  The luminous high-redshift (z=2.17) quasar S50836+710 has been observed in
October 1997 with the VSOP at 1.6 GHz and 5 GHz. We report here a previously
unpublished image made from the data at 1.6 GHz and compare the structure of a
relativistic jet in this quasazr at the two frequencies. We present a spectral
index image tracing spectral properties of the jet up to ~40 milliarcsecond
distance from the nucleus. The curved jet ridge line observed in the images and
the spectral index distribution can be described by Kelvin-Helmholtz
instability developing in a relativistic outflow with a Mach number of ~6. In
this description, the overall ridge line of the jet is formed by the helical
surface mode of Kelvin-Helmholtz instability, while areas of flatter spectral
index embedded into the flow correspond to pressure enhancements produced by
the elliptical surface mode of the instability. An alternative explanation
involving a sequence of slowly dissipating shocks cannot be ruled out at this
point.
