Experimental evaluation of active-member control of precision structures

The results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure are described. These experiments are directed toward the development of high-performance structural systems as part of the Control/Structure Interaction (CSI) program at JPL. The focus of CSI activity at JPL is to develop the technology necessary to accurately control both the shape and vibration levels in the precision structures from which proposed large space-based observatories will be built. Structural error budgets for these types of structures will likely be in the sub-micron regime; optical tolerances will be even tighter. In order to achieve system level stability and local positioning at this level, it is generally expected that some form of active control will be required

Vibration suppression by stiffness control

The feasibility of using piezoelectric ceramics as both sensors and actuators for vibration suppression in a lightweight, flimsy structure was demonstrated. Multimode control was achieved using one sensor and actuator pair. The Positive Position Feedback control strategy requires only knowledge of the natural frequencies of the structure. Implementation of the Positive Position Feedback used only strain measurements to achieve damping, no velocities, or acceleration are needed. All spillover is stabilizing for sufficient small gains

Paper Session II-A - SIRTF: The Fourth Great Observatory

The Space Infrared Telescope Facility (SIRTF), the fourth of the Great Observatories, will look through a new window on the universe. Using SIRTF, the astronomical community will be able to explore the infrared universe with a depth and precision complementary to that achieved by NASA’s other Great Observatories-the Hubble Space Telescope (HST), the Advanced X-ray Astrophysics Facility (AXAF), and the Compton Gamma Ray Observatory (GRO). SIRTF will achieve unprecedented infrared sensitivity by fully utilizing a new generation of infrared detector arrays. The new detectors, combined with an 85-cm cryogenic telescope, will allow SIRTF to provide scientific capabilities so impressive that SIRTF was designated the highest priority major new mission for all US astronomy in the 1990s. This paper will provide a review of the SIRTF program—the science, mission design, facility, and the instruments. Emphasis will be placed on those features of the program which will allow us to realize the great scientific potential of the Observatory in a resource-constrained environment

The On-Orbit Performance of the Galaxy Evolution Explorer

We report the first year on-orbit performance results for the Galaxy Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter modified Ritchey-Chretien telescope with a 1.25 degree field of view, selectable imaging and objective grism spectroscopic modes, and an innovative optical system with a thin-film multilayer dichroic beam splitter that enables simultaneous imaging by a pair of photon counting, microchannel plate, delay line readout detectors. Initial measurements demonstrate that GALEX is performing well, meeting its requirements for resolution, efficiency, astrometry, bandpass definition and survey sensitivity.Comment: This paper will be published as part of the Galaxy Evolution Explorer (GALEX) Astrophysical Journal Letters Special Issu

A Road Map for the Exploration of Neighboring Planetary Systems (ExNPS)

A brown dwarf star having only 20-50 times the mass of Jupiter is located below and to the left of the bright star GL 229 in this image from the Hubble Space Telescope. At the 19 light year distance to GL 229, the 7.7-arcsec separation between the star and the brown dwarf corresponds to roughly the separation between Pluto and the Sun in our Solar System. The goal of the program described in this report is to detect and characterize Earth-like planets around nearby stars where conditions suitable for life might be found. For a star like the Sun located 30 light years away, the appropriate star-planet separation would be almost 100 times closer than seen here for GL 229B

The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission

We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on April 28, 2003. GALEX is performing the first space UV sky-survey, including imaging and grism surveys in two bands (1350-1750 Angstroms and 1750-2750 Angstroms). The surveys include an all-sky imaging survey (m[AB] ~ 20.5), a medium imaging survey of 1000 square degrees (m[AB] ~ 23), a deep imaging survey of 100 square degrees (m[AB] ~ 25), and a nearby galaxy survey. Spectroscopic grism surveys (R=100-200) are underway with various depths and sky coverage. Many targets overlap existing or planned surveys. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the UV-global star formation rate relationship in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 < z < 1.5, and probe the physical drivers of star formation in galaxies. The GALEX mission includes a Guest Investigator program supporting the wide variety of programs made possible by the first UV sky survey.Comment: This paper will be published as part of the Galaxy Evolution Explorer (GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS/ after November 22, 200

The Occurrence of Rocky Habitable-zone Planets around Solar-like Stars from Kepler Data

We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define η⊕ as the HZ occurrence of planets with radii between 0.5 and 1.5 R⊕ orbiting stars with effective temperatures between 4800 and 6300 K. We find that η⊕ for the conservative HZ is between 0.37^(+0.48)_(−0.21) (errors reflect 68% credible intervals) and 0.60^(+0.90)_(−0.36) planets per star, while the optimistic HZ occurrence is between 0.58^(+0.73)_(−0.33) and 0.88^(+1.28)_(−0.51) planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ~6 pc away and there are ~4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun

The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data

We present occurrence rates for rocky planets in the habitable zones (HZ) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define $\eta_\oplus$ as the HZ occurrence of planets with radius between 0.5 and 1.5 $R_\oplus$ orbiting stars with effective temperatures between 4800 K and 6300 K. We find that $\eta_\oplus$ for the conservative HZ is between $0.37^{+0.48}_{-0.21}$ (errors reflect 68\% credible intervals) and $0.60^{+0.90}_{-0.36}$ planets per star, while the optimistic HZ occurrence is between $0.58^{+0.73}_{-0.33}$ and $0.88^{+1.28}_{-0.51}$ planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates using both a Poisson likelihood Bayesian analysis and Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with $95\%$ confidence that, on average, the nearest HZ planet around G and K dwarfs is about 6 pc away, and there are about 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.Comment: To appear in The Astronomical Journa

An Experimental Investigation of Vibration Suppression in Large Space Structures Using Positive Position Feedback

A new technique for vibration suppression in Large Space Structures is demonstrated in laboratory experiments on a thin cantilever beam, resulting in substantially reduced dynamic response. This technique, called Positive Position Feedback, makes use of generalized displacement measurements to accomplish vibration suppression. The concept of a piezoelectric active-member is developed in relation to controlling space-truss type structures. The active-member functions dually as a structural member and a control actuator. Piezoelectric ceramic material is adhered to a thin cantilever beam and simulates the use of an active-member. This space-realizable control scheme makes use of strain measurements, a preferred measurement quantity for vibration suppression, and internal control forces which completely decouple the rigid-body motion from the elastic motion. A simple necessary and sufficient condition for stability with Positive Position Feedback is presented. This condition is non-dynamic and is in general easily satisfied. As a result, Positive Position Feedback is demonstrated to have superior robust stability properties. It is also demonstrated that with Positive Position Feedback, all control and observation spillover is stabilizing. Five experiments are described in which the first six modes of vibration of the cantilever beam are controlled.</p