Point of no return – when MASCOT separates from Hayabusa2
The date has been set! On 3 October 2018, after almost four years in space, the Franco-German MASCOT asteroid lander will separate from its Japanese mother craft Hayabusa2 and free-fall onto the surface of the asteroid Ryugu. The separation, driven by a small mechanism, will be a pivotal moment on which much depends. Once triggered, it will create a mechanically coupled chain reaction that will irrevocably initiate the mission. This is the point of no return. The way in which the mechanism functions and the possible risks of separation are briefly outlined here.##markend##
It all sounds pretty simple. The MASCOT landing module is attached to its supporting frame on Hayabusa2 via a central bolt in a clamping mechanism (non-explosive actuator). When MASCOT is ejected, this clamping device is disengaged, the bolt is released and a specially shaped push-off plate gives the landing module a small push using a compression spring, so that it slides out of the supporting frame. Ryugu's low gravitational pull does the rest of the work, pulling the lander towards it for the remaining metres in 'free-fall'.
To put this ostensibly simple principle into practice, however, the MASCOT team analysed various options during the development phase in Bremen to come up with a design that would be both compact and reliable. For this reason, the clamping mechanism was also designed as a one-shot device. This means that the operation is irreversible – there is no turning back once it has been initiated. It is triggered via a short but high pulse current, divided by two parallel channels in the form of two independent fuse wires. It is rather like triggering an electric fuse in a car. If, in the event of a short circuit, the amount of current exceeds a certain level, the fuse blows, thus protecting the other components from damage. In the case of MASCOT, these fuse wires also hold the clamping device together, which in turn keeps the bolt in place. If the wires (or at least one of them) burn through, the clamping jaws spring apart. These one-shot devices have often proven useful in space travel, as they are impervious to mechanical shocks and thermal impacts. As such, they are frequently used for releasing or disengaging rocket and satellite components.
With regard to MASCOT, the new features are the spring mechanism, the special push-off plate and the separable electrical connector that supplies MASCOT with electricity during the flight and ultimately transmits the ignition impulse. The interaction of these individual components had to be tested in a realistic, microgravity environment. We carried this out in successive test campaigns, first in parabolic flight in Bordeaux, France, and then in the Drop Tower in Bremen. Other functional tests were also performed at different test facilities at DLR in Bremen. The biggest challenge was to ensure that the landing module would be released at a defined, replicable and moderate speed of around five centimetres per second, and that it would not get stuck in the supporting frame during the separation process. If MASCOT were to get stuck, the mission would be over. Moreover, should MASCOT land on Ryugu too fast, it would bounce back up like a rubber ball due to the very low gravitational pull, and then soar off into space. The difficulty was to find and calibrate the adjustable parameters in order to set the mechanism to its precise and unalterable mode of operation on the asteroid before the launch itself.
The signal for separation is pre-programmed and given manually via a command by the Japanese team on the specified day. As such, the focus is on the 'ignition box', which is inside Hayabusa2 and triggers the impulse. This is then reliant on another pre-set event in the mission. For the separation to take place, Hayabusa2 must travel to a relative altitude of 60 metres above the asteroid's surface. When this 'trigger altitude' is reached, an automatic countdown (approximately two minutes) begins. At this point, Hayabusa2 is slowed down again, all control manoeuvres are switched off, the observation cameras are turned on and the systems involved are checked once more. Hayabusa2 now starts its 'free-fall' to the surface.
Once the countdown runs out, the last automatic sequence is initiated and the ignition box then sends the signal to MASCOT. As soon as the retaining clamp has opened and the pressure plate has pushed out a few millimetres, the electrical contacts of the connector come loose. This essentially functions as a switch, so we see this contact sensor monitor 'turn off' in our telemetry data. Whether MASCOT has actually come out of the supporting frame should then be revealed by the readings from our photoelectric cells (small solar cells) – which are illuminated by direct sunlight on the outside of Hayabusa2 – as well as the changing signal strength of our radio connection. In addition, Hayabusa2 will attempt to capture images of MASCOT during the separation, using its two wide-angle cameras. After the separation, MASCOT will fall approximately 50 metres, completely unpowered, onto the asteroid, accelerated slightly but evenly by the low gravitational field. After about 5 to 10 minutes, it will touch down on the selected landing zone (MA-9) at a speed of approximately 15 to 20 centimetres per second (seven times slower than walking pace) and is expected to come to rest after a prolonged bouncing phase. What exactly will happen during this phase and especially thereafter will be covered in blog reports by other team members in the coming days.