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
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,
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
= 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
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.
References
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
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.
