  Aims. We present observations of the first coronal mass ejection (CME)
observed at the Solar Orbiter spacecraft on April 19, 2020, and the associated
Forbush decrease (FD) measured by its High Energy Telescope (HET). This CME is
a multispacecraft event also seen near Earth the next day. Methods. We
highlight the capabilities of HET for observing small short-term variations of
the galactic cosmic ray count rate using its single detector counters. The
analytical ForbMod model is applied to the FD measurements to reproduce the
Forbush decrease at both locations. Input parameters for the model are derived
from both in situ and remote-sensing observations of the CME. Results. The very
slow (~350 km/s) stealth CME caused a FD with an amplitude of 3 % in the
low-energy cosmic ray measurements at HET and 2 % in a comparable channel of
the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar
Reconnaissance Orbiter, as well as a 1 % decrease in neutron monitor
measurements. Significant differences are observed in the expansion behavior of
the CME at different locations, which may be related to influence of the
following high speed solar wind stream. Under certain assumptions, ForbMod is
able to reproduce the observed FDs in low-energy cosmic ray measurements from
HET as well as CRaTER, but with the same input parameters, the results do not
agree with the FD amplitudes at higher energies measured by neutron monitors on
Earth. We study these discrepancies and provide possible explanations.
Conclusions. This study highlights that the novel measurements of the Solar
Orbiter can be coordinated with other spacecraft to improve our understanding
of space weather in the inner heliosphere. Multi-spacecraft observations
combined with data-based modeling are also essential to understand the
propagation and evolution of CMEs as well as their space weather impacts.
