Charley Noecker

StarLight mission: a formation-flying stellar interferometer

The StarLight mission is designed to validate the technologies of formation flying and stellar interferometry in space. The mission consists of two spacecraft in an earth-trailing orbit that formation-fly over relative ranges of 40 to 600m to an accuracy of 10 cm. The relative range and bearing of the spacecraft is sensed by a novel RF sensor, the Autonomous Formation Flyer sensor, which provides 2cm and 1mrad range and bearing knowledge between the spacecraft. The spacecraft each host instrument payloads for a Michelson interferometer that exploit the moving spacecraft to generate variable observing baselines between 30 and 125m. The StarLight preliminary design has shown that a formation-flying interferometer involves significant coupling between the major system elements - spacecraft, formation-flying control, formation-flying sensor, and the interferometer instrument. Mission requirements drive innovative approaches for long-range heterodyne metrology, optical design, glint suppression, formation estimation and control, spacecraft design, and mission operation. Experimental results are described for new technology development areas.

TPF-C: Status and recent progress

The Terrestrial Planet Finder Coronagraph (TPF-C) is a deep space mission designed to detect and characterize Earth-like planets around nearby stars. TPF-C will be able to search for signs of life on these planets. TPF-C will use spectroscopy to measure basic properties including the presence of water or oxygen in the atmosphere, powerful signatures in the search for habitable worlds. This capability to characterize planets is what allows TPF-C to transcend other astronomy projects and become an historical endeavor on a par with the discovery voyages of the great navigators.

Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets Executive Summary of a Report of the ExoPlanet Task Force Astronomy and Astrophysics Advisory Committee Washington, DC June 23, 2008

WE STAND ON A GREAT DIVIDE in the detection and study of exoplanets. On one side of this divide are the hundreds of known massive exoplanets, with measured densities and atmospheric temperatures for a handful of the hottest exoplanets. On the other side of the divide lies the possibility, as yet unrealized, of detecting and characterizing a true Earth analogue—an Earth-like planet (a planet of one Earth mass or Earth radius orbiting a Sun-like star at a distance of roughly one astronomical unit). This ExoPlanet Task Force Report describes how to bridge the divide. The recommendations emphasize immediate investment in technology and space mission development that will lead to discovering and characterizing Earth analogues. We recognize that setting a goal of detecting planets like Earth sets the bar high. It is important that the program target such objects if we are to determine whether the conditions we find on our own world are a common outcome of planetary evolution. The only example of a habitable world we have is our own one-Earth-mass planet; and indeed our nearest neighbor, Venus, is nearly the same mass but uninhabitable by virtue of closer proximity to the Sun. Searching for planets, for example, five times the mass of Earth is easier. But should they turn out to lack habitable atmospheres, we would not know whether this is by chance or whether it is a systematic effect of the higher mass. The connection of

External occulters for direct observation of exoplanets: an overview

Perhaps the most compelling piece of science and exploration now under discussion for future space missions is the direct study of planets circling other stars. Indirect means have established planets as common in the universe but have given us a limited view of their actual characteristics. Direct observation holds the potential to map entire planetary systems, view newly forming planets, find Earth-like planets and perform photometry to search for major surface features. Direct observations will also enable spectroscopy of exoplanets and the search for evidence of simple life in the universe. Recent advances in the design of external occulters - starshades that block the light from the star while passing exoplanet light - have lowered their cost and improved their performance to the point where we can now envision a New Worlds Observer that is both buildable and affordable with todays technology. We will summarize recent studies of such missions and show they provide a very attractive alternative near term mission.

The New Worlds Observer: using occulters to directly observe planets

We discuss the progress that has been made in the understanding of the use of external occulters to observe exoplanetary systems. We show how a starshade can be designed and built in a practical and affordable manner to fully remove starlight and leave only planet light entering a telescope. When coupled to a powerful observatory like the James Webb Space Telescope, an occulter can extinguish the starlight and reveal basic details of the planetary systems around our closest, neighboring stars.

The New Worlds Observer: scientific and technical advantages of external occulters

Perhaps the most compelling piece of science and exploration now under discussion for future space missions is the direct study of planets circling other stars. Indirect means have established planets as common in the universe but have given us a limited view of their actual characteristics. Direct observation holds the potential to map entire planetary systems, view newly forming planets, find Earth-like planets and perform photometry to search for major surface features. Direct observations will also enable spectroscopy of exoplanets and the search for evidence of simple life in the universe. Recent advances in the design of external occulters - starshades that block the light from the star while passing exoplanet light - have lowered their cost and improved their performance to the point where we can now envision a New Worlds Observer that is both buildable and affordable with todays technology. In this paper we explore the comparison of scientific capability of external occulters relative to indirect means and to internal coronagraph missions. We conclude that external occulters logically provide the architecture for the next space mission for exoplanet studies.

Starshade Scaling Relations

The New Worlds Observer enables high-contrast imaging by placing a space telescope in the dark shadow cast by an apodized starshade. Depending on the science requirements, we consider starshades that provide a wide range of contrast (from ~10-4 to more than 10-15) over an octave of wavelength (from UV to Visible) at a variety of inner working angles (from a few milliarcseconds to several arcseconds). The starshade-telescope system is described by many parameters, including starshade diameter, telescope diameter, starshade-telescope separation, and wavelength range, that interact non-linearly. In this paper, we show how the different parameters contribute to the starshades performance and discuss the selection process for different science requirements.

The Fourier-Kelvin Stellar Interferometer: a practical interferometer for the detection and characterization of extrasolar giant planets

The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near-to-mid-infrared spectral region (3-8µm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets: Σ What are the characteristics of the known extrasolar giant planets? Σ What are the characteristics of the extrasolar zodiacal clouds around nearby stars? Σ Are there giant planets around classes of stars other than those already studied? We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5m telescopes on a 12.5m boom feeding a Mach-Zender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution.

Optical systems engineering for a terrestrial planet finder coronagraph

Optical coronagraphy is a promising possibility for finding and characterizing Earth-like planets that orbit nearby stars. This approach begins with a large unobscured conventional telescope, but significant modifications are needed to achieve adequate suppression of the glare of the star. Three techniques are under consideration for suppression of the aperture diffraction which redirects starlight into the planets pixel; once this is satisfied, extraordinary precision and stability in the wavefront are needed as well to suppress scatter of the starlight into that pixel. We discuss the central choices in the setting of error budgets, a summary of key allocations in that budget, optical model results that demonstrate the operation and performance of the system, and key hardware requirements.

Error analysis of a compound nulling interferometer

Nulling interferometry at mid-infrared wavelengths holds promise for finding and characterizing Earth-like planets that orbit nearby stars. By strongly suppressing light from a nearby star, the instrument becomes sensitive enough for direct detection of planets orbiting that star. A compound nulling interferometer (combining light from more than 2 telescopes) is needed for these searches, in order to achieve adequate light suppression across the full disk of the star. We present an error analysis of quasi-static and chopping variants of a four element nulling interferometer, including the dependence on amplitude, delay, baseline length, and telescope pointing errors.