When the Hayabusa2 sample return capsule entered the Earth’s atmosphere and lit up as a fireball, scientific observations were made using a variety of instruments. In this article, we take a look the close-up image of the fireball captured with the super-telephoto imaging system.
The purpose of acquiring super-telephoto images is to understand the aerodynamic heating environment that was encountered by the capsule as it entered the Earth’s atmosphere through observing the capsule and the surrounding flow of hot gas, and to confirm the behaviour of the thermal protection system that protects the capsule from the heat. Based on the findings gained from the flight result, it may be possible to improve on the design when developing new capsules for future sample return missions.
The Vixen astronomical telescope VC200L (primary mirror effective diameter is 200 mm, focal length 1,800 mm) was used as the primary optical system in acquiring the super-telephoto images, with the ZWO ASI224MC 1.2 MP CMOS camera attached. The resolution of the optical system is 0.58 arcsecond, and the image viewing angle is 0.16° horizontal x 0.12° vertical.
Accurate tracking of the fireball is essential in such super-telephoto imaging, but since the fireball moves across the sky at high speed, tracking is not possible with a commonly used equatorial mount or auto-guide. In order to track the fireball, we therefore developed an original altazimuth (two-axis) mount that can control both the horizontal and vertical axis with high precision and high speed using stepping motors. The tracking altazimuth mount uses harmonic gears to reduce backlash, and can achieve a resolution of 1.3 arcseconds on each axis and a maximum speed of 7.2° per second.
Dedicated to tracking the fireball was a telephoto camera consisting of the AstroStreet guide scope with effective diameter of 60 mm / F4, equipped with the AstroStreet color CMOS camera ToupCam. The captured image was analysed by the control computer which automatically tracked the fireball by driving the stepping motor so that the fireball was always located in the image center (PID control). However, the tracking telephoto camera had a viewing angle of only 1.1° horizontal x 0.86° vertical, so if the fireball failed to be caught in the first frame or missed any frame in the middle, then the tracking would be lost. To solve this problem, we added the Watec CMOS WAT-933 camera with a wide-viewing angle, equipped with the Computar AG3Z2812FCS-MPWIR megapixel lens. If the fireball was out of frame for the tracking telephoto camera, a function was added that allowed the altamimuth mount to be manually rotated, and the fireball could once again caught in the frame using the image from the wide-angle camera.
During the recovery operation for the Hayabusa2 sample return capsule in Australia, we attempted the super-telephoto imaging from a ground station located in front of the capsule’s planned atmospheric re-entry trajectory. Figure 1 shows the super-telephoto imaging system installed at the ground station.
We predicted the location appearance of the fireball based on the estimated trajectory of the capsule entering the atmosphere. Then we aimed the super-telephoto imaging system in that direction and waited for the appearance of the fireball. However, there seemed to be a slight deviation and we were unable to initially capture the fireball with the dedicated telephoto tracking camera. We therefore used the wide-angle camera and rotated the altazimuth mount so that the fireball could be captured in the frame of the telephoto tracking camera and successfully lock onto the fireball. We succeeded in taking super-telephoto images of the fireball until the fireball brightness became too weak and could no longer be tracked. Figure 2 shows a typical snapshot taken with the super-telephoto system.
Although the light emission from the fireball is too strong to directly capture the appearance of the capsule, the tail of the fireball could be clearly seen, and the tail size and structure of the flow confirmed. We are currently analysing all the super-telephoto images, and expect to gather new insights into the aerodynamic heating environment encountered by the re-entry capsule and the behaviour of the thermal protection systems.
Hayabusa2 Sample Return Capsule Optical Observation Team
2021.9.17 (2021.8.23 Japanese version)