  The number of satellites on low orbit has dramatically increased over the
past years, raising concerns among the astronomical community about their
impact on observations. Spectroscopic observations represent a large fraction
of professional data, and spectrographs lack spatial information that can
reveal the presence of a satellite. We simulated how often satellites
contaminate spectrograph observations by using realistic constellations with
over 400,000 objects. We also measured how a spectrum is affected by using real
data from different scientific targets and a scaled solar analogue as the
satellite, and using standard tools to measure astrophysical parameters and
compare them with the clean spectrum. The fraction of affected spectra varies
dramatically with the elevation of the sun, with a maximum of 10% at twilight
and a nightly average of about 2%. Because of the fast motion of the satellites
and the limiting magnitude of the spectrographs, high-resolution instruments
are essentially blind to most satellites. For lower resolution spectrographs,
the effect on the measured astrophysical parameters depends strongly on the
signal-to-noise of the exposure, longer exposures on brighter targets being the
least affected at <=1%. Satellites that are brighter and/or higher than the
constellation satellites, while less numerous, can also contaminate spectra.
While the fraction of affected spectra is likely to remain low, some of these
contaminated spectra will be difficult to identify, as it is already the case
with existing satellites and asteroids. The best mitigation is to ensure that
their brightness is fainter than V=7, that their absolute magnitude V1000km is
also fainter than 7, and, whenever possible, to shoot multiple exposures.
