Infrared space interferometers can surpass the spatial resolution limitations of single-dish space telescopes. However, stellar interferometers from space have not been realized because of technical difficulties. Two beams coming from individual satellites separated by more than a few tens of meters should precisely interfere such that the optical-path and angular differences between the two beams are reduced at the wavelength level. Herein, we propose a novel beam combiner for space interferometers that records the spectrally-resolved interferometric fringes using the densified pupil spectroscopic technique. As the detector plane is optically conjugated to a plane, on which the two beams interfere, we can directly measure the relative phase difference between the two beams. Additionally, when an object within the field of view is obtained with a modest signal-to-noise ratio, we can extract the true complex amplitude from a continuous broadband fringe (i.e., one exposure measurement), without scanning a delay line and chopping interferometry. We discovered that this spectral imaging method is validated for observing the solar system objects by simulating the reflected light from Europa with a small stellar interferometer. However, because the structure of the object spectrum may cause a systematic error in the measurement, this method may be limited in extracting the true complex amplitude for other astronomical objects. Applying this spectral imaging method to general astrophysics will facilitate further research. The beam combiner and spectral imaging method are applied to a formation-flying stellar interferometer with multiple small satellites in a Sun-synchronous orbit for observation of the solar system objects in visible and near-infrared. We present an overview of SEIRIOS and the optimized optical design for a limited-volume spacecraft.
Space interferometry is the inevitable endpoint of high angular resolution astrophysics, and a key technology that can be leveraged to analyse exoplanet formation and atmospheres with exceptional detail. However, the anticipated cost of large missions such as Darwin and TPF-I, and inadequate technology readiness levels have resulted in limited developments since the late 2000s. Here, we present a feasibility study into a small scale formation flying interferometric array in Low Earth Orbit, that will aim to prove the technical concepts involved with space interferometry while still making uniq…
This paper presents a strategy for optimal manoeuvre design of multi-satellite formation flying in low Earth orbit environment, with the aim of providing a tool for mission operation design. The proposed methodology for formation flying manoeuvres foresees a continuous low-thrust control profile, to enable the operational phases. The design is performed starting from the dynamic representation described in the relative orbital elements, including the main orbital perturbations effects. It also exploits an interface with the classical radial-transversal-normal description to include the maximum…
The ultimate astronomical observatory would be a formation flying interferometer in space, immune to atmospheric turbulence and absorption, free from atmospheric and telescope thermal emission, and reconfigurable to adjust baselines according to the required angular resolution. Imagine the near/mid-infrared sensitivity of the JWST and the far-IR sensitivity of Herschel but with ALMA-level angular resolution, or imagine having the precision control to null host star light across 250m baselines and to detect molecules from the atmospheres of nearby exo-Earths. With no practical engineering limit…
The Darwin and TPF-I missions are Infrared free flying interferometer missions based on nulling interferometry. Their main objective is to detect and characterize other Earth-like planets, analyze the composition of their atmospheres and their capability to sustain life, as we know it. Darwin and TPF-I are currently in study phase. A number of mission architectures of 3 and 4 free flying telescopes are evaluated on the basis of the interferometer's response, ability to distinguish multiple planet signatures and starlight rejection capabilities. The characteristics of the new configurations are…
The capability of maintaining two satellites in precise relative position, stable in a celestial coordinate system, would enable major advances in a number of scientific disciplines and with a variety of types of instrumentation. The common requirement is for formation flying of two spacecraft with the direction of their vector separation in inertial coordinates precisely controlled and accurately determined as a function of time. We consider here the scientific goals that could be achieved with such technology and review some of the proposals that have been made for specific missions. Types o…
We discuss two geosynchronous gravitational wave mission concepts, which we generically name gLISA. One relies on the science instrument hosting program onboard geostationary commercial satellites, while the other takes advantage of recent developments in the aerospace industry that result in dramatic satellite and launching vehicle cost reductions for a dedicated geosynchronous mission. To achieve the required level of disturbance free-fall onboard these large and heavy platforms we propose a "two-stage" drag-free system, which incorporates the Modular Gravitational Reference Sensor (MGRS) (d…
Double-Fourier interferometry is the most viable path to sub-arcsecond spatial resolution for future astronomical instruments that will observe the universe at far-infrared wavelengths. The double transform spatio-spectral interferometry couples pupil plane beam combination with detector arrays to enable imaging spectroscopy of wide fields, that will be key to accomplishing top-level science goals. The wide field of view and the necessity for these instruments to fly above the opaque atmosphere create unique characteristics and requirements compared to instruments on ground-based telescopes. I…
PRISMA is a demonstration mission for formation-flying and on-orbit-servicing critical technologies that involves two spacecraft launched in low Earth orbit in June 2010 and still in operation. Funded by the Swedish National Space Board, PRISMA mission has been developed by OHB Sweden with important contributions from the German Aerospace Centre (DLR/GSOC), the French Space Agency (CNES), and the Technical University of Denmark (DTU). The paper focuses on the last CNES experiment achieved in September 2012 that was devoted to the preparation of future astrometry missions illustrated by the NEA…
We analyze the trajectories of three geostationary satellites forming the GEOstationary GRAvitational Wave Interferometer (GEOGRAWI)~\cite{tinto}, a space-based laser interferometer mission aiming to detect and study gravitational radiation in the ($10^{-4} - 10$) Hz band. The combined effects of the gravity fields of the Earth, the Sun and the Moon make the three satellites deviate from their nominally stationary, equatorial and equilateral configuration. Since changes in the satellites relative distances and orientations could negatively affect the precision of the laser heterodyne measureme…
We present an analytic model to estimate the capabilities of space missions dedicated to the search for biosignatures in the atmosphere of rocky planets located in the habitable zone of nearby stars. Relations between performance and mission parameters such as mirror diameter, distance to targets, and radius of planets, are obtained. Two types of instruments are considered: coronagraphs observing in the visible, and nulling interferometers in the thermal infrared. Missions considered are: single-pupil coronagraphs with a 2.4 m primary mirror, and formation flying interferometers with 4 x 0.75 …
Entangled light can provide a seminal improvement in resolution sensitivity even without achieving Heisenberg limit in a single channel. In this paper, based on the back-of-the-envelope type calculations, I demonstrate an alternative path to space based long-arm interferometer. Its advantage with respect to LISA is that it does not require complex satellites with many active components to achieve similar resolution.
Radio astronomical observation below 30 MHz is hampered by the refraction and absorption of the ionosphere, and the radio frequency interference (RFI), so far high angular resolution sky intensity map is not available. An interferometer array on lunar orbit provides a perfect observatory in this frequency band: it is out of the ionosphere and the Moon helps to block the RFIs from the Earth. The satellites can make observations on the far side of the Moon and then send back the data on the near side part of the orbit. However, for such array the traditional imaging algorithm is not applicable: …
An X-ray Interferometer (XRI) has recently been proposed as a theme for ESA's Voyage 2050 planning cycle, with the eventual goal to observe the X-ray sky with an unprecedented angular resolution better than 1 micro arcsec (5 prad) [1]. A scientifically very interesting mission is possible on the basis of a single spacecraft [2], owing to the compact 'telephoto' design proposed earlier by Willingale [3]. Between the practical demonstration of X-ray interferometry at 1 keV by Cash et al. [4] with a 1 mm baseline and 0.1 arcsec effective resolution to a mission flying an interferometer with a bas…
Tata Institute of Fundamental Research (TIFR) has a very long tradition of conducting space astronomy experiments. Within a few years of the discovery of the first non-solar X-ray source in 1962, TIFR leveraged its expertise in balloon technology to make significant contributions to balloon-borne hard X-ray astronomy. This initial enthusiasm led to extremely divergent all-round efforts in space astronomy: balloon-borne X-ray and infrared experiments, rocket and satellite-based X-ray experiments and a host of other new initiatives. In the early eighties, however, TIFR could not keep up with the…
The radio sky at lower frequencies, particularly below 20 MHz, is expected to be a combination of increasingly bright non-thermal emission and significant absorption from intervening thermal plasma. The sky maps at these frequencies cannot therefore be obtained by simple extrapolation of those at higher frequencies. However, due to severe constraints in ground-based observations, this spectral window still remains greatly unexplored. In this paper, we propose and study, through simulations, a novel minimal configuration for a space interferometer system which would enable imaging of the radio …
University of Sydney（悉尼大学）；Macquarie University（麦考瑞大学）；University of Victoria（维多利亚大学）；Flatiron Institute（平顶山研究所）；Space Telescope Science Institute（太空望远镜科学研究所）
We have constructed a five station 12 GHz atmospheric phase interferometer (API) for the Submillimeter Array (SMA) located near the summit of Mauna Kea, Hawaii. Operating at the base of unoccupied SMA antenna pads, each station employs a commercial low noise mixing block coupled to a 0.7 m off-axis satellite dish which receives a broadband, white noise-like signal from a geostationary satellite. The signals are processed by an analog correlator to produce the phase delays between all pairs of stations with projected baselines ranging from 33 to 261 m. Each baseline's amplitude and phase is mea…
Ground-based observation at frequencies below 30 MHz is hindered by the ionosphere of the Earth and radio frequency interference. To map the sky at these low frequencies, we have proposed the Discovering the Sky at the Longest wavelength mission (DSL, also known as the "Hongmeng" mission, which means "Primordial Universe" in Chinese) concept, which employs a linear array of micro-satellites orbiting the Moon. Such an array can be deployed to the lunar orbit by a single rocket launch, and it can make interferometric observations achieving good angular resolutions despite the small size of the a…
We propose an atom interferometer gravitational wave detector in low Earth orbit (AGIS-LEO). Gravitational waves can be observed by comparing a pair of atom interferometers separated over a ~30 km baseline. In the proposed configuration, one or three of these interferometer pairs are simultaneously operated through the use of two or three satellites in formation flight. The three satellite configuration allows for the increased suppression of multiple noise sources and for the detection of stochastic gravitational wave signals. The mission will offer a strain sensitivity of < 10^(-18) / Hz^(1/…
In this paper are reviewed various designs of advanced, multi-aperture optical systems dedicated to high angular resolution imaging or to the detection of exo-planets by nulling interferometry. A simple Fourier optics formalism applicable to both imaging arrays and nulling interferometers is presented, allowing to derive their basic theoretical relationships as convolution or cross correlation products suitable for fast and accurate computation. Several unusual designs, such as a super-resolving telescope utilizing a mosaicking observation procedure or a free-flying, axially recombined interfe…
The Laser Interferometer Space Antenna (LISA) is a proposed space mission for the detection of gravitational waves. It consists of three drag-free satellites flying in a triangular constellation. A gravitational reference sensor is used in conjunction with a laser interferometer to measure the distance between test masses inside the three satellites. Other future space mission such as DECIGO and BBO also require a gravitational reference sensor. The Modular Gravitational Reference Sensor (MGRS) is being designed for these purposes and consists of two different optical sensors and a UV LED char…
The Capella Program: Toward A Space-only High-frequency Radio VLBI Network Formed by Small Satellites in Low Earth Orbits
Seoul National University（首尔大学）；Korea Astronomy and Space Science Institute（韩国天体物理与空间科学研究所）；Max Planck Institute for Radio Astronomy（马克斯·普朗克射电天文研究所）；University of Science and Technology（科学技术联合大学院大学）；SpaceBeam Inc.（SpaceBeam公司）；LeO Space Inc.（LeO Space公司）；Hanbat National University（韩南国立大学）；Joint Institute for VLBI ERIC（甚长基线干涉测量欧洲研究基础设施联合体）；Ulsan National Institute of Science and Technology（蔚山科学技术院）；Kyung Hee University（庆熙大学）；Sejong University（世宗大学）
The concept for space interferometry from Polar or Equatorial Circular Medium Earth Orbits (the PECMEO concept) is a promising way to acquire the image of the "shadow" of the event horizon of Sagittarius A* with an angular resolution of circa 5 microarcseconds. The concept is intended to decrease the size of the main reflector of the instrument to about 3 m using a precise orbit reconstruction based on Global Navigation Satellite System (GNSS) navigation, inter-satellite range and range-rate measurements, and data from the Attitude and Orbit Determination System (AODS). The paper provides the …
The Laser Interferometer Space Antenna (LISA) is a future space-based interferometric gravitational-wave detector consisting of three spacecraft in a triangular configuration. The interferometric measurements of path length changes between satellites will be performed on optical benches in the satellites. Angular misalignments of the interfering beams couple into the length measurement and represent a significant noise source. Imaging systems will be used to reduce this tilt-to-length coupling. We designed and constructed an optical test bed to experimentally investigate tilt-to-length couplin…
We present a short history of the TAUVEX instrument, conceived to provide multi-band wide-field imaging in the ultraviolet, emphasizing the lack of sufficient and aggressive support on the part of the different space agencies that dealt with this basic science mission. First conceived in 1985 and selected by the Israel Space Agency in 1989 as its first priority payload, TAUVEX is fast becoming one of the longest-living space project of space astronomy. After being denied a launch on a national Israeli satellite, and then not flying on the Spectrum X-Gamma (SRG) international observatory, it wa…
The far-infrared (FIR) is one of the few wavelength ranges where no astronomical data with sub-arcsec resolution exist yet. Neither of the medium-term satellite projects like SPICA, Millimetron or OST will resolve this malady. Information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, highly excited CO, and especially from water lines would, however, open the door for transformative science. This calls for interferometric concepts. We present first results of our feasibility study IRASSI (Infrared Astronomy Satellite Swarm Interferometry) for a FIR …
LISA is a proposed space-based laser interferometer detecting gravitational waves by measuring distances between free-floating test masses housed in three satellites in a triangular constellation with laser links in-between. Each satellite contains two optical benches that are articulated by moving optical subassemblies for compensating the breathing angle in the constellation. The phase reference distribution system, also known as backlink, forms an optical bi-directional path between the intra-satellite benches. In this work we discuss phase reference implementations with a target non-recipr…
The Virtual Telescope for X-ray Observations (VTXO) will use lightweight Phase Frensel Lenses (PFLs) in a virtual X-ray telescope with 1 km focal length and with nearly 50 milli-arcsecond angular resolution. Laboratory characterization of PFLs have demonstrated near diffraction-limited angular resolution in the X-ray band, but they require long focal lengths to achieve this quality of imaging. VTXO is formed by using precision formation flying of two SmallSats: a smaller, 6U OpticsSat that houses the PFLs and navigation beacons while a larger, ESPA-class DetectorSat contains an X-ray camera, a…
We present a mission concept, the Geostationary Antenna for Disturbance-Free Laser Interferometry (GADFLI), for a space-based gravitational-wave interferometer consisting of three satellites in geostationary orbit around the Earth. Compared to the nominal design of the Laser Interferometer Space Antenna (LISA), this concept has the advantages of significantly decreased requirements on the telescope size and laser power, decreased launch mass, substantially improved shot noise resulting from the shorter 73000 km armlengths, simplified and less expensive communications, and an overall lower cost…
The Saratoga transfer protocol was developed by Surrey Satellite Technology Ltd (SSTL) for its Disaster Monitoring Constellation (DMC) satellites. In over seven years of operation, Saratoga has provided efficient delivery of remote-sensing Earth observation imagery, across private wireless links, from these seven low-orbit satellites to ground stations, using the Internet Protocol (IP). Saratoga is designed to cope with high bandwidth-delay products, constrained acknowledgement channels, and high loss while streaming or delivering extremely large files. An implementation of this protocol has n…
