Radio Interferometers Larger than Earth: Lessons Learned and Forward Look of Space VLBI Leonid I. Gurvitsa,b a Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands b Dept. of Astrodynamics and Space Missions, TU Delft, Kluyverweg 1, 2629 HS Delft, The Netherlands [email protected] Accepted for publication in the Proceedings of the 69th International Astronautical Congress (IAC-18-A 7.2.8), Bremen, 1–5 Oct 2018 Abstract Extension of radio interferometric baselines into space is inevitable if a diffraction-limited angular resolution defined by the Earth diameter at a given observing wavelength limits a pursuit of specific scientific goals. This was understood in the early1960s, at the very dawn of the era of Earth-based Very Long Baseline Interferometry (VLBI). Since then, three VLBI missions operated in Space thus enabling baselines longer than the Earth diameter. These are the Tracking and Data Relay Satellite Orbital VLBI experiment (1986–1988), the VLBI Space Observatory Program (VSOP, 1997–2003) and RadioAstron (2011– present time). These Space VLBI (SVLBI) systems enabled studies of celestial radio sources with an unprecedentedly sharp angular resolution reaching 0.1 nanoradian (tens of microarcseconds) and sharper. Space VLBI is a conglomerate of diverse technologies and arguably one of the most sophisticated techniques of space-based astronomy. It involves deployment of large space-borne electro-mechanical structures, state of the art analogue and digital instrumentation, precise attitude control and orbit determination, high capacity data downlink, simultaneous and highly coordinated operations of space-borne and Earth-based facilities, advanced processing of large (tens of terabytes) volumes of data. The first generation SVLBI missions provided cutting-edge results in several topics of modern radio astronomy. These include discoveries of ultra-compact galactic hydroxyl and water vapor masers, radio emission in active galactic nuclei with the brightness exceeding conventional theoretical limits, and detection of pulsar emission at meter wavelengths shedding a new light on the properties of the interstellar medium. One of the Space VLBI missions, the RadioAstron, also ventured into the domain of fundamental physics by sing its onboard Hydrogen maser local oscillators for experimental verification of the Einstein Equivalence Principle. The paper provides a brief review these discoveries and lessons learned over the first half a century of Space VLBI. This comparison, together with the review of technological achievements of the three SLBI missions, creates a fundament for projecting the development of space-based radio interferometry into the next decades. Keywords: (radio astronomy, angular resolution, VLBI) radio astronomers to sharpen the angular resolution 1. Introduction defined by the same simple formula mentioned above, with substitution of the antenna diameter to the Radio astronomy, born in 1933 [1], exemplifies the projection of the baseline, connecting two elements of multi-wavelength nature of the modern astrophysics. the interferometer, on the picture plane. Later, in the During the first two decades of its development, radio second half of the 1960s, the concept of radio astronomy observations were conducted with antennas interferometry got its ultimate extension to the limited in their angular resolution (“sharpness of baselines, comparable to the Earth diameter. This was vision”). In the case of reflecting antennas (in a widely the beginning of Very Long baseline Interferometry used professional slang – the “dishes”), the resolution is (VLBI) [3]. VLBI offered a breathtaking sharpness of defined by the diffraction limit, λ/D, where λ is the vision, reaching milliarcseconds and even sub- wavelength, and D – diameter of the reflector, just as is milliarcseconds. However, very soon radio astronomers the case in the “traditional” optical astronomy. For realized that even this record-high angular resolution, typical in radio domain decimeter to meter wavelengths, that exceeded best optical values by three orders of affordable dishes of tens of meters in diameter can reach magnitude, was not sufficient: there were celestial radio angular resolution of tens of arcminutes, far worse than sources that remained point-like (unresolved) even in a typical angular resolution of Earth-based optical observations with the global VLBI systems. In fact, the telescopes, which is of the order of an arcsecond. A necessity of even higher angular resolution was concept of interferometers was introduced in radio understood at the very dawn of the era of Earth-based astronomy in the beginning of the 1950s [2]. It enabled Very Long Baseline Interferometry (VLBI). As is clear 1 from the simple formula at the beginning of this source can be resolved with a given baseline only up to introduction, the only solution of getting an even a certain brightness temperature. If the brightness is sharper resolution at a given wavelength than offered higher for a given total flux density of the source, a with a baseline equal to the Earth diameter is to place at longer baseline is needed for resolving the source’s least one element of an interferometer in Space. This is image. The TDRSS Orbital VLBI experiment indicated Space VLBI (SVLBI). that in several quasars the brightness temperature is higher than can be measured with Earth-based VLBI 2. SVLBI proof of concept – TDRSS experiment systems. The first ad hoc demonstration of SVLBI experiment 2. The first generation SVLBI missions: VSOP and was conducted with the NASA’s geostationary Tracking RadioAstron and Data Relay Satellite (TDRS) in 1986 [4]. The satellite is shown in Fig. 1. It was equipped with two 4.9 In the wake of the success of the TDRSS VLBI m antennas, one of which was used as a radio telescope. demonstration experiment, the VLBI Space Observatory The observations were conducted at 13 and 2 cm Program (VSOP) was conceived at the Institute of together with a network of large Earth-based antennas in Space and Astronautical Science in Japan in the end of Australia, Japan and the US. the 1980s [5]. The concept presumed a launch on the new four-stage solid fuel rocket M-V and use as the mission platform the engineering test satellite Muses-B. Although the project was formally aimed primarily at the test of the new rocket and new satellite platform, it did have a core science program that defined the science payload. Fig. 1. TDRSS communication satellite in orbit, an artist’s impression. Credit: NASA. The space-borne component of this first SVLBI system was not designed as a radio telescope, and was not supposed to operate as such. It was the ingenuity of Fig. 2. VSOP HALCA satellite in orbit, an the NASA JPL engineers who were able to re-configure artist’s impression. Credit: JAXA. and re-program the on-board instrumentation in the way making it compatible with the Earth-based VLBI radio The main component of the mission was the space- telescope. borne radio telescope with the parabolic reflector of 8.8 The main achievement of the TDRSS Orbital VLBI m effective diameter (Fig. 2). Its surface was formed by experiment was the confirmation of a principle a gold-coated molybdenum wire mesh. Three receivers possibility of obtaining an interferometric response (so enabled observations at 18, 6 and 1.3 cm in left-hand called “interferometric fringes”) on baselines exceeding circular polarisation (LCP). The satellite was on a the Earth diameter and involving a radio telescope not medium eccentricity orbit with the apogee of about fixed on the surface of Earth. The maximum projected 22,000 km above Earth and orbital period of about 6 baseline achieved in the demonstration was equal to 2.2 hrs. The mission had a sizeable involvement of Earth diameters (ED). It has to be noted, that the international community coordinated by the group sensitivity of an interferometer to the source’s called VISC (VSOP International Science Council). brightness, traditionally measured in radio astronomy in Among other things, VISC coordinated such the mission degrees of Kelvin (of a hypothetic black body of the implementation issues as operations of five dedicated same brightness as the source), depends on the physical Earth-based tracking and data acquisition stations (in length of the projected baseline. In other words, the Australia, Japan and Spain, one in each country, plus 2 two stations in the US), orbit determination, VLBI data Science and Production Association lead the mission. A processing, Earth-based VLBI network arrangements modification of the universal satellite platform and policy of access to the mission by the worldwide Navigator, called Spektr-R, carries the space-borne 10 science community. After successful launch on 12 m radio telescope (Fig. 3). February 1997, the satellite was renamed HALCA (Highly Advanced Laboratory for Communication and Astronomy). VSOP operated with the aggregate data rate of 128 Mbit/s – the highest sustainable data downlink rate of a space science mission at the time. The signal coherency – the major requirement of a VLBI system – was supported by a phase-locked loop (PLL), fed by a hydrogen maser frequency standard at a tracking station. Two of the three observing bands, 18 and 6 cm, operated successfully. The signal in the third, the highest frequency band of 1.35 cm, had a very high system noise, preventing science operations. It is believed that the reason was a damage of the waveguide of this band due to a high