Alberto Cellino, Mario Di Martino, Elisabetta Dotto, Paolo Tanga, Vincenzo Zappala, Stephan Price, Mike Egan, Edward Tedesco, Andrea Carusi, Andrea Boattini, Paolo Persi, Karri Muinonen, Alan Harris, Marco Castronuovo, Mark Bailey, Johan Lagerros, Luigi Bussolino, Antonella Ferri, Pietro Merlina, Andrea Mariani, Stefano Brogi, Thomas Murdock
We investigate a broad system design for a space-based observatory operating at mid-infrared and visible wavelengths to perform physical characterization and discovery of near-Earth objects (NEOs) in the inner solar system. Our goals require measurements that are much more efficiently done from space. The mission objectives are to obtain accurate diameters, albedos and multiband reflectance properties for the known NEOs, and to conduct a search for objects spending most or all their orbital period inside Earth's orbit. The purpose is to observe a large fraction of the existing population during a mission operational lifetime of two years. A rather modest sized telescope (70 cm primary mirror and Ritchey-Chretien optical configuration) is found to be adequate to meet the objectives.
The Cosmic Background Explorer (COBE) satellite was launched on November 18, 1989 from Vandenberg Air Force base on a Delta rocket. It carried two superfluid liquid-helium-cooled (LHe) infrared (IR) instruments in a 600 liter dewar, and three microwave radiometers mounted on the outside of the dewar. One of the LHe-cooled instruments is a ten-band photometer covering the spectral range from 1.2 to 240 micrometers - the Diffuse Infrared Background Experiment (DIRBE). A goal of the DIRBE program is to obtain full-sky infrared observations that can be used to model accurately the IR contributions arising from the interplanetary dust (IPD) and the Galaxy. Using such models, the foreground can be removed to expose and underlying extragalactic IR component produced early in formation of the universe. The nature of the IPD IR foreground detected by the DIRBE is found to be quite complex, but amenable to modelling.
The Diffuse InfraRed Background Experiment (DIRBE) onboard the cosmic Background Explorer (COBE) was designed to conduct a search for a cosmic infrared background (CIB), which is expected to be the fossil radiation from the first luminous objects in the universe. The instrument, a ten-band cryogenic absolute photometer and three-band polarimeter with a 0.7 degree(s) beam and a wavelength range from 1 - 240 micrometers , scans the sky redundantly and samples half the sky each day. During the ten month lifetime of the cryogen, the instrument achieved a nominal sensitivity on the sky of 10-9 W/m2/sr at most wavelengths, or approximately 1% of the natural background at wavelengths where the sky is very luminous. The short wavelength bands from 1 - 5 micrometers continue to operate after exhaustion of the cryogen, although at reduced sensitivity. In this paper, we review the design, testing, and in-flight performance of the DIRBE.
The Diffuse Infrared Background Experiment (DIRBE) on board NASA's Cosmic Background Explorer (COBE) satellite has surveyed the entire sky in 10 broad photometric bands covering the wavelength region from 1 to 240 micrometers , at an angular resolution of 0.7 degree(s) (Boggess et al. 1992). the extensive spectral coverage of the DIRBE observations offers an unprecedented opportunity to undertake comprehensive large-scale studies of the content, structure, and energetics of the stellar and interstellar components of the Galaxy. Understanding the Galactic emission is not only a task of scientific value in its own right, but also a necessary step in the accurate extraction of faint cosmological emission from the DIRBE data.
Because the thermal history of an isothermal metal sphere with an emissive coating can be accurately modelled, such a reference sphere is a suitable calibration object for a space-based IR sensor. To achieve high quality calibration, the uncertainty in the sphere's material parameters must be constrained by relating calibration requirements to design tolerances in the sphere parameters. A methodology for doing this will be presented and applied to an orbiting reference sphere. For clarity, the approach will be illustrated with a gray-body model of the sphere thermal behavior, but results for a non-gray-body sphere will also be given. Sources of uncertainty in the sphere signature will be identified and estimated. In particular, earth flux scatters from the sphere and contaminates the sphere's thermal signature. While the scattered earth flux constitutes a small fraction of a highly emissive sphere's IR signal, it will be shown that the uncertainty in the scattered flux is a significant fraction of the uncertainty in the signal. In sunlight the uncertainty in scattered earthflux reduces the total uncertainty by cancelling other error terms, but in darkness the total uncertainty can be increased by scattering effects.
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