Search for the re-entry capsule using the beacon
The expected landing area for the capsule covers over a 100 kilometer-squared region. This large space could accommodate multiple Tokyo metropolitan areas, but then the separation of the capsule was performed at a distance of about 220,000 km from the Earth. Since the distance to the Moon is about 380,000 km, this is the equivalent of throwing a ball from more than half the distance to the Moon to a 100 km-sq spot in Australia, so the control is really great.
However, a capsule on the ground in the dark will be very difficult to find. Therefore, we will search for the beacon signal, as mentioned in the previous article. Five antennas will be installed around the expected landing site, which will each record the direction of the signal. These directions will be reported to the headquarters and when plotted on a map, the intersection will reveal the position of the signal source (principal of triangulation). This method is the same as used for Hayabusa, but for Hayabusa2, considering the area we needed to cover and potential troubles, we decided to increase the number of stations by one and search with a total of five stations.
But even when the signal source is identified by a ground station, its location can only be narrowed down to a few kilometres square. Additionally, the capsule may be hidden from a ground station by the horizon and that station would be unable to receive the signal after the landing. Therefore, a helicopter is also equipped with a beacon receiver.
Backup capsule search method
While we hope everything will go well, we have also prepared for problems to arise. For Hayabusa2, we introduced some additional methods that can be used to search for the capsule. One of these is marine radar. The capsule is too small to detect by this method, but the parachute can be located. This can be searched for even if the beacon signal is not being emitted due to a problem. For this method, four stations will be deployed around the expected landing area.
Also, as with Hayabusa, optical observations of the light emitted during re-entry will be performed. The direction of the emitted light can be measured from multiple locations to determine the re-entry trajectory using the principal of triangulation. If the sky is cloudy, then this will not be possible to measure so we will also observe by aircraft. By extending the orbit of the trajectory, you can predict the landing point by assuming that the wind has little effect. If there is an abnormality during the heat shield separation, the parachute will not open and no beacon signal will be transmitted, so the results of the optical observations will be the most reliable.
Hayabusa2 will also introduce a drone search. The drone can fly at a programmed latitude and longitude and take a continuous sequence of pictures. If none of the previous methods have worked, or if the capsule cannot be found when searching an area, the huge number of photographs from the drone can be analysed to find the capsule.
Work after capsule discovery
Once the capsule is found, it will not be approached immediately. Pyrotechnics are used to open the parachute and separate the heat shield, so first a safety check is needed for any non-ignition. After safety verification and confirming there has been no damage to the capsule or leakage of the sample and with permission from an Australian safety officer, the capsule will be taken to headquarters by helicopter for cleaning and gas sampling in a clean room. Since the sample container may contain a small amount of gas emitted from the Ryugu sample, a simply analysis will be conducted on-site before it is contaminated by the Earth’s atmosphere. If the sample itself is able to be collected before it is oxidized, it will be brought back to Japan by charter flight before the team members. In Australia, the capsule itself will not be opened.
I am anxious and excited about how much sample has been collected, but if we find something in the gas analysis, my expectations for the results will grow greatly. From now on, the on-site work will begin.
Hayabusa2 Project Satoru Nakazawa