Far-Infrared/Submillimeter Astronomical Interferometry ... - Springer Link

Astrophys Space Sci (2006) 302:225–239 DOI 10.1007/s10509-006-9038-7

O R I G I NA L A RT I C L E

Far-Infrared/Submillimeter Astronomical Interferometry with Spaceborne Tether Formations Enrico Lorenzini · Claudio Bombardelli · Mario Cosmo · Martin Harwit · David Leisawitz · Rodger Farley · Stephen Rinehart · David Quinn · David Miller

Received: 2 December 2005 / Accepted: 10 January 2006 C Springer Science + Business Media B.V. 2006

Abstract Through the continuing development of improved detectors and detector arrays, far-infrared/submillimeter astronomical space missions have had enormous successes in recent years. Despite these advances, the diffraction-limited angular resolving power has remained virtually constant. The advent of telescopes with apertures of several meters will improve this capability, but will still leave image resolution many orders of magnitude poorer than in most other spectral ranges. Here we point out that the only foreseeable way to improve image quality to rival that of modern optical telescopes will be with interferometers whose light collectors are connected by tethers. After making the scientific case for high spatial resolution far-infrared/submillimeter imaging and the use of interferometry as the most immediate way of producing results, we discuss recent advances in dynamic analysis and control of tethered formations, and argue that the further development and testing of tethers in space is a first step toward providing improved far-infrared/submillimeter angular resolution and astronomical image quality. E. Lorenzini Department of Mechanical Engineering, University of Padua, Padova, Italy and Harvard-Smithsonian Center for Astrophysics, Cambridge, MA C. Bombardelli · M. Cosmo Harvard-Smithsonian Center for Astrophysics, Cambridge, MA M. Harwit () 511 H st, SW, Washington, DC 20024, also Cornell University, Ithaca, NY e-mail: [email protected] D. Leisawitz · R. Farley · S. Rinehart · D. Quinn NASA Goddard Space Flight Center, Greenbelt, MD D. Miller Massachusetts Institute of Technology, Cambridge, MA

Keywords Instrumentation: Miscellaneous · Space vehicles: Miscellaneous · Techniques: Miscellaneous · Astronomical data bases: Miscellaneous

1. Introduction A compelling challenge to far-infrared/submillimeter (FIR/SMM) astronomical space observations, today, is to find a way to improve angular resolving power and image quality. Roughly half of the energy generated in the Universe since radiation decoupled from matter – i.e. half of the radiation not confined to the microwave background radiation – is currently observed at FIR/SMM wavelengths. This prevalence was strikingly confirmed with instrumentation aboard the Cosmic Background Explorer, COBE, but had been suspected far earlier from observations conducted with balloons, high-flying aircraft and the Infrared Astronomical Satellite, IRAS. The physical insights that can be obtained with FIR/SMM studies have been further illustrated through photometric, polarimetric and spectroscopic observations conducted with the Kuiper Airborne Observatory, KAO, the Japanese Infrared Telescope in Space, IRTS, the Infrared Space Observatory, ISO, the Submillimeter Wave Astronomy Satellite, SWAS, and – most recently – the Spitzer Infrared Telescope Facility, SIRTF. The striking advances in FIR/SMM astronomy since the 1960s have come about largely through a roughly millionfold increase in detector sensitivity accompanied by the development of sizeable detector arrays that have improved the effective sensitivity for mapping and imaging by further orders of magnitude. Throughout these decades, however, telescope apertures have remained almost constant. The telescope aboard NASA’s Lear Jet, in the late 1960s, had an aperture of 30 cm. The KAO, which began flying in the mid-1970s Springer

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Astrophys Space Sci (2006) 302:225–239

Fig. 1 Artist’s rendition of the currently considered configuration of SPECS

increased this to ∼90 cm. But none of the FIR/SMM telescopes flown in space, to date, have had an aperture exceeding 85 cm. At the long wavelengths, beyond 200 μ m, the diffraction-limited image quality obtained by these space missions has inevitably fallen short of observations that Tycho Brahe was able to make at visual wavelengths with the naked eye in the late 16th century. The advent of the 3.5-meter aperture on the ESA/NASA Herschel mission will improve on this, but diffraction will still limit the FIR/SMM image quality to a level comparable to the visual images that Galileo was able to obtain with his first attempts at improving the spyglass. In order for FIR/SMM astronomical observatories to provide diffraction-limited images with an angular resolution comparable to that delivered by the Hubble Space Telescope, HST, at optical wavelengths, effective apertures of the order of a kilometer will be required. These apertures by no means need to be filled. As amply illustrated by the performance of the small space telescopes already launched, the light gathering power of FIR/SMM telescopes is now somewhat of a secondary concern, given the strong signals and high number of photons obtained from astronomical sources and the exquisite sensitivity of detectors developed to date. However, just as in the radio wavelength regime, large effective apertures are needed to provide improved diffraction-limited images. Sparse apertures, and interferometers in particular, are fully adequate and economically viable for this purpose. We have been studying the potential scientific capabilities of a particular interferometer design comprising two 4-meter aperture light collectors, whose output is steered to a central beam combiner (see Figure 1). The entire assembly will be launched into space and placed at the second Lagrange point, L2, where the interferometer orbits the Sun in synchrony with Earth. Here, disruptive tidal forces on the interferometer assembly are minimal and the distance that telemetry signals have to traverse to return data to Earth remains relatively short

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and constant. This interferometer has become known as the “Submillimeter Probe of the Evolution of Cosmic Structure,” with the acronym “SPECS” (Harwit et al., 2005). In Section 2 of this paper, we briefly highlight the astronomical promise of SPECS. Section 3 outlines technical requirements and explains why dynamics require a tether connection between the light collecting apertures and beam combiner to effectively operate a high-angular-resolution, imaging interferometer in space. Section 4 describes the operation and control of tethered interferometer architectures and briefly depicts experience, to date, with tethers. A final Section 5 summarizes steps that should be taken for an orderly progression toward a kilometer-baseline FIR/SMM interferometer in space.

2. The need for high angular resolution 2.1. The first stars A key question of current concern is how the first stars formed and when the first heavy elements were synthesized and dispersed into the intergalactic medium (IGM). Results obtained with the Wilkinson Microwave Anisotropy Probe (WMAP) suggest that massive stars were formed in sufficient numbers to ionize the ambient medium by the time the Universe was 200 Myr old, corresponding to a redshift z = 17 ± 5 (Bennett et al., 2003; Spergel et al., 2003). The Sloan Digital Sky Survey has shown the existence of fully-formed galaxies and quasars only 600 Myr later, at redshifts z > 6, indicating that the epoch of reionization was, by then, coming to an end and stars and galaxies were becoming visible (Fan et al., 2001). However, even before the availability of these results, theoretical studies by Abel, Bryan, and Norman (2002) had predicted that the first stars should have been very


massive and formed when the Universe was only ∼150 Myr old. The collapse of individual Population III protostars appears to have involved radiative cooling by molecular hydrogen, at flux densities of 10−5 μJy, levels too faint for even the most sensitive currently planned missions to detect (Mizusawa et al., 2004). However, Schneider et al. (2004) have suggested that Population III stars with masses in the 140–260 M range would evolve into pair-instability supernovae, converting ∼15–30% of the progenitor mass into ejecta that eventually should form circumstellar Mg2 SiO4 and SiO2 silicate dust shells surrounding the exploded stars. Since massive primordial stars are believed to form in binaries (Saigo et al., 2004), some of these shells will be heated by luminous unexploded companion stars as well as by turbulent interaction with ambient gas as the shells expand, and appear as bright silicate emission rings red-shifted to ∼10(z + 1) microns in the submillimeter range. The rings should typically subtend 20–100 mas, resolvable by an interferometer with 50 mas resolving power, which calls for a FIR/SMM interferometer with a baseline ranging up to 1 km. The redshifted spectral features of the resolved shells will reveal the era of these stars’ birth. While Tumlinson et al (2004) invoke a different Population III supernova mass distribution, the chemical abundances of the ejecta are not appreciably affected. Neither the James Webb Space Telescope, JWST nor the Atacama Large Millimeter Array, ALMA will have access to the wavelength regime required for observing the redshifted silicate features.

2.2. The evolution of galaxies HST surveys show that most large galaxies have formed through the merger of smaller progenitors, whose angular diameters of are ∼0.8 arcsec, almost independent of redshift for redshifts z >1. The planned Single Aperture Far Infrared Telescope, SAFIR, (Benford et al., 2004) with an aperture of 10 meters, though highly sensitive, will be unable to resolve such small sources. A FIR angular resolution of ∼50 mas or better will be needed at 250 μm to determine the extinction-independent morphology, distribution of dust and heavy chemical elements in such galaxies, or assess whether their radiation emanates from active nuclei or star-forming regions. Spectral features of redshifted polycyclic aromatic hydrocarbons (PAHs), atomic or ionic fine-structure transitions, as well as rotational and/or vibrational transitions from H2 O, CO and small hydrides at high redshifts, will lead to detailed insight into enrichment histories. Many of these spectral features will lie at wavelengths left untouched by JWST or ALMA, but accessible from space with a farinfrared/submillimeter interferometer. Two further examples may be cited.

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By far the most luminous FIR spectral line observed in many distant galaxies is the fine structure transition of singly ionized carbon, C+ , at a rest-frame wavelength of 158 μm. In the Galaxy, the power in this single line amounts to ∼0.3% of the Milky Way’s luminosity. At a redshift z = 4, a C+ line sensitivity of ∼10−22 W m−2 will permit detection of a line luminosity as low as ∼3 × 106 L from a galaxy with a continuum as faint as ∼109 L . With a sensitivity of this order, SPECS will spectroscopically analyze extragalactic parsec-scale molecular clouds. The interferometer will also measure the luminosity function of galaxies down to 0.1 L∗ at z < 6 and determine spectroscopic redshifts for the objects it finds. In order to detect line radiation efficiently from galaxies a minimum spectral resolution R ∼ 1000 is required. This will be possible with a beam-combiner effective delay line stroke of the order of 30 cm. A folded delay line reduces the mechanical travel to a small fraction of this distance. 2.3. Star and planet formation A long-standing problem in star formation is the lack of detail on protostellar collapse. At low angular resolution, the signature of infall is marred by competing effects of cloud core rotation, turbulent and thermal velocity dispersion, outflow, the intricacies of spectral line radiative transfer, and the depletion of gas-phase elements onto dust grains. Only the FIR/SMM spectrum provides the wealth of molecular lines that will permit unraveling the chemical and the intimately linked dynamical evolution that produces stars, disks and planets. Shocks are believed to trigger the collapse of massive protostars. Shock-heating to temperatures exceeding ∼400 K lead to gas-phase reactions of oxygen and hydrogen to form water vapor (e.g. Harwit et al., 1998). Subsequent freeze-out onto grain surfaces is believed to be the source of interstellar ice. Cold water vapor radiates primarily in low-lying transitions at 538, 269, and 179 μm, while the strongest emissions from warm water lie in the 40–180 μm spectral region. With a spectral range of 40–640 μm, SPECS will permit observation of all these lines and also emission and absorption features of light hydrides, such as OH, CH, and FH, and other molecular species inaccessible to either JWST or ALMA. Such observations will clarify the interplay between dynamics and chemistry in star and planet formation. The interferometer’s high angular and spectral resolution will enable competing dynamical effects in protostellar envelopes, outflows and disks to be both spatially and kinematically disentangled. In studies of protoplanetary disks, SPECS will make direct observations of the “snow line” delineating where water vapor abundances result from the vaporization of ice. This should elucidate the pathways by which terrestrial exoplanSpringer

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Table 1 SPECS science requirements Item

Value

Max. baseline Min. baseline Field of view Field of regard Wavelength range Photometry Spectroscopy Number of targets

1 km as short as feasible 1 arcmin ±20 deg from ecliptic 40–640 μm λ/λ ∼ 3 λ/λ ∼ 3000 ∼ 1500 over 5 years

ets might be supplied with water – a question of great interest to astrobiology. After young stars lose their gas disks, they still retain debris disks of dust, rocks, and young planets. Like ALMA, SPECS will require a spatial resolution of 50 mas (0.5 AU at 10 pc), to detect the gaps and other structural features that are expected to result from gravitational interactions between disks and protoplanets.

3. SPECS: An imaging interferometer in space Historically, angular resolution requirements in the FIR/SMM wavelength range have been satisfied by increasing the size of telescopes. The diffraction limit at wavelength λ is 1.2λ/Dtel , where Dtel is the telescope aperture. To obtain spatial resolution comparable to that of HST, a FIR/SMM observatory would require an aperture several hundred times larger than the Hubble mirror, or about 1 km. This makes a filled aperture telescope impractical. An interferometer rather than a single aperture telescope is the natural solution. Table 1 summarizes the key requirements for the SPECS interferometer. Astronomical interferometry dates back to Michelson’s experiments in the early 1900s. The angular resolution of an interferometer is determined by the separation, or “baseline,” between the light collecting mirrors. To produce images with an interferometer a challenging requirement must be met: the interferometric data from baselines of many different lengths and orientations must be combined to form an image. Each baseline provides information about the source on a particular spatial frequency. The longer the baseline, the higher the frequency. The resolution in the final image is given by λ/2Bmax , where Bmax is the maximum mirror separation. A complex scene, such as the Hubble Deep Field, contains information on essentially all spatial frequencies. Accordingly, the synthetic aperture must be densely sampled, though possibly not all at once, as most astronomical sources change on timescales that are much longer than even a space mission’s lifetime and normally far longer than the time it would take to gather the data.

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When the resolution requires baselines Bmax > 100 m, traditional space structures (e.g., deployable booms or solid structures assembled in space) are not sufficiently stable to provide useful platforms. Instead, the components of an interferometer (light collecting mirrors and beam combiner) may be stationed on separate spacecraft that are repositioned to potentially sample all baselines up to Bmax , The challenge then is to provide the means to move the spacecraft at will. 3.1. The need for tethers To sample all possible combinations of baselines while observing an astronomical source, say, in the course of a 24-hour day, 4-meter aperture light collectors will be needed to provide adequate sensitivity. When these light collectors are on free-flying spacecraft, a prohibitive amount of fuel would be needed to move them to different positions solely through use of even the most efficient thrusters. For SPECS as it is currently conceived, with its maximum baseline Bmax = 1 km, the two light collectors mounted on separate spacecraft are, instead, joined by tethers to a central beam combiner and rotate about it in a plane normal to the collector line of sight on the sky. Without tethers the circular motion, or some equivalent raster pattern, would have to be supplied by thrusters demanding an inordinate amount of fuel. The tension the tethers can apply to a rotating configuration of light collectors side-steps this need. It also permits the collectors to be reeled in or out at constant angular momentum about the beam combiner. In principle, this permits in-plane motion requiring no thruster fuel at all once the initial motion is established. In practice, thruster fuel is still used, mainly to keep tangential velocities within useful bounds, to reposition the orbital plane of the light collectors to acquire new astronomical targets, and to correct the attitudes of the spacecraft and beam combiner. The consumption of thruster fuel in these activities, however, is far lower than would be required by rastering or any other motion unaided by tethers.

4. Tethers for space-based interferometry 4.1. Fundamental requirements No matter what does the work, tethers or thrusters, the need to stabilize and accurately measure interferometric fringes imposes a series of requirements on the spacecraft motions and attitudes of any distributed space interferometer. First, the positions of the collecting mirrors and combiner unit must be maintained within a range such that the zero optical path difference is within the span of the optical delay lines. For SPECS, keeping the relative positions of the spacecraft to within 10 cm of the ideal relative position, is acceptable. Also, oscillations around the ideal relative positions


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should not have frequencies higher than the fringe tracker bandwidth, which depending on the specific design and conditions of operation can be as low as a few Hz. Secondly, the attitude of the individual spacecraft has to be controlled to a fraction of an arcmin to allow overlapping the fields of view of the two telescopes and to accurately relay the collimated beams from each collector to the central combiner. Given the limited bandwidth of both the bearing angle sensors and the combiner beam combination angle sensor disturbances on the spacecraft attitude should not exceed a fraction of a Hz. Additional requirements are imposed on the tethers. The latter should survive the space environment at L2 over the nominal mission duration of 5 years with a probability of survival better than 99%. Finally, each tether mass should be a small percentage of a spacecraft mass (let us say