Infrasound and Seismic Observation of the Hayabusa Reentry: Burst Signals and Air-to-Ground Coupling Process

The Hayabusa, the world's first sample-return minor body explorer, reentered the Earth's atmosphere on June 13, 2010. This was the third direct reentry event from the interplanetary transfer orbit to the Earth at a velocity of over 11.2 km/s, and was the world's first case of a direct reentry of the spacecraft itself from the interplanetary transfer orbit. This was the very good and rare opportunity to study bolide class meteor phenomena by various aspects. Multi-site ground observations of the Hayabusa reentry were carried out in the Woomera Prohibited Area, Australia (Fujita et al., 2011). The observations were configured with optical imaging, spectroscopies, and shockwave detection with infrasound and seismic sensors. At three main stations (GOS2, GOS2A, and GOS2B), we installed small aperture infrasound/seismic arrays, as well as three single component seismic sub stations (GOS2B-sub1, to GOS2B-sub3) (Yamamoto et al., 2011; Ishihara et al., 2012). The infrasound and seismic sensors clearly recorded sonic-boom-type shockwaves from the Hayabusa sample return capsule (Ishihara et al., 2012). In addition, following capsule signal, lots of signals that probably correspond shockwave from disrupted fragments of spacecraft and energetic bursts of the spacecraft were also recorded (Yamamoto et al., 2011). In this study, we analyze signals generated by hypersonic motion of the disrupted fragments and energetic burst of the spacecraft. In addition, we examine the air-to-ground coupling process by comparing the waveforms computed by finite difference scheme with the actual ones. At all three arrayed main stations, after the capsule's shockwave arrival, we detect multiple shockwave signals by both infrasound and seismic sensors. For some of these signals arrive within 10 seconds after capsule's signal, we can identify one to one correspondence with optically tracked disrupted fragments of the spacecraft. Far after the capsule's signal, we also detect some arrivals of wave packets. Based on the arrival time and the slowness of the signals, we identified one of these signals as the shockwave signal that corresponds to the energetic burst of the spacecraft at altitude of 57.3 km, which was recorded by video. However, we could not identify a lot of signals as the direct arrivals of the burst and sonic-boom-type shockwave. Some of those unidentified signals were probably related the multipath phases of the burst and sonic boom shockwaves. To study the air-to-ground coupling, we compare the observed waveforms to synthetic waveforms computed by 2-D finite difference scheme. For the actual seismic data, we can find precursor wave packets slightly prior to the direct-coupled wave from the capsule at GOS2 and GOS2A station. On the other hand, GOS2B and sub stations did not recorded distinct precursor wave. According to the synthetic waveforms, apparent velocity of the incident air-pressure wave controls the existence of the precursor wave prior to the direct coupling. When the apparent velocity of the incident pressure wave is almost identical to the phase velocity of ground surface wave, the surface wave is excited efficiently as precursor wave. Namely, for GOS2B station, the elevation angle of the incident shockwave is high. Therefore, the apparent velocity of the shockwave is too fast to generate the precursor surface wave.