Radiative and hybrid cooling of infrared space telescopes

The designs of cold space telescopes, cryogenic and radiatively cooled, are similar in most elements and both benefit from orbits distant from the Earth. In particular such orbits allow the anti-sunward side of radiatively-cooled spacecraft to be used to provide large cooling radiators for the individual radiation shields. Designs incorporating these features have predictedT _tel near 20 K. The attainability of such temperatures is supported by limited practical experience (IRAS, COBE). Supplementary cooling systems (cryogens, mechanical coolers) can be advantageously combined with radiative cooling in hybrid designs to provide robustness against deterioration and yet lower temperatures for detectors, instruments, and even the whole telescope. The possibility of such major additional gains is illustrated by the Very Cold Telescope option under study forEdison, which should offerT _tel5 K for a little extra mechanical cooling capacity.     

The Edison infrared space observatory

For five years, theEdison program has had the goal of developing new designs for infrared space observatories which will break the cost curve by permitting more capable missions at lower cost. Most notably, this has produced a series of models for purely radiative and radiative/mechanical (hybrid) cooling which do not use cryogens and optical designs which are not constrained by the coolant tanks. Purely radiatively-cooled models achieve equilibrium temperatures as low as about 20 K at a distance of 1 AU from the sun. More advancedEdison designs include mechanical cooling systems attached to the telescope assembly which lower the optical system temperature to 5 K or less. Via these designs, near-cryogenic temperatures appear achievable without the limitations of cryogenic cooling. OneEdison model has been proposed to the European Space Agency as the next generation infrared space observatory and is presently under consideration as a candidate ESA Cornerstone mission. The basic design is also the starting point for elements of future infrared space interferometers.     

Optimised radiative cooling of infrared space telescopes and applications to possible missions

General principles are outlined for the design of space infrared telescopes intended to cool by radiation to the lowest temperatures attainable without the use of on-board cryogens, and assuming on-orbit cooling after a warm launch. Maximum protection from solar and earth heating, maximum radiating area and efficiency and minimum absorbing area and absorptivity are the obvious basic criteria. The optimised design is a short, fat telescope surrounded by a series of radiation shields, each cooled by its own radiator. Maximising the longitudinal conductivity of the radiation shields and of the telescope tube itself is important both to the on-orbit cooling time and the final achieveable temperature. Realistic designs take between 80 and 200 days to cool to within a few degrees of equilibrium temperatures, depending on the materials used. Great advantages accrue from the use of an orbit distant from earth. Both simple models and detailed simulations suggest that temperatures of 30 to 40 K are attainable in high earth orbits. Placing a radiatively cooled telescope in a halo orbit around the Lagrangian point L2 is a particularly attractive option and significantly lower temperatures can be achieved there than in Earth orbit. Optimised radiative cooling is an important element of the small Japanese mission SMIRT. We suggest that a combination of an ESA Medium-sized Mission with a NASA Explorer to send a 2m+ telescope to an L2 halo orbit would provide a cost-effective and powerful long-duration facility for the early 21st century.     

Human space exploration and human spaceflight: Latency and the cognitive scale of the universe

The role of telerobotics for space exploration in placing human cognition on other worlds is limited almost entirely by the speed of light, and the consequent communications latency that results from large distances. This latency is the time delay between the human brain at one end, and the telerobotic effector and sensor at the other end. While telerobotics and virtual presence is a technology that is rapidly becoming more sophisticated, with strong commercial interest on the Earth, this time delay, along with the neurological timescale of a human being, quantitatively defines the cognitive horizon for any locale in space. That is, how distant can an operator be from a robot and not be significantly impacted by latency? We explore that cognitive timescale of the universe, and consider the implications for telerobotics, human spaceflight, and participation by larger numbers of people in space exploration. We conclude that, with advanced telepresence, sophisticated robots could be operated with high cognition throughout a lunar hemisphere by astronauts within a station at an Earth-Moon L1 or L2 venue. Likewise, complex telerobotic servicing of satellites in geosynchronous orbit can be carried out from suitable terrestrial stations.     

Edison and radiatively-cooled IR space observatories

Radiative cooling of IR space telescopes is an alternative to embedding within massive cryostats and should offer advantages for future missions, including longer life, larger aperture for a fixed spacecraft size, lower cost due to less complex engineering, and easier ground handling. Relatively simple analyses of conventional designs show that it is possible to achieve telescope temperatures in the range of 25 to 40 K at distances from the sun of about 1 AU. Lower temperatures may be possible with “open” designs or distant orbits. At 25 K, an observatory will be limited by the celestial thermal background in the near- and mid-IR and by the confusion limit in the far-IR. We outline here our concept for a moderate aperture ( 1.75 m; Ariane 4 or Atlas launch) international space observatory for the next decade.     

EDISON: The next generation infrared space observatory

EDISON, a large-aperture, radiatively-cooled telescope, is proposed as the major international mission to follow the current generation of cryogenically-cooled infrared space telescopes. It is being studied at present as a 2.5–3.5 m mixed radiatively- and mechanically-cooled facility optimized to investigate the wavelength range 3–100+ m. This paper outlines the status of the project, discusses some aspects of a smaller-aperture precursor mission, and describes a portion of the baseline science mission.     

Far-infrared and submillimeter survey of the galactic center and nearby galactic plane

Maps are presented with 12′ resolution of the Galactic Center and adjacent galactic plane, from ?^II = 359° to ?^II = 5°. The data were obtained with the Steward Observatory cryogenically-cooled, balloon-borne telescope. The data are from channels filtered for a bandpass of 70 μm < γ < 110 μm and for a longpass of γ > 80 μm. For the typical effective temperature of 25 K of a galactic HII region at this spatial resolution, the effective wavelength of the channels are 93 μ and 145 μm. Continuous emission is mapped along the galactic plane in both wavelengths. There are two contrasts between the immediate vicinity of SgrA (?^π < 1°) and the galactic plane in general. Firstly, for ?^π > 1° the galactic plane narrows dramatically at 93 μm, while retaining its width at 145 μm. Secondly, the individual sources at ?^π > 1° (which we associate with HII regions) have greater peak brightness in the 145 μm channel than the 93 μm channel, while SgrA hasapproximately equal peak brightness in each. The maps demonstrate the importance of submillimeter wavelengths to galactic surveys.