The continuous viewing needed for a high detection
efficiency for planetary transits requires that the field-of-view
(FOV) of the photometer be out of the ecliptic plane so as not
to be blocked periodically by the Sun or the Moon. A star field
near the galactic plane that meets these viewing constraints
and has a sufficiently high star density has been selected, with
galactic coordinates centered on l=70° and b=5° (RA=19h
45m, dec=+35°).
An Earth-trailing heliocentric orbit with
a period of 372.5 days provides the optimum approach to meeting
of the combined Sun-Earth-Moon avoidance criteria within the Boeing
D2925-10 (Delta-II) launch vehicle capability
(launch videos).
In this orbit the spacecraft
slowly drifts away from the Earth and is at a distance of 0.5
AU (worst case) at the end of four years. Telecommunications
and navigation for the mission are provided by NASA's Deep Space
Network (DSN).
Another advantage of this orbit is that it
has a very-low disturbing torque on the spacecraft, which leads
to a very stable pointing attitude. Not being in Earth orbit
means that there are no torques due to gravity gradients, magnetic
moments or atmospheric drag. The "largest" external
torque then is that caused by solar pressure. This orbit also
avoids the high radiation dosage associated with an Earth orbit,
but from time to time is subject to solar flares.
Orbit of Spacecraft Relative to the Sun-Earth
Heliocentric Orbit for Kepler Mission
Mission Lifetime
The mission must last long enough to detect
and confirm the periodic nature of the transits of planets in
or near the HZ. A four year mission is proposed which enables
a four-transit detection of all orbits up to one year in length
and a three-transit detection of periods up to 1.33 years. This
mission duration also provides three-transit detections for 50%
of 1.6 year orbits and 10% of 1.9 year orbits.
We have also proposed a two year mission extension
which greatly enhances the ability to detect planets smaller
than Earth and reliably detect Earth-size planets in orbits corresponding
to that of Mars (2 year periods).
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