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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
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
operation between 1997 and 2003. Information concerning many aspects of this mission
can be found at http://www.vsop.isas.ac.jp.
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
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,
http://www.asc.rssi.ru/radioastron/index.html. 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
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,
ftp://ftp.cv.nrao.edu/pub/NRAOstaff/efomalon/IARISE/iarise.pdf, 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
- The resolution of space VLBI will be sufficient to resolve and
image the accretion disk of M87, the nearby supermassive black hole
candidate. Only with this instrument can the
formation of radio beams and the interaction of the accretion disk and the black
holes be directly studied.
- Water megamasers orbiting several nearby galaxies, such as NGC 4258, are
unique tracers of the dynamics, stability, warping and clumpiness of accretion
disk material around a black hole. Over several years many such galaxies can
monitored. These data also provide a distance measurement for the galaxies.
- From a sample of hundreds of luminous AGN, the evolution of the shocks, the
magnetic field configuration, and the material flowing from the accretion disk to
the lobes can be monitored. The radio imaging of these objects will complement
observations of GLAST and ConstellationX
which will monitor the higher
energy, variable emission.
- Astrometric accuracy at the microarcsec level will be competitive with that
expected from optical space VLBI programs. The space motion of hundreds of
objects, including planetary systems, will be measured.
- Space VLBI can monitor the evolution of microquasars. These objects may be
the key to understanding the physics in more distant and slowly evolving AGN.
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Last modified: Sat Feb 12 16:48:56 EST 2005