Title: Kinematics effects of atmospheric friction in spacecraft flybys,
Abstract: Gravity assist manoeuvres are one of the most succesful techniques in
astrodynamics. In these trajectories the spacecraft comes very close to the
surface of the Earth, or other Solar system planets or moons, and, as a
consequence, it experiences the effect of atmospheric friction by the outer
layers of the Earth's atmosphere or ionosphere.
In this paper we analyze a standard atmospheric model to estimate the density
profile during the two Galileo flybys, the NEAR and the Juno flyby. We show
that, even allowing for a margin of uncertainty in the spacecraft cross-section
and the drag coefficient, the observed -8 mm/sec anomalous velocity decrease
during the second Galileo flyby of December, 8th, 1992 cannot be attributed
only to atmospheric friction. On the other hand, for perigees on the border
between the termosphere and the exosphere the friction only accounts for a
fraction of a millimeter per second in the final asymptotic velocity.