Radar Investigations of Asteroid 33342 (1998 WT24) in December 2001 with Evpatoria-Medichina and Goldstone-Medichina Bistatic Systems

In order to test two bistatic radar systems Evpatoria (70-m dish, 150-kW continuous transmitter at 6-cm wavelength) - Medichina (32-m dish, two polarization channel low-noise receiver) and Evpatoria - Svetloe (32-m dish, low noise receiver) before forthcoming radar research of NEA 1998 WT24, in August-September 2001 the radar of Russian calibrating satellite ETALON-1 was made. This ordinary fact had a rich and unexpected response in Italian mass media as a first radar astronomy experiment in this country - at least dozen newspaper articles on that and related, asteroid hazard, topics were published.

After, seemingly under the impression of above Italian publications, in Russia at the front page of the central newspaper "Izvestia" (“News” in English, No 164 (25602) on Sept 7, 2001) turned up an article under the loud sub-heading "Unprecedented experiment destined to save world". But actually it was the usual job, which, in addition, was made under very trying conditions in the absence thereof of any financial support from Russian budgetary or grant financing, and its implementation was possible due to ardor of people, only.

Abstract for the International Conference ACM-2002 "Asteroid, Comets, Meteors", Berlin, Germany, 29 July 2 August, 2002

Radar Detection of NEA 33342 (1998 WT24) with Evpatoria => Medichina System at 6 cm

A. L. Zaitsev (IRE RAS), M. di Martino (OATO), A. A. Konovalenko (IRA NASU), S. Montebugnoli (IRA), S. P. Ignatov (RISDE), Y. F. Kolyuka (MCC), A. S. Nabatov (IRA NASU), I. S. Falkovich (IRA NASU), A. L. Gavrik (IRE RAS), Y. A. Gavrik (MPTI), C. Bortolotti (IRA), A. Cattani (IRA), A. Maccaferri (IRA), G. Maccaferri (IRA), M. Roma (IRA), M. Delbo (OATO), L. Saba (SAO).

We present the first radar detection of near-Earth asteroid 33342 1998 WT24 that was performed with bistatic system Evpatoria ® Medichina at 6 cm wavelength during WT24 close approaching with Earth at a minimal distance of 0.0125 AU in mid of December, 2001. The 70-m parabolic antenna and power S-band transmitter (central frequency 5.01 GHz, effective aperture 2500 m², continuous power 150 kW and 90 kW in two and one-klystron modes, respectively) were used in Evpatoria Space Center (Crimea, Ukraine), and 32-m parabolic antenna and low-noise two-channel polarization receiver (effective aperture 500 m², system noise temperature 50 K) – in Medichina Radio Astronomical Station (near Bologna, Italy). In order to test Evpatoria ® Medichina system before WT24 radar experiment, the Etalon-1 spherical calibrating satellite as radar target was used. Doppler ephemerides were introduced into transmitted frequency in order to avoid spectral smearing during receiving. Receiving signals after analog to digital conversions and CD recording were mailed to IRE RAS for off-line data processing.

Radar echo from 1998 WT24 was detected during observational days on December 16 and 17, 2001. The signal to noise ration in the opposite sense of circular polarization (OC) were about 3.4 for filter, matched with echo bandwidth, which results in OC radar cross section of 0.08 km², [*]. Doppler trend of spectral center during receiving was about 0.3 mHz/sec, and the echo spectrum bandwidths, estimated at first cross of background level, are 6.5 Hz and 6.2 Hz, for December 16 and 17. So, the lower bounds on the WT24’s maximum pole-on breadth for rotation period 3.7 hours are 0.41 km and 0.39 km, respectively. However, the more realistic, optical based size of WT24 is about 1 km. In that case the formalized estimation of radar albedo gives ~ 0.1. The most interesting result of our radar measurements is a near-unity circular polarization ratio both for December 16 and 17. The same value for WT24 was revealed earlier by Ostro [1] at 3.5 cm in Goldstone. Such combination of low radar albedo and near-unity circular polarization ratio would be interpreted as a property of comet-like surface of 33342 1998 WT24.

[*] It was an invalid estimation of radar cross section, sorry. The correct values is about 0.02 km², which for 410-m asteroid size gives the upper limit on radar albedo estimation < 15 %.

[1] S. Ostro (2001) Personal e-mail communication.

Paper for ACM-2002 Proceedings, accepted on August, 2002

RADAR DETECTION OF NEA 33342 (1998 WT24) WITH EVPATORIA => MEDICINA SYSTEM AT 6 CM

Alexander L. Zaitsev ⁽¹⁾, Mario di Martino ⁽²⁾, Alexander A. Konovalenko ⁽³⁾, Stelio Montebugnoli ⁽⁴⁾, Sergei P. Ignatov ⁽⁵⁾, Yury F. Kolyuka ⁽⁶⁾, Alexander S. Nabatov ⁽³⁾, Igor S. Falkovich ⁽³⁾, Anatoly L. Gavrik ⁽¹⁾, Yury A. Gavrik ⁽⁷⁾, Claudio Bortolotti ⁽⁴⁾, Alessandro Cattani ⁽⁴⁾, Andrea Maccaferri ⁽⁴⁾, Giuseppe Maccaferri ⁽⁴⁾, Mauro Roma ⁽⁴⁾, Marco Delbo ⁽²⁾, Laura Saba ⁽⁸⁾, Tatiana Afanas’eva ⁽⁶⁾, Tatiana Gridchina ⁽⁶⁾ Simona Righini ⁽⁸⁾.

⁽¹⁾ IRE RAS, Vvedensky Square 1, 141190, Fryazino, Russia, alzaitsev@ms.ire.rssi.ru

⁽²⁾ Osservatorio Astronomico di Strada Osservatorio, Torino, Italy.

⁽³⁾ Radioastronomical Institute, National Academy of Science, Ukraine, Kharkiv.

⁽⁴⁾ Institute of Radio Astronomy, Medicina, Italy.

⁽⁵⁾ Russian Institute of Space Device Engineering, Moscow, Russia.

⁽⁶⁾ Mission Control Center, Koroliov, Russia.

⁽⁷⁾ Moscow Institute of Physics and Technology, Dolgoprudny, Russia.

⁽⁸⁾ Sardinian Astronomical Observatory, Italy.

ABSTRACT

We present the first radar detection of near-Earth asteroid 1998 WT24 that was performed with the bistatic system Evpatoria ® Medicina operating at 6 cm wavelength during WT24 close approaching to Earth at a distance of 0.0125 AU. Radar echo from WT24 was detected during observational days on Dec. 16, and 17, 2001. The maximum value of signal to noise level ratio was about 0.4 for filter, matched with echo bandwidth, which results in OC radar cross section of 0.02 km². Doppler trend of spectrum center doesn’t exceed 0.44 mHz/sec, the echo spectrum bandwidth varies from 6.2 to 6.7 Hz and the WT24’s pole-on breadth is ~0.41 km, the estimation of radar albedo is ~0.1. Of most interest is the measurement result giving near-unity circular polarization ratio. Such combination of low radar albedo and near-unity circular polarization ratio may be interpreted as a property of comet-like surface.

1. INTRODUCTION

Near-Earth asteroid (NEA) 33342 (1998 WT24) was discovered by LINEAR Group, New Mexico on 25 Nov. 1998. This asteroid, a Mercury/Venus/Earth-crosser, is one of only few known objects with such small perihelion and aphelion distances (one of other is 2000 EE14). When 1998 WT24 was discovered, its approach was expected as an unprecedented opportunity for radar investigation of a small body. On Dec. 16 and 17, 2001 asteroid 1998 WT24 passed at the distances of 0.0125 AU and 0.0139 AU respectively from Earth at a declination of 42 deg and therefore was visible by both bistatic radar systems Evpatoria ® Medicina and Goldstone ® Medicina. In order to test Evpatoria ® Medicina system before 1998 WT24 radar experiment, the Etalon-1 spherical calibrating satellite was used as radar target.

2. OBSERVATIONS

Observational and data reduction techniques were very similar to those described by Zaitsev et al. [1, 2]. Table 1 lists nominal parameters of the transmitting and receiving systems used in the observations. The radar detection of NEO WT24 was performed using bistatic system Evpatoria ® Medicina at l=6 cm wavelength during the WT24 close approach to Earth at a minimal distance of 0.0125 AU in mid of December, 2001. The 70-m parabolic antenna and power S-band transmitter (continuous power 150 kW and 90 kW in two and one-klystron modes, respectively) were used in Evpatoria Space Center (Crimea, Ukraine), also a 32-m parabolic antenna with a low-noise two-channel polarization receiver (in the same sense of circular polarization as transmitted SC and in the opposite sense OC) – in Medicina Radio Astronomical Station (near Bologna, Italy) was used.

Table 1. System parameters

	Evpatoria	Medicina
Antenna diameter	70 m	32 m
Effective aperture	2500 m²	390 m²
System temperature		50 K
Continuos power	90 kW
Polarization	Right Circular	Right and Left Circular
Central frequency	5010.024 MHz

Evpatoria radiated an unmodulated continuous-wave (cw), circularly polarized signal with extremely high frequency stability. To avoid spectral smearing of the radar echoes in Medicina, the transmitted frequency was 5010 + D_shift(t), where D_shift(t) is an a priori predicted Doppler shift designated to place Evpatoria ® Medicina echoes at 5010 MHz. The ephemeredes for blind pointing of Evpatoria antenna and Doppler shift prediction were calculated employing three different software programs in the Mission Space Center, Koroliov, Russia, the Institute of Applied Mathematics, Moscow, Russia, and the Institute of Radio Astronomy, Bologna, Italy. The calculations mentioned above gave the comparable results with negligible variances within a few arc seconds for angle coordinates (besides, the Evpatoria antenna’s beam width is about 2.6 arc minutes at 6 cm).

The Medicina receiver used the essential part of existing system for radioastronomy application. During reception, the low-noise-amplified signal was mixed with lower frequencies and filtered, and then digital samples of the signal’s voltage was recorded directly into a format more amenable for post-real-time processing. The radar echo from 1998 WT24 at 6 cm was not detected on-line in Evpatoria ® Medicina system with real-time spectral analysis, while the same radar echo from this asteroid was successfully detected in Goldstone ® Medicina system at 3.5 cm. The detailed off-line analysis of time-domain information, recorded in Medicina in order to try detecting the 1998 WT24 radar echo at 6-cm wavelength was carried out in the Institute of Radio Engineering and Electronic, Fryazino, Russia. The echo was detected in six time intervals (see Table 2).

Table 2. Observations

No	Time (UTC)	Dist. (AU)	Phase* (deg)
December 16
1	13:00 - 13:55	0.0125	0 – 89
2	18:10 - 19:10	0.0126	143 – 24
3	19:35 - 20:10	0.0126	280 – 337
4	20:35 - 21:00	0.0127	18 – 58
December 17
5	12:18 - 13:08	0.0138	109 – 190
6	13:15 - 14:15	0.0139	201 – 298

* We assumed that the phase angle of rotated asteroid equals to zero at the time of the first detection of echo from WT24 and its spin rate equals 3.6977 h.

3. RADAR DATA ANALYSIS

The average properties of the asteroid were measured using a weighted average of the power spectra. At first spectra over longer time intervals were summed, the measured frequency drift F(t) was compensated by multiplying the digital voltage samples by a complex factor exp[ij(t)], where j(t) is a phase factor corresponding to F(t). Then the phase-rotated samples were fast-Fourier-transformed and power spectra were formed. The raw frequency resolution was 0.15 Hz and the echo is situated near the middle of the spectral window 156 Hz. The integration time per spectrum was 90 sec, during which WT24 rotated through 2.4^o of rotation phase. Fig. 1 shows the signature-time-plots OC (top panel) and SC (bottom panel) echoes.

Fig. 1. The echoes signature-time-plots (OC - top panel and SC - bottom panel) from cw observations for six time intervals are given according to Table 2. Black color corresponds to maximum spectral power, white one corresponds to the absence of echo.

The signatures from Fig.1 show considerable variations in the spectral power, some of which (Dec. 16, 13:41 – 13:45 UTC and 18:32 – 18:48 UTC) might be due to inexact orientation of transmitting antenna. The spectral power for 2-4 time intervals is greater than other that might be due to mistakes in the sampling frequency (less than Nyquist frequency) for 1,5,6 time intervals.

The signatures show that echoes are centered about 0 Hz from the on-line ephemeris prediction and the off-line frequency drift compensation. The echo center frequencies were obtained with the accuracy 0.15 Hz that permits their use for more precise determination of the 1998 WT24 orbit. Echo bandwidths, which depend on size, spin period and aspect angle, show small variations (indicative of WT24’s shape is almost sphered) and maximum bandwidth not exceeding the value 6.7 Hz.

4. ANALYSIS OF SPECTRA

The minimal radar cross section can be estimated using equation [3]:

s = 4pl²(R_TR_R)²(A_TA_RP_T)^-1kT_sDf (P_s/P_n).

Here R_T, R_R– transmitter-asteroid and asteroid-receiver distances, A_T, A_R – transmitter and receiver effective aperture, P_T – transmitted power, T_s – receiver noise temperature, Df - frequency resolution matched to the echo spectrum, P_s/P_n – signal to noise level ratio (it doesn’t depend on time integration) for filter, matched with echo bandwidth. The parameters of total spectra with frequency resolution ~9 Hz are represented in the Table 3.

Table 3. Data for filter, matched with echo bandwidth.

No	P_s/P_n	DP_n	SNR	m_c	s, km²
1. OC	0.24	0.005	43	1.07	>0.013
SC	0.25	0.007	36	-	>0.014
2. OC	0.38	0.008	48	0.99	0.020
SC	0.37	0.005	73	-	0.020
3. OC	0.40	0.008	48	1.05	0.021
SC	0.42	0.007	56	-	0.022
4. OC	0.40	0.006	59	0.95	0.021
SC	0.38	0.009	41	-	0.020
5. OC	0.14	0.005	25	1.04	>0.010
SC	0.14	0.007	19	-	>0.010
6. OC	0.19	0.005	37	1.01	>0.014
SC	0.19	0.005	34	-	>0.014

Standard deviation DP_nwas determined by the random noise fluctuation relatively to the average background noise level. The value of DP_n for OC and SC can strongly differ from each other. Signal-to-noise ratio SNR depends on time integration and it is equal to SNR = P_s/DP_n. The high value SNR > 50 was observed on 16 Dec. 18:10 – 21:00 only.

Polarization ratio [3] is equal to m_c = s_sc / s_oc, where s – radar cross section for OC or SC polarization. The value of polarization ratio for all data is nearly 1 without any significant variations. The big differences of radar cross sections in time are presumably due to differences in the recording conditions. The more reliable value s was obtained on 16 Dec. 18:10 – 21:00 only. But the ratio of radar cross sections for observations 1 and 6 is in proportion to the ratio of (R_TR_R)²for these times.

The echo’s Doppler frequency dispersion (bandwidth)

B = (4p D/lT) sina

depends on the asteroid’s size D, the synodic rotation period T and the aspect angle a between the spin vector and the line of sight [3]. Fig. 2 shows the OC and SC echo spectra for six time intervals (Table 2). The frequency resolution was 0.15 Hz, random noise fluctuations were small enough and B can be defined from Fig.2.

Fig. 2. Weighted sums of cw echo spectra obtained for WT24 in six time intervals in the OC and SC polarization (thick and thin curves). Value f = 0 Hz corresponds to hypothetical echoes from the asteroid’s center of mass. Echo spectra are normalized to the average noise level.

The echo spectrum bandwidths, estimated at first cross of background level, are 6.6 Hz and 6.3 Hz for Dec. 16 and 17 respectively. So, the lower bounds on the WT24’s maximum pole-on breadth for rotation period 3.7 hours are 0.42 km and 0.40 km, respectively. The central frequency of spectra on Fig.2 is derived as a spectrum’s center of mass and its value is equal to 0 after rms Doppler linear trend Df was removed.

Table 4. The main parameters of echo spectra

No	P_max	DP_n	P_int	B(Hz)	Df(mHz/s)
1.OC	0.43	0.025	12.8	6.7	0.436
SC	0.48	0.026	13.9	6.7	-
2. OC	0.78	0.027	20.4	6.7	0.208
SC	0.72	0.028	20.2	6.7	-
3. OC	0.89	0.030	21.5	6.2	0.330
SC	0.93	0.030	22.2	6.2	-
4. OC	0.80	0.034	21.7	6.6	0.304
SC	0.73	0.033	20.4	6.6	-
5. OC	0.29	0.023	7.4	6.0	0.342
SC	0.30	0.024	7.8	6.0	-
6. OC	0.43	0.023	10.1	6.3	0.158
SC	0.40	0.022	10.0	6.3	-

The main parameters of WT24’s spectra with frequency resolution ~0,15 Hz are represented in the Table 4. The unexpected results are that the spectra for OC and SC polarization have coinciding parameters for the different rotating phases: the differences between the bandwidths, the maximum powers P_max and the integral powers P_int of echo spectra for OC and SC polarization are extremely small.

5. SUMMARY

The Evpatoria ® Medicina data, considered along, offer joint constraints on WT24’s dimension and any properties. The echo energy achieved for 1998 WT24 was high enough for estimating its size: the hull of polar silhouette obtained from extent of the spectra is approximately sphere with diameter 410 m. The unexpected results are that the spectra for OC and SC polarization have coinciding parameters for the different rotating phases. The estimation of radar cross section obtained from echo power is 0.02 km²on both circular polarization. The formalized estimation of radar albedo gives ~0.1. The most interesting result of our radar measurements is a near-unity circular polarization ratio both for December 16 and 17. The same value for 1998 WT24 was revealed earlier by Ostro S. J. [4] at 3.5 cm in Goldstone. Such a combination of low radar albedo and near-unity circular polarization ratio can be interpreted as a property of comet-like surface of 33342 (1998 WT24).

The first asteroid detected by European radar was 4179 Toutatis; its observations were carried out on 8 and 9 Dec. 1992 with bistatic radar system Evpatoria ® Effelsberg [1]. The second asteroid of Goldstone ® Evpatoria radar was 6489 Golevka (13, 14, 15 June 1995) [2]. The Evpatoria ® Medicina radar is the third case of European observation of asteroid while U.S. radar detected nearly 190 asteroids and comets [5]. But our experiment can be thought of as the next step toward a European radar network for continuous, comprehensive investigations of near-Earth asteroids and comets.

REFERENCES

1. Zaitsev A. L., et al. Radar Investigations of the Asteroid 4179 Toutatis at Wavelength 6 cm. J. Comm. Tech. Electronics, vol. 38, No 16, 135-143, 1994.

2. Zaitsev A. L., et al. Intercontinental bistatic radar observations of 6489 Golevka (1991 JX), Planet. Space Sci. Vol.45, No 7, 771-778, 1997.

3. Ostro S. J. Planetary Radar. Encyclopedia of the Solar System, Academic Press, 773-807, 1999.

4. Ostro S. J. (2001) Personal e-mail communication.

5. Ostro S. J. Asteroid Radar Research [online]. California Institute of Technology, Pasadena, 2002, http://echo.jpl.nasa.gov

Paper, submitted on October 2002 to the Planetary and Space Science

Results of the first Italian planetary radar experiment

M. Di Martino ⁽¹⁾, S. Montebugnoli ⁽²⁾, G. Cevolani ⁽³⁾, C. Bortolotti ⁽²⁾, A. Cattani ⁽²⁾, M. Delbò ⁽¹⁾, C. Guidi ⁽²⁾, A. Maccaferri ⁽²⁾, G. Maccaferri ⁽²⁾, J. Monari ⁽²⁾, A. Orlati ⁽²⁾, M. Poloni ⁽²⁾, S. Poppi ⁽²⁾, S. Righini ⁽²⁾, M. Roma ⁽²⁾, L. Saba ^{(1, 6)}, S. Ostro⁽⁴⁾, A. Zaitsev ⁽⁵⁾, A. Gavrik⁽⁵⁾, Y. Gavrik⁽⁵⁾

(1) INAF-Osservatorio Astronomico di Torino, strada Osservatorio 20, 10025 Torino (Italy)

(2) CNR-Istituto di Radioastronomia, Via Fiorentina Aia Cavicchio, 40059 – Villafontana, Bologna (Italy)

(3) ISAC-CNR, Via P. Gobetti, 101 - 40129 Bologna

(4) JPL, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, (818) 354-4321

(5) IRE-RAS, Institute of Radioengineering and Electronics, Vvedensky Square 1, 141190, Fryazino, Russia

(6) INAF-Osservatorio Astronomico di Cagliari, strada 54, loc. Poggio dei Pini, 09012 Capoterra , Cagliari (Italy)

Correspondence to M. Di Martino,

Fax: +39-0118101930, E-mail:dimartino@to.astro.it

Abstract

We describe the first intercontinental planetary radar initiative undertaken in Italy.

We present the results of the observations of Near-Earth Asteroid (NEA) 33342 (1998 WT24), performed in December 2001 using the bistatic configurations Goldstone-Medicina and Evpatoria-Medicina.

The experiment goal was to characterize the system for radar follow-up observations of NEA and artificial orbiting debris, in the framework of a feasibility study which aims at using the Sardinia Radio Telescope, at present under construction, also as a planetary radar facility.