The VSOP missionaries

In February 1997 a collection of wise-heads in VLBI astronomy could be found in Kagoshima, Japan. They were there to observe the Institute of Space and Astronautical Science (ISAS) launch the first satellite dedicated to Space VLBI as part of the VLBI Space Observatory Programme (VSOP). After the successful launch the satellite was named HALCA (Highly Advanced Laboratory for Communication and Astronomy) — which is an anglicisation of the Japanese word Haruka: far away. HALCA's orbit gave a maximum baseline of 30,000 km so it is a very suitable name.

HALCA was equipped with three wavelength bands: 18 cm, 6 cm and 13 mm, the latter which would have provided the highest VLBI resolutions possible at the time. In check-out phase it was found that the 13-mm system had very high attenuation, probably due to alignment damage during the launch. The other two bands worked perfectly and over its seven-year lifetime HALCA made more than 700 observations of quasars, blazars, masers and pulsars.

In its later years HALCA has been getting arthritic, as two of the four momentum wheels – which keep the pointing constant – have slowly worn out and seized up. Heroic efforts succeeded in recovering attitude control of the satellite after several failures. Before launch, the satellite was expected to have a three to five year life-expectancy, with degradation of the solar panels by cosmic radiation foreseen as the major limitation. With HALCA's final observations made over 6.5 years after launch, the satellite has certainly exceeded expectations.

The last observations were made in October 2003, shortly after which control was lost again. Since then all attempts to restart a third reaction wheel have failed. In September 2004 the decision was taken to down grade the monitoring of HALCA from six to one pass each week. This essentially is acceptance that further efforts at restarting are unlikely to succeed.

The Australian connection to HALCA is implicit in the VLBI baselines formed for southern sources, but also through the close association of Australian researchers: Primarily D Jauncey (ATNF), J Reynolds (ATNF), A Tzioumis (ATNF), S Tingay (JPL/ATNF/Swinburne), S Horuichi (NAOJ/JPL/Swinburne), P McCulloch (Tasmania), J Lovell (ISAS/ATNF), P Edwards (ISAS), and R Dodson (Tasmania/ISAS). The latter three have benefited from the Japan Society for the Promotion of Science fellowships.

A selection of research highlights are:

♦ AGN brightness temperatures greater than the inverse-Compton limit (Tingay et al. 2001; Scott et al. 2004);
♦ distribution of spot sizes in OH maser sources (Slysh et al 2001);
♦ the superluminal quasar 3C345 (Klare et al 2000) and 3C273 (Lobanov & Zensus, 2001);
♦ multi-epoch imaging of the quasar 1928+738 (Murphy et al 2003);
♦ the M87 jet (Junor et al 2000);
♦ the detection of high redshift quasars (e.g., Lobanov et al. 2001); and
♦ observations of the Vela pulsar (Gwinn et al. 2000).

A special issue of PASJ is planned to bring together a collection of more results. (And with the proprietary period now having elapsed for all VSOP observations, the data archive is starting to be mined!)

Following the success of the VSOP, a next generation space VLBI mission, currently called VSOP-2, is being planned. For VSOP-2, higher observing frequencies, cooled receivers and increased bandwidths will result in gains in resolution and sensitivity by factors of 10 over the VSOP mission. The possibility of including a rapid slewing capability for the spacecraft is also being pursued, so that observations using the phase-referencing technique will enable the sensitivity to be improved even further. The VSOP-2 proposal will be formally submitted to ISAS in September this year. (A call for proposals last year had to be rescinded after a reassessment of ISAS's long term budget profile.)

HALCA has been the excuse for the purchase of far too many bottles of Very Special Old Pale brandy, and many (but not enough) trips to Sushi bars and Karaoke. It will be missed.

References

Gwinn, C., et al, 2000, Proceedings of the VSOP Symposium, Japan, January 19 – 21, 2000, eds.: H. Hirabayashi, P.G. Edwards, and D.W. Murphy, ISAS, p117
Junor, W., Biretta, Proceedings of the VSOP Symposium, Japan, January 19 – 21, 2000, eds.: H. Hirabayashi, P. G. Edwards, and D. W. Murphy, ISAS, p13
Klare, J., Zensus, J. A., Ros, E., Lobanov, A. P., 2000, Proceedings of the VSOP Symposium, Japan, January 19 – 21, 2000, eds.: H. Hirabayashi, P. G. Edwards, and D. W. Murphy, ISAS, p21
Lister, M. L., Tingay, S. J., Murphy, D. W., Piner, B. G., Jones, D. L., Preston, R. A., 2001, ApJ 554, 948.
Lobanov, A., Gurvits, L. I., Frey, S., Schilizzi, R. T., Kawaguchi, N., Pauliny-Toth, I. I. K, 2001, ApJ 547, 714
Lobanov A., Zensus, J., 2001, Science 294, 128
Murphy, D. W., Preston, R. A., Hirabayashi, H., 2003, New Astronomy Review 47, 633.
Slysh, V. I., et al. 2001, MNRAS, 320, 217
Scott, W., et al, 2004, ApJS, 155, 33
Tingay, S. J. et al. 2001, ApJ, 549, L55

Richard Dodson for the VSOP group
(rdodson@vsop.isas.ac.jp)