VLBI Solar System Observations.                          
 
        T. M. Eubanks  (U.S. Naval Observatory, 
        Washington, D.C., 20392-5100)
 
 Although Very Long Baseline Interferometry (VLBI) observations of radio
 transmitters inside the solar system commenced soon after the initial developme
nt of
 the VLBI technique, at present such observations have made a fairly minor
 contribution to solar system celestial dynamics. The DE402 ephemeris, for examp
le,
 uses only 16 VLBI observations in all, with typical accuracies of order 3 milli
 arc
 seconds (mas), although even this limited amount of information provides an acc
urate
 tie between the extragalactic radio reference frame and the dynamical solar sys
tem
 frame.  By contrast, geodetic and astrometric VLBI can determine the position o
f
 natural radio sources to within 0.3 mas in a single observing session, and the 
accuracy
 and stability of the radio reference frame is approaching parts in 10-10 and 10
-11
 year-1. Solar system VLBI observations at this level of accuracy would make a
 considerable improvement to the dynamical reference frame, as well as providing
 new and sensitive tests of general relativity and Mach's principle. 
 
 The primary limitation in the application of VLBI to solar system dynamics has 
been
 the shortage of suitable space missions during the period of extensive developm
ent of
 VLBI in the last 15 years. The cost of operations, and the accuracy of the resu
lts,
 would be considerably enhanced by the development of VLBI beacons, specifically
 designed to match the capabilities of current VLBI systems. Routine tracking of
 lunar
 beacons would complement and enhance results from Lunar Laser Ranging, and
 could provide a two orders of magnitude improvement in the determination of lun
ar
 inclination changes and the lunar geodesic precession. Beacons placed on select
ed
 Earth crossing asteroids, or on Mars or Mercury, could provide a considerable
 improvement in determinations in the solar J2 and the relativistic b parameter.
 
 
 This talk will provide a review of past VLBI observations of spacecraft, and de
scribe
 the radio signal structure and preliminary design details of a proposed lunar b
eacon.