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.

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 7925-10L (Delta-II) launch vehicle capability. 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.

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).

The spacecraft must execute a 90 degree roll every 3 months to reposition the solar panels to face the Sun while keeping the instrument aimed at the target field of view. See animation.