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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2009 Symposium The Kepler Mission: A Search for Terrestrial Planets RILEY DUREN Jet Propulsion Laboratory Pasadena, California The Kepler mission, which was launched by the National Aeronautics and Space Administration (NASA) on March 6, 2009, is the first mission capable of finding Earth-size planets orbiting in the habitable zones (HZs) of other stars. Kepler will also determine the distribution of these planets near solar-like stars.1 After a two-month commissioning activity, Kepler science observations began; these observations will monitor more than 100,000 dwarf stars simultaneously over the primary mission life of 3.5 years, with a capability of extending observations for a total of 7 years. Precision differential photometry will be used to detect the periodic signals of transiting planets via small changes in the light of their host stars. Kepler will also support asteroseismology by measuring the pressure-mode (p-mode) oscillations of selected stars. Transits of a solar-size star by an Earth-size planet causes a reduction in the stare light of 84 parts per million (ppm) or 0.008 percent. For a statistically significant detection, the minimum single-transit signal-to-noise ratio (SNR) is taken to be 4 sigma (σ), leading to a combined average significance of 8σ for 4 transits. The Kepler combined differential photometric precision must therefore be less than 21 ppm binned at 6.5 hours (half the duration of a central transit). The 1 Earth-size planets are terrestrial or rocky planets with masses ranging from 0.5 to 10 Earth masses. Smaller planets cannot maintain a life-sustaining atmosphere, and larger planets retain primordial atmospheres. The habitable zone for a given star is defined as the region in which orbiting planets have the potential for liquid surface water, a key building block of life.
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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2009 Symposium detection threshold for terrestrial HZ planets in the Kepler data pipeline is set at 7σ, yielding a detection rate of 84 percent and controlling the number of expected false alarms to no more than one for the entire experiment. Given this design, the mission will be capable of detecting not only Earth analogs, but also a wide range of planetary types and characteristics, ranging from Mars-size objects and orbital periods of days to gas-giants and decade-long orbits. In fact, the mission is designed to survey the full range of spectral types of dwarf stars. The criteria for an Earth-size planet transiting a solar-size star—three or more transits of a star with a statistically consistent period, brightness change, and duration—provide a rigorous detection method. The size of the planet can be calculated from the relative change in brightness. The size (semi-major axis) of the orbit can be calculated from the observed orbital period using Kepler’s 3rd law of planetary motion and the planet’s location relative to the HZ. A key consideration when looking for planetary transits is the probability that the orbital plane is aligned along the line of sight. The probability of random orbital alignment is simply the ratio of the stellar diameter to the orbital diameter. For example, the probability of orbital alignment for a solar system like our own where the earth’s orbital diameter is about 300 million kilometers (km) and the Sun’s diameter is 1.4 million km is approximately 0.005 or 0.5 percent. Hence, many thousands of stars need to be monitored before a statistically meaningful result can be drawn. Confirmation of the existence of a planet will also require a sequence of transits with a consistent period, depth, and duration. The Kepler observatory, or Flight Segment of the mission, consists of a large field-of-view (FOV) photometer and a spacecraft bus. This observatory was launched on a Delta II rocket into an Earth-trailing, heliocentric orbit, which will mean that after 3.5 years the spacecraft will have drifted less than 0.5 astronautical units away from the Earth. The photometer is a Schmidt camera design consisting of a graphite-cyanate ester metering structure, a sunshade, a 95 centimeter diameter Schmidt corrector, a 1.4 meter diameter primary mirror, field-flattening lenses, and an array of 42 charge-coupled devices (CCDs) with an active FOV greater than 100 square degrees. The CCDs, back-illuminated devices with dual outputs and 1024 × 2200 27-μm pixels, are passively cooled by a radiator panel. The CCDs measure the brightnesses of 103,000 planetary target stars which are each accumulated (summed) for 30 minutes before being stored on the spacecraft’s solid-state recorder. Approximately 512 targets assigned to a succession of different target stars are co-added at a one minute cadence to support p-mode asteroseismology and other non-transit science. Since the targets are pre-selected, only the pixels relevant to each star (rather than the entire image) will be stored for downlink. This means that only about 3 percent of the pixels will be stored, thus saving a tremendous amount of on-board storage and communications link time. In addition, a data-compression scheme is used to further reduce storage and transmission requirements. The photometer does not have a shutter, and the only moving parts
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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2009 Symposium are three focus and tilt adjustment mechanisms under the primary mirror and a one-time deployable dust cover. The spacecraft provides necessary support for the photometer, including power, pointing control, and data systems. The spacecraft has three-axis stabilization and fine guidance sensors mounted on the scientific focal plane. The entire mission will be spent viewing a single star field centered near the constellation Cygnus. Attitude maneuvers will only occur every three months when the spacecraft is rotated about the photometer axis by 90 degrees to keep the solar array pointed toward the Sun and the radiator pointed toward deep space. Given Kepler’s heliocentric orbit, no disturbances will be caused by geomagnetic moments, gravity gradients, or atmospheric drag. The only disturbance will be a steady torque due to solar pressure. Stability for precision pointing is provided by reaction wheel assemblies for which periodic momentum-de-saturation maneuvers are provided by the same hydrazine-reaction control system that was used to de-spin and de-tumble the spacecraft following separation from the launch vehicle. No propulsive capability for orbital delta-V correction is required because the spacecraft’s drift-away rate was limited by the launch vehicle’s injection dispersion errors. The transmission of uplink commands and downlink real-time engineering data are performed by omni-directional X-band antennas. All stored science and engineering data are downlinked using a high-gain Ka-band system. The solar arrays, which are rigidly mounted on the spacecraft, also provide some shielding of the photometer from the Sun. The only moving parts on the spacecraft bus are the reaction wheels. All spacecraft sub-systems are fully redundant. During science operations, regular “housekeeping” communications with the spacecraft are planned to occur approximately twice weekly, and playback of the stored science data is downlinked once a month. The antennas of NASA’s Deep Space Network (DSN) support Kepler’s uplink and downlink communications. The Kepler Ground Segment includes a collection of facilities, software, processes, and procedures for operating the Flight Segment and analyzing data. Overall mission direction is provided from the Mission Management and Science Offices hosted by the Science Operations Center (SOC) at NASA Ames Research Center in Mountain View, California. Strategic mission planning and target selection are performed at SOC. Target selection to separate smaller solar-type dwarf stars from giant stars,2 is based on a Kepler-unique input catalog produced by a pre-launch Stellar Classification Program—based on ground-based observations. Operations management of the Flight Segment, tactical mission planning, sequence validation, and engineering trend analyses are provided by a flight 2 Primary interest is on main-sequence dwarf stars, which are longer lived than giant stars and hence more likely to host habitable planets. In addition, transit signals from Earth-size planets are easier to detect around smaller stars, which have a larger obscuration ratio.
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Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2009 Symposium planning center (FPC) at Ball Aerospace Technologies Corporation in Boulder, Colorado. Command and data processing, monitoring of the health and status of the Flight Segment, and scheduling of the DSN, are performed by the Mission Operations Center (MOC) at the Laboratory for Astronomy and Solar Physics (LASP) in Boulder, Colorado. Uplink and downlink telecommunications use NASA’s DSN 34 meter antennas located in Goldstone, California; Madrid, Spain; and Canberra, Australia. Navigation and orbit propagation are provided by the Jet Propulsion Laboratory in Pasadena, California. The Data Management Center (DMC) at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, receives the “raw” telemetry data and calibrates pixel levels. The resulting calibrated data set is archived by DMC and forwarded to SOC for further processing, which includes generating calibrated photometric light curves and transit detection. STScI also provides p-mode analysis. After an extensive data-validation process, follow-up observations on each planetary candidate will be performed (the Follow-up Observing Program [FOP]) to eliminate intrinsic false positives caused by grazing eclipsing binaries and extrinsic false positives caused by background eclipsing binaries and to discriminate between terrestrial transits of the target star and transits of giant-planet background stars. In some cases, FOP should also be able to use ground-based observations to measure the mass of the largest planets, which together with estimates of planet diameters derived from Kepler transit-depth measurements, can be combined to determine the density of the planets. ACKNOWLEDGEMENTS Project management and systems engineering for the Kepler mission development and commissioning was provided by the NASA Jet Propulsion Laboratory. The NASA Ames Research Center, which provides mission management during science operations, led the development of the Ground Segment and is the home of the Science Principal Investigator and Science Operations Center. The Flight Segment was designed and built by the Ball Aerospace Technology Corporation. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was sponsored by NASA.