I found this information about the Kepler telescope. It will be in an Earth-trailing heliocentric orbit at approximately the same distance as the earth is from the sun.
2009 May 29. Kepler Project Manager Update
- Kepler remains safe and stable in its "drift-away" heliocentric orbit. The space-craft is over 8.4 million kilometers from Earth. Kepler has been collecting science data since 12 May. The operations team has had nearly daily contacts using the Deep Space Network to check the spacecraft health.
This is NASA’s first mission capable of finding Earth-sized and smaller planets around stars other than our Sun. Unlike the Hubble telescope which orbits Earth, this telescope is in orbit around the Sun. It is roughly at Earth’s distance from the Sun, but on an orbit where it lags slightly more behind Earth’s position as time passes. After 4 years, Kepler will be about 0.5 AU, or half the Earth-Sun distance, behind Earth on its orbit.
Kepler will stare continuously at the same small region of the sky for three and a half years. Scientists did not want this steady gaze interrupted by day-night cycles or by passage behind the Earth, as would happen if the telescope were in Earth’s orbit. Further, Kepler is looking at a region of space far above the plane of our solar system, so the Sun, Moon, and other solar system bodies never come near the field of view. That area of space is also in the galactic plane roughly in the direction the Sun itself is traveling. This means we are observing stars at the Sun’s approximate distance from the galactic core.
Kepler will detect extrasolar planets using the transit method. This method involves looking at stars continually for long periods of time to see if the light ever gets slightly dimmer. If the slight dimming occurs on a regular basis, it might be because a planet is orbiting the star and regularly passing in front of it from our perspective. Such a passage is called a transit. When a planet as small as our Earth transits its star, the star dims by only a factor of 1/10,000. Only now, with Kepler, do we have an instrument powerful enough to detect such a tiny change in a star’s brightness. Of course, we need to be fortunate enough to observe the planetary system edge-on, otherwise no transit will occur. However, the chosen field of view contains about 100,000 stars, so odds are at least a few are oriented favorably.
It sure would be exciting to make contact with life from another planet in our lifetime.