Asteroids, Comets, Meteors Conference, Cornell University, USA, July 26-30, 1999

Presentation #: 01.16-P


 

ASTEROIDíS MOONS: DISCOVERY BY RADAR

 Alexander L. Zaitsev

Institute of Radio Engineering and Electronics

Russian Academy of Science

 

Moons of known asteroids have a low position ambiguity, which makes their discovery with powerful ground-based radar much easier than the discovery of new asteroids or other objects of Solar System. Also, unlike dedicated space missions, transmitting and receiving a radar beam is a cheap, flexible and reliable way to detect asteroid moons which allows to survey thousands of known asteroids during a reasonable time.

 

There are two techniques, which can be used to discover asteroid moons. The first technique is indirect, based on accurate delay and Doppler astrometry of a parent asteroid, searching for perturbations caused by invisible satellite. This method requires long series of radar astrometry, preferably, without periodic gaps, which obstruct when moon's orbital period is commensurable with the period of Earth rotation.

 

The other technique is a direct detection of radar echo from an asteroid moon. For a direct detection of Dactyl-like moon the radar potential (production of transmitted power P, effective aperture of transmitter and receiver antennas, divided by the receiver system temperature and L^1.5, where L is the radar wavelength) should be several thousand times greater than it is for the upgraded Arecibo radar (fig. 1). To achieve such enormous radar potential not only a powerful transmitter but also a very large parabolic antenna are necessary. However, the diameter of a ground-based fully-steerable reflector is limited by 120-150 m to make it possible to perform two different functions: focusing and tracking (fig. 2). Building a large radar antenna with separated focusing and tracking (fig. 3) can be implemented by an integrated ground/orbital-based facility.

 

The basic concept is to construct a fixed ground-based focusing reflector, permanently directed to a definite point of the geostationary orbit (GEO), where the other, flat and steerable reflector, will be located (fig. 4). The size of the ground-based reflector is limited only by troposphere turbulence and by the strength of the Earth's crust, while the size of the flat steerable reflector, placed in windless and zero-gravity conditions, depends only on space technology level. This reflector can be assembled by astronauts of a near-Earth space station and then towed to the GEO. For example, detection of Dactyl-like moon at a distance about 2 AU needs a radar with GEO-based flat reflector of diameter 600 m and a transmitter of P = 10 MW at L = 3.5 cm, and a fixed ground-based focusing reflector of diameter 5.7 km, (fig. 5). Other designs of the large antenna, e.g. antennae array, are also possible. This powerful radar can be also used for visualization of thousands of MBAs, early precise astrometry of nuclei of new long-period comets, broadcast for extra-terrestrial intelligence (BETI), etc 

 



Fig. 1. Required Radar Potential

 



Fig. 2.
Upgraded Arecibo v.s. Asteroid Moon Radar (AMR)

 



Fig. 3.
Separation of Focussing (1) and Tracking (2) Functions:

 



Fig. 4.
Integrated Ground/Orbital Antenna

 

 

 



Fig. 5.
Integrated Ground/Orbital Antenna