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KEPLER MISSION OVERVIEW NASA/Ames Research Center/Kepler Mission “Is Earth unique, and if not, how many Earth-size planets might there be in our galaxy, orbiting their parent stars at just the right distances to have liquid water on their surfaces?” “What are the distributions in planet size, in planet orbits, and the types of stars hosting planets”? (modified from NASA, Press Kit/Feb. 2009) As one mission scientist put forth: “while not searching for ET, the Kepler Mission may potentially discover ET’s home” Kelper Mission Overview (outline) • • • • • • • Paradigm Changing Mission and Overview Queries being addressed through Kepler What the Kepler mission is designed to do Kepler’s unique features Kepler’s target Kepler’s orbit Kepler’s spacecraft o Dimensions o Command and control o The Kepler instrument – Photometer o The sunshade o CCD radiator o Solar array o Thruster modules o High gain antenna o Data compilation Kepler Mission Overview (cont.) • • • Transit method o Characteristics of a planetary transit o Probability of transits o Why does Kepler need so many (6.5 million) stars o Disadvantages in method o Advantages Follow-up to potential target Summary NASA’s 1st mission to identify: Earth-size and smaller planets Credit: NASA Kepler’s unique features Largest Schmidt telescope in space 170,000 multi-channel photometer 95 mega-pixel focal plane Instrument precision of 10 ppm Largest field of view for such an instrument While incredibly powerful astronomical instruments, when compared to the Kepler mission, the Hubble Space Telescope and other space telescopes are not optimized for planet reconnaissance. They typically point at many different areas of the sky, have very small fields of view, and rarely look continuously at just one star field. HST Kepler targeting Credit: Jon Lomberg The Kelper telescope is detecting exoplanet candidates by monitoring the Cygnus Lyra region of the Milky Way galaxy and measuring the brightness of more than 100,000 stars every 30 minutes, including Earthsize planets that can be detected by the telescope Kepler’s orbit Earth-trailing heliocentric (http://www.youtube.com/watch?v=54fnbJ1hZik) Orbital period equaling 371 days Credit: NASA Dimensions: Diameter = ~ 3 m Height = ~ 5 m Credit: NASA The command manager performs command processing of both stored-sequence and real-time commands. The command and data handling system is the spacecraft’s brain. It can operate the spacecraft either with commands stored in computer memory or via real-time commands radioed from Earth for immediate execution. In addition, it handles engineering and science data destined to be sent to Earth. The sole Kepler instrument is a photometer. It has a Schmidt telescope with a 95 cm clear aperture and 140 cm primary mirror; it has a 105 degrees2 field of view Credit: NASA The photometer features a focal plane array with 95 million pixels. The focal plane array is the largest camera NASA has flown in space. The Kepler focal plane consists of 42 science CCD and 4 fine guidance CCD. Each science CCD is 2200 columns by 1024 rows, thinned, backilluminated, anti-reflection coated, 4-phase devices manufactured by e2v. Each CCD has two outputs with the serial channel on the long edge. The pixels are 27 µm2, corresponding to 3.98 arcsec on the sky Credit: NASA Interior perspective of the Kepler Photometer Credit: NASA Bandpass = 430-890 nm Dynamic range = 9th to 15th magnitude stars Science data storage = ~ 2 months The Kepler photometer uses a pointing control system to orient itself in deep space (i.e., to determine and control the spacecraft’s attitude). The two star trackers, which provide the spacecraft with inertial attitude data, are part of an attitude determination and control system. The system includes fine guidance sensors, reaction wheels, and coarse sun sensors. Credit: NASA Credit: NASA and Ball Aerospace The sunshade provides a 55 degree sun avoidance angle for the photometer, while at the same time allowing for a 16 degree field of view. The sunshade provides continuous viewing of the star field in Cygnus throughout the lifetime of the mission. The CCD array is cooled by heat pipes connected to an external radiator. Kepler’s thermal control system includes heat pipes, thermally conductive adhesives, heaters, and temperature sensors. Propane and ammonia flowing through pipes embedded in the spacecraft’s exterior panels cool the focal plane. The electrical power system provides power for all onboard systems, including the photometer. Power is provided by the solar arrays and an onboard battery. The solar array is rigidly mounted on the spacecraft’s upper deck, providing both power and a shield for the photometer from direct solar heating. The solar array is expected to generate up to 1,100 Watts of electrical power Firing of the thrusters removes the excess momentum from the reaction wheels. There is enough fuel to last for ~ 6 years. Once the hydrazine is exhausted, the reaction wheels are expected to spin up to rates that exceed their design capacity. The high-gain antenna is part of the telecom subsystem designed to operate out to a distance of 96 million kilometers. The system also uses two receiving low-gain antennas and two transmitting low-gain antennas. The system can receive commands from Earth at speeds ranging from ~7 to 2,000 bits per second, and can transmit data to Earth from 10 to 4.3 million bits per second, the highest data rate of any NASA mission to date. Data Compilation The 95 megapixels of data can’t be stored continuously for 30 days, thus the science team has pre-selected 5% of the total pixels of interest associated with each star of interest, approximating. These data are then recompiled, compressed, and stored. The on-board photometer flight software compiles the science and ancillary pixel data and stores them in a 16 GigaByte solid-state recorder. Porter et al., 2012, Ecological and evolutionary informatics 2, 121-129 Method - Transit Method Credit: NASA Tiny dims (“winks”) in star brightness (which can last anywhere from an hour to half a day) occur when a planet passes in front of a star (planetary transit). The amount the star dims depends on the relative sizes among the star and planet. Credit: NASA AMES Characteristics of a Planetary Transit Period of recurrence of the transit Duration of the transit Fractional change in brightness of the star Credit: NASA Exoplanets are confirmed by observing several transits that have the same decrease in star light, time to transit the star, and total amount of time between successive transits. It takes ~ 1000 people-hours to confirm an exoplanet Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. The probability of a transit depends on the size of the planet’s orbit relative to the size of the star. For a planet in an Earth-sized orbit, the possibility of it being aligned to produce a transit is less than 1%. H-alpha image (view into the chromosphere) of (left) a Jupiter transit superimposed to scale, as if viewed from outside our solar system, and (right) an Earth transit to scale. Credit:NASA 3 or more transits with a consistent period, brightness change, and duration collectively provide ample evidence that an extra-solar planet has been identified. Probability of transits i = inclination of planet’s orbit to the plane of the sky Ɵ0 = angle of planet’s orbit with respect to the observer (= 90˚ - i) a = planet’s semi-major axis Rs = stellar radius Then, the probability that a planet will transit is given by: Modified form http://www3.geosc.psu.edu/~ruk15/Transits/Transits.ppt Probability of detecting Jupiter in the solar system = 700000km (solar radius) /5.2 AU (semi major axis of 778 million km) = ~0.1% To find one Jupiter at 5.2 AU from a Sun like star, one needs to look at ~ 1/(0.1%) ~ 1/(0.1%) ~ 1000 stars • Beatty, T.G., and Seager, S. (2010) Transit probabilities with stellar inclination constraints. The Astrophys. Journ. 712, 1433-1442. • Brown, R.H. (1968) Measurement of stellar diameters. Ann.Rev.AstrA&Astro. 6, 13-38. • Cassen, P., Guillot, T., and Quirrenbach, A. (2006) Extrasolar Planets. Swiss Soc. For Astr. &Astr. 31, DOI: 10.1007/978-3-540-31470-7 . ~ 6.5 million stars occupy the target region between Cyngnus and Lyra. Why are so many necessary for the Kepler mission? Due to the way in which the Kepler instrument searches for planets, there has to be planetary systems that are lined up so that the planet actually passes between the star and Kepler telescope and its orbit. The probability of that is estimated to be only 1-10%. So of those 6.5 million scattered over the target region, only about 200,000 our of interest to the Kepler team. Of those of interest, the team selects 170,000 or so that are most suitable to perform reconnaissance for planets. The team expects to end up with somewhere between a few hundred and few thousand signals that are really planets around the stars that are being looked at. The transit method makes it possible to: determine the size of the planet through the lightcurve (will be discussed in the coming weeks). study the atmosphere of the transiting planet. When the planet transits the star, light form the star passes through the upper atmosphere of the planet. By studying the high-resolution stellar spectrum carefully, one can detect elements present in the planet’s atmosphere. measure the planet’s radiation when the planet is blocked by its star (secondary eclipse) The transit method has two major disadvantages: 1. Planetary transits are only observable for planets whose orbits are perfectly aligned with the instrument’s vantage point. o The probability of a planetary orbital plane being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit. ~ 10% of planets with small orbits have such alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a sun-sized star at 1AU, the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot answer the question of whether any particular star is a host to planets. The transmit method has two major disadvantages (cont.) 2. The method results in a high rate of false detections. A transit detection requires additional confirmation, typically from the radialvelocity method. Follow-up to a potential target Once Kepler’s candidate planetary transit events are identified, a team of ground-based observers perform follow-up observations to rule out false positive events that may mimic a sequence of transits. The follow-up observations provide additional information about the characteristics of the parent stars, their size, mass, age, etc., and should lead to the detections of other planets in the systems. If a planet has been detected by the transit method, for example, the Transit Timing Variation method (TTV) can be applied based on the variations in the timing of the transit. The method is capable of detecting additional planets in the system with sizes potentially as small as Earth-sized planets (see Holman & Murray (2005) Science). Results of the Paradigm-changing Kepler mission will be highlighted in the coming weeks Credit: http://orbiterchspacenews.blogspot.com/2010/12/kepler-mission-manager-update.html