Terrestrial Planet Finder Research Articles

Characterizing extrasolar terrestrial planets with reflected, emitted and transmitted spectra.

NASA and ESA are planning missions to directly detect and characterize terrestrial planets outside our solar system (nominally NASA-Terrestrial Planet Finder and ESA-DARWIN missions). These missions will provide our first opportunity to spectroscopically study the global characteristics of those planets, and search for signs of habitability and life. We have used spatially and spectrally-resolved models to explore the observational sensitivity to changes in atmospheric and surface properties, and the detectability of surface biosignatures, in the globally averaged spectra and light-curves of the Earth. Atmospheric signatures of Earth-size exoplanets might be detected, in a near future, by stellar occultation as well. Detectability depends on planet's size, atmospheric composition, cloud cover and stellar type. According to our simulations, Earth's land vegetation signature (red-edge) is potentially visible in the disk-averaged spectra, even with cloud cover, and when the signal is averaged over the daily time scale. Marine vegetation is far more difficult to detect. We explored also the detectability of an exo-vegetation responsible for producing a signature that is red-shifted with respect to the Earth vegetation's one.

Comparison of beam combiners for a synthetic aperture imaging telescope

Future astronomical space missions will comprise a constellation of several optical telescopes to detect exo-planets by interferometric nulling of starlight. The Darwin mission of the European Space Agency (ESA) and NASAs Terrestrial Planet Finder Interferometer both consist of a free-flying collection of telescopes and a beam combiner. As such, the constellation provides a co-phased array of telescopes that can also be used for aperture synthesis imaging. This imaging technique relies on recording intensity interference patterns, in which the layout of the beam combination optics and the detector play a key role. Several designs for beam combination have been proposed in the literature. In this article, we compare these beam combiners by rigorously simulating the imaging process of a weak stellar source, taking into account the photon arrival statistics, an imperfect detection process and the image reconstruction from the recorded data. The results are presented as the to be expected reconstruction error in the luminous intensity distribution function of a wide-field stellar source versus the provided amount of photons. Using these results, the optimum design of the combination beam combiner and detector array can be identified.

Thin-Film Piezoelectric Unimorph Actuator-Based Deformable Mirror With a Transferred Silicon Membrane

This paper describes a proof-of-concept deformable mirror (DM) technology, with a continuous single-crystal silicon membrane reflecting surface, based on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ PbZr _0.52 Ti_0.48 O _3$</tex> (PZT) unimorph membrane microactuators. A potential application for a terrestrial planet finder adaptive nuller is also discussed. The DM comprises a continuous, large-aperture, silicon membrane “transferred” onto a 20 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$,times,$</tex> 20 piezoelectric unimorph actuator array. The actuator array was prepared on an electroded silicon substrate using chemical-solution-deposited 2- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu m$</tex> -thick PZT films working in a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$d _31$</tex> mode. The substrate was subsequently bulk-micromachined to create membrane structures with residual silicon acting as the passive layer in the actuator structure. A mathematical model simulated the membrane microactuator performance and aided in the optimization of membrane thicknesses and electrode geometries. Excellent agreement was obtained between the model and the experimental results. The resulting piezoelectric unimorph actuators with patterned PZT films produced large strokes at low voltages. A PZT unimorph actuator, 2.5 mm in diameter with optimized PZT/silicon thickness and design showed a deflection of 5.7 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$~mu m$</tex> at 20 V. A DM structure with a 20- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu m$</tex> -thick silicon membrane mirror (50 mm <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$times,$</tex> 50 mm area) supported by 400 PZT unimorph actuators was successfully fabricated and optically characterized. The measured maximum mirror deflection at 30 V was approximately 1 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$~mu m$</tex> . An assembled DM showed an operating frequency bandwidth of 30 kHz and an influence function of approximately 30%. 1738

Linear and Bayesian Planet Detection Algorithms for theTerrestrial Planet Finder

Current plans call for the first Terrestrial Planet Finder mission, TPF-C, to be a monolithic space telescope with a coronagraph for achieving high contrast. The coronagraph removes the diffracted starlight allowing the nearby planet to be detected. In this paper, we present a model of the planet measurement and noise statistics. We utilize this model to develop two planet detection algorithms, one based on matched filtering of the PSF and one using Bayesian techniques. These models are used to formulate integration time estimates for a planet detection with desired small probabilities of false alarms and missed detections.

Detectability of Red-Edge-shifted Vegetation on Terrestrial Planets Orbiting M Stars

We have explored the detectability of exovegetation on the surface of a terrestrial planet orbiting an M star. The exovegetation is responsible for producing a pigment-derived surface signature that is redshifted with respect to the Earth vegetation's red edge. The redshift was estimated using a model of leaf optical property spectra (Jacquemoud & Baret) combined with a 3 photon photosynthetic scheme calculated by Wolstencroft & Raven for a possible exovegetation growing on an M star planet. To study the detectability of this surface biosignature on an M star terrestrial planet, we have used the three-dimensional model developed by Tinetti et al. for the case of the Earth. This model can generate disk-averaged spectra and broadband integrated fluxes, which will be useful for future terrestrial planet exploration missions, such as the NASA Terrestrial Planet Finder Coronagraph. Input to this model were the atmospheric profiles and cloud distributions predicted by Joshi and coworkers for a synchronous planet orbiting an M dwarf and the distinctive surface reflectance of the exovegetation. While on Earth this pigment-derived surface feature would be almost completely masked by water absorption, even in a cloud-free atmosphere, we found that the strength of the edge feature on our simulated M star terrestrial planet can exceed that on Earth, given the right conditions. Obviously, the detectability of such biosignatures would be highly dependent on the extent of vegetation surface area, cloud cover, and viewing angle.

Detection Limits from the McDonald Observatory Planet Search Program

Based on the long-term radial-velocity surveys carried out with the McDonald Observatory 2.7m Harlan J. Smith Telescope from 1988 to the present, we derive upper limits to long-period giant planet companions for 31 nearby stars. Data from three phases of the McDonald Observatory 2.7m planet-search program have been merged together and for 17 objects, data from the pioneering Canada-France-Hawaii Telescope (CFHT) radial-velocity program have also been included in the companion-limits determination. For those 17 objects, the baseline of observations is in excess of 23 years, enabling the detection or exclusion of giant planets in orbits beyond 8 AU. We also consider the possibility of eccentric orbits in our computations. At an orbital separation of 5.2 AU, we can exclude on average planets of M sin i > 2.0+/-1.1 Mjup (e=0) and M sin i > 4.0+/-2.8 Mjup (e=0.6) for 25 of the 31 stars in this survey. However, we are not yet able to rule out true Jupiters, i.e. planets of M sin i ~ 1 Mjup in 5.2 AU orbits. These limits are of interest for the Space Interferometry Mission (SIM), Terrestrial Planet Finder (TPF), and Darwin missions which will search for terrestrial planets orbiting nearby stars, many of which are included in this work.

Jitter Correction Algorithms for the COROT Satellite Mission

ABSTRACTThe COROT satellite contains a highly accurate stellar photometer with two channels that are respectively optimized for asteroseismology and terrestrial planet finding. The asteroseismology channel can observe 10 bright objects per field for 150 days (long run). At least five different long‐run fields will be observed over the spacecraft's lifetime. In order to sample every second, the data that are downloaded must be restricted to fit within the telemetry rate. Each object is therefore assigned a digital mask. The downloaded data will consist of the total electron count within this mask and the barycenter of each star image. Small satellite motions (or jitter) cause some of the flux to fall outside the mask and also cause changes in the position of the barycenter. Here we provide a methodology for using the change in position to correct for flux loss. Different methods are presented, their quality is examined, and the results of the best method(s) are discussed. While our methodology is designed for COROT, it is sufficiently general to be used for any satellite mission with aspirations to long‐term, high‐precision photometry, such as Kepler.

Expected productivity-based risk analysis in conceptual design

To be effectively used in design decisions, it is important to bring risk into the design process as a quantitative parameter that engineers can both understand and trade. One parameter that is often used when making design decisions is the productivity of a design. If instead of only the nominal value, it were possible to examine the expected value of the productivity metric, taking into account failures and risk items, the concept of risk could be brought into the design process in a very quantitative and tradable way. While the expected productivity is easy to calculate for a simple system, it is more complex if the system has a path-dependent productivity function. An approach has been developed to model the expected productivity of systems with path-dependent productivities, in an accurate and efficient manner. This approach has been tested against Monte Carlo simulations with excellent results. This paper discusses the need for the new modeling methodology, the details of the methodology that has been developed, and an example of how to use expected productivity in a trade study for a real mission, the Terrestrial Planet Finder Interferometer.

NASA's new budget; Titan will brighten; 200 years in geology; Magnetic fields fire jets; PPARC economizes; M101 at large scale; Astronomical arms.

Sue Bowler, Editor It’s a cheery time for solar system science: UK work in this field has had an international stamp of approval, ESA’s Aurora programme is moving forwards, and existing missions such as Cassini and Mars Express are producing superb quality data that are full of scientific surprises. But it’s a bit less cheery for other branches of the science, faced with PPARC’s and NASA’s budget plans for the coming years. NASA is giving human spaceflight – in the form of the International Space Station, the shuttle and its replacement – priority over planned science missions such as Dawn and the Terrestrial Planet Finder. PPARC is struggling to cover the higher than anticipated costs of paying universities a fair rate for the work researchers do, and paying postgraduate students and research assistants a living wage. The result, on both sides of the Atlantic, is a squeeze on successful areas of research that will mean some of them stopping for good. Space missions are being cancelled or indefinitely postponed, research groups will have to find something else to research, and hardpressed physics departments will feel the pinch of less research in the long run. It’s not an enticing prospect, but this is what is happening. Science and scientists are expensive, and those running science have to decide what parts of it are worth paying for. There is no alternative: money is not going to magically appear from the public purse, nor should it. Instead, people are going to have to make the case for science, loudly and repeatedly, at all levels. Outreach has become a necessity, not an extra. The challenge is to show that research – in space, Earth sciences and astrophysical sciences, as well as in planetary exploration – matters and is worth paying for. And we have to reach voters, not just the idealized “interested layperson”. Compare the current public enthusiasm for the ISS with the fervour of the early days of the Apollo programme: we have a very long way to go. EDITORIAL

Planet Signal Extraction for theTerrestrial Planet FinderInterferometer

Signal extraction is a vital link between science and systems-engineering requirements. In this paper we present a Terrestrial Planet Finder (TPF) interferometer planet-signal-extraction algorithm and demonstrate the performance of several nulling-interferometer designs on canonical TPF astronomical scenes. We create the output response of a linear phase-chopping dual Bracewell nulling interferometer and a matrix version of the correlation method employed to generate dirty images. We derive general and specific map parameters, such as signal-to-noise ratio, signal-to-artifact ratio, and detection confidence, used for individual maps or comparing array architectures. We implement a matrix form of CLEAN that removes map artifacts, produces reconstructed images, and retrieves planetary signals. Monte Carlo simulations show that some fixed-length structurally connected interferometer configurations can detect Earth-like planets for systems at 10 pc in the presence of stellar Poisson noise. Since angular resolution depends on baseline length, a design that can vary array configuration for each specific scene is superior to an interferometer with a fixed array length. Thus, a flexible free-flying architecture should satisfy the science requirements for more TPF candidates, compared to a fixed-length structurally connected architecture.

Performance study of ground-based infrared Bracewell interferometers

Nulling interferometry, a powerful technique for high-resolution imaging of the close neighbourhood of bright astrophysical objets, is currently considered for future space missions such as Darwin or the Terrestrial Planet Finder Interferometer (TPF-I), both aiming at Earth-like planet detection and characterization. Ground-based nulling interferometers are being studied for both technology demonstration and scientific preparation of the Darwin/TPF-I missions through a systematic survey of circumstellar dust disks around nearby stars. In this paper, we investigate the influence of atmospheric turbulence on the performance of ground-based nulling instruments, and deduce the major design guidelines for such instruments. End-to-end numerical simulations allow us to estimate the performance of the main subsystems and thereby the actual sensitivity of the nuller to faint exozodiacal disks. Particular attention is also given to the important question of stellar leakage calibration. This study is illustrated in the context of GENIE, the Ground-based European Nulling Interferometer Experiment, to be installed at the VLTI and working in the L' band. We estimate that this instrument will detect exozodiacal clouds as faint as about 50 times the Solar zodiacal cloud, thereby placing strong constraints on the acceptable targets for Darwin/TPF-I.

LIVIO, M., REID, N. &amp; SPARKS, W. (eds) 2005. Astrophysics of Life. Space Telescope Science Institute Symposium Series no. 16. xi +110 pp. Cambridge, New York, Melbourne: Cambridge University Press. Price £55.00, US $100.00 (hard covers). ISBN 0 521 82490 7

LIVIO, M., REID, N. &amp; SPARKS, W. (eds) 2005. Astrophysics of Life. Space Telescope Science Institute Symposium Series no. 16. xi +110 pp. Cambridge, New York, Melbourne: Cambridge University Press. Price £55.00, US $100.00 (hard covers). ISBN 0 521 82490 7 - Volume 143 Issue 1

Exploring the Potential of Integral Field Spectroscopy for Observing Extrasolar Planet Transits: Ground‐based Observations of the Atmospheric Na in HD 209458b

ABSTRACTWe explore the use of integral field spectroscopy (IFS) for observing extrasolar planet transits. Although this technique should find its full potential in space‐based observations (e.g., James Webb Space Telescope, Terrestrial Planet Finder), we have tested its basics with ground‐based time‐series observations of HD 209458b obtained with the William Herschel Telescope optical fiber system INTEGRAL during a transit in 2004 August 17/18. For this analysis we have used 5550 spectra (from a potential of ∼30,000) obtained in 150 exposures during a period of more than 7 hr. We have found that IFS offers three fundamental advantages with respect to previously used methods (based on imaging or standard slit spectroscopy). First, it improves the effective signal‐to‐noise ratio in photon‐limited observations by distributing the light coming from the star into the two dimensions of the detector. Second, this type of IFS data allows us to “autocalibrate” instrumental and background effects. Third, since the star image characteristics (i.e., seeing, spatial shifts, etc.) as well as its photometric properties are extracted from the same data cube, it is possible to decorrelate photometric instabilities induced by point‐spread function (or instrument) variations. These data have also allowed us to explore the accuracy limits of ground‐based relative spectrophotometry. This was done using a photometric index that probes the Na D lines, for which we obtained a nominal 1 σ error of ∼1.0 × 10−4. This result, based on observations of only one transit, indicates that this type of ground observation can constrain the characterization of the transmission spectrum of extrasolar planets, especially if they cover multiple transits under good weather conditions. The present observations are compatible with no extra Na D depression during the transit. Although this result seems to be inconsistent with the recently reported Hubble Space Telescope STIS findings, we point out its limited statistical meaning: the results disagree within 1 σ, but agree within 2 σ. This method requires careful and specific reductions, and details about this process are given. We also give some recommendations to instrument developers in order to enhance the efficiency of the method.

Advanced thermo-structural technologies for the NASA terrestrial planet finder mission

The primary objective of the NASA Terrestrial Planet Finder flight missions will be to detect and characterize Earth-like planets around nearby stars. Because of the complexity of the design concepts, the project will rely heavily on the use of analytical simulations to predict on-orbit performance. Furthermore, current understanding of these missions indicates that the 3–8 m class optical systems need to be as stable as picometres in wavefront and sub-milli arcsec in pointing. These extremely small requirements impose on the models a level of predictive accuracy heretofore never achieved, especially in the area of microgravity effects, material property accuracy, thermal solution convergence, microdynamic stability and all other second-order modelling effects typically ignored. New modelling tools and analysis paradigms are developed which emphasize computational accuracy and fully integrated analytical simulations. The process is demonstrated on sample problems using a TPF Coronagraph design concept. The TPF project is also planning a suite of testbeds through which various aspects of the models and simulations will be verified. Copyright © 2005 John Wiley & Sons, Ltd.

Decentralized Formation Flying Control in a Multiple‐Team Hierarchy

In recent years, formation flying has been recognized as an enabling technology for a variety of mission concepts in both the scientific and defense arenas. Examples of developing missions at NASA include magnetospheric multiscale (MMS), solar imaging radio array (SIRA), and terrestrial planet finder (TPF). For each of these missions, a multiple satellite approach is required in order to accomplish the large-scale geometries imposed by the science objectives. In addition, the paradigm shift of using a multiple satellite cluster rather than a large, monolithic spacecraft has also been motivated by the expected benefits of increased robustness, greater flexibility, and reduced cost. However, the operational costs of monitoring and commanding a fleet of close-orbiting satellites is likely to be unreasonable unless the onboard software is sufficiently autonomous, robust, and scalable to large clusters. This paper presents the prototype of a system that addresses these objectives-a decentralized guidance and control system that is distributed across spacecraft using a multiple team framework. The objective is to divide large clusters into teams of "manageable" size, so that the communication and computation demands driven by N decentralized units are related to the number of satellites in a team rather than the entire cluster. The system is designed to provide a high level of autonomy, to support clusters with large numbers of satellites, to enable the number of spacecraft in the cluster to change post-launch, and to provide for on-orbit software modification. The distributed guidance and control system will be implemented in an object-oriented style using a messaging architecture for networking and threaded applications (MANTA). In this architecture, tasks may be remotely added, removed, or replaced post launch to increase mission flexibility and robustness. This built-in adaptability will allow software modifications to be made on-orbit in a robust manner. The prototype system, which is implemented in Matlab, emulates the object-oriented and message-passing features of the MANTA software. In this paper, the multiple team organization of the cluster is described, and the modular software architecture is presented. The relative dynamics in eccentric reference orbits is reviewed, and families of periodic, relative trajectories are identified, expressed as sets of static geometric parameters. The guidance law design is presented, and an example reconfiguration scenario is used to illustrate the distributed process of assigning geometric goals to the cluster. Next, a decentralized maneuver planning approach is presented that utilizes linear-programming methods to enact reconfiguration and coarse formation keeping maneuvers. Finally, a method for performing online collision avoidance is discussed, and an example is provided to gauge its performance.

Control Strategies for Formation Flight In the Vicinity of the Libration Points

The concept of formation flight of multiple spacecraft offers many promising possibilities, both for space exploration and the associated technology development. Past studies on formation flight have focused primarily on Earth-orbiting clusters. However, space-based observatory and interferometry missions, such as the proposed Terrestrial Planet Finder and the Micro Arc-second X-ray Imaging Mission, reflect plans for formation flight in multibody regimes, particularly in the vicinity of the sun-Earth/moon libration points. two specific tasks are accomplished in this study. First, in a dynamically sensitive regime such as that near the libration points, baseline propulsive requirements are established. A decentralized control strategy based on existing linear and nonlinear control techniques is employed and the results are presented through a series of sample mission configurations. Note that the control problem is formulated to facilitate future tradeoff studies in the libration point environment. The analysis is presented within the context of both the circular restricted three-body problem and the more complete ephemeris model. Once baseline costs are available, the second task is the introduction of potential constraints. These constraints may affect not only the formation control strategies but also the conceptual design of the mission, and their influence on the cost is evaluated.

Spectroscopy with the Terrestrial Planet Finder Coronagraph

We describe the rationale and requirements for an integral-field spectrograph aboard NASA's Terrestrial Planet Finder Coronagraph.

Towards 1010 Contrast for NASA's Terrestrial Planet Finder Mission: Demonstration of High Contrast in a Shaped-Pupil Coronagraph at Princeton

Our group at Princeton University is developing the Shaped-Pupil Coronagraph (SPC) as a solution to the high contrast imaging requirements for NASA's Terrestrial Planet Finder mission. At the heart of the SPC is a specially designed shaped mask at the pupil plane, and a star occulter at the image plane. We report a measurement of , with no adaptive optics corrections. This contrast is maintained at laser wavelengths of 532, 594, and 632nm, and for broadband light from at least 550nm to 750nm. The contrast is almost certainly limited by wavefront aberrations in the mirrors. Indeed, the level and general structure of the speckles in the high contrast region is consistent with statistical simulations of our optics.

Transit, Astrometric, Coronagraphic and Interferometric Exo-planet Studies - Synergy and Complementarity

The goals of the Navigator Program at NASA are to find Earth-like planets around nearby stars, to determine if they are habitable, and to search for signs of life. Three strategic missions are planned to carry out this program: the Space Interferometer Mission Planetquest (SIM), the Terrestrial Planet Finder Coronagraph (TPF-C), and the Terrestrial Planet Finder Interferometer (TPF-I). These missions, along with the PI-class Kepler project, will each discover unique knowledge about extrasolar planets, synergistically building on the other missions.

Atmospheric Biomarkers and their Evolution over Geological Timescales

The search for life on extrasolar planets is based on the assumption that one can screen extrasolar planets for habitability spectroscopically. The first space born instruments able to detect as well as characterize extrasolar planets, Darwin and terrestrial planet finder (TPF-I and TPF-C) are scheduled to launch before the end of the next decade. The composition of the planetary surface, atmosphere, and its temperature-pressure profile influence a detectable spectroscopic signal considerably. For future space-based missions it will be crucial to know this influence to interpret the observed signals and detect signatures of life in remotely observed atmospheres. We give an overview of biomarkers in the visible and IR range, corresponding to the TPF-C and TPF-I/DARWIN concepts, respectively. We also give an overview of the evolution of biomarkers over time and its implication for the search for life on extrasolar Earth-like planets. We show that atmospheric features on Earth can provide clues of biological activities for at least 2 billion years.