Ken-ichiro MAKI

Microwave emissions due to hypervelocity impacts have been detected by heterodyne receivers in the ground experiments using an accelerator. We aim to establish the detection system of space debris impacts on a space structure via microwave. The emitted powers at several frequencies due to the impact velocity of 10km/sec are estimated from the characteristics at the experimental impact velocity. Considering the emitted power, the receiving antennas and the microwave frequencies, the maximum distances for detecting the microwave emission are 85m at 2GHz-band and 24m at 22GHz-band. The type and location of receiving antennas which can detect the impact on all habitation and experiment modules on the International Space Station are discussed. It is concluded that the impact can be sufficiently detected via a low gain antenna at 2GHz band.

Microwave emission was found when materials were destroyed by a hypervelocity impact or by a static pressure. This paper describes the experimental setup to observe the phenomena, the obtained terms and results, and the possibility to apply the phenomena to geophysical explorations. As this field may not be familiar to most readers and the special techniques to receive and measure impulsive microwaves are required, the overall understanding is pursued instead of detailed description. In the receiving system, microwave signal is first amplified by a low noise amplifier, digitized in a sampling frequency high enough for the observed frequency, and then stored as data. The observed frequency was 22GHz, 2GHz, 300MHz and 1MHz. If the data storage capacity is too small to keep the data, namely at 22GHz and 2GHz, the signal is converted to a lower frequency by a heterodyne receiver and then processed to data.<br> In the impact experiment, the velocity is 7 km/sec at maximum. Target material was selected from metal such as aluminum or iron, ceramic, brick or rubber. In the destruction experiment due to a static pressure, four kinds of rocks were pressed with a compressor. The observed microwave is intermittent quite narrow pulses in every destruction mode. In the rock destruction due to a static pressure, 22 GHz was detected only from quartzite. As the waveforms thus obtained are almost sinusoidal in shape, we can calibrate the power through the receiving system. As a result, the average emitted power at 2 GHz was 2.7x10^-5mW and 2.7x10^-8mW in the hypervelocity and static pressure experiments, respectively. The cause of microwave emission is inferred to be the dissociation of atoms or molecules, but is not yet completely confirmed. Currently, the phenomena are expected to be applied to geophysical explorations in the following fields:<br> (1) Research of material characteristics: celestial body impacts, material science, space debris issues.<br> (2) Change of the underground structure: rock crush.<br> (3) Earthquake detection.<br>

Electromagnetic emissions observed in a series of rock fracture tests are described. Four kinds of rocks, basalt, gabbro, granite and quartzite were pressed by uniaxial compression to fracture, for all of which many signals were detected at two microwave bands (2GHz and 300MHz). These detected signals consist of intermittent pulses of a short duration. Comparing the microwave records and the observation with a high-speed digital video camera, we found that the pulse signals were generated after the decrease of the axial load, and even after the macroscopic fracture (deformation) was completed. This differs from the occurrence of lower frequency emissions (0.3-300kHz) monitored as well, which became active and was strongest during the load decrease. The occurrence of signals at the two microwave bands did not always coincide, but a signal at 300MHz often followed a signal at 2GHz with a short interval of 50-100ns. An additional detector at 22GHz picked up emissions only for quartzite, which occurred exclusively during the decrease of axial load.