Japan today announced it will be accepting proposals for payloads to fly aboard the third in a series of experimental satellites designed to test new satellite and science technology.

The Japanese space agency will accept "onboard mission applications" from August through October. This is the first step in an evaluation process to determine what components and experiments will be placed aboard the planned orbiter. The agency will consider proposals from Japanese corporations and research groups, and foreign proposals affiliated with Japanese organizations.

The satellite, to be called Mission Demonstration test Satellite 3 (MDS-3), is being suggested for launch into a low- or medium-altitude orbit in 2004. It is expected to weigh somewhere between 1,100 and 1,760 pounds (500 to 800 kilograms), although its specifications will be determined by the experiments chosen for the mission.

MDS-3 will follow Mission Demonstration test Satellites 1 and 2, which are tentatively scheduled for launch in 2001 and 2002, respectively. The test-satellite series is geared toward developing technology for Japan's emerging space and satellite programs.

The first of these orbiters, MDS-1, will carry several packages designed to find the best way to build a satellite. Among these is an experiment that will test the impact of solar radiation on various semiconductor devices. Other components will test an assortment of solar cells, batteries, and a spacecraft data recorder. Even the performance of various computer systems and data processing schemes are being put to the space test. Finally, the satellite will take solar radiation and magnetic field measurements.

MDS-2 is completely different. It is essentially a science mission that will test a laser radar system for characterizing the three-dimensional distribution of clouds in Earth's atmosphere. This is important to scientists studying the global warming, and others who need information about the heat budget of the atmosphere.

Japan's demonstration satellite series is the most unconstrained program in its space agency. The objectives are wide open, so it is possible that the third in the series will be something altogether different again.

The window for submitting onboard mission applications for MDS-3 will close Oct. 15.