M. Bester

THE BERKELEY INFRARED SPATIAL INTERFEROMETER: A HETERODYNE STELLAR INTERFEROMETER FOR THE MID-INFRARED

A detailed description is given of the Infrared Spatial Interferometer (ISI), developed at the Space Sciences Laboratory of the University of California at Berkeley, which is a high spatial resolution interferometer for mid-infrared wavelengths. The instrumentation, its capabilities and performance, data analysis, science program, and future plans are all discussed. The systems use of heterodyne detection, analogous to that of a modern radio interferometer, is also compared with the homodyne or direct methods more commonly encountered in the visible and infrared. The ISI has been operating productively on Mount Wilson for the past 10 years measuring materials immediately surrounding stars and their changes as well as some stellar diameters. The new spectral capabilities described here, a recent increase in baseline length, and the upcoming expansion to a closure-phase imaging array provide important additional types of measurements.

NONSPHERICAL STRUCTURES AND TEMPORAL VARIATIONS IN THE DUST SHELL OF o CETI OBSERVED WITH A LONG BASELINE INTERFEROMETER AT 11 MICRONS

Visibility observations at 11 km of o Ceti have been made with the University of California (Berkeley) Infrared Spatial Interferometer during the time period 1988E1995. The observed visibilities change dra- matically from one epoch to another and are not consistent with simple heating or cooling of the dust with change in luminosity as a function of stellar phase. Instead, large temporal variations in the density of dust within a few stellar radii of the photosphere of o Ceti have occurred. Spherically symmetric models of the dust distribution with two dust shells, one within three stellar radii of the photosphere of the star, the other approximately 10 stellar radii from the star, can account reasonably well for the observed changes. Four types of axially symmetric radiative transfer models were also compared with the dataEa spherical shell with an ellipsoidal inner cavity, a disk, a spherical shell with one or two inhomogeneities or clumps, and a set of thin partial shells with a -xed distance between them. Of the four models, only the one with the ellipsoidal inner cavity is excluded. The data were best--tted with the last two models, which emphasize inhomogeneities or clumps. To -t the observed temporal changes in the visibility data, all models must include a change in the densityEincreasing and decreasingEof dust close to the photosphere of the star. The axially symmetric models had clumps placed at distances from the star in agreement with distances of the spherical models. Good -ts to the observed broadband spec- trum of the star were also obtained with these models. Subject headings: circumstellar matter E infrared: stars E stars: individual (o Ceti) E stars: mass loss E stars: variables: long-period variables E techniques: interferometric

Atmospheric fluctuations : empirical structure functions and projected performance of future instruments

The extension of high quality astronomical observations towards larger apertures, adaptive optics, and infrared wavelengths leads to extrapolation of present knowledge of astronomical «seeing» by means of theoretical models, such as Kolmogorov turbulence combined with Taylors «frozen atmosphere» swept past the observer by winds. Observations of path length fluctuations from a star to a two-telescope spatial interferometer at 11 μm wavelength, and also measurements of path length fluctuations 3 m above the ground by laser distance interferometers, show substantial deviations from such a model. Intermittent turbulence may be involved and relatively short outer scales are frequently indicated

Nonuniform Dust Outflow Observed around Infrared Object NML Cygni

Measurements by the University of California Berkeley Infrared Spatial Interferometer at 11.15 μm have yielded strong evidence for multiple dust shells and/or significant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8-13 μm spectra taken from 1994-1995 and N-band (10.2 μm) photometry from 1980-1992. These and past measurements are analyzed and fitted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both near- and mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 μm brightness distribution suggests a double-shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth (τ11 μm ~ 1.9) plus an outer shell (τ11 μm ~ 0.33), is consistent not only with the 11 μm visibility data but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 μm silicate feature. The outer shell, or large-scale structure, is revealed only by long-baseline interferometry at 11 μm, being too cold (~400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis proved superior to the Draine & Lee (1984) constants in fitting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of H2O maser emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with the location of the two dust shells and provide further evidence for the two-shell model.

MULTIPLE DUST SHELLS AND MOTIONS AROUND IK TAURI AS SEEN BY INFRARED INTERFEROMETRY

A visibility curve of IK Tau has been measured with the ISI, an 11 μm stellar interferometer, over a period of several years. Time variations in 11 μm flux were also measured. The results indicate an approximately periodic distribution of dust shells around the star, with shells separated by 200-250 mas and a diameter of about 200 mas for the innermost shell. Some shell motion has been detected, and if velocities are the same as those measured for CO gas and OH masers, the motion implies that the distance to IK Tau is about 265 pc and that shells have been emitted at times separated by about 12 yr, which is considerably longer than the stars luminosity period of 470 days.

Lunar magnetic field measurements with a cubesat

We have developed a mission concept that uses 3-unit cubesats to perform new measurements of lunar magnetic fields, less than 100 meters above the Moon’s surface. The mission calls for sending the cubesats on impact trajectories to strongly magnetic regions on the surface, and transmitting measurements in real-time to a nearby spacecraft, or directly to the Earth, up until milliseconds before impact. The cubesats and their instruments are partly based on the NSF-funded CINEMA cubesat now in Earth orbit. Two methods of reaching the Moon as a secondary payload are discussed: 1) After launching into geostationary transfer orbit with a communication satellite, a small mother-ship travels into lunar orbit and releases the cubesats on impact trajectories, and 2) The cubesats travel to the Moon using their own propulsion after release into geosynchronous orbit. This latter version would also enable other near-Earth missions, such as constellations for studying magnetospheric processes, and observations of close-approaching asteroids.

Ground Systems and Flight Operations of the THEMIS Constellation Mission

THEMIS, a five-spacecraft constellation mission to study magnetospheric phenomena leading to auroral outbursts was launched on February 17, 2007 on a single Delta II rocket into a 31.4-hour, low-inclination insertion orbit. After an initial on-orbit check-out and science instrument commissioning period, the five spacecraft called probes were maintained in temporary coast phase orbits to control orbital dispersions. Beginning in early September 2007, four of the five probes were maneuvered into their highly elliptical, synchronized mission orbits with 1, 2 and 4-day periods in preparation for the primary winter observing season. The fifth probe, acting as an on-orbit spare, was maneuvered into its 4/5-day period orbit, once the four primary probes were completely deployed. This paper describes the concept of constellation operations including a description of the flight and ground systems, as well as mission, science and flight dynamics operations, and discusses challenges encountered and lessons learned during the first year of on-orbit operations.

Long-baseline interferometer for the mid-infrared

The Infrared Spatial Interferometer (ISI) is a high-resolution aperture synthesis imaging system for the 10-micron region. As in many radio interferometers, heterodyne signal detection and lobe rotation for fringe tracking are employed. A Helium-Neon laser metrology system is used for monitoring critical distances within the telescope optics. Although designed for baselines up to 1000 m, initially the interferometer will have baselines ranging from 4 to 34 m, yielding angular resolutions as fine as 0.030 arcsec. First fringes were detected in June 1988. Since then the system has been successfully operated on 4 m and 13 m baselines.

Multi-Mission Flight Operations at UC Berkeley - Experiences and Lessons Learned

The University of California, Berkeley has conducted flight operations for multiple NASA-funded spacecraft from its multi-mission operations center at Space Sciences Laboratory for more than a decade. All ground systems were designed and implemented by members of the multi-mission operations team who are involved in all phases of mission life cycles from the early proposal stages through mission design and development, integration, launch and on-orbit operations. Operational task areas include mission and science operations, mission design and navigation, ground station operations, and hardware and software systems support. Team members are trained across missions and across support disciplines to provide a breadth of knowledge and redundancy within the team. This paper describes the ground system design and summarizes experiences, challenges, and lessons learned with conducting complex multi-mission spacecraft operations in an academic environment.

Navigating THEMIS to the ARTEMIS Low-Energy Lunar Transfer Trajectory

THEMIS – a NASA Medium Explorer (MIDEX) mission – is a five-spacecraft constellation launched in February 2007 to study magnetospheric phenomena leading to the aurora borealis. During the primary mission phase, completed in the summer of 2009, all five spacecraft collected science data in synchronized, highly elliptical Earth orbits. Both mission design and efficient navigation and flight operations during the primary mission resulted in appreciable fuel reserves. Therefore, an ambitious mission extension, ARTEMIS, became feasible. ARTEMIS involves transferring the outer two spacecraft from Earth to lunar orbits where they will conduct measurements of the Moon’s interaction with the solar wind and its crustal magnetic fields. Earth departure of these two spacecraft is accomplished by successively raising the apogees of their orbits until lunar perturbations become the dominant forces significantly altering their trajectories. This orbit raise sequence requires over forty maneuvering events, with multiple lunar approaches and fly-bys, before setting the two spacecraft on low-energy transfer trajectories to lunar orbit in February and March 2010. This paper addresses overcoming the navigation and operational challenges presented by the ARTEMIS mission, consisting of two spacecraft that were not designed to leave Earth orbits.