Kepler Mission

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Kepler Mission
Conceptual drawing of the Kepler Spacecraft
Organization:	NASA
Major Contractors:	Ball Aerospace & Technologies Corp.
Mission type:	Space observatory
Satellite of:	Sun
Launch Date:	November 2008
Launch Vehicle:	Delta II
Mission Duration:	Four years
Webpage:	Kepler Home
Mass:	1039 kilograms (Jan. 2006 est.)
Orbital elements
Semimajor Axis:	1.01319 AU
Eccentricity:	0.03188
Orbital Period:	372.5 days
Orbits:	Earth-trailing heliocentric
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The Kepler Mission is a space observatory being developed by NASA that will search for extrasolar planets and will only be the second space-based telescope particularly constructed for that task, the first one being COROT. For this purpose, it will observe the brightness of about 100,000 stars over four years to detect periodical transits of a star by its planets. Kepler will not be in an Earth orbit but in an Earth-trailing solar orbit so that Earth will not occult the stars which are to be observed continuously and the photometer will not be influenced by stray light from Earth. This orbit also avoids gravitational perturbations and torques inherent in an Earth orbit, allowing for a more stable viewing platform. The photometer, built by Ball Aerospace & Technologies Corp. of Boulder Colorado, will point to a field in Cygnus, which is well out of the ecliptic plane, so that sun light never enters the photometer as the spacecraft orbits the Sun. Cygnus is also a good choice to observe because it will never be obscured by Kuiper_Belt objects or the asteroid belt. The observatory is currently scheduled for launch in November 2008. In January 2006, it was delayed eight months because of budget cuts and consolidation at NASA. It was delayed again by 4 months in March 2006 due to fiscal problems. At this time the high-gain antenna was changed from a gimballed design to one fixed to the frame of the spacecraft to reduce cost and complexity, at the cost of one observation day per month.

The spacecraft, being built by Ball Aerospace & Technologies Corp., is estimated to have a mass of 1,039 kilograms (2,290 lb.), have a 0.95 meter (3.11 ft) aperture, a 1.4 meter (4.59 feet) primary mirror (the largest on any telescope outside of Earth orbit), have a 105 deg² (about 12 degree diameter) field of view (FOV), equivalent to roughly two hands held at arm's length. The photometer will have a soft focus to provide excellent photometry, rather than sharp images. The combined differential photometric precision (CDPP) for a m(V)=12 solar-like star for a 6.5 hour integration will be 20 ppm, including an expected stellar variability of 10 ppm. An earth-like transit produces a brightness change of 84 ppm and lasts for 13 hours when it crosses the center of the star. The focal plane is made up of 42 CCDs with 1024 rows by 2200 columns and 27 micrometre pixels. The array will be cooled by a Cryotiger closed-cycle cooling system. The CCDs are readout every 3 seconds and co-added on board for 15 minutes. Only the pixels of interest from each of the target stars are stored and telemetered to the ground. The mission will cost an estimated $467 million.

The following is an extract from the official website of the Kepler Mission, detailing in summary its mission objectives:

The scientific objective of the Kepler Mission is to explore the structure and diversity of planetary systems. This is achieved by surveying a large sample of stars to achieve several goals:

Determine how many terrestrial and larger planets there are in or near the habitable zone of a wide variety of spectral types of stars
Determine the range of sizes and shapes of the orbits of these planets
Estimate how many planets there are in multiple-star systems
Determine the range of orbit size, brightness, size, mass and density of short-period giant planets
Identify additional members of each discovered planetary system using other techniques
Determine the properties of those stars that harbor planetary systems

The random probability of a planetary orbit being along the line-of-sight to a star is the diameter of the star divided by the diameter of the orbit. For an Earth-like planet at 1 AU transiting a solar-like star the probability is 0.47%, or about 1 in 210; it's slightly larger at 0.72 AU (the orbital distance of Venus), 0.65%; such planets would be Earth-like if the host star is a late G-type star such as Tau Ceti. In addition, because planets in a given solar-system tend to orbit in similar planes, the possibility of multiple detections around a single star is actually rather high. For instance, if an alien Kepler-like mission observed Earth transiting the Sun, there is a 12% chance of also seeing Venus transit.

The Kepler Mission has probably the best chance of detecting Earth-like planets at current technology levels. One important advantage it has is that it is designed to observe 100,000 stars simultaneously. This provides a much better chance for seeing a transit. In addition, the 1 in 210 probability means that if 100% of stars observed had Earth-like terrestrial planets, Kepler would find about 480 of them. The mission is therefore ideally suited to determine the frequency of Earth-like planets around other stars.