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The Forefront of Space Science

Exploring the Interior of the Moon and Mars with Seismometers
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The next challenge is vibration and temperature variation. The pendulum needs to be sustained softly without friction, but robust to vibration. To find a solution to these contradictory requirements “soft and robust,Ewe checked various types of pendulums by vibrating them in launch waveforms. At the beginning, our attempts failed. Plate springs to softly sustain the pendulum bent or sheared due to vibration. By investigating the cause, we found a solution: eliminate the force that bends the plate spring and the large vibration to shear it. Thus, a spring works softly in the range of small vibration while a stopper controls it in the range of large vibration. In this way, we were able to solve the pendulum problem. A laser source is vulnerable to heat, so our solution was to send light to the seismometer via optical fiber. We tested to verify that the laser interferometer and optical fiber can operate in the temperature range of E0 deg. C to +300 deg. C. Figure 2 (b) shows the second model, which combines the “soft and robust pendulumEand the “optical fiber-type interferometer.EWith this model we confirmed the performance in the quiet seismic observation tunnel by comparing with the conventional broadband seismometer. We were also able to perform its initial adjustments automatically by a built-in computer, which had been previously manual. We have moved a step closer to the seismometer for the Moon and Mars.

Future development

At present, we are manufacturing and testing the third small-size seismometer (approx. 10cm cubic model) as shown in Fig. 2 (c) assuming the actual exploration. Until now we have performed tests for each function or performance. With this model, we now plan to confirm whether or not vibration and temperature variation affect the entire seismometer. We also need to devise how to deploy our seismometers. For the Moon, some methods are under consideration, including putting the seismometer inside a thermal insulation cover to prevent temperature changes or installing it in a penetrator to dig and burrow itself underground like a mole. For Mars, some means are naturally required, including burying the seismometer in the ground using penetrator or, when deploying it on the surface, using a cover to reduce the influence of the wind and temperature changes. Figure 3 shows our test to study how the wind effect varies depending on shape of cover. Applying wind of approx. 30 m/s speed under 0.1 atm to measure the force imposed on the cover, we found the shape with the highest wind resistance.

Figure 3
Figure 3. Wind-tunnel test simulating the surface of Mars
Force imposed on the cover (center) was measured when the wind of approx. 30 m/s under 0.1 atm flowed from upper left to lower right.

Before actually implementing the seismic exploration on the Moon and Mars, we need to clear many hurdles. With the cooperation of ISAS staff, we were able to use ISAS facilities for vibration, temperature, and wind tunnel tests. Detection performance was assessed at the observatory of the Earthquake Research Institute, University of Tokyo. Domestic manufacturers fabricated trial models of the laser interferometer and seismometer. Research on background free oscillation and R&D on penetrator/mole-type mechanisms are underway by Japanese researchers. I believe that, with the collaboration of experts in respective fields, the target seismometer can be produced and an original Japanese exploration plan can be set up by a domestic team. This “seismometer technology for extreme environmentsEcan be applied not only to the Moon or Mars, but also to terrestrial seismic observations in high-temperature environments of deep underground near the quake sources. Bringing together knowledge and technology, I wish to develop the core technology for future planetary explorations and earthquake studies.

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