VLBI Space Observatory Programme

Space Very Long Baseline Interferometry (SVLBI) telescopes represent a spectacular achievement in the quest for extremely high resolution telescopes for astronomy. The first SVLBI mission is the VLBI Space Observatory Programme (VSOP), led by the Japanese Institute of Space and Astronautical Science, in collaboration with the National Astronomical Observatory of Japan . The first VSOP satellite, Muses-B, was launched on the new ISAS M-V rocket from the Kagoshima Space Center on 12 February 1997. The rocket placed the spacecraft in an elliptical orbit where it was deployed to form an 8-metre diameter radio telescope.

The telescope depicted in the accompanying figures has combined a space-borne antenna with its ground-based counterparts around the world to form a telescope about 32000 km in size (effectively 3 times larger than the Earth), achieving a maximum resolving power of about 90 micro-arcseconds at radio wavelengths. (For comparison this is about 100 times greater than the Hubble Space Telescope).

This is an international mission with contributions from several countries and organizations. The National Research Council's role in the project is part of a partnership within Canada, set up in September, 1995, between the Canadian Space Agency ( CSA - the major funding agency), the National Research Council (NRC), Natural Resources Canada (NRCan), and the Centre for Research in Earth and Space Technology (CRESTech).

Canadian scientists participate in the VSOP mission using a VLBI system developed in Canada , mainly by two groups - the National Research Council's facility at the Dominion Radio Astrophysical Observatory (DRAO), and CRESTech, based at York University . The S2 VLBI recording systems, developed by ISTS, are combined with a VLBI correlator developed at DRAO. For the VSOP program, S2 recorders purchased by the CSA are used in a network of 5–7 Ground Radio Telescopes (GRT 's) and NASA telemetry stations.

The participating GRTs will be at Arecibo (Puerto Rico), Green Bank (West Virginia), Noto (Italy), Hartebeesthoek (South Africa), Usuda (Japan), Shanghai (China), Mopra, Narrabri, Hobart, Ceduna, Tidbinbilla (Australia), Bear Lakes and Kalyazin (Russia). Other telescopes in the United States , Japan, and several other countries have compatible recording systems and will also participate. Calibration and image qualification is carried out at the University of Calgary Radio Astronomy Laboratory.

The following table gives more of the VSOP telescope's technical details:

* With a 70 m GRT at 8 times rms noise on a single baseline (8 sigma) in 300 s of integration.

The scientific questions to be tackled by VSOP are governed by the frequencies available, the sensitivity, angular resolution, u-v coverage, spectral line capability, and monitoring capability. Table 1 contains the basic observing parameters of the space VLBI system, assuming 70 m GRTs.

The VSOP orbit produces rapidly changing interferometer baselines that provide a good synthesis of the interferometer aperture in a single 24-hour period. VSOP is therefore an “imaging mission” that will supply good quality images of high brightness radio sources in the sky.

Although the space radio antenna is small compared to the large ground radio telescopes that are currently operational, the wide recording bandwidths yield interferometer sensitivities that allow for a large range of scientific purposes. With a 40 m ground station equipped with state-of-the-art receivers, rms noise levels on space-ground baselines as low as a few mJy are achievable.

The main scientific reason that space antennas are required is that radio sources at cm wavelengths cannot be resolved at the longest Earth baselines. Also, many of these sources are very bright, thus allowing observations with relatively small antennas. Eventually larger antennas will be launched so that weaker sources can be observed, and in fact, one is already in the planning stages at NASA/JPL (ARISE). In the long term very large antennas on the ground may be used in conjunction with antennas in space. This will also achieve improvements in sensitivity. A number of scientific goals have been identified in advance of the SVLBI missions. These include:

• Tests of cosmological models and determination of fundamental cosmological parameters via ultra-high resolution imaging to measure angular sizes and proper motions of components in the cores of high red-shift Active Galactic Nuclei and Quasi-stellar Objects.
• Direct detection of "super-Compton" emission from compact radio cores by measurements of intrinsic brightness temperatures.
• Imaging of the cores of Active Galactic Nuclei to study the nature of the central energy source and the collimation mechanism for superluminal jets.
• Ultra high resolution imaging of the polarization structure of Quasars and BL Lac objects.
• Dynamical imaging of outburst phenomena in Active Galactic Nuclei on time scales of days.
• Studies of the structure and dynamics of maser systems and the measurement of parallactic distances to nearby galaxies.
• Imaging of outburst from nearby stellar systems with linear resolution comparable to stellar diameters.
• Investigations of the small spatial scale properties of the interstellar medium by measurements of the scattering of images of strong extragalactic sources on micro-arcsec scales.

Because of the very small linear scales imaged by the resolution of these telescopes (either in the sources themselves or in the intervening medium through which the sources will be observed), almost all of the observed phenomena will be time variable on the scale of the mission lifetimes (often much shorter). Of course, time variability is a well known phenomenon in ground-based VLBI, but the effect will be enhanced because the increased resolution will allow imaging of the flux responsible for the variation. This adds an entire dimension to the scientific work, and means that multiple observations of most sources will be required to allow full interpretation of the results.

Access to the Mission

The total observing time on the spacecraft has been divided roughly into three parts - 50% for General Observing Time (GOT), 25% for a survey of high brightness temperature (high-Tb) sources and maser sources (the VSOP Survey), and 25% for engineering time. The GOT is by open application; the Survey is by invitation of the VSOP mission under the auspices of the Survey Working Group. The first round of applications for GOT occurred in 1995 and covers the period from about July, 1997 to September, 1998. The second call for observing proposals will probably occur in August, 1997. The total lifetime of the spacecraft is 3–5 years.