Alexander L. Zaitsev

IRE RAS, Vedensky Square 1, 141190 Fryazino, Russia

ABSTRACT

Radar astronomy paradox (RAP): practically everybody agree with essential contributions of active radar observations to Solar System and especially to near-Earth object (NEO) explorations, but despite everything prefer to develop new and new passive telescopes and disposable space missions, only, and nobody want to build at least one dedicated multipurpose radar telescope (neither Arecibo nor Goldstone and Evpatoria radars were created as dedicated radar astronomy instruments). Also, as of June 2002, among of 188 radar detected asteroids and comets [1] there are only 3 NEOs, which were investigated in Europe, with single European radar facility, sited in Evpatoria, [2, 3, 4]. The main reason of such deep gap is a low sensitivity of Evpatoria radar, which is in 10 and 300 times less powerful than Goldstone and Arecibo.

Therefore, I guess the first dedicated European NEO Radar (ENEOR) is earnestly needful now. From time to time we discuss this problem [5, 3, 6], but it is not solve for the present moment, perhaps because of above formulated RAP. Origin and concept of the ENEOR, as well as the ENEOR project, based on the being under construction 64-m Sardinia Radio Telescope, will be presented below.

 

1. INTRODUCTION

All thinkable multiplicity of today’s assets and techniques employed to research outer space could be reduced to the following three categories: (1) proper or induced emission from celestial bodies registered by ground or orbital-based telescopes throughout a different bands from radio to gamma, (2) coherent man-made sounding signal returns picked up by radars or lidars, and (3) direct investigations of targeted objects "in situ", i.e. during unmanned and then manned space missions.

Ground-based space research, involving purpose-built radars, appears a far more effective approach than the long-practiced strategy. Such radars are not constrained by space launches and are obviously less costly than operating space-based systems. As we launch high-powered beams of electromagnetic energy, rather than automatic probes, towards the selected celestial bodies and review the echo signals, we can have access to data about all objects in the Solar System continuously and in real time, too. Radar techniques have been particularly effective in researching the multiplicity of asteroids and comets, because of impossibility to direct usual space probes to all of Solar System’s minor bodies.

Unfortunately, only (1) and (3) techniques historically use in Europe. As a result, there is the deep lack of development in European radar astronomy. For example, the harvest of asteroid and comet radar research during the 1969 – mid 2002 interval is 188 objects in USA [1], and only 3 – in Europe [2, 3, 4], see Table 1.

Table 1. Harvest of asteroid and comet radar research during the [1969 – mid 2002] interval

1969 – 06.2002	USA	Europe
Main Belt Asteroids	75	0
Near-Earth Asteroids	107	3
Comets	6	0
Total Amount	188	3

To overcome such anomalous state of affairs, in the first instance it is necessary to create here, in Europe, the first dedicated radar telescope. There are 11 large reflectors, with diameter 64 or more meters, in the worlds, but only three of them – Arecibo, Goldstone, and Evpatoria, have power water-cooling heavyweight transmitters, which namely convert usual radio telescope into radar telescope. So, to create modern radar telescope in the first instance we must create at the minimum one power radar transmitter. Also, and it is important to emphasize usual transmitter which is using for up-link radio control of space crafts is not fit for radar purposes because the power of radar transmitter should be in hundred times more than of first one.

 

2. CONCEPTUAL DESIGN

In order to choose the prototype of future ENEOR, let us view current planetary radar systems. Arecibo Radar Telescope (ART, http://www.naic.edu) sited at the Puerto Rico Island, its antenna’s diameters is 300 m and transmitting power is one megawatts at 12.6 cm wavelength. ART is the most powerful radar in the world, but the main Arecibo defect is fixed antenna, and so, narrow declination window, in plus/minus 20 degrees with respect to local vertical, only.

The Goldstone Solar System Radar (GSSR, http://deepspace.jpl.nasa.gov/technology/95_20/gold.htm) sited in Mojave Desert, California, and usually presents two-dish configuration, with 70-m dish, X-band half-megawatt transmitter and beam wave guide 34-m dish with 14 K two-channel receiver. The distance between these antennas is 22 km, so transmitter does not jam the receiver and continuous mode, without any gaps in measurements, is possible. The GSSR is a coherent precise radar owing to fiber optic communication between H-maser and both antennas, which provides a 50-nanosecond resolution in delay measurements, or 7.5-m in range. It can not say anything about defects of this perfect system, except of very hard occupancy of 70-m dish by spacecraft’s operations, and so, very scant time provided for radar experiments.

The third system is Evpatoria Planetary Radar (EPR, http://evpatoria.asteroids.ru/indexd-eng.html), sited in Crimea Peninsula. It has 70-m dish and C-band 0.15 MW transmitter. For radar study of near-Earth objects this radar is needed to use the second remote antenna for receiving of echo signals, because of very slow switching from transmitting to receiving mode in EPR. There are two significant defects of EPR: (1) rather modest power of transmitter, which permits to detect only very close targets within 0.02 AU, (2) in bistatic configuration it is not a coherent system, thus the precise delay/Doppler astrometry is more difficult than it is for the GSSR coherent system.

So, the most acceptable prototype for future ENEOR would be the Goldstone-like system. Now let us try to estimate the parameters of this proposed radar. The main operating character of radar telescope is System Factor SF, which is defined as:

SF = P_t ´ S_t ´ S_r / (T_s ´ l ^3/2 ),

where P_t – transmitted power, S_t and S_r – effective aperture of transmitting and receiving antennas, respectively, T_s – receiver system temperature, and l – radar wavelength. And the Radar Delectability, or maximal distance R_max, at which we can detect the celestial target with given diameter D, is

R_max = 1.26× 10^-6× [SF× s_0× D^3/2× (P× T_i)^1/2/SNR_min]^1/4,

and it depends of SF and some other parameters: s₀ – radar albedo, P – target’s spin period, T_i – integration time, SNR – signal-to-noise ratio, (here R evaluated in Astronomical Unites).

Further, let us choose the reference point [R_max, D] is equal to [0.1 AU, 300 m], i.e. let us demand so as our estimated radar will be able to detect NEO with diameter 300 m at a maximal distance 0.1 AU. For this case, if we assume s ₀ = 10%, T_i = P = 8 hours, we get that the System Factor of European near-Earth object radar should be equal 2.5 × 10¹³ [W× m^2.5/K], or 134 dB.

Table 2. Maximal distances R_max, at which NEOs with a given diameters D can be detected by radar with SF = 134 dB.

D, km	0.05	0.1	0.2	0.3	0.5	0.8	1.0	1.8
Rmax, AU	0.05	0.07	0.09	0.1	0.12	0.15	0.16	0.2

 

Now let us estimate, for three points of distance: 0.2. 0.1 and 0.05 AU, the number of NEOs N, larger than a given diameter D, which may be annually investigated by proposed ENEOR with SF = 134 dB. The Table of future close approaches (CA) of known NEOs, accessible at http://neo.jpl.nasa.gov/cgi-bin/neo_ca, gives 2500, 725, and 240 CAs of 1618 known (at a date 01.11.01) NEOs at near centennial interval [2001-2100], or 25, 7.25, and 2.4 CAs per year, respectively. This statistic demonstrates that annually 1.5% of NEO cross the geosphere with 0.2 AU radius, 0.45% – with 0.1 AU radius, and 0.15% – with 0.05 AU radius. As we already estimated early (see Table 2), the minimal size of target at this distances for our 134-dB radar are 1800 m, 300 m, and 50 m, respectively. On the other hand, there is an estimation of Earth-crossing asteroid population larger than given size D, [7]. So, we may apply CA’s statistic to whole population by multiplying of "Percent" (fourth line of Table 3) on "Population" (sixth line) in order to get N. You see, our 134-dB ENEOR will be not a jobless during a few centuries (last line of Table 3). Of course, the total number of NEOs, in continual band of sizes and distances, not only for three points on distance and size, as were in our example, is yet more than N.

 

Table 3. Number of successive NEOs N larger than a given diameter D, annually researched by estimated 134-dB Radar

Distance, AU	£ 0.2	£ 0.1	£ 0.05
Number of Close Approaches of known NEOs / century	2500	725	240
CA per year	25	7.25	2.4
CA per year / number of known NEOs (1618 NEOs)	1.5%	0.45%	0.15%
Dmin at a given Distance for 134-dB Radar	³ 1800 m	³ 300 m	³ 50 m
Population of ECAs larger than Dmin , [7]	» 600	» 20,000	» 1,000,000
N = Population ´ Percent, number per year	9	90	1500
Population / N, year	67	222	667

 

On November 9, 2001 at SRT Meeting (Cagliary, Sardinia) we have suggested to build up this radar in Sardinia, near the pre-arranged 64-m Sardinia Radio Telescope (SRT), in order to use SRT for receiving of echo signals. This second, also 64-m antenna SRT2 and power transmitter should be sited not very far from SRT, in order to connect both antennas with fiber optic cable and create coherent, GSSR-like, radar system.

Finally, let us estimate the power of transmitter, in order to get SF = 134 dB in SRT2 => SRT configuration. These estimations for four combinations of ENEOR parameters are listed in Table 4.

 

Table 4. Power of transmitter for goal attainment of SF = 2.5× 10¹³ W× m^2.5/K, or 134 dB.

Band, frequency	Beam width	Proposed parameters of SRT and SRT-2	Power of transmitter
X, 8.5 GHz	1.9¢	St = Sr = 1900 m2 or 60% Efficiency and Ts = 15 K	680 kW
Ka, 32 GHz	0.5¢	St = Sr = 1600 m2 or 50% Efficiency and Ts = 35 K	310 kW
		St = Sr = 1600 m2 or 50% Efficiency and Ts = 30 K	270 kW
		St = Sr = 1900 m2 or 60% Efficiency and Ts = 30 K	190 kW

 

So, if we shall prefer the X-band radar, assumed the 60% efficiency for both antennas and 15K noise temperature for receiving system, the power of transmitter should be equal about seven hundred kilowatts. At present the project of such transmitter already exists in the Jet Propulsion Laboratory. For the new, Ka-band, if will be achieved the 60% efficiency and T_s = 30 K, power of transmitter should be about two hundred kilowatts, only.

 

3. ECOLOGICAL PROBLEMS WITH SRT-2 POWER TRANSMITTER

As was emphasized above, it is impossible to create ENEOR by building of only new and new radio astronomical instruments and various VLBI-nets – at least one power radar transmitter is needful as well. The ecological problems, which appear with this subject, were successfully solved in such densely populated and tourist areas, as California, Puerto Rico and Crimea, so will be not a barrier in Europe, too. Obviously, the multitude of mobile phones with its omnidirectional antennas near the people’s heads is much more dangerous radio source than single high-directivity antenna of the ENEOR in thinly populated Sardinian highlands, in ten kilometers far off people settlement.

To resume, we may state that X-band ENEOR is more easily for implementation now, but Ka-band one is more preferable because it has narrower beam and less power of transmitter. The world first telescope was created by Galileo in Italy, the world first asteroid was discovered by Piazzi in Italy, and the ENEOR – first dedicated asteroid and comet radar telescope (neither Arecibo, nor Goldstone and Evpatoria were created especially for radar astronomy) might be also realize in Italy.

 

REFERENCES

1. Ostro S. J. Asteroid Radar Research [online]. California Institute of Technology, Pasadena, 2002,  .

2. 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.

3. Zaitsev A. L., et al. Intercontinental bistatic radar observations of 6489 Golevka (1991 JX). Planetary and Space Science, vol. 45, 771-778, 1997.

4. Zaitsev A. L., et al. Radar Detection of NEA 33342 (1998 WT24) with Evpatoria-Medichina System at 6 cm. Abstract at Asteroids Comets Meteors Conference 2002, Berlin, and This Proceedings.

5. Ostro S. J. and Zaitsev A. L. The Vulcano Workshop “Beginning the Spaceguard Survey”, 1995.

6. Zaitsev A. L. EuroRadar: Origin and Concept. Tumbling Stone, Number 10, http://spaceguard.ias.rm.cnr.it/tumblingstone/issues/num10/euroradar.htm .

7. Rabinowitz D. L., et al. The Population of Earth-crossing Asteroids. In Hazards due to Comets & Asteroids, Arizona Press, 1994, 285-312.