A NEW SCIENCE STRATEGY OF THE RADIOASTRON MISSION WITH A HIGH APOGEE ORBIT
Astro Space Center, Moscow, February 1998
The Radioastron mission has been in the development by the international community for about 20 years, and is aimed at the significant breakthrough in the astronomical research. The cost of the project realization is much higher than for the largest ground based telescopes. This implies that the scientific output be much larger than that from modern ground based observations in the same scientific field.
1. Since regular ground based observations with VLBI network are carried out with the wavelength l g = 3 mm with the baseline Bmax ~ 10,000 km, that is, with fringe size j g = l g / Bmax ~ 60 m as, a ten-fold decrease of the fringe size (j s ~ 6 m as ) with Radioastron at the shortest wavelength l = 1.35 cm will be possible with a baselineÂ = l / j s = 450,000 km. In what follows we will discuss an orbit with the apogee radius ra ~ 320,000 km, that is the gain is forseen by a factor of 7.
2. The implementation of the higher orbit must be adjusted to the sensitivity and to results of measurements with smaller base lines, as well as with existing models of objects in study. The anticipated sensitivity of the Radioastron - phased VLA system at 22 GHz (bandwidth 32 MHz, integration time 100s) is Fc = 7s = 100 mJy of correlated flux. A Gaussian source with the total flux density Ft ~ 1 Jy at the projected baselineÂ ~ 320,000 km will have the brightness temperature at the peak Tmax = p Ft B2 / 2kln (Ft / Fc) @ 5 * 1013 K. For synchrotron sources with particle outflow one can have Tmax ~ g 3 * 1012 = 1015 Ê with Lorentz factor g ~ 10 and flat spectrum.
According to the 22 GHz survey  132 extragalactic objects of 140 have correlated flux higher than above mentioned Fc; for 126 objects only a lower limit of the brightness temperature (> 3 * 1010 K) was found, for 21 objects the temperature was > 1012 K, and sources 1611+343 and 1921-293 have a lower limit 5.7 * 1012Ê and 7.0 * 1012 Ê respectively. Since that survey more candidates with high brightness temperature have been found. For example, from observations of the variability and its correlation between different frequencies such objects might be 0405-385 , 0537-441 [Ç], 0716+714 , 1308+328 . The brightness temperature estimates go up to 1021 K.
Examples of H2O masers unresolved at largest ground base lines are star forming region IC1396N (flux 100 Jy, line width 20 kHz, angular size less than 100 m as  and nucleus of the galaxy NGC 4285 (flux 2 Jy, line width 200 kHz) .
Among OH masers at 1665 MHz unresolved galactic objects are W48 (25 Jy, 2 kHz) and G 34.26+0.18 (16 Jy, 2 kHz) .
Note that the majority of pulsars are unresolved objects for ground base lines.
3. Many of the mentioned sources are rapidly varying sources. For some sources significant variations occur during days or even hours. In order to obtain a full picture of the variable structure of such objects with the very high angular resolution it is necessary to build in future multi-element space systems similar to ground arrays. The minimum number of satellite telescopes is ~ 6. However at the beginning it is necessary to confirm the existence of the superhigh brightness temperature components and to estimate their size. This can be achieved with the single baseline interferometer with application of model fitting, similar to what was done at the earliest stages of ground based radio astronomy. These studies will enable to obtain important scientific results, which are unachievable for ground based observations, and to determine specification for future multi-element space arrays (frequency bands, sensitivity, antenna size, optimal orbits).
4. Description of a new orbit: period 8.5-10 days, ra ~ 260,000 - 350,000 km, rp ~10,000 - 100,000 km. The orbit parameters are chosen such that the gravitational perturbation from the Moon and the Sun provided the largest rotation of the orbital plane around the axis of apsides, and a motion of the axis.
The final selection is optimized such that the largest coverage of the UV-plane be secured with high angular resolution in about 3 years due to the orbit variation. A faster orbit variation is impossible. The orbit selected provides a high coverage (~ 70 per cent) for sources inside a 30° radius around galactic poles, and ~ 50 per cent in other directions. At the same time the orbit allows to synthesize images with the resolution of VSOP using small projections of the baseline (£ 30,000 km) for objects within = ± 5° from the plane of the orbit, with UV-coverage ~ 50 per cent in 12 hours, as well as for all objects during observations in time of the passage of the perigee. The UV-coverage configuration with the small baseline projections is maintained for each source for at least 3 to 5 orbits.
5. Examples of the most important scientific programs are given in the Appendix.
6. The upgrading of the space payload has been done by the ASC in such a way that no change of the ground tracking stations is needed.
7. It is required to consider an increase of the sensitivity of the SRT, especially for 22 GHz band (LNA and the feed-LNA connection).
8. It is required to consider a possibility to build a special unit for frequency tuning for 22 GHz band in order to provide observations of megamasers with red shift.
9. It is necessary to consider a possibility to have large radio telescopes (~ 3 simultaneously) with low noise temperature and with a possibility of adding signals in order to provide high sensitivity of the interferometer in the apogee portion of the orbit, and about 10 telescopes for observations of objects near the plane of the orbit and during the passage of perigee.
10. It is necessary to carry out special observations on the ground telescops and with VSOP (or extract necessary data from completed observations) in order to correct key science programs, related to the scientific program of the RadioAstron mission.
11. A scientific program of the Radioastron mission for the new orbit must be developed, which takes into consideration a regular evolution of the orbit in space, use of small baseline projections for rapidly varying sources, model fitting of the objects from observations with the super high resolution, and study of permanent structure with the highest UV-coverage for long periods of time.
12. Discussion on the adoption of the new orbit, preparation of the scientific program must be completed in 1998. Approval of the new profile has to be done at the RISC meeting in January 1999.
Appendix. Key programs: