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General Overview

Since 1970 VLBI observations have provided unprecedented astrophysical knowledge of the evolution of quasars from the enormous energies provided by black holes and the environment surrounding them, to the energy flow in beams from the galactic nucleus. Observations of the highest redshift objects are discovering the conditions of the universe when galaxies were first forming. From the study of binary stars, often called microquasars, the evolution can be followed in hours to months to provide a living laboratory associated with the energy balance in energetic phenomenon. The astrometric precision, now approaching 10 microarcsec, gives the distances, motion and orbital parameters of many objects in our galaxy and even in nearby galaxies, and are important for establishing the cosmological distance scale.

Space-VLBI, by extending the resolution of conventional ground VLBI, is a natural extension of this technology. After the first experimental tests of space-VLBI using the TDRSS satellite, a Japanese-led project, VSOP, launched the HALCA VLBI satellite in 1997 and produced images of unprecedented resolution at 1.4 and 5 GHz of extragalactic objects. This mission clearly showed that space-VLBI was technologically no more difficult than ground VLBI, and that many extragalactic objects have submilliarcsec structure that only space-VLBI resolutions can probe. The major observational programs included: observations of intraday variable sources; the superluminal motions in jets; direct observations of brightness temperatures greater than 1013K; the determination of the submicroarcsec structure of radio galaxies, and a catalog of sources for future space-VLBI missions.

Several space-VLBI projects are currently planned outside of the USA, but they are limited in scope and ambition and do not reach the goals to which space-VLBI is capable in understanding many energetic phenomenon.

Current Non-US Space VLBI Missions

The VSOP mission, led by Japan, was the first successful space-VLBI mission, in operation between 1997 and 2003. Information concerning many aspects of this mission can be found at It success has generated enthusiasm in Japan for a followup mission, VSOP2, which will be similar to VSOP, but operating at a higher frequency, with more sensitivity and better resolution. Information can be found at Although the proposed VSOP2 mission will give improved sensitivities and resolution over VSOP, the scientific returns will be limited to what is possible.

A Russian mission, RadioAstron, is in nearly final development with a possible launch of a 10m telescope into a high orbit of 300,000 km in 2007, Astronomers and engineers at NRAO, NASA, JPL, Europe and Australia have been in close contact with the RadioAstron group, but there is concern that RadioAstron will not be launched on time, and has a low probability of success. The major reasons are: the system has too low a sensitivity to detect many radio sources, the electronic development is lagging, and the necessary ground services (correlator and spacecraft telemetry) are not yet developed and will be difficult to be provided from Russian sources alone. In the next 6 months, this mission will pass through a critical period where its status will be decided. Support from NASA (which supported VSOP extensively) for a tracking station in North America and support of the correlation and data reduction facility are necessary, but perhaps not sufficient to insure a meaningful scientific return from RadioAstron.


The ARISE mission was first proposed in 1998 and was recommended by the 2000 AASC report. Its fundamental design is summarized in

ARISE is an orbiting 25m telescope with observing capability up to 43 GHz, with excellent sensitivity. It would probe the black holes and accretion disks in the nearest galaxies with a resolution of 20 microarcsec. ARISE is not included in the current NASA planning.

In 2002, astronomers from JPL, SAO and NRAO proposed an updated ARISEtype mission, called iARISE, to the NASA SEUS meeting in December 2002 in Cocoa Beach, FL. The 'i' stands for "international" since a bold jump in space-VLBI will undoubtedly need strong international cooperation. The major change in design was the inclusion of two spacecrafts instead of one. As is described in the two documents, and, two smaller space telescopes provide much better imaging quality and astrometry, and are less expensive for 43 GHz observations than the ARISE onespacecraft mission. Furthermore, the imaging capability of iARISE would complement the high spectral capabilities of ConstellationX. The cost of iARISE is estimated to be about $400M. The collaboration with Japan and/or Russia with their interest and expertise in space-VLBI has been discussed at several NASA, RadioAstron and VSOP2 meetings. There was overwhelming agreement that any space mission with more than one spacecraft was far superior than a one spacecraft mission. However, the collaboration between the space agencies of several countries and the blending of their somewhat independent space-VLBI desires will be needed.

Potential of Space VLBI

The main astronomical goals of space-VLBI are:

This page created and maintained for the RMSPG by Martha Haynes
Last modified: Sat Feb 12 16:48:56 EST 2005