In order to mitigate the overcrowding problem of geosynchronous orbits, we propose a monitoring satellite placed in a sub-synchronous, retrograde circular orbit. The monitoring satellite has an on-board optical sensor, and observes look-angles of the target geosynchronous satellites that come into the field of view one after another. This kind of monitoring makes it possible to determine the orbit of every target satellite in a short term. Covariance analyses show that the target position determination can be accurate to 350m. The monitoring satellite's orbit determination will be obtained from ranging at two ground stations, with a sufficient accuracy because range biases can be estimated.

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In this paper we discuss the use of a monitoring camera on board a satellite to detect unknown satellites coming so close as to cause collision risk. We assume that a camera on board our satellite tracks direction angles of an unknown target satellite and try simulations of determining the target's trajectory relative to our satellite. Simulations show that we cannot determine uniquely the target's trajectory, while showing that we can decide whether we need a collision avoidance maneuver and can determine its strategy when we need it. The on-board camera does not need precise alignment, as the bias in direction angles can be estimated as an unknown parameter. On-board monitoring can thus be practical for orbital risk avoidance.

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In order to estimate the velocity field induced by a trailing vortex which is located along the free-stream direction in supersonic flow, we extend the Biot-Savart law, that is originally proven for subsonic flow, to supersonic flow. During the derivation we use the linearized perturbation potential equation that can be applied to both subsonic and supersonic (inviscid) flows. It is shown that our current formula reduces to the original Biot-Savart law when the flow speed reduces from supersonic to subsonic one. It is also shown in the appendix that the Kutta-Joukowski theorem is still applicable to supersonic flows within the supersonic thin-airfoil approximations.

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In future space developments, the welding in space may be required for the repairs of the ISS and the constructions of lunar base and space structures. The authors have studied the space Gas Hollow Tungsten Arc (GHTA) welding process since 1993. This paper describes the results for space applying the space Diode Laser (DL) welding process which the authors proposed in 2002. It is necessary to prevent the metal deposition on optical devices in order to utilize the space DL welding process in space. The authors studied the preventing technique of metal deposition which covered optical devices with the nozzle and blew the shielding gas out from nozzle outlet. The metal deposition can be reduced by supplying the nozzle with inert gas and blowing the gas out from nozzle outlet. The shielding gas argon perfectly prevents the metal deposition on optical devices when argon pressurizes the nozzle to over 19.9 Pa and spouts out from the nozzle outlet.

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