TOP > Report & Column > The Forefront of Space Science > 2006 > Next Space VLBI Mission "VSOP-2" by HALCA
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We intend to realize high sensitivity at 22 and 43GHz observational bands by cooling the receivers up to 30K with Stirling-cycle refrigerator. Today, cooling of important receivers has become the norm, as applied to the AKARI satellite observing infrared region, the SUZAKU satellite observing X-ray region, and the ASTRO-G observing radiowave region. In space VLBI, wave form in the observed bands are sampled and transmitted at high speed to tracking stations on the ground. In this process, the broader the transmission band is, the higher sensitivity becomes. The VSOP-2 will realize data transmission rate of 1Gbps, eight times that of HALCA, and this in itself contributes to about three times improved sensitivity. In order to conduct switching-observations of two bodies located within 3 deg. at an interval of within one min., two control moment gyros (CMG) with high-torque capability will be added on the satellite in addition to four reaction wheels (RW). Since highly accurate orbit determination (i.e., within 5cm in position accuracy) is also requested, we are considering the use of on-board GPS receivers. We still have as many challenges that must be cleared as at the time of MUSES-B (HALCA) development. We hope that many people will join our project. From proposal to project start We proposed the VSOP-2 in October 2005 in response to a call for proposals for the ISAS’s 25th scientific satellite. The hard X-ray astronomical satellite NeXT was also proposed to the Space Science Committee of ISAS. The Committee selected the VSOP-2 plan as its primary project on February 1, 2006. On the other hand, the Space Engineering Committee selected the solar sail project. Later, on March 1, the VSOP-2 was chosen by two decision-making councils of ISAS. Further, the Steering Committee and Board of Councilors of ISAS approved the project in April followed by approval by JAXA in May. In July, the Space Activities Commission (SAC) assessed and approved the project. In this bottom-up process, we received much advice and many comments. A budgetary request was made by JAXA assuming that the project will officially start from FY2007. The ASTRO-G’s launch is planned for FY2011 and a mission extending over five years after launch is expected. Operation of space VLBI and international cooperation Overall, the plan and flow of VSOP-2 observation and operation is the same as the VSOP. The satellite will be operated from the command station in Kagoshima. It will function by referencing the frequency transmitted from the tracking stations and return observational data to the stations for recording. The satellite and tracking stations will function together to act as a single space radio telescope. Observational data obtained from the satellite and ground radio telescopes will be sent to a correlation center where data correlation are made. And image processing are made to produce radio images by researchers. HALCA’s tracking network comprised five tracking stations, and such are also needed for VSOP-2. A correlator for the VSOP-2 is planned to be manufactured and operated jointly by the National Astronomical Observatory of Japan (NAOJ) and Korea. Correlators in Europe and in US are also expected to join. The VSOP operated successfully with the establishment of large and close international cooperation. The experience and international understanding gained with the VSOP will be fully utilized for the VSOP-2. In addition to VSOP-participant China, Korea will build a dedicated Korean VLBI Network (KVN) composed of three stations by 2007. The VSOP-2 project will be joined by international VLBI telescopes including VLBA (U.S.), EVN (Europe, China), and AT (Australia). VSOP-2 born from HALCA Through the implementation of projects such as VSOP and VERA, the number of researchers in Japan involved in VLBI has increased steadily compared to the time of VSOP development. The Japanese VLBI community has a common understanding on the roadmap of VSOP, VERA, and VSOP-2. The observational frequencies of NAOJ’s four VERA stations are the same as those of the VSOP-2. Moreover, since they are radio telescopes featuring phase-compensation observation, unique results can be expected by joint observations. Having cooperated on the VSOP/HALCA project, ISAS and NAOJ will now form a close partnership. The “Space VLBI Promotion Office” was founded at NAOJ in April 2004 aimed at the VSOP-2. The VSOP-2 project was reviewed and assessed by various NAOJ committees and was approved as a joint project with ISAS. HALCA bringing birth the VSOP-2 ended its operation in November 2005. HALCA established new observation method “space VLBI.” Our team endeavored to establish a full-fledged scientific mission. We are determined that our project following HALCA will not perish like toki, a beautiful bird that once flew in the sky over Japan. There are always teething troubles when we start something new. We are determined to implement the VSOP-2 project successfully. (Hisashi HIRABAYASHI)
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