One of PLATO's top level requirements is the possibility of conducting
very long observations in a stable environment, with as few
interruptions as possible. This requirement naturally rules out
low-Earth orbits, for which there are strict limitations in terms of
"continuous viewing zone" (see CoRoT or MOST mission scenarios). Also,
low-Earth orbits suffer from high radiation levels as well as from very
high scattered light contamination for the telescope from the sunlit
Earth. Finally, ground-station contacts are short. These three issues
are well-known major limitations of the CoRoT satellite, located on
such a low-Earth orbit. Geostationary orbits or highly eccentric orbits
with a high perigee, have better characteristics, but are very costly,
and may still suffer from high levels of scattered light contamination
as well as from eclipse seasons.
We are led to choose a large Lissajous orbit around the L2 point, as
Large Lissajous orbit
around L2, as seen from the ecliptic plane.
The Earth is at (0,0) coordinates, and the
Sun is to the right.
A large Lissajous features a limited cost of transfer from the Earth
and is not affected by too many Earth-Sun or Moon-Sun eclipses.
Besides, in the specific case of a reasonable satellite-to-ground
telemetry rate (200 kb/s) as proposed for PLATO, one can easily manage
the antenna lobe geometry with respect to the half-year parallax on the
radio frequency line of sight (steering function). Another advantage of
a L2 orbit is to reduce dramatically the sources of noise related to
the interaction between the spacecraft and its environment.
Radiation impacts at L2 will affect in a limited way the photometric
performances of PLATO. This problem is well known for missions in
low-earth orbits, such as CoRoT, which suffer repeated passages in the
South Atlantic Anomaly, but still succeed in providing ultra-high
precision photometry. The radiation environment on a large Lissajous
orbit around L2 will be less hostile, and dominated by solar eruptions.
Since the first Cosmic Vision M-class mission will be flown around
solar minimum, the effect of radiation impacts should remain limited,
and in any case much lower than that of CoRoT. We also expect a lower
impact of radiations on the mission performances than e.g. for Gaia, because PLATO will work at
much higher photo-electron flux, thus minimizing the effect of electron
traps produced by radiation impacts.
The current assumption is to launch PLATO with a Soyuz/Fregat from
Kourou. This is the same rocket that launched CoRoT in December 2006
(see image below). This launcher, available at a reasonable price, is
extremely reliable, and its performance is well matched to PLATO's
requirements. The mission strategy foresees that the Soyuz rocket will
inject the PLATO/Fregat assembly in a low parking orbit, from which a
firing of the Fregat upper stage will inject the PLATO spacecraft into
an escape track toward the final L2 orbit.