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ICAPS

INTRODUCTION

The different steps in the formation of planetary systems are still badly known; especially the accretion of tiny dust particles to small pebbles and rocks is a badly understood phenomenon. On earth, dust particle aggregation experiments are seriously hampered by sedimentation due to gravity. This can be compensated for single sized particles, but it is impossible to compensate this for collections of particles with different sizes, as will be the case for growing particle aggregates. In order to get a better understanding of the early stages of planet formation, these kinds of experiments should be performed in an environment where sedimentation can be eliminated. The ideal place for this is the International Space Station (ISS) that offers long periods of microgravity.

The only way to compare results of experiments run in the laboratory (either in space or on earth) with astronomical observations is by looking at the effects that grains have on the scattering of the light from the central star. However, since it is not yet known what kind of aggregates will form, and because the light scattering calculations of ensembles of irregular grains are not easy to solve, the study of light scattering measurements from grains formed by accretion will significantly improve our knowledge not only in our own solar system, but also around other planet forming systems.  

The ICAPS Precursor Experiment (IPE) is based on a part of the microgravity experiment proposal (Ref. AO-99-018), which a European expert team submitted to ESA in response to the AO 98-99 on Microgravity Research and Applications in Physical Sciences and Biotechnology.

The final presentation of Phase A/B of ICAPS-Sounding Rocket Experiment (ICAPS-SRE) was given on 19 September 2003. Later the decision has been made by ESA to transfer this experiment to the ISS (either in the Microgravity Science Glovebox (MSG), or as a free standing experiment in the Russian module). The results of the above-mentioned study have been used as guideline for the document at hand.

The complete IPE set-up shall be integrated into the Microgravity Science Glove Box (MSG) on-board the International Space Station (ISS) or in the Russian Segment of the ISS. The decision shall be taken within three months after the start of the project.

 IPE shall be uploaded to the ISS in PROGRESS, ATV, MPLM or US Space Shuttle.

 

MISSION DEFINITION

Presently there are three types of experiments foreseen for IPE that are denoted as Type 1, Type 2, and Type 3 experiments.


Type 1 experiments: CODAG-type experiments.

Type 1 experiments are foreseen to augment the knowledge on Brownian motion driven dust agglomeration in molecular clouds and star-forming regions. From the CODAG missions it became clear that Brownian agglomeration studies in dust clouds not only require high-quality microgravity conditions but also the exclusion of other external accelerations. In particular, the thermophoretic motion, which was responsible for the rather rapid loss of the dust clouds in the CODAG experiments, must be reduced. Thus, an active temperature control is required for an extension of previous experiments. There is a strong need to gain additional information about

  • Dust agglomeration with different particle materials, particle sizes, particle morphologies as well as on non-monodisperse initial size distributions (e.g. bimodal size distribution for inverse runaway growth simulations)
  • Brownian motion and Brownian rotation of irregular particles (e.g. needles, agglomerates).

It is estimated that the average runtime for experiments of Type 1 will be 10 minutes. This does not take into account experiment preparation and data handling after the end of the experiment. Therefore, these 10 minutes is the expected time between dust injection and the last scientific measurement (image or LSU data).

The order of the main steps in experiment Type 1 is:

  1. Select dust sample
  2. Start cogwheel to bring the sample in the chamber
  3. When there is (enough) dust stop the cogwheel
  4. Observe the sample with the DHM/LDM,
  5. Observe the sample with the overview camera’s
  6. Observe the sample with the light scattering unit
  7. End of the experiment and clean the chamber (if necessary).

Step 4, 5 and 6 might be operated simultaneously or separately.

 
Type 2 experiments: Paul-trap experiments.

Experiments on dust clouds suffer from diffusion- and thermophoresis-driven particle loss even in the case of active temperature control for an estimated timescale of longer than 10 minutes. Thus, active capturing of dust clouds is required for long-duration agglomeration experiments (in which, e.g., very large dust agglomerates are formed). Preliminary work has shown that a 4-ring electrical quadrupole trap efficiently captures clouds of slightly charged dust particles and prevents them from being lost to the walls of the experiment chamber. With the use of a Paul trap scientifically important long-duration dust agglomeration experiments can be performed with several additional advantageous properties:

  • unlimited observation time
  • non-Brownian motion velocity field
  • further concentration of dust cloud reduces experimental timescales
  • dust cloud is always concentrated towards the same point in space (but only if other disturbing factors like thermophoresis can be minimized) so that a fixed observational volume (for microscopes and LSU) can be used
  • ideally one large agglomerate will form in the center of the Paul trap which can be investigated by microscopy and LSU.

The runtime for Type 2 experiments is much harder to estimate because it denotes the time between dust injection and the termination of the experiment. The latter might, in some cases, be determined by the formation of single, large agglomerate in the center of the Paul trap with some subsequent scientific measurements of its properties. For low concentration efficiencies of the Paul trap (wanted or unwanted), the experiment duration is basically unlimited. Thus, we give as a rough estimate 30 minutes runtime for Type 2 experiments. A better estimation of the mean duration of Type 2 experiments will only be available when IPE is operational.

 The order of the main steps in experiment Type 2 is:

  1. Select dust sample
  2. Start cogwheel to bring the sample in the chamber
  3. Turn on the Paul trap
  4. When there is (enough) dust stop the cogwheel
  5. Observe the sample with the DHM/LDM,
  6. Observe the sample with the overview camera’s
  7. Observe the sample with the light scattering unit
  8. Turn off the Paul-trap
  9. End of the experiment and clean the chamber (if necessary).

Step 5, 6 and 7 might be operated simultaneously or separately and/or might require the temporarily switching off of the Paul trap.

 
Type 3 experiments: Photophoretic experiments.

Manipulation of light-absorbing dust agglomerates is feasible and desirable. Photophoretic drift velocity increases with increasing size of dust agglomerates so that with the use of four collimated light sources (with a 1 cm diameter), large dust agglomerates can be arbitrarily moved and manipulated within the embedding dust cloud. Note that much smaller dust agglomerates, however, experience a much smaller photophoretic drift so that the embedding dust cloud is not too much distorted. Thus, experiments can be realized with the following objectives:

  • Realistic simulation of preplanetary runaway growth
  • Formation of growing single agglomerates and active positioning of these agglomerates in the field of view of the microscope and LSU

Since dust coagulation in Type 3 experiments is expected to be faster than in Type 2 experiments due to forced run-away growth, we expect that the runtime of a single experiment will be in the order of 10 minutes.

 The order of the main steps in experiment Type 2 is:

  1. Select dust sample
  2. Start cogwheel to bring the sample in the chamber
  3. Turn on the Paul trap
  4. When there is (enough) dust stop the cogwheel
  5. Observe the sample with the DHM/LDM,
  6. Observe the sample with the overview camera’s
  7. Observe the sample with the light scattering unit
  8. Turn off the Paul-trap and select a target grain
  9. Move the target grain with the optical manipulation device back and forwards
  10. Repeat step 5, 6, 7 and 9
  11. Stop the optical manipulation device
  12. End of the experiment and clean the chamber (if necessary).

Step 5, 6 and 7 might be operated simultaneously or separately and/or might require the temporarily switching off of the Paul trap. During step 10 it might be necessary to temporarily switch of the optical manipulation device (e.g. for step 7) and it also might be necessary to turn on the Paul-trap to confine the cloud again.

 

Estimation of the number of individual experiments to be performed with IPE

A conservative estimate of the amount of dust required for a single dust-cloud injection is 0.1 g. The total amount of dust expected to be dispersed in a 1000 cm 3 vacuum chamber with a gas pressure of 100 Pa is ~1mg, assuming an efficiency of only 1% which, by experience, is a lower limit to the efficiencies expected for IPE. Taken into account that a tray of dust samples will contain about 50g, at least 500 dust-cloud experiments can be performed.

Estimation of the distribution of the dust-agglomeration experiments and the resources required for their performance

The following table summarizes the estimated distribution of experimental parameters and resources within the three types of dust-agglomeration experiments in IPE.

 
Experiment overview
 
These are the initial set of experiments that the scientists would like to perform. It is envisioned that the results from this experiment will trigger new requests for experiments. In conclusion, the estimated total runtime for the first 250 dust-agglomeration experiments will be around 100 hours. Therefore the total runtime for the 500 dust agglomeration experiments will be about 200 hours (Note that this only counts the real observing time, and not the experiment preparation time).
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Created by: D. Langkowski, 23.03.2001
Last modified: 28.07.2005