Takahiro Iwata

Selenodetic experiments of SELENE: Relay subsatellite, differential VLBI, and laser altimeter

Abstract Since 1960s, the gravitational potential of the Moon has been extensively studied from Doppler tracking data between a ground station and spacecraft orbiting in front of the Moon (e. g., Lorell and Sjogren, 1968; Bills and Ferrari, 1980; Konopliv et al. , 1993; Lemoine et al. , 1997). Because direct radio communication is interrupted while spacecraft is orbiting behind the Moon, however, the coverage of tracking data has been limited mostly to the nearside of the Moon so far. In order to compensate for such lack of tracking data, we propose satellite-to-satellite Doppler measurement by using a relay subsatellite in Japanese mission to the Moon in 2003. A complete coverage of Doppler tracking from an orbiter at sufficiently low altitude will significantly improve lunar gravity model and will contribute for future geophysical study of interior and tectonics on the Moon. Further, we propose differential VLBI experiment between the subsatellite and a propulsion module landed on the surface of the Moon. The differential VLBI is about 10 times more accurate than conventional Doppler measurement for long-wavelength gravity field. Besides, differential VLBI is sensitive to the displacement perpendicular to the line of sight. Thus the VLBI experiment provides precise estimates of the lunar gravity potential at low degree. The last proposal for selenodetic experiments is a laser altimeter. Global topography model has been already developed from the analysis of Clementine LIDAR data (Zuber et al. , 1994), but it is suggested that the model includes appreciable anisotropy between NS and E-W directions due to highly eccentric orbit of Clementine spacecraft (Bills and Lemoine, 1995). The laser altimeter experiment from an orbiter in nearly circular orbit will provide a new reference for the isotropic lunar topography model.

Application of a PZT telescope to In situ Lunar Orientation Measurement (ILOM)

We are proposing a selenodetic mission, e.g. In situ Lunar Orientation Measurement (ILOM) to study lunar rotational dynamics by direct observations of the lunar physical liberation and the free librations from the lunar surface with an accuracy of 1 millisecond of arc in the post-SELENE project. Year-long trajectories of the stars provide information on various components of the physical librations and they can also be used to possibly detect the lunar free librations in order to investigate the lunar mantle and the liquid core. The PZT on the moon is similar to that used for latitude observations of the Earth except the half mirror above the objective, a CCD with high well capacity, and the heater beneath the mercury pool. Although a star position on the focal plane does not change even if the telescope inclines in principle, the tilt of the telescope affects the star position due to aberrations of the objective in the actual case. We obtained the relation between the deviation of the star position in CCD and the tilt of the telescope by ray tracing method and found that the effect of the tilt less than 100 arc seconds upon the star position does not exceed 1 mas. Thermal test of a tube made of CFRP showed that the tube did not incline by more than 100 seconds of arc even in the conditions of the lunar surface. We have a prospect to attain an accuracy of positioning of better than 1 mas from simulated experiments in laboratory using a CCD.

Lunar laser ranging by optical transponder collocated with VLBI radio sources on the Moon

The lunar laser ranging (LLR) has been dependent on the retro-reflectors placed on the Moon since the past three decades before. In spite of the technical improvement, regular observations are performed by a limited number of stations due to the weak echo. To allow more opportunities for observations, it is most effective to place modern device on the Moon. Since a Japanese second lunar probe is planned to be launched in 2006, the ideas are collected for scientific purposes. It is described here that an optical transponder on the Moon would enhance the possibility of LLR observations to a large degree. It is also proposed to have three radio sources on the Moon to determine the angular component perpendicular to the Earth-Moon vector and librations. One of the most important scientific targets for the proposed mission is a relativistic experiment which was not attained by a conventional LLR.

Four-Way Doppler Measurement System for Selenodesy Using SELENE Relay Satellite

Four-way Doppler measurements toward SELENE Main Orbiter (Mstar) above the lunar far side will be executed using SELENE Relay Satellite (Rstar) to obtain global maps of lunar gravity field. Carrier waves are extracted from the up-linked ranging signals on Rstar, relayed to Mstar, Rstar, and the ground station with coherent frequency conversion. The properties of each link for four-way have been confirmed to be adequate for signal acquiring and tracking processes.

Possibilities of lunar polar orbiter

Abstract It appears very likely that early next century a lunar base and a manned outpost on Mars will be established through international collaboration to explore and utilize the Moon and Mars. In order to contribute to these projects, National Space Development Agency of Japan (NASDA) has conducted the preliminary studies. As for the lunar projects, possibilities of a lunar polar orbiter (LPO), robotic rover, and sample return mission have been investigated. This paper describes the concept of the LPO based on the H-II rocket and earth observation technology of Japan. The LPO will map the global surface, especially the polar region and the farside of the Moon from 100 km lunar polar orbit and send the precise images of various wave length, acquired by onboard sensors to the Earth. The primary purpose of the LPO is to identify global and local structures of lunar resources and topography and to search for a suitable site for the lunar base.

A VLBI Study of H2O Maser Spots Associated with a Molecular Outflow ρ Oph-East

A molecular outflow is one of the most conspicuous active phenomena associated with protostars, and the kinetic energy of its outflowing mass is as large as that of random motions of ambient molecular cloud, which suggests that outflow has dynamically influence on ambient molecular gas. Possible observational evidence which suggests the existence of dynamical interaction between molecular outflow and ambient molecular cloud has been detected in several star forming regions (Fukui et al. 1986; Iwata et al. 1988). Recent detections of H 2 O maser emission associated with low-mass protostars (e.g. Comoretto et al. 1990) also suggest that there still exist active phenomena in the low-mass star forming regions. Molecular outflow ρ Oph-East, discovered toward a low-mass protostar IRAS 16293-2422 (Fukui et al. 1986), has been known as a site of dynamical interaction between molecular outflowing gas and ambient molecular cloud by CO and NH 3 observation (Mizuno et al. 1990). Existence of several strong H 2 O maser spots (Wilking & Claussen 1987; Wotten 1989; Terebey et al. 1992) also suggests that active phenomena are occurring in this region. In this paper, we report our result of H 2 O maser observation for molecular outflow ρ Oph-East with milli-arcsecond resolution by VLBI.

The CRL 34-m radio telescope at Kashima — A new strong tool in maser research — and the first results of a 22 GHz H2O maser survey

The CRL 34-m radio telescope at Kashima was completed in 1988. The telescope is equipped with low noise receivers from 1.4 GHz to 44 GHz and is able to observe most of the important astronomical masers in this frequency range. We made an automatic maser survey software and started an H20 maser survey at November 1991 for IRAS color selected objects. Until the end of January 1992, about 930 sources were observed with the rms noise level of 0.1 Jy and about 300 sources were detected.