|Annu. Rev. Astron. Astrophys. 2014. 54:
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6.1. Space VLBI
Space-VLBI involves using a radio antenna in orbit around the Earth to obtain baselines lengths greater than an Earth diameter. As is clear from Equation 2, for fixed delay uncertainty one gains astrometric accuracy as the baseline length increases. For space VLBI to improve on Earth-based astrometry, the satellite position (orbit determination) must be determined to ~ 1 cm accuracy, which is very challenging. However, the complementary application of using VLBI to accurately track spacecraft holds great promise (Duev et al. 2012).
VSOP/HALCA was a space-VLBI mission launched in 1997. Using VSOP, Guirado et al. (2001) conducted space-VLBI astrometry of the radio QSO pair B1342+662 / B1342+663 (which are only 5 arcmin apart) and demonstrated that the satellite position error was ~ 3 meters and that the useful astrometry could be accomplished only for very close pairs. Because of the poor performance of the VSOP 22 GHz receiving system, maser astrometry was not attempted.
Currently the Russian space-VLBI satellite, RadioAstron, is in orbit (with a maximum interferometric baseline of ~ 300,000 km, comparable to the Earth-Moon separation). Fringes from space-baselines have been obtained (Kardashev et al. 2013) and, perhaps, high-accuracy space-VLBI astrometry can be realized.
6.2. The Event Horizon Telescope
At millimeter and sub-mm wavelengths, VLBI can achieve an angular resolution sufficient to resolve event-horizon scales for nearby super-massive black holes. A prediction of general relativity is that at this scale one should see the "shadow" of the black hole (Falcke, Melia & Agol 2000). Impressive results have been achieved using ad hoc arrays of antennas that can observe at the short wavelengths required to "see through" a screen of electrons that blurs the image of Sgr A*, the super-massive black hole at the center of the Milky Way (Doeleman et al. 2008).
The Event Horizon Telescope is a world-wide collaboration to realize a powerful VLBI array operating at 1 mm or shorter wavelengths, anchored by the phased-ALMA (acting as a single telescope with great collecting area). In addition to imaging event horizon scales for Sgr A* and M87, one should be able to explore the detailed structure of accretion disks and jet launching points in the vicinity of these super-massive black holes using multi-frequency astrometry. Broderick, Loeb & Reid (2011) investigated the possibility of such observations by using the sub-array mode of ALMA, SMA, CARMA and other telescopes and demonstrated that astrometry at the ~ 3 μas level is possible. With such accuracy one can trace positional variations of the black holes owing to perturbations from surrounding stars and/or a black hole companion. In addition, one should be able to monitor structural variations occurring on dynamical timescales of the innermost stable circular orbit, which for Sgr A* would be ~ 10 min.
6.3. Square Kilometer Array
The Square Kilometer Array (SKA) is a world-wide project to build and operate the next generation of radio interferometers with an aggregate collecting area ~ 1 km2. Achieving the SKA will likely require three independent arrays, using different technologies to cover the frequency range of ~ 100 MHz to ~ 20 GHz. Micro-arcsecond astrometry can be achieved at frequencies above ~ 3 GHz, provided baselines of several thousand km are an integral part of the design. For astrometric observations, the SKA holds the promise of significantly increased relative positional accuracy, because its great sensitivity allows the use of weak calibrators much closer in angle on the sky to the astrometric target. Compared to the VLBA, for example, the increased sensitivity of the full SKA should allow the use of background compact radio sources at least an order of magnitude closer to the target, resulting in astrometric accuracy better than ± 1 μas! In addition, if multi-beam feeds are used on individual antennas, one could simultaneously observe many sources, greatly increasing astrometric survey speed.
MH would like to thank Nobuyuki Sakai, Naoko Matsumoto, Kazuhiro Hada, Hikaru Chida, Osamu Kameya, Fuyuhiko Kikuchi, and Yuka Oizumi for their support in preparing the manuscript. MH also acknowledges financial support by the MEXT/JSPS KAKENHI Grant Numbers 24540242 and 25120007.