The first Space-VLBI mission, VSOP, started successfully with the launch of the dedicated space-VLBI satellite HALCA in 1997. The
mission has been in scientific operation in the 1.6 GHz and 5 GHz bands, and studies have been done mainly of the jet phenomena related to active galactic nuclei. Observing at higher frequencies has the advantage of less absorption through the ambient plasma and less
contribution from scattering, and also has the merit of resulting in higher angular resolution observations. A second generation space-VLBI mission, VSOP-2, has been planned by the working group formed at ISAS/JAXA with many collaborators. The spacecraft is planned to observe in the 8, 22 and 43 GHz bands with cooled receivers for the two higher bands, and with a maximum angular resolution at 43 GHz
(7 mm) of about 40 micro-arcseconds. The system design, including the spacecraft and ground facilities, will be introduced, and the impact for sub-mm space-VLBI further into the future will be discussed.
We have carried out the experiment of real-time space VLBI by using a high-speed ATM network. A space VLBI program is carried out with the HALCA satellite, which has an 8-m diameter radio telescope. Downlink data is transmitted to Usuda station, which is a Japanese data link station. And Usuda 64-m telescope is used as a ground radio telescope. They were used for this experiment. It is the first experiment that an optical fiber network was applied for a real-time space VLBI. The ATM optical-fiber network has 2.4 Gbps transmission capability. For the real-time space VLBI experiment, 128 Mbps data are transmitted. The VLBI correlator at NAOJ is used, which is usually used for the tape based ground and space VLBI observations. And we have succeeded to detect fringes using this network with a satellite downlink station and a ground radio telescope by test data, which are play-backed with recorded tapes. Unfortunately we have not tested actual observations because of a serious trouble of the satellite.
The Institute of Space and Astronautical Science (ISAS) launched the first space VLBI (Very Long Baseline Interferometry) satellite, HALCA, in February 1997. After completing a series of engineering experiments to verify space-VLBI observations, the first VLBI fringes and images were obtained in May and in June, respectively. HALCA has now been operated for science observations at 1.6 and 5 GHz for the VSOP (VLBI Space Observatory Programme) project in cooperation with many organizations and radio telescopes around the world. In this paper the current science activities of the mission are reviewed and results presented.