The second International Microgravity Laboratory (IML-2) mission was a truly international event, paving the way for cooperation in the scientific utilisation of the forthcoming International Space Station. ESA provided several major facilities for conducting research in microgravity, as did France and Germany. The facilities were used jointly with scientists from the US; in exchange, NASA provided the mission itself. Several experiments measured and characterised the microgravity environment and the astronauts' response to spaceflight conditions. Others were in the fields of biology, biotechnology, fluid dynamics, crystal growth and alloy solidification, and near-critical-point investigations.
Another important objective of the mission was to demonstrate remote payload operations or telescience'. Using that method, principal investigators monitored and controlled their flight experiments from various user centres and laboratories across Europe. This approach is proving to be a very efficient and cost-effective way to conduct and optimise scientific research, and it will become increasingly important as long-duration access to space becomes available.
The Space Shuttle Columbia with the second International Microgravity Laboratory (IML-2) with Spacelab on board was launched on 8 July 1994 at 12:43 a.m. Eastern Daylight Time. Columbia returned safely on 23 July at 6:38 a.m. after a very successful mission. With a flight duration of nearly 15 days, it was the longest Shuttle mission thus far and was also a milestone for microgravity research and international cooperation.
NASA had invited its international partners to participate in this mission under very favour-able conditions, namely the joint utilisation of flight experiment facilities delivered by the partners and of the mission itself provided by NASA, as was the case with the IML-1 mission which took place in January 1992.
The crew of seven astronauts (Fig. 1A), with Robert D. Cabana as commander, James D. Halsell as pilot and Richard J. Hieb as payload commander, faced a very complex mission. The mission specialists, Carl E. Walz, Leroy Chiao, Donald A. Thomas and the Japanese Chiaki Naito-Mukai, a medical doctor, had an extremely tight schedule. They worked in teams in shifts around the clock to accomplish the large number of diverse experiments. The flight crew was supported from the ground by the alternate payload specialist, J.J. Favier (Fig. 1B) from the Centre d'Etudes Nucleaires (CEA) in Grenoble.
Some 82 experiments from 15 different coun-tries and involving about 200 scientists were conducted. The experiments covered a wide range of scientific domains such as human physiology, biology, biotechnology, crystal growth and alloy solidification, fluid dynamics, near-critical-point phenomena and technology.
Microgravity, the key parameter of these experiments was well characterised. Different sensors measuring residual accelerations and vibrations (g-jitter) were attached to the payload at various locations. By orbiting the Earth at an altitude of almost 300 km and with operational conditions optimised with regard to g-level perturbations, low residual accelerations were obtained. Data measured with the German accelerometer QSAM is shown in Figure 2. The overall 'noise level' between 0 and 50 Hz was of the order of one milli-g (1 milli-g = 10 to -3 go ,g o being the gravitational acceleration experienced on the surface of the Earth) (Fig. 2A). Between 0 and 20 Hz, which is the most critical frequency range for experiments, the perturbations did not exceed a few micro-g (1 micro-g =10 to -6 go ) (Fig. 2B). These were almost perfect conditions for investigations in life sciences and physical sciences under near-weightlessness, the primary objective of this mission.
The facilities used in the IML-2 mission are listed in Table 1. Europe's contribution to this mission was very important. ESA provided the Bubble, Drop and Particle Unit (BDPU), the Critical Point Facility (CPF), the Automated Protein Crystallisation Facility (APCF) and the Biorack. France provided RAMSES for free-flow electrophoresis, and Germany contributed TEMPUS for electromagnetic levitation processing, NIZEMI for microscopic observation of biological samples in a slow-rotating centrifuge, Biostack for investigating the response of biological samples exposed to the radiation environment, and QSAM for measuring residual accelerations.
A first review of the preliminary results of the experiments, which was organised by NASA's Mission Scientist R.S. Snyder, took place at the European Space Operations Centre (ESOC) in Darmstadt, Germany, on 1 and 2 November. The feedback from the principal investigators was very positive, and it seems that most of the science objectives were accomplished. However, a thorough evaluation of the data and samples obtained will require at least 8 to 12 months and, after the final results are known, a full assessment of the scientific accomplish-ments can be made.
Figure 1A. The flight crew. Top (left to right): R. Hieb, C. Mukai, R. Cabana, L. Chiao, J. Halsell. Bottom: C. Walz, D. Thomas
Figure 1B. J.J. Favier, the payload specialist who assured the link between the flight crew and the investigators on the ground
Figure 2. Residual accelerations and g-jitter measured with the German accelerometer QSAM
A. The overall amplitude in milli-g (10 3 g) between 0 Hz and 50 Hz as a function of time
B. Perturbations as a function of frequency. The peak at 22 Hz corresponds to the eigenfrequency of the experiment rack, while the peak at 49.1 Hz is due to an as yet unknown source, such as a mechanical pump (Courtesy of H. Hamacher, DLR, Cologne, Germany)
The primary emphasis in the Life Sciences was on Space Biology. Of the 33 experiments using biological specimens, 19 were performed in ESA's Biorack, a multi-purpose facility designed to investigate the response of cells, tissues and plants to weightlessness and cosmic radiation (Table 2). The Biorack has three incubators for experimentation, a glove-box for handling samples, and a cooler-freezer for preserving samples for analysis in the laboratory after the mission for comparison with reference samples produced on Earth.
Both the Biorack and the coolers, which flew for the first time, performed very well. There were only two problems. One was in the control electronics of a centrifuge in one of the incubators; it was solved by activating the back-up operations mode. The other was a malfunction of the Biorack videocamera; that videocamera was replaced by one of the Spacelab videocameras.
The crew devoted a total of 42 hours to the operation of the Biorack. They transferred experiment containers between incubators, cooler-freezer, glovebox and stowage about 1100 times.
Of the remaining 17 biological experiments, seven were carried out in DARA's NIZEMI, a slow-rotating centrifuge equipped with a microscope; four were performed in NASDA's Aquatic Animal Experiment Unit and three others were carried out in NASDA's Cell Culture Kits. Investigations on human spinal changes in microgravity and radiation studies were also undertaken, using NASDA's real-time radiation monitoring device and DLR's Biostack.
Investigations concerned with fluid dynamics and capillarity made use of ESA's Bubble, Drop and Particle Unit (BDPU). This instrument, which flew for the first time on IML-2, is dedicated to the study of the behaviour of bubbles, drops and particles in transparent liquids. The core unit is equipped with sophisticated optical diagnostics. In addition, there are modular containers with specific diagnostics and stimuli tailored to the needs of the individual experiments. These slide-in units are interchangeable in orbit.
Seven experiments were performed using the BDPU (Table 3). They investigated:
There were problems in executing two of the seven experiments. The problems, however, were related to the experiments themselves; the BDPU performed flawlessly.
The BDPU ground team, with engineers and scientists at NASA's Marshall Space Flight Center, was responsible for the BDPU operations. The team was in contact with the investigators in their laboratories in Europe who were evaluating data and video images in real time in order to assess the performance of an experiment and to define further procedures to optimise an experiment run. This 'telescience' operation adds great flexibility to experimentation, and enhances considerably the scientific return since the investigators, who are the real experts, are actively involved in the experiment. Initially, 104 hours of operation of the BDPU were foreseen. That number was later extended to 146 hours.
The evaluation of the experiments conducted with the BDPU is complex and the principal investigators have not yet received all the data and images. However, some preliminary conclusions can be drawn:
In summary, these highly specialised experiments were very successful. The full results are expected to be presented at the IXth European Symposium on Gravity-dependent Phenomena in 'Physical Sciences' in May 1995 in Berlin.
Near-critical-point phenomena in fluids ESA's Critical Point Facility (CPF) consists of a very precisely controlled thermostat with interchangeable fluid cells, cameras for observation and light-scattering diagnostics. It was first flown on the IML-1 mission in January 1992. The experiment results obtained during that mission encouraged NASA to include the CPF in the IML-2 payload as well.
A critical fluid is in a physical state that is distinctly different from a gas or a liquid. It is very interesting scientifically since its physical behaviour is unique. A critical fluid is highly compressible. Conse-quently, on Earth, one can achieve the critical state at a precise critical pressure and temperature in a very thin layer of the fluid only. The weight of the liquid itself prohibits establishing constant pressure conditions vertically. This problem is eliminated in microgravity and bulk samples of critical fluids can be investigated.
As is the case with the BDPU, the CPF has modular slide-in units, which are tailored to provide the optimum conditions for each individual experiment. Five of the units accommodated experiments from American and European investigators (Table 4). The facility performed flawlessly. It was operated for 312 hours and the objectives of the experiments were fully accomplished. The information obtained was greatly enhanced by the facility being operated in a telescience mode. The ground team at NASA's MSFC transmitted about 1200 commands to the payload crew and the principal investigators had access in real time to the important data including video images.
The evaluation of the data is underway and substantial new insight into the physical behaviour of fluids near their critical point is expected.
One example of a new physical phenomenon that was discovered through experimentation in microgravity is concerned with the heat transfer in a supercritical fluid (SF 6 ). Following a heat pulse from a point source, the temperature rise in the bulk fluid is monitored at various locations with thermistors, as a function of time (Fig. 3). The classical conduction of heat by thermal diffusion occurs and, in addition, a much faster heat transfer mechanism, the piston effect', is observed. The latter is due to the expansion of a thermal boundary layer acting as a piston, which pressurises and heats adiabatically a confined fluid.
Figure 3. Thermalisation in microgravity of a supercritical fluid (SF 6 ) after a heat pulse. T1, T2 and T3 are measuring thermistors. The thermalisation by the piston effect is fast and complete, whereas the peak temperature due to heat conduction is reached much later. (Courtesy of D. Beysens, CEA, Saclay, France)
Crystals of biological macromolecules, such as proteins and enzymes, are required to determine a substance's precise molecular structure and atomic arrangement using X-ray diffraction. That information is necessary to understanding the biochemical reactions and mechanisms, including diseases, in living organisms. The obstacle in determining the structure is the preparation of single crystals of sufficient size (0.1 to 0.5 mm) and sufficient quality. Although the underlying mechanisms are not yet understood, there are strong indications that crystallisation is enhanced in microgravity. Since protein crystallisation is an extremely important issue both scientifically and for medical applications, scientists around the world have endeavoured to evaluate and exploit microgravity.
ESA's Advanced Protein Crystallisation Facility (APCF), which had already flown on Spacehab-1 in June 1993, fits into the Shuttle middeck lockers. Two units were flown on IML-2, each held 48 crystallisation reactors contained in a thermostat at 20 degC. Video-cameras were used to observe the crystallisation process in selected reactors (12 per unit) and a simple light-scattering device was used to detect nucleation. Both APCF units performed nominally and some 7000 video images were recorded. The units were active for 12 days. After landing, the reactors were returned to the investigators and the first crystals were harvested. Although the diffraction analysis is just beginning, photo-graphs of crystals are already available (Fig 4 and Fig 5). Based on a preliminary evaluation, the crystals collected appear to be very promising. Numerous crystals were obtained and some appear to be of excellent quality.
Figure 4. A flawless single crystal of lysozyme, grown in the APCF on IML-2. This model substance is investigated to quantify improvements in protein crystal growth in microgravity. Approx. 1.8 mm in length. (Courtesy of J.R. Helliwell, University of Manchester, UK)
Figure 5. A single crystal of ribonuclease S grown in the APCF on IML-2. The first, qualitative assess-ment made was it is one of the best crystals ever seen'. (Courtesy of L. Sjoelin, Chalmers Technical University, Sweden)
Table 5. APCG experiments
Building on experience gained in Europe with the remote operation of experiments on sounding rockets, Eureca, Spacelab D-1 and D-2, as well as Atlas 1 and 2, an important element of the IML-2 mission was the demonstration of telescience or remote opera-tions. ESA is keen to develop these operational techniques and procedures for science. They will take on great significance when the International Space Station becomes operational. A dedicated article on Implementation of a Communications Infrastructure for Remote Operations' is included in this issue. Only a brief summary is therefore presented here. It emphasises the experimenter's point of view.
The telescience operations on IML-2 were not just another series of experiments or technology demonstrations, but rather a service to scientists - the role of remote operations is to support scientific experiments. Remote operations give scientists and engineers in Europe the opportunity to participate actively in payload operations during the performance of their experiments. Such direct participation, where commands can be issued from User Centres, means that modifications and changes in schedules can be made in real time, while the experiment is underway. Another significant advantage is that experimental data can be received directly at the laboratory or institute, giving the scientists immediate access to the data for evaluation and reaction.
Communications - including video, voice, low-rate and high-rate - were based on the Interconnection Ground Subnetwork (IGS) for telescience or remote operations, set up by ESA's Space Operations Centre (ESOC) at Darmstadt, Germany. The IGS network connects the NASA Operations Support Center (HOSC) in Huntsville, Alabama, with the IGS Control Centre at ESOC. For scientists, the HOSC represents the access point for real-time experiment data and command.
At ESOC, an integrated Network Management Facility represented the European centre for monitoring and controlling the services for all user sites. It was the communications hub, linking the NASA Payload Operations Control Center (POCC) at the MSFC in Huntsville to the European User Centres. Those User Centres are:
All staff in Europe and the US were connected via a direct voice link and there were also video links, allowing those in Huntsville to view those working in the User Centres in Europe. Immediate access to data gave co-investi-gators in Remote Centres the opportunity to make evaluations and modifications during the course of an experiment. There was also the possibility of repeating an experiment.
For future space station missions, where a series of experiments could run over many months, telescience will bring many advan-tages. It will mean that scientists can monitor and control their experiments from their own laboratory, which is a much more efficient and cost-effective way to conduct scientific research during long-duration space missions.
The IML-2 mission was thus a major and successful step forward in the undertaking of decentralised science and payload operations. From low-rate monitoring to full data-receiving and commanding capabilities, the concept has matured and its usefulness has been demonstrated. The scientific users were extremely satisfied with this service. The possibility of their active participation in the experimentation in an orbiting facility is a great asset that considerably improves the scientific return.