Title:

The measurement of the solar gravitational deflection with the spacecraft Cassini

Dr. Luciano Iess,
Dipartimento di Ingegneria Aerospaziale ed Astronautica
   Universita' La Sapienza, Rome

Solar gravity has three main effects on the propagation of photons in the solar
system: a deflection, a delay and a Doppler shift. In an experiment carried out
in the summer of 2003 while Cassini was in superior solar conjunction at an
heliocentric distance of 7.3 AU, the Doppler shift of radio signals propagating
between the spacecraft and the Earth was measured to an accuracy of about one
part in 10**14 over time scales of 1000-10000 s. This corresponds to a range rate
accuracy of about 1 micron/s. Thanks to the favorable location of the spacecraft
in the solar system, a quiet dynamical state and an extremely stable radio
system, the relativistic Doppler shift (about 6 10**-10) was measured with
excellent accuracy. The predictions of General Relativity have been confirmed to
one part over 50000, pushing the search for violations at levels accessible only
to dedicated experiments and instrumentation.