- Email: [email protected]
submillimeter kilometer-baseline interferometer in space
New Astronomy Reviews 50 (2006) 228–234 www.elsevier.com/locate/newastrev
A far-infrared/submillimeter kilometer-baseline interferometer in space Martin Harwit a
a,b,* ,
David Leisawitz c, Stephen Rinehart
c
511 H Street, SW, Washington DC 20024-2725, United StatesU:/ES/DTD501/Astrev/998 b Cornell University, Ithaca, New York, United States c NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States Available online 5 January 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 the situation, but will still leave image resolution many orders of magnitude poorer than in most other spectral ranges. After making the scientific case for high spatial resolution imaging in this spectral range, and the use of interferometry as the most immediate way of producing results, we review the use of farinfrared/submillimeter interferometers to provide insight on the formation of the first stars. 2005 Elsevier B.V. All rights reserved. Keywords: Cosmology; Instrumentation; Interferometry; Far-infrared/submillimeter
Contents 1. 2. 3. 4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Formation of the first stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The need for high angular resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Architecture and implementation approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Space systems architecture: approach through a decision tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Science requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Ensuing engineering requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Architectural approach: a path based on a decision tree. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A challenge to far-infrared/submillimeter astronomical space observations, today, is to find a way to improve angular resolving power and image quality. The needs are *
Corresponding author. Tel.: +1 202 479 6877; fax: +1 202 484 2654. E-mail address: [email protected] (M. Harwit).
1387-6473/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2005.11.030
. . . . . . . . . .
228 229 230 231 231 231 231 232 234 234
compelling. Roughly half of the energy generated in the Universe since the decoupling of radiation 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 the Cosmic Background Explorer (COBE) but had been suspected far earlier from observations conducted with balloons, high-flying aircraft and the Infrared
M. Harwit et al. / New Astronomy Reviews 50 (2006) 228–234
Astronomical Satellite (IRAS). The physical insights that can be obtained with FIR/SMM studies have been further illustrated through photometric 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 great strides in FIR/SMM astronomy that have been witnessed since the 1960s have come about largely through a roughly million-fold 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. NASAÕs Lear Jet, flying in the late 1960s had an aperture of 30 cm. The KAO, which began routine service in the mid-1970s increased this to 90 cm. But none of the FIR/SMM telescopes flown in space have had an aperture exceeding 85 cm. At the long wavelengths beyond 200 lm the best image quality obtained by these missions has consistently 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-m aperture on the ESA/NASA Herschel mission will improve this, but the image quality in the FIR/SMM will still remain merely comparable to the visual images that Galileo was able to obtain with his first spy glasses. 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 do not need to be filled. As amply illustrated by the performance of the small FIR/SMM space telescopes already launched, the light gathering power of 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. Like in the radio wavelength regime, large effective apertures are needed primarily to provide improved diffraction-limited images. For this purpose sparse apertures and, in particular, interferometers, are fully adequate and economically viable. We have been studying the potential scientific capabilities of a particular interferometer design comprising two 4-m aperture light collectors, whose output is steered to a central beam combiner. The entire assembly is to be launched into space and placed at the second Lagrangian 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 and constant. We have coined the name ‘‘Submillimeter Probe of the Evolution of Cosmic Structure,’’ for this interferometer, with the acronym ‘‘SPECS.’’
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2. Formation of the first stars A current belief is that the first cosmic condensations gave rise to Population III stars having several hundred solar masses (Abel et al., 2002). During initial gravitational collapse at an epoch of red shift z 20–30 a shock ensues as infalling matter piles up. The temperature abruptly rises, reaching 200–500 K. Molecular hydrogen H2 is the only effective coolant at this epoch, and H2 collisionally excited states radiate away sufficient energy to assure continuing protostellar collapse and the onset of hydrogen burning in the newly formed star. While this suggests a search for H2 emission to directly observe the initial collapse, Mizusawa et al. (2004) estimate that the expected H2 flux would not exceed 1029 W m2 or 105 lJy — far below measurable levels. To search for traces of these first stars, we need to look at their later evolutionary phases. Because the stars are so massive, the proton–proton reaction is unable to produce sufficient heat to stop contraction before central temperatures rise to >108 K, where the triple-a process sets in. With carbon formed, the CNO cycle can start to supply sufficient heat to stem further contraction. Hydrogen keeps burning in the compact hot core until fully exhausted. For stars with masses in the range 100–140 Mx, further contraction leads to central temperatures so high that collisions between photons begin to form electron–positron pairs. At threshold, the photonsÕ energy is converted into electron–positron masses with little kinetic energy. The pressure suddenly drops, and the ensuing collapse raises the temperature to where even–even nuclei form through the successive addition of alpha particles and carbon to oxygen. The heat released as neon, silicon, sulfur and magnesium are formed produces a rebound and initiates pulsations that eject the outer layers of the star before its interior collapses to form a black hole. In stars with masses 140–260 Mx elements as massive as the iron group are formed and a single pair-production pulse disrupts the star, ejecting all matter and leaving no remnant (Heger and Woosley, 2002). Schneider et al. (2004) believe that as much as 30% of the starÕs mass may then be ejected in the form of these heavy elements. While stars of all other masses below 500 Mx, appear to retain much of their mass, explosions of stars with masses in the 100–260 Mx range appear sufficiently powerful to eject a significant fraction of their initial mass into extragalactic space. Traces of excess silicon and sulfur that may have originated in such explosions have been recently detected in X-ray emission from the hot gases pervading large clusters of galaxies, whose deep gravitational potentials retain any matter ejected from member galaxies (Baumgartner et al., 2005). Schneider, Ferrara, and Salvaterra have suggested that immediately after a pair-instability supernova explosion, the ejecta condense into silicate grains. To test this, SPECS will search for silicate 10 lm emission red shifted out to 200 lm and emitted by silicate dust shells streaming out
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Fig. 1. Creation of the first metals and dust.
of the stellar explosion. SPECS will have the angular resolution to coarsely image these shells, they should look rather similar to the spherical Galactic dust shells imaged by the MSX and Spitzer missions Fig. 1. The explosive energy of these massive supernovae ranges up to 1053 erg, most of which is ejected as kinetic energy. If the initial ejection velocity is far greater than the final velocity after the ejecta sweep up ambient gas, energy and momentum conservation dictate that most of this energy must go into cooling. Since the most effective coolants are H2 and silicate grains, much of this energy will be emitted in the mid-infrared at 2(z + 1) 40 lm in H2 rovibrational, or 10(z + 1) 200 lm far infrared silicate grain emission. Heger et al. (2001) estimate the rate at which such supernovae explode to be 4 · 106 s1 per square degree, with an emission plateau lasting of order a year in the observerÕs rest frame for z 20. This suggests that 100 remnants per square degree should be visible at any given time, with a total flux at the observer of 4 · 1018 W m2 per square degree. The flux from individual supernovae would be 4 · 1020 W m2. On average, there would be one such remnant in every 36 square arc minutes. Individual sources will be bright enough to detect in searches with SAFIR and then imaged by SPECS to confirm their identities. These are heady prospects. The question, however, is ‘‘How can a kilometer-baseline be constructed and deployed in space?’’ 3. The need for high angular resolution A team of more than 30 astronomers and 20 engineers recently completed a NASA-sponsored year-long study to identify technological advances that would be required to make a FIR/SMM interferometer in space a reality. Our study identified compelling needs for observations with an angular resolution of 50 milliarcseconds (mas), a
requirement that led to a number of technological consequences. Resolving powers at this level led us to examine how headway might be made on the construction of a kilometer-baseline FIR/SMM Michelson stellar interferometer in space, operating in the 40–640 lm wavelength range with fully cryogenically cooled optics and photon-noise limited detectors. The range of scientific applications calls for photometric studies, as well as observations at intermediate spectral resolution, R 1000–3000, and at extremely high resolving powers, R J 3 · 105. The most favorable orbit for an interferometer enabling these scientifically compelling observations lies in the Sun– Earth Lagrangian region L2. The total light collecting area of the interferometer will have to be J 25 m2, most economically attained with a combination of two 4-m light collectors and one central beam combiner. Tethered formation flying provides the only foreseeable means for conducting the mission with an affordable mass of electrodynamic thruster propellant, mandated both by cleanliness and thrust requirements. A FIR/SMM interferometer will provide unique new insights that cannot be obtained at other wavelengths, nor with other facilities operating in the FIR/SMM wavelength band. We anticipate that the James Webb Space Telescope (JWST) may offer complementary information at wavelengths shorter than 40 lm, and the Atacama Large Millimeter Array (ALMA) may do the same at wavelengths longer than 640 lm; but the phenomena to be studied by a kilometer-baseline FIR/SMM interferometer, operating in an energy regime covering four full octaves, from 40 to 640 lm, i.e., photons with energies of 2– 16 meV, cannot be duplicated by instruments sensitive to radiation at significantly higher or lower energies. The thermal regimes of dust and gases to be studied are quite different and complementing, so that these three observatories will observe quite different physical aspects of stars, planets and galaxies.
M. Harwit et al. / New Astronomy Reviews 50 (2006) 228–234
4. Architecture and implementation approach A wide variety of approaches present themselves as one begins to consider the most promising architectures for attaining high-angular-resolution images at FIR/SMM wavelengths in space. Our team began its deliberations by defining the most compelling scientific questions that could be answered today if such a capability were currently at hand. While the technical requirements that would permit delving into the many scientific investigations spanned a rather wide range, a central set of capabilities emerged to satisfy a large majority of the proposed studies. These science requirements thus gave rise to a well-defined set of technical specifications and engineering requirements. In turn, those began to show the way to narrow down architectures that would satisfy the scientific goals. The decision tree we followed had many branches, but we found that numerous practical challenges emerged along the way, steadily narrowing down acceptable alternatives, channeling our choices down a relatively well-definable road. In the following subsections, we depict the path we followed in arriving at the architecture we ultimately adopted, Fig. 2. 4.1. Space systems architecture: approach through a decision tree The SPECS decision tree (Fig. 3) is based on a series of scientific requirements:
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4.1.1. Science requirements (a) A field of view of 1-arc-min, (b) Wavelength coverage from 40 to 640 lm, (c) Spatial resolution of 50 mas across this entire waveband, (d) Ability to provide images in broad spectral bands, in photometry, at intermediate spectral resolution R 1000–3000, and at high resolution R > 100,000 in at least one diffraction-limited collector beam, (e) Ability to obtain typically 300 images in 1 year over a 5-year lifetime, (f) Ability to survey at least two, but preferably more nearby star-forming regions, such as the Taurus and q Ophiuchus molecular clouds, (g) Sufficient sensitivity to map extragalactic sources at high red shifts.
4.1.2. Ensuing engineering requirements (a) An interferometer or sparsely filled aperture sampling many baselines up to a maximum baseline of 1 km. (b) Light collectors having a total collecting area of 25 square meters. (c) An optical system cooled to 4 K. (d) Direct detector arrays cooled to <0.1 K. (e) Direct detectors with NEP 1019–1020 Watt Hz1/2. (f) Heterodyne receivers with sensitivities close to the quantum noise limit.
Fig. 2. SPECS architecture decision tree. The flow starts at top left and follows the arrows. The terminology and rationale for choices made are given in Section 4.1.3.
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Fig. 3. SPECS architecture.
(g) An environment free of significant differential forces on the light collectors. (h) The ability to readily communicate large data streams from space back to Earth. (i) A clean environment to prevent deposition of contaminants on cold surfaces. (j) Metrology at submicron levels along all optical paths. (k) An ability to control optical alignment and path length. (l) An ability to point the light collectors with an accuracy and jitter [10 mas.
4.1.3. Architectural approach: a path based on a decision tree (A) Location of SPECS: Three orbits were considered: Near-Earth, Drift-away, and L2. We settled on an L2 orbit where the perturbing forces are low but a constant and acceptable distance from Earth permits a high data stream to be maintained. (B) A two-beam interferometer can be considered to be the simplest unfilled aperture. But the beams can be combined in two different ways. A Fizeau interferometer produces an image very much in the way that a single filled aperture would if the primary light collector were masked off with only two apertures contributing to the final image. For the long baselines considered and the 1-arc-min required field of view, this would produce images requiring excessively large detector arrays. A Michelson stellar interferometer is not similarly limited. From the beginning, we opted for the Michelson architecture (Fig. 3). (C) Any number of light collectors can be employed in constructing a Michelson interferometer. When there are N such collectors, the number of beams requiring combination is N(N 1)/2. With N = 3, it is possible to monitor performance through phase closure. However, the wavelengths at which the FIR/SMM interfer-
ometer is to work are so long that currently available metrological precision can provide information that would be redundant with phase closure. These considerations led us to opt for a two-beam interferometer as simplest to construct as well as least costly. (D) The movement of light collectors in space to cover the uv-plane, and yield – through so-called aperture synthesis – all possible combinations of baselines that a filled aperture would provide, can be accomplished in two ways. One is through use of free-flying light collectors conveying beams of light to a similarly free-flying beam combiner. The other, depicted in Fig. 3, is to employ tethers that connect the light collectors to the beam combiner, the tethers being kept taut by the centrifugal force of the collectors circling the beam combiner. We examined the amount of thruster fuel that would be required to move the light collectors in a uv-plane filling motion, and determined that the required mass of thruster fuel would be so high as to prohibit the imaging of even as few as 100 sources in a 5-year mission. The use of tethers, on the other hand, reduces the total amount of fuel required for a nominal 5-year mission to manageable scales. (E) The control of tethers is still an untested art. The single most important problem to be investigated in a continuing study leading to a kilometer-baseline interferometer in space is the control of tethered configurations in space. While analytic studies are well advanced, and laboratory studies are ongoing, a small-scale mission in space will be needed to provide assurance that attitude control of light collectors for pointing at the arc second level as well as precise beam overlap at the combiner and control of the separation between spacecraft at a level of roughly ±5 cm, can be readily managed over a range of rotational velocities of the light collectors about the beam combiner.
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(F) Tethered configurations can be arranged in a variety of ways. Angular momentum can be conserved through the use of dummy masses that are reeled out as light collectors are reeled in and vice versa. Judicious use of such masses can reduce the amount of thruster propellant required. We examined a number of these configurations, but concluded that the added mass of the dummies was greater than that of the thruster fuel needed to provide the required astronomical performance over a 5-year mission. Were a mission of considerably longer duration desirable, however, the use of dummy masses should be revisited. (G) Thrusters come in two varieties: with chemical or electrodynamic propulsion. Electrodynamic propulsion provides higher thrust for less propellant, undergoes less beam spreading and, in the current application would make use of chemically inert propellants like xenon to reduce contamination of cryogenically cooled optical surfaces to a minimum. (H) Light collectors also come in two varieties, making use either of optical flats, or reflecting telescopes. For the FIR/SMM wavelength range under consideration, shielding against stray light is essential to lower detector background noise and to reduce heat incident on cryogenically cooled optics. This means minimizing the apertures through which radiation enters the beam combiner. For this purpose reflecting telescopes are superior to optical flats. (I) A number of telescope configurations may be considered. Telescopes with elements that obscure part of the beam produce undesirable diffraction patterns and chromatic effects that need to be avoided. Through judicious optical design, these diffraction patterns can be largely made to fall outside the beams that ultimately reach the heart of the beam combiner. (J) Heat shielding by passive means is a major consideration in the construction of SPECS if cryocoolers are to efficiently cool all optical surfaces down to 4 K. Solar heating must be reduced to an absolute minimum with passive radiators if cryocoolers are to efficiently cool all optical surfaces down to 4 K. On one hand, sunshields have to be kept sufficiently small to both reduce solar radiation pressures that would result in undesirable spacecraft motion and stray light from the sunshields into the optical path. Sunshields that are too small, on the other hand, require the collector field of view to be pointed into a narrow region around the anti-sun direction, which would excessively restrict the accessible region of the sky to a narrow band about the ecliptic. The compromise we chose was to assume sunshields that permit a range of angles ±20 from the anti-sun line. This region allows one to observe several young star-forming regions, as well as some of the darkest portions of the extragalactic sky.
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(K) To minimize stray radiation from the collector spacecraft onto the beam combiner, the sunshields have to be designed to maximally cool the external surfaces of both the collector and the beam combiner spacecraft facing each other. This requires large sunshields constructed so that the radiated heat is channeled away from the optical line of sight joining the spacecraft. The three spacecraft, however, also have to approach each other closely to provide the smallest possible baseline. Otherwise, astronomical features that subtend angles between 1.22k/D and k/2Bmin will not be registered. If the closest permitted approach between spacecraft is 8 m, and the telescope apertures are D = 4 m, then for a linear configuration of spacecraft and beam combiner, Bmin = 16 m, and the missing spatial Fourier components in the map will range from 0.37500 to 3.6600 at 60 lm wavelength. At 600 lm the missing angular components range from 3.7500 to 36.600 . The closest approach problem is one of the outstanding questions to be more fully investigated in a follow-on study. (L) We considered but also rejected a variant of the triangular spacecraft configuration in which all three spacecraft are identical, each housing both a collector that feeds light to the other two spacecraft and a combiner that receives light from the other two. This has two advantages: It permits the construction of three identical units, which may be cost effective, and it will permit interferometry to be conducted if one of the three units fails, thus avoiding single point failure. Nevertheless, this option appeared to be more expensive than the simpler linear construction, and had the disadvantages of the triangular configuration already cited in paragraph K. (M) The requirement that 300 separate targets be investigated in 1 year leads to detector read-out rates that are incompatible with existing detector capabilities, unless a number of specific strategies are adopted. A related concern is that, during a delay-line scan, different portions of the field of view pass through the zero path difference in succession, making the effective dwell time on each target in the field of view much shorter than the duration of the delay line stroke. At the shortest wavelengths, where the 1-arc-min field of view is imaged onto a 30 · 30 array, the effective dwell time on the sky is reduced by a factor of 30, yielding a viewing efficiency of only 3%. While the wide field of view of 1-arc-min square still provides significant advantages over single pixel imaging, a strategy to reduce the readout rate and increase the dwell time significantly is clearly needed. No simple solutions exist, but a combination of different steps can be envisaged. For any given observations, the choice of steps is likely to depend most strongly on the required spectral resolving power. The considerations are detailed and will more appropriately have to be described in a dedicated publication.
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(N) At short baselines, the tangential velocity at which the light collectors circle the beam combiner can be reduced to prevent excessive smearing of the scene during the delay line stroke. We have investigated several strategies for controlling the speed of rotation of the interferometer. An efficient program is to start out at very low velocities at short baselines and to apply a constant thrust that terminates at very long baselines with the light collectors moving with tangential velocities of 2 m/s. An electrodynamic propellant mass of 500 kg per light collector will suffice to service a 5-year mission involving 500 separate targets. Additional, more efficient strategies based on maintaining constant angular momentum for prolonged periods can also be implemented as described in paragraph K. (O) We investigated the best ways to assure accurate pointing of the collector telescopes and precise superposition of beams in the beam combiner. A combination of laser metrology and star tracking promises to provide the required precision. Accurate guiding on stars will require near infrared viewing of stellar images directly through the primary telescope. The SPECS optics will have to be fabricated to tolerances required for metrology at wavelengths of 1 lm. Sparse coverage of the uv-plane, and/or larger telescope apertures than the 4-m telescopes envisaged will permit the use of faint guide stars to provide sufficient guide star availability.
5. The future Four technologies will require development to make the interferometer a reality: first, controllable tethered flight, preferably tested on a small precursor mission in space. Second, a vigorous program of detector development leading to background limited detector arrays and close to quantum noise limited heterodyne receivers, both compatible with millisecond readout rates. Third, a massive interferometric cryo-delay mechanism capable of optical path sweeps at speeds of 50 cm s1, direction reversal at 1 s intervals, and reliable operation over 108– 109 cycles. Fourth, a program of detailed end-to-end computer simulations of the complex interplay between metrological, attitude control, and data gathering systems.
References Abel, T., Bryan, G.L., Norman, M.L., 2002. Science 295, 93. Baumgartner, W.H., Loewenstein, M., Horner, D.J., Mushotzky, R.F., 2005. ApJ 620, 680. Heger, A., Woosley, S., 2002. ApJ 567, 532. Heger, A., Woosley, S.E, Baraffe, I., Abel, T., 2001. Available from:. Mizusawa, H., Nishi, R., Omukai, K., 2004. PASJ 56, 487. Available from: . Schneider, R., Ferrara, A., Salvaterra, R., 2004. MNRAS 351, 1379.
A far-infrared/submillimeter kilometer-baseline interferometer in space Martin Harwit a
a,b,* ,
David Leisawitz c, Stephen Rinehart
c
511 H Street, SW, Washington DC 20024-2725, United StatesU:/ES/DTD501/Astrev/998 b Cornell University, Ithaca, New York, United States c NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States Available online 5 January 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 the situation, but will still leave image resolution many orders of magnitude poorer than in most other spectral ranges. After making the scientific case for high spatial resolution imaging in this spectral range, and the use of interferometry as the most immediate way of producing results, we review the use of farinfrared/submillimeter interferometers to provide insight on the formation of the first stars. 2005 Elsevier B.V. All rights reserved. Keywords: Cosmology; Instrumentation; Interferometry; Far-infrared/submillimeter
Contents 1. 2. 3. 4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Formation of the first stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The need for high angular resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Architecture and implementation approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Space systems architecture: approach through a decision tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Science requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Ensuing engineering requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3. Architectural approach: a path based on a decision tree. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A challenge to far-infrared/submillimeter astronomical space observations, today, is to find a way to improve angular resolving power and image quality. The needs are *
Corresponding author. Tel.: +1 202 479 6877; fax: +1 202 484 2654. E-mail address: [email protected] (M. Harwit).
1387-6473/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2005.11.030
. . . . . . . . . .
228 229 230 231 231 231 231 232 234 234
compelling. Roughly half of the energy generated in the Universe since the decoupling of radiation 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 the Cosmic Background Explorer (COBE) but had been suspected far earlier from observations conducted with balloons, high-flying aircraft and the Infrared
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Astronomical Satellite (IRAS). The physical insights that can be obtained with FIR/SMM studies have been further illustrated through photometric 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 great strides in FIR/SMM astronomy that have been witnessed since the 1960s have come about largely through a roughly million-fold 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. NASAÕs Lear Jet, flying in the late 1960s had an aperture of 30 cm. The KAO, which began routine service in the mid-1970s increased this to 90 cm. But none of the FIR/SMM telescopes flown in space have had an aperture exceeding 85 cm. At the long wavelengths beyond 200 lm the best image quality obtained by these missions has consistently 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-m aperture on the ESA/NASA Herschel mission will improve this, but the image quality in the FIR/SMM will still remain merely comparable to the visual images that Galileo was able to obtain with his first spy glasses. 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 do not need to be filled. As amply illustrated by the performance of the small FIR/SMM space telescopes already launched, the light gathering power of 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. Like in the radio wavelength regime, large effective apertures are needed primarily to provide improved diffraction-limited images. For this purpose sparse apertures and, in particular, interferometers, are fully adequate and economically viable. We have been studying the potential scientific capabilities of a particular interferometer design comprising two 4-m aperture light collectors, whose output is steered to a central beam combiner. The entire assembly is to be launched into space and placed at the second Lagrangian 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 and constant. We have coined the name ‘‘Submillimeter Probe of the Evolution of Cosmic Structure,’’ for this interferometer, with the acronym ‘‘SPECS.’’
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2. Formation of the first stars A current belief is that the first cosmic condensations gave rise to Population III stars having several hundred solar masses (Abel et al., 2002). During initial gravitational collapse at an epoch of red shift z 20–30 a shock ensues as infalling matter piles up. The temperature abruptly rises, reaching 200–500 K. Molecular hydrogen H2 is the only effective coolant at this epoch, and H2 collisionally excited states radiate away sufficient energy to assure continuing protostellar collapse and the onset of hydrogen burning in the newly formed star. While this suggests a search for H2 emission to directly observe the initial collapse, Mizusawa et al. (2004) estimate that the expected H2 flux would not exceed 1029 W m2 or 105 lJy — far below measurable levels. To search for traces of these first stars, we need to look at their later evolutionary phases. Because the stars are so massive, the proton–proton reaction is unable to produce sufficient heat to stop contraction before central temperatures rise to >108 K, where the triple-a process sets in. With carbon formed, the CNO cycle can start to supply sufficient heat to stem further contraction. Hydrogen keeps burning in the compact hot core until fully exhausted. For stars with masses in the range 100–140 Mx, further contraction leads to central temperatures so high that collisions between photons begin to form electron–positron pairs. At threshold, the photonsÕ energy is converted into electron–positron masses with little kinetic energy. The pressure suddenly drops, and the ensuing collapse raises the temperature to where even–even nuclei form through the successive addition of alpha particles and carbon to oxygen. The heat released as neon, silicon, sulfur and magnesium are formed produces a rebound and initiates pulsations that eject the outer layers of the star before its interior collapses to form a black hole. In stars with masses 140–260 Mx elements as massive as the iron group are formed and a single pair-production pulse disrupts the star, ejecting all matter and leaving no remnant (Heger and Woosley, 2002). Schneider et al. (2004) believe that as much as 30% of the starÕs mass may then be ejected in the form of these heavy elements. While stars of all other masses below 500 Mx, appear to retain much of their mass, explosions of stars with masses in the 100–260 Mx range appear sufficiently powerful to eject a significant fraction of their initial mass into extragalactic space. Traces of excess silicon and sulfur that may have originated in such explosions have been recently detected in X-ray emission from the hot gases pervading large clusters of galaxies, whose deep gravitational potentials retain any matter ejected from member galaxies (Baumgartner et al., 2005). Schneider, Ferrara, and Salvaterra have suggested that immediately after a pair-instability supernova explosion, the ejecta condense into silicate grains. To test this, SPECS will search for silicate 10 lm emission red shifted out to 200 lm and emitted by silicate dust shells streaming out
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Fig. 1. Creation of the first metals and dust.
of the stellar explosion. SPECS will have the angular resolution to coarsely image these shells, they should look rather similar to the spherical Galactic dust shells imaged by the MSX and Spitzer missions Fig. 1. The explosive energy of these massive supernovae ranges up to 1053 erg, most of which is ejected as kinetic energy. If the initial ejection velocity is far greater than the final velocity after the ejecta sweep up ambient gas, energy and momentum conservation dictate that most of this energy must go into cooling. Since the most effective coolants are H2 and silicate grains, much of this energy will be emitted in the mid-infrared at 2(z + 1) 40 lm in H2 rovibrational, or 10(z + 1) 200 lm far infrared silicate grain emission. Heger et al. (2001) estimate the rate at which such supernovae explode to be 4 · 106 s1 per square degree, with an emission plateau lasting of order a year in the observerÕs rest frame for z 20. This suggests that 100 remnants per square degree should be visible at any given time, with a total flux at the observer of 4 · 1018 W m2 per square degree. The flux from individual supernovae would be 4 · 1020 W m2. On average, there would be one such remnant in every 36 square arc minutes. Individual sources will be bright enough to detect in searches with SAFIR and then imaged by SPECS to confirm their identities. These are heady prospects. The question, however, is ‘‘How can a kilometer-baseline be constructed and deployed in space?’’ 3. The need for high angular resolution A team of more than 30 astronomers and 20 engineers recently completed a NASA-sponsored year-long study to identify technological advances that would be required to make a FIR/SMM interferometer in space a reality. Our study identified compelling needs for observations with an angular resolution of 50 milliarcseconds (mas), a
requirement that led to a number of technological consequences. Resolving powers at this level led us to examine how headway might be made on the construction of a kilometer-baseline FIR/SMM Michelson stellar interferometer in space, operating in the 40–640 lm wavelength range with fully cryogenically cooled optics and photon-noise limited detectors. The range of scientific applications calls for photometric studies, as well as observations at intermediate spectral resolution, R 1000–3000, and at extremely high resolving powers, R J 3 · 105. The most favorable orbit for an interferometer enabling these scientifically compelling observations lies in the Sun– Earth Lagrangian region L2. The total light collecting area of the interferometer will have to be J 25 m2, most economically attained with a combination of two 4-m light collectors and one central beam combiner. Tethered formation flying provides the only foreseeable means for conducting the mission with an affordable mass of electrodynamic thruster propellant, mandated both by cleanliness and thrust requirements. A FIR/SMM interferometer will provide unique new insights that cannot be obtained at other wavelengths, nor with other facilities operating in the FIR/SMM wavelength band. We anticipate that the James Webb Space Telescope (JWST) may offer complementary information at wavelengths shorter than 40 lm, and the Atacama Large Millimeter Array (ALMA) may do the same at wavelengths longer than 640 lm; but the phenomena to be studied by a kilometer-baseline FIR/SMM interferometer, operating in an energy regime covering four full octaves, from 40 to 640 lm, i.e., photons with energies of 2– 16 meV, cannot be duplicated by instruments sensitive to radiation at significantly higher or lower energies. The thermal regimes of dust and gases to be studied are quite different and complementing, so that these three observatories will observe quite different physical aspects of stars, planets and galaxies.
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4. Architecture and implementation approach A wide variety of approaches present themselves as one begins to consider the most promising architectures for attaining high-angular-resolution images at FIR/SMM wavelengths in space. Our team began its deliberations by defining the most compelling scientific questions that could be answered today if such a capability were currently at hand. While the technical requirements that would permit delving into the many scientific investigations spanned a rather wide range, a central set of capabilities emerged to satisfy a large majority of the proposed studies. These science requirements thus gave rise to a well-defined set of technical specifications and engineering requirements. In turn, those began to show the way to narrow down architectures that would satisfy the scientific goals. The decision tree we followed had many branches, but we found that numerous practical challenges emerged along the way, steadily narrowing down acceptable alternatives, channeling our choices down a relatively well-definable road. In the following subsections, we depict the path we followed in arriving at the architecture we ultimately adopted, Fig. 2. 4.1. Space systems architecture: approach through a decision tree The SPECS decision tree (Fig. 3) is based on a series of scientific requirements:
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4.1.1. Science requirements (a) A field of view of 1-arc-min, (b) Wavelength coverage from 40 to 640 lm, (c) Spatial resolution of 50 mas across this entire waveband, (d) Ability to provide images in broad spectral bands, in photometry, at intermediate spectral resolution R 1000–3000, and at high resolution R > 100,000 in at least one diffraction-limited collector beam, (e) Ability to obtain typically 300 images in 1 year over a 5-year lifetime, (f) Ability to survey at least two, but preferably more nearby star-forming regions, such as the Taurus and q Ophiuchus molecular clouds, (g) Sufficient sensitivity to map extragalactic sources at high red shifts.
4.1.2. Ensuing engineering requirements (a) An interferometer or sparsely filled aperture sampling many baselines up to a maximum baseline of 1 km. (b) Light collectors having a total collecting area of 25 square meters. (c) An optical system cooled to 4 K. (d) Direct detector arrays cooled to <0.1 K. (e) Direct detectors with NEP 1019–1020 Watt Hz1/2. (f) Heterodyne receivers with sensitivities close to the quantum noise limit.
Fig. 2. SPECS architecture decision tree. The flow starts at top left and follows the arrows. The terminology and rationale for choices made are given in Section 4.1.3.
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Fig. 3. SPECS architecture.
(g) An environment free of significant differential forces on the light collectors. (h) The ability to readily communicate large data streams from space back to Earth. (i) A clean environment to prevent deposition of contaminants on cold surfaces. (j) Metrology at submicron levels along all optical paths. (k) An ability to control optical alignment and path length. (l) An ability to point the light collectors with an accuracy and jitter [10 mas.
4.1.3. Architectural approach: a path based on a decision tree (A) Location of SPECS: Three orbits were considered: Near-Earth, Drift-away, and L2. We settled on an L2 orbit where the perturbing forces are low but a constant and acceptable distance from Earth permits a high data stream to be maintained. (B) A two-beam interferometer can be considered to be the simplest unfilled aperture. But the beams can be combined in two different ways. A Fizeau interferometer produces an image very much in the way that a single filled aperture would if the primary light collector were masked off with only two apertures contributing to the final image. For the long baselines considered and the 1-arc-min required field of view, this would produce images requiring excessively large detector arrays. A Michelson stellar interferometer is not similarly limited. From the beginning, we opted for the Michelson architecture (Fig. 3). (C) Any number of light collectors can be employed in constructing a Michelson interferometer. When there are N such collectors, the number of beams requiring combination is N(N 1)/2. With N = 3, it is possible to monitor performance through phase closure. However, the wavelengths at which the FIR/SMM interfer-
ometer is to work are so long that currently available metrological precision can provide information that would be redundant with phase closure. These considerations led us to opt for a two-beam interferometer as simplest to construct as well as least costly. (D) The movement of light collectors in space to cover the uv-plane, and yield – through so-called aperture synthesis – all possible combinations of baselines that a filled aperture would provide, can be accomplished in two ways. One is through use of free-flying light collectors conveying beams of light to a similarly free-flying beam combiner. The other, depicted in Fig. 3, is to employ tethers that connect the light collectors to the beam combiner, the tethers being kept taut by the centrifugal force of the collectors circling the beam combiner. We examined the amount of thruster fuel that would be required to move the light collectors in a uv-plane filling motion, and determined that the required mass of thruster fuel would be so high as to prohibit the imaging of even as few as 100 sources in a 5-year mission. The use of tethers, on the other hand, reduces the total amount of fuel required for a nominal 5-year mission to manageable scales. (E) The control of tethers is still an untested art. The single most important problem to be investigated in a continuing study leading to a kilometer-baseline interferometer in space is the control of tethered configurations in space. While analytic studies are well advanced, and laboratory studies are ongoing, a small-scale mission in space will be needed to provide assurance that attitude control of light collectors for pointing at the arc second level as well as precise beam overlap at the combiner and control of the separation between spacecraft at a level of roughly ±5 cm, can be readily managed over a range of rotational velocities of the light collectors about the beam combiner.
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(F) Tethered configurations can be arranged in a variety of ways. Angular momentum can be conserved through the use of dummy masses that are reeled out as light collectors are reeled in and vice versa. Judicious use of such masses can reduce the amount of thruster propellant required. We examined a number of these configurations, but concluded that the added mass of the dummies was greater than that of the thruster fuel needed to provide the required astronomical performance over a 5-year mission. Were a mission of considerably longer duration desirable, however, the use of dummy masses should be revisited. (G) Thrusters come in two varieties: with chemical or electrodynamic propulsion. Electrodynamic propulsion provides higher thrust for less propellant, undergoes less beam spreading and, in the current application would make use of chemically inert propellants like xenon to reduce contamination of cryogenically cooled optical surfaces to a minimum. (H) Light collectors also come in two varieties, making use either of optical flats, or reflecting telescopes. For the FIR/SMM wavelength range under consideration, shielding against stray light is essential to lower detector background noise and to reduce heat incident on cryogenically cooled optics. This means minimizing the apertures through which radiation enters the beam combiner. For this purpose reflecting telescopes are superior to optical flats. (I) A number of telescope configurations may be considered. Telescopes with elements that obscure part of the beam produce undesirable diffraction patterns and chromatic effects that need to be avoided. Through judicious optical design, these diffraction patterns can be largely made to fall outside the beams that ultimately reach the heart of the beam combiner. (J) Heat shielding by passive means is a major consideration in the construction of SPECS if cryocoolers are to efficiently cool all optical surfaces down to 4 K. Solar heating must be reduced to an absolute minimum with passive radiators if cryocoolers are to efficiently cool all optical surfaces down to 4 K. On one hand, sunshields have to be kept sufficiently small to both reduce solar radiation pressures that would result in undesirable spacecraft motion and stray light from the sunshields into the optical path. Sunshields that are too small, on the other hand, require the collector field of view to be pointed into a narrow region around the anti-sun direction, which would excessively restrict the accessible region of the sky to a narrow band about the ecliptic. The compromise we chose was to assume sunshields that permit a range of angles ±20 from the anti-sun line. This region allows one to observe several young star-forming regions, as well as some of the darkest portions of the extragalactic sky.
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(K) To minimize stray radiation from the collector spacecraft onto the beam combiner, the sunshields have to be designed to maximally cool the external surfaces of both the collector and the beam combiner spacecraft facing each other. This requires large sunshields constructed so that the radiated heat is channeled away from the optical line of sight joining the spacecraft. The three spacecraft, however, also have to approach each other closely to provide the smallest possible baseline. Otherwise, astronomical features that subtend angles between 1.22k/D and k/2Bmin will not be registered. If the closest permitted approach between spacecraft is 8 m, and the telescope apertures are D = 4 m, then for a linear configuration of spacecraft and beam combiner, Bmin = 16 m, and the missing spatial Fourier components in the map will range from 0.37500 to 3.6600 at 60 lm wavelength. At 600 lm the missing angular components range from 3.7500 to 36.600 . The closest approach problem is one of the outstanding questions to be more fully investigated in a follow-on study. (L) We considered but also rejected a variant of the triangular spacecraft configuration in which all three spacecraft are identical, each housing both a collector that feeds light to the other two spacecraft and a combiner that receives light from the other two. This has two advantages: It permits the construction of three identical units, which may be cost effective, and it will permit interferometry to be conducted if one of the three units fails, thus avoiding single point failure. Nevertheless, this option appeared to be more expensive than the simpler linear construction, and had the disadvantages of the triangular configuration already cited in paragraph K. (M) The requirement that 300 separate targets be investigated in 1 year leads to detector read-out rates that are incompatible with existing detector capabilities, unless a number of specific strategies are adopted. A related concern is that, during a delay-line scan, different portions of the field of view pass through the zero path difference in succession, making the effective dwell time on each target in the field of view much shorter than the duration of the delay line stroke. At the shortest wavelengths, where the 1-arc-min field of view is imaged onto a 30 · 30 array, the effective dwell time on the sky is reduced by a factor of 30, yielding a viewing efficiency of only 3%. While the wide field of view of 1-arc-min square still provides significant advantages over single pixel imaging, a strategy to reduce the readout rate and increase the dwell time significantly is clearly needed. No simple solutions exist, but a combination of different steps can be envisaged. For any given observations, the choice of steps is likely to depend most strongly on the required spectral resolving power. The considerations are detailed and will more appropriately have to be described in a dedicated publication.
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(N) At short baselines, the tangential velocity at which the light collectors circle the beam combiner can be reduced to prevent excessive smearing of the scene during the delay line stroke. We have investigated several strategies for controlling the speed of rotation of the interferometer. An efficient program is to start out at very low velocities at short baselines and to apply a constant thrust that terminates at very long baselines with the light collectors moving with tangential velocities of 2 m/s. An electrodynamic propellant mass of 500 kg per light collector will suffice to service a 5-year mission involving 500 separate targets. Additional, more efficient strategies based on maintaining constant angular momentum for prolonged periods can also be implemented as described in paragraph K. (O) We investigated the best ways to assure accurate pointing of the collector telescopes and precise superposition of beams in the beam combiner. A combination of laser metrology and star tracking promises to provide the required precision. Accurate guiding on stars will require near infrared viewing of stellar images directly through the primary telescope. The SPECS optics will have to be fabricated to tolerances required for metrology at wavelengths of 1 lm. Sparse coverage of the uv-plane, and/or larger telescope apertures than the 4-m telescopes envisaged will permit the use of faint guide stars to provide sufficient guide star availability.
5. The future Four technologies will require development to make the interferometer a reality: first, controllable tethered flight, preferably tested on a small precursor mission in space. Second, a vigorous program of detector development leading to background limited detector arrays and close to quantum noise limited heterodyne receivers, both compatible with millisecond readout rates. Third, a massive interferometric cryo-delay mechanism capable of optical path sweeps at speeds of 50 cm s1, direction reversal at 1 s intervals, and reliable operation over 108– 109 cycles. Fourth, a program of detailed end-to-end computer simulations of the complex interplay between metrological, attitude control, and data gathering systems.
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