C. De Beule, T. Kelling, G. Wurm, J. Teiser, T. Jankowski, (2013), "From planetesimals to dust: Low gravity experiments on recycling solids at the inner edge of protoplanetary disks", Astrophysical Journal, 763, 1, DOI: 10.1088/0004-637X/763/1/11, pp. 11.
This experiment was performed by the Dustbrothers team within ESA's educational programme: "Fly Your Thesis! - An Astronaut Experience". The Fly Your Thesis! programme gives university students the possibility to fly their scientific experiment in microgravity, as part of their Masters thesis, PhD thesis or research programme, by participating in a series of parabolic flights. In total, three teams of postgraduate students were flying their experiments during the 2012 'Fly Your Thesis!' campaign.
BACKGROUND Planet formation takes place in a protoplanetary disk around a young star. It consists mainly of gas and about 1 % of dust and dust aggregates. These dust aggregates are very porous in the first steps of their formation which is very important for the understanding of the forces they are affected by.
One force is delivered by the Knudsen Compressor effect (KCE). It describes a gas flow through small tubes. If the walls have a temperature gradient along the axis of the tube there is a gas flow called thermal creep along the walls directed to the warmer side. Making the channels very small in diameter all the flow consists only of thermal creep but no pressure driven backflow. Because of the conservation of inertia a porous aggregate having microchannels heated on one side is subject to the KCE and thus experiences a force away from the heat source.
OBJECTIVE It is very important to quantify the influence of transport mechanisms for particles in protoplanetary disks to understand the process of planet formation. It depends on these mechanisms whether a particle falls into the star and gets lost as planetary material or stays in the disk and is available for collisions forming a planet.
In this experiment we want to measure the magnitude of this kind of microrocketdrive acting on a porous aggregate consisting of μm-sized particles.
Because the Knudsen Compressor effect only works at low pressure (as present in a protoplanetary disk) we use a vacuum chamber for the experiment. Inside are two surfaces which we can heat or cool to establish a temperature gradient. The sample is placed between these surfaces. It is a plate of 40 mm diameter, a thickness of around 1 mm and consists of sintered glass beads with a diameter of 40-60 μm so they are still a highly porous aggregate with many small microchannels. At 1g the plate will just lie on the bottom surface which will be heated. The force is much too small to lift the sample during normal gravity, but as soon as the acceleration force of the residual gravity gets smaller than the force delivered by the KCE it will begin to float in direction to the cooler surface which is at the top of the chamber. During each parabola there are usually a few changes in direction of residual gravity so we have positive and negative accelerations and each time it changes we can take a measurement.
During the whole parabola we will use a camera to observe the movement of the sample and at the same time an accelerometer measures the current residual gravity. By linking both data we can calculate the acting force.
During the three flights we will switch the samples, the pressure in the chamber and the strength of the temperature gradient between the two surfaces.
APPLICATIONS OF THE RESEARCH This project is a small part to get a better understanding of the process of planet formation. Only if we are able to predict which kind of material takes part in the different stages this can be achieved, so it is very important to know how the transport mechanisms in a protoplanetary disk work.
Figure 1: Illustration of the Knudsen Compressor Effect - KCE.
Figure 2: Sketch of the experiment set-up.
Photo 1: Dustbrothers student team and their experiment. ESA Fly Your Thesis 2012 Campaign. Credits: ESA
Photo 2: The Dustbrother's experiment installed in the plane. Credits: ESA-J.Makinen
Photo 3: The Dustbrothers team with their experiment rack inside the aircraft. Credits: ESA-J.Makinen
Photo 4: The Dustbrothers team during a microgravity phase of the flight. Credits: ESA-A.Le Floc'h
Photo 5: Dustbrothers' computer receiving data during flight. Credits: Dustbrothers team