The exploration of the universe and thus also of celestial bodies is progressing steadily. Although the public is primarily familiar with research work on celestial bodies close to Earth, such as the Moon and Mars, the investigation of small cosmic bodies is also becoming increasingly important. In the recent past, there have already been several missions to study comets and asteroids, such as the DART and Rosetta missions – but the planning and costs involved are very high.
Comet research in the Einstein-Elevator
However, a space mission is not always necessary to research the properties of these small bodies and investigate their interaction with external environmental conditions.
The Institute of Transport and Automation Technology (ITA) at Leibniz University Hannover, together with the Institute of Geophysics and Extraterrestrial Physics (IGEP) at Technische Universität Braunschweig, has launched the AKUS project. The acronym AKUS stands for “Aktivität von Kometen unter partieller Schwerkraft” which translates as “Activity of Comets under Partial Gravity”. This is intended to enable comet research in the Einstein-Elevator.
The aim is to better understand the surface activity of comets, which results in the tail, among other things (see Figure 1). In particular, the loss of mass due to solar radiation, considering the prevailing gravitational levels of these celestial bodies, is to be taken into account. This will allow conclusions to be drawn about the formation of planets in our solar system on the one hand and about changes in the orbits of comets on the other.
Investigation of comet-like samples under realistic gravitational conditions
In order to be able to simulate the behaviour of small bodies in the AKUS project, the scientists from Braunschweig are using the Comet Physics Laboratory (CoPhyLab) to produce comet-like samples. One such simulation sample consists of a solid ice surface covered with a self-made layer of dust in order to best reproduce the surface of a comet.
The ITA, on the other hand, is working on the development of a propulsion system to realize gravitational conditions in the range of 10-2 to 10-4 g, similar to the conditions that prevail on comets and other small cosmic bodies. Depending on the increasing temperature and the resulting gas pressure of the sample and the gravitational level, the dust ejection is to be observed.
Basis for research into the mining of small cosmic bodies
The fact that the exploration of asteroids and comets is becoming increasingly important is also demonstrated by the asteroid sample collected by NASA’s OSIRIS-REx on September 24, 2023. The probe collected a rock sample on the asteroid Bennu, which contains water-binding clay minerals, among other things.
With the successful implementation of the AKUS project, Hannover can also become a location for research into small cosmic bodies. However, this would not only enable basic physical research. Research projects dealing with the mining of raw materials on these celestial bodies – so-called space mining – would also be conceivable in the future thanks to the implementation of the AKUS project.
The Einstein-Elevator and its special features
The Einstein-Elevator at Leibniz University Hannover provides the necessary basis for the AKUS joint project. This is a third-generation free-fall tower and enables the simulation of microgravity (“weightlessness”) as well as other adjustable gravitational conditions. The three main core components of the Einstein-Elevator are the linear drives, the gondola guide and the gondola itself, in which the experiment carrier and thus also the experiments are located.
To generate microgravity, the gondola is first accelerated at 5 g for 0.5 s. The experiment carrier is then decoupled from the gondola floor and the so-called free fall occurs, in which microgravity is simulated for 4 s. This is followed by the deceleration phase with 5 g, which in turn also lasts 0.5 s.
In addition to the tower-in-tower construction, which separates the guidance of the gondola from the drive guidance, the quality of the microgravity is additionally increased by generating a vacuum of 10-2 mbar within the gondola. This enables acoustic decoupling of the experiment from the gondola environment. The experimental setups mounted in the experiment carrier, on the other hand, can be used in a normal atmosphere, as the experiment carrier can be fitted with a pressure envelope if required.
Minimum accelerations during weightlessness
In addition to microgravity, only simulated gravitational conditions in the range of 0.1 to 5 g can be implemented in the Einstein-Elevator. The samples are therefore to be accelerated during the free-fall phase with the aid of an additional drive concept. The successful implementation of this project would extend the usable range of simulated gravitational conditions in the Einstein-Elevator to very small gravitations.
The AKUS experiment itself consists of a vacuum chamber in which the comet-like sample is located (see Figure 2). The chamber is designed for a vacuum of up to 10-6 mbar. The sample is connected inside the chamber with vacuum-compatible spindle axes. Moreover, the axes are driven by motors located outside the vacuum chamber and are connected to the associated rotary feedthroughs. This avoids the need to use vacuum-compatible motors. Another advantage is that the size of the vacuum chamber and thus the total weight of the experiment is reduced. This is because the permissible total mass of the experiment of 1000 kg must not be exceeded. For this reason, the chamber and the internal components must be as compact as possible.
In this context, the loads and vibrations that arise and affect the components, particularly during the 5 g phase, must also be taken into account and minimized. Furthermore, the centre of gravity of the overall structure must be in the centre in order to avoid rotations or translations of the experiment carrier. If these influences are not reduced well or early enough, the 4-second time window of the experiment is shortened.
Examining the sample with cameras and sensors
An accelerometer designed for the low acceleration range is used to measure and control the acceleration. The dust ejection of the sample is observed using two cameras, one on the side and the other above the sample. In addition to these, a temperature sensor is used to continuously measure the sample temperature.
Besides the simulative works, a vacuum test with the planned rotary feedthroughs will be carried out this year once the vacuum chamber (see Fig. 3) has been received. Further preliminary tests, in particular of the drives, are planned for 2024.