained by integrating ${\cal P}(\Delta f)$ can then be evaluated as

where the integration should be done for the frequency range where radar echo signal is present. The total power of the radar echo signal was evaluated as 20.5 X N. This value gives rhorA = 0.11 (km2). This radar cross section shows good agreement with Arecibo 1991 results and monostatic Goldstone 1995 results. The effective diameter of Golevka has been estimated to be no greater than 600 m from a preliminary inversion of Goldstone delay-Doppler images (Mottola et al., 1995; Hudson and Ostro, 1995). Thus the estimated effective diameter implies a radar albedo of at least 0.39, or several times larger than the typical values reported to date for small asteroids (Ostro et al., 1991; Ostro et al., 1996). From this result, it is expected that this object's surface is unlikely to be porous, that is, it probably lacks a regolith.

3. Future Asteroid Radar Observations

The success of the first asteroid radar experiment from Japan is quite encouraging. In the near future, radar observations to 1982TA and 1994PC1 are being considered. In these experiments, the phase-modulated signal will be transmitted to the asteroids to make the ranging observations possible. Also, the technical feasibility of radar interferometric observations will be investigated by using two large antennas at Kashima and at Usuda.


Hudson R. S. and Ostro S. J. 1995, Bull. Amer. Astron. Soc. 27, 1063

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

Updated on October 28, 1996. Return to CONTENTS