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The Forefront of Space Science

Next Space VLBI Mission
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Performance enhancement for VSOP-2

To realize significant improvement compared to the VSOP, three challenging objectives were set for the VSOP-2.
  • Up to 10 times higher frequency: Improve resolution by raising observation frequencies up to 43GHz, which is far higher than VSOP’s 5GHz (note that many VSOP results were obtained in the 5GHz). With higher frequency, observation with good visibility in the plasma region is also expected.
  • Up to 10 times higher resolution: Due to higher frequency observation and effect of increase of the satellite’s apogee height, angular resolution will be 0.040 milli arc sec. at 43GHz, 10 times better than the VSOP’s 0.4 milli arc sec. at 5GHz.
  • Up to 10 times higher-sensitivity: Improve sensitivity up to about 10 times at continuum observation. This will be realized by low noise cooled receivers (22 and 43GHz bands), increased transmission rate of 1Gbps, and increased antenna gain.

In addition, two important capabilities will be added. In the VLBI observations due to the errors in “phase”, it becomes difficult to achieve high sensitivity or high accuracy. To solve this problem, we plan to introduce a “phase compensation observation” mode where two objects, the targeted source and a reference source located near it, are switched to be observed at an interval of about one minute. This function enables further improvement of sensitivity and astrometry observation.

Moreover, by actively conducting polarization observation, we intend to obtain information on magnetic-field direction and Faraday Rotation. These are very important information to understand plasma region, because magnetic field and radiation are closely related.

ASTRO-G satellite and technological development

One major problem was the design of the ASTRO-G’s radio astronomical antenna. The conclusion we reached was a large deployable antenna (9mø, 0.4mm rms) of the module-structured, offset-parabola type, which differs from that of HALCA. A one-wing solar paddle will be mounted on the opposite side of the antenna. The satellite will adopt three-axis attitude control. Its weight will be about 910kg and generation power about 1.8kW (Fig. 2).

Figure 2
Figure 2. Papercraft of the ASTRO-G satellite (made by Dr. Yoshiharu Asaki and presented on the ISAS Open Day in 2006)

The observation orbit is planned to be elliptical with parameters of apogee height 25,000km, perigee height 1,000km, orbital inclination 31 deg., and orbital period 6.5 hours.

We decided to introduce the LDR (Large-scale Deployable Reflector) structure, which is adopted for the ETS-III satellite, as the large antenna’s deployable structure. The antenna will comprise seven antenna modules. In order to improve mirror surface accuracy of individual modules, newly developed radial ribs will be employed, with a concept identical to the umbrella. A two-axis adjustment mechanism will be installed on its main mirror and three-axis adjusting mechanism on its sub mirror so that we can adjust to keep antenna gain maximum in orbit.

Since FY2000, the development budget for the satellite had been allocated in the name of strategic developmental budget prior to its formal selection. We are confident that the deployable antenna will be successfully developed as a result of our intensive study over the past six years including prototype module fabrication (Fig. 3).

Figure 2
Figure 3. Test Model of large deployable antenna module

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