Both the Hayabusa and Hayabusa2 missions required a long time and high cost to return samples of material from asteroids. The sample returned is by no means large, and it is also true that we have many examples of meteorites –whose origins is thought to be the same as the asteroids—that have fallen to Earth. You might therefore wonder if we should instead analyse meteorites. However, meteorites consist of the material that is left over after re-entry through the atmosphere and their structure, along with easily volatile substances, have been lost. They will also be contaminated with substances from the Earth. On the other hand, samples from the asteroids are brought back to Earth in the same condition as they were in space, with the “re-entry capsule” protecting the sample through the atmosphere and landing.
Capsule separation:
The Hayabusa2 spacecraft is currently in the mission phase between leaving the asteroid and returning to the Earth. As the spacecraft approaches the Earth, the ion engines are used to adjust the orbit to one that just passes by the side of the planet. During the final Earth approach, a total of five orbit corrections (TCM: Trajectory Control Manoeuvres) are made using the chemical population system. TCM-1, TCM-2 and TCM-3 adjust the orbit of the spacecraft towards Australia. Then after TCM-4 and about 12 hours before re-entry, the capsule is separated. Since the separation speed of the capsule is small compared to the speed of the spacecraft, the capsule re-enters the Earth on almost the same orbit as the spacecraft. Therefore, if nothing is done, the spacecraft would also re-enter the Earth’s atmosphere. So one hour after capsule separation, the final TCM-5 is performed to put the spacecraft back into an orbit that passes by the side of the Earth. The capsule will meanwhile re-enter the atmosphere, and land in the Woomera Prohibited Area in Australia. TCM-3 and capsule separation will be carried out with the permission of the Australian Safety Officer for operations regarding the re-entry of spacecraft onto Australian land.
Figure: Spacecraft operation during the approach to Earth (image credit: JAXA)
Capsule re-entry:
The capsule has no attitude control or orbit control capabilities. The accuracy of the separation is therefore important, as no adjustments can be made after the capsule has detached from the spacecraft. The capsule is accelerated by the Earth’s gravity and eventually re-enters the Earth’s atmosphere at a speed of 12 km/s. During re-entry, the surface of the capsule becomes extremely hot, shining brightly as it reaches temperatures of about 3000°C. To protect the inside of the capsule from the heat, the outside of the capsule is encased in a heat resistant material referred to as the heat shield, which is designed to keep the inside temperature below 80°C. At an altitude of about 10km, the front and rear heat shields are separated. A parachute attached to the main body (I/M: Instrument Module) that contains the sample is deployed, and the beacon transmitting antenna is exposed. The capsule then slowly descends and lands, while transmitting the beacon signal. The parachute may be swept away by the wind, but the separated heat shield will fall without much influence from the wind to land in a different location.
Figure: Overview of the capsule re-entry (image credit: JAXA)
Once landed, the capsule should be found as soon as possible and brought back to Japan.
Reference: Although this is described as “re-entry” is does not mean the entry is actually being performed twice. Since the spacecraft was launched from the Earth and has now returned, we refer to entering the atmosphere as “re-entry”. As a converse example, the fall of a meteorite that is not of Earth origin is not called “re-entry”.
