\documentstyle{article}

\title{RADAR ASTROMETRY OF ASTEROIDS, COMETS AND PLANETARY SATELLITES}
                \author{Steven J. Ostro, JPL}
\date{}

\begin{document}
\maketitle


Groundbased radar has considerable astrometric potential for asteroids, 
comets and natural satellites that enter the detectability windows of 
available telescopes.  For very closely approaching asteroids and comets, 
measurements of the distribution of echo power in time delay (range) 
and Doppler frequency (radial velocity) can achieve a fractional 
precision between $10^{-5}$ and $10^{-9}$, and consequently are invaluable 
for refining orbits and prediction ephemerides.  Even for mainbelt 
asteroids and the more readily detectable planetary satellites, whose 
orbits are relatively well known, radar can collapse 
range uncertainties from a few target radii to a few percent of a 
target radius.  Radar aperture synthesis can, in principle, 
complement delay-Doppler astrometry of small bodies by providing
angular astrometry at a level approaching 0.01 arcsec. 

Radar astrometry of a near-Earth asteroid (NEA) during its discovery 
apparition can ensure its optical recovery.  Observational 
experience has demonstrated that radar astrometry commonly improves 
upon the accuracy of optical-only ephemerides of newly discovered 
NEAs by one to three orders of magnitude.  Even for asteroids with 
very long astrometric histories and secure orbits, radar measurements 
can significantly shrink positional error ellipsoids for decades, with 
direct implications for the navigation of spacecraft to asteroids and 
predictions of extremely close approaches of asteroids to Earth. 

Radar can image NEOs if the echoes are strong enough, and if the 
orientational coverage of the images is adequate, inversion of the data 
can yield a three dimensional model and an estimate of the delay-Doppler 
trajectory of the center of mass with very fine precision: a few 
decameters for work reported so far and a few meters for anticipated 
results of observations planed for the next few years.


\end{document}
