Airbus A310 Zero-G
Airbus A310 Zero-G (Experimental Microgravity Flights for Research)
In April 2014, the refitted Airbus A310 aircraft is on the runway and ready for its first flight for weightless research. Although the aircraft can weigh up to 157 tons, skilled pilots will angle its nose 50º upwards to create brief periods of weightlessness. At the top of each curve, the forces on the passengers and objects inside cancel each other out, causing everything to float in weightlessness. 1) 2)
The French company Novespace has conducted these ‘parabolic flights' for more than 25 years. In 2014, the company acquired a new aircraft to replace their trusty Airbus A300. Most seats were removed to provide as much space as possible inside, while padded walls provide a soft landing for the researchers – the changes in ‘gravity' can be hard to handle. Extra monitoring stations have been installed for a technician to monitor the aircraft system's as it is pushed to its limits – this is no transatlantic cruise.
Experiments include understanding how humans sense objects under different gravity levels, investigating how the human heart and aorta cope, looking at how plants grow, testing new equipment for the International Space Station, trying out new techniques for launching nanosatellites, investigating whether pharmaceutical drugs will work without ‘gravity', understanding Solar System dust clouds and planet formation as well as investigating potential propulsion for martian aircraft.
Conducting hands-on experiments in weightlessness and hypergravity is enticing for researchers in fields as varied as biology, physics, medicine and applied sciences.
Figure 1: Photograph of the newly refitted Airbus Zero-G A310 aircraft and ready for its first flight for weightless research (image credit: ESA, CNES, DLR)
The inaugural scientific campaign will start on May 5, 2015, a collaboration between Novespace's three main research partners: ESA, France's CNES space agency and DLR (German Aerospace Center).
Key parameters of the Zero-G
The following is a list of the main characteristics and features of the Airbus A310 "ZERO-G" aircraft: 3)
• The aircraft is a two-engine modified Airbus A310 "ZERO-G" aircraft
• It is based at the Aéroport International de Bordeaux–Mérignac
• Aircraft maximum mass: 157 tons
• Overall length: 46.4 m; Wingspan: 43.9 m; Fuselage diameter: 5. 64 m
• Total cabin volume: 300 m3
• Dimensions of testing volume inside cabin: 20 x 5 x 2.3 m (L x W x H)
• Total testing volume: 230 m3
• The cabin walls, floor and ceiling are specially padded
• The interior is continuously illuminated by LED lights
• The aircraft can accommodate 40 passengers
• There are 4 passenger doors
• The door through which equipment is loaded has a height limit of 1.80 m based on the capabilities of the loading truck and a width of 1.06 m. For experiments larger than this, the equipment must be designed to be taken apart.
Some background: "Zero-G", an Airbus A310 that previously served the German air force VIP fleet as "Konrad Adenauer" (registration 10+21, now F-WNOV), is one of just a handful of aircraft in the world capable of letting scientists and astronauts work, for brief stretches, in microgravity without travelling to space. A US company, Zero G, operates a modified Boeing 727-200 and Russia's state astronautics agency has an Ilyushin Il-76; the National Research Council of Canada has a much smaller Falcon 20. 4)
The trick is to fly a parabolic flight path, pulling up to 1.8 g in a steep climb that gains some 2,500 m in altitude in 30 s, then cruising over the hump and diving back down to resume level flight. A typical outing – from Bordeaux, Novespace flies out over the Bay of Biscay – can include 15 to 30 of these parabolic maneuvers, providing many valuable minutes of effective weightlessness, for science experiments or astronaut training.
But while the aircraft is operating inside the normal 2.5 g design limit of a civil airliner, its radical flight pattern is very hard on the structure. More than 13,000 parabolas in nearly 18 years of service wore out the old Zero-G, an A300 that was the third built by Airbus, so it was retired in October 2014.
The A310 will be much easier to maintain than the A300 aircraft and bring with it other safety advantages characteristic of newer models. With a glass cockpit and a new flight control system, Novespace expects to be able to provide its customers with more precise control of what were already "excellent" microgravity conditions during the parabolic maneuver – clearly important on scientific missions, when researchers need to reproduce conditions as closely as possible on each trial.
In June 2014, the A310 was converted from a German VIP plane configuration to a research aircraft by Lufthansa Technik AG in Hamburg. Various frames and other structures had been weakened by the original VIP conversion, so before a new interior could be installed for Novespace, Lufthansa had to bring the aircraft back to its original condition and verify its structural integrity – a job that involved over 1,300 airframe modifications (Ref. 2).
In 2014, the Airbus A310 was purchased by the French company Novespace, which is based at Bordeaux-Mérignac. Since March 2014, the metropolis' airport in south-western France has also been home to the A310 ZERO-G.
Parabolic flight maneuver
The Airbus A310 "ZERO-G" aircraft generally executes a series of 31 parabolic maneuvers during flight. From a steady horizontal flight, the aircraft gradually pulls up its nose and starts climbing up to an angle of approximately 50º . This "pull-up" phase lasts for about 20 seconds, during which the aircraft experiences an acceleration of around 1.8 times the gravity level at the surface of the Earth, i.e. 1.8 g. The engine thrust is then strongly reduced to the minimum required to compensate for air-drag, and the aircraft then follows a free-fall ballistic trajectory, i.e. a parabola, lasting approximately 20 seconds, during which weightlessness is achieved (Figure 2).
Figure 2: Schematic of parabolic flight maneuvers (image credit: ESA, Novespace)
Alternatively, for reduced gravity parabolas, the pull-up phase is reduced to a lower angle, and the engine thrust is reduced sufficiently to a point where the remaining vertical acceleration in the cabin is approximately 0.16 g for approximately 23 seconds or 0.38 g for approximately 30 seconds. — At the end of this period, the aircraft must pull out of the parabolic arc, a maneuver which gives rise to another 20 second period of 1.8 g on the aircraft, after which it returns to normal level flight attitude.
These maneuvers are flown repeatedly, with a period of 3 minutes between the start of two consecutive parabolas, i.e. approximately a one-minute parabolic phase (20 seconds at 1.8 g + 20 seconds of weightlessness + 20 seconds at 1.8 g), followed approximately by a two-minute "rest" period at 1 g. After every group of five parabolas however, the rest interval is increased from 5 to 8 minutes.
Throughout the flight, all personnel are kept continuously informed of the flight status, i.e. indication of how many seconds to the next parabola, number of minutes of rest period, etc.
The short descriptions in the following chapters are presented in reverse order.
Two student teams successfully performed their studies in microgravity during ESA Academy's Fly Your Thesis! 2018 campaign.
November 8, 2018: Aboard Novespace's Zero-G Airbus A310 scientists have the unique opportunity to ‘fly' their experiment in micro- and hypergravity conditions, generated by four intensively trained pilots who carefully control the aircraft into a parabolic trajectory. At about 6 km altitude, the pilots increase the thrust and gradually pull up the nose of the aircraft to reach an angle of approximately 50 degrees. For about 20 seconds, the local acceleration increases up to 1.8g and passengers in the cabin experience an extreme gravitational force. ‘It's as if there's glue underneath my shoes and my arms are pulled down with strings' – passenger onboard of the Zero-G A310. 5)
In order not to get ill in the hypergravity phase, passengers are instructed to keep their gaze straight and not move their eyes. However, some scientists are specifically interested in these hypergravity phases: In the back of the cabin, participants of a professional medical experiment team are jumping up and down on a small platform. By not moving their heads horizontally, the participants hope to prevent nausea.
The four Dutch students from team G-Reach (MC Erasmus, NL) and the Spanish, Italian and two Swedish students from team PVT-Gamers (Luleå Technical University, SE) were selected by the Fly Your Thesis! Selection Board about a year ago. Since then, both teams worked on their projects during which they were guided by experts of Novespace and members of ESA Academy. A Gravity Related Training Week was organized in order to enhance the student's knowledge on system engineering, project management, outreach and communication, and the students had to update both Novespace and ESA continuously on their developments throughout the year.
‘It is an amazing experience. You really get the full package, from the very beginning of designing your own project to project and financial management to analyzing your results. Everything that you accurately plan beforehand will need extra work and time. But that's exactly what distinguishes the Fly Your Thesis program from ordinary thesis projects,' said one student from PVT-Gamers.
Figure 3: PVT-Gamers team in flight (image credit: ESA)
When the maximum pitch angle is reached the engine power is reduced to only compensate for the air drag and the aircraft freely falls through the air, following a natural parabolic path. Similar to astronauts in the International Space Station, the passengers in the cabin experience weightlessness. ‘It's amazing to see our experiments working in the aircraft and the whole sensation of weightlessness is a complete new experience for me.' said a member of G-REACH.
Contrary to astronauts who sometimes remain in space for several months, these passengers only get to experience the microgravity sensation for 22 seconds before re-entering a second hypergravity phase. To ensure that scientists retrieve enough data within this short time frame, the parabolic flight campaign contains three flights with 31 parabolic maneuvers per flight, leading up to 93 parabolas in total.
Within an aircraft cabin hosting and powering many different and difficult experiments, ranging from medical and biological topics to fluid dynamics and technology demonstrations, the risks and hazards are high. Experiments chosen to fly on board of the aircraft therefore have to face the many requirements set by both Novespace and ESA and use creativity and ‘out-of-the-box thinking' to prepare and built their set up in the most efficient and safe way. This also applied to the student teams selected by ESA Academy.
Figure 4: Students in the cockpit (image credit: ESA)
During this campaign, team G-Reach hoped to uncover what sensory information is relied upon by the brain to detect errors and adapt movements. Each flight, two participants performed a fairly simple motor task, moving their arms up and down vertically while relying on proprioceptive and/or visual feedback. Team PVT-Gamers aimed to validate a new gauging method developed for accurately determining the remaining propellant mass in spacecraft tanks used for electric propulsion systems. By applying this new method to specially designed gas containers under micro- and hypergravity conditions, it could be possible to determine levels of propellant within the tanks. Even though both teams are only at the beginning of the analysis phase, it seems that both teams are on their way to produce some useful and interesting results.
The Fly Your Thesis! Selection Board selected two new teams for the FYT 2019 campaign: team Grain Power 3D Printing from Germany and team PHP3from the UK. Keep up to date with their progress on our ESA Education twitter and facebook accounts!
Gravity for the loss
June 12, 2018: Last week, ESA, the German Aerospace Center (DLR) and French space agency CNES joined forces to run a special parabolic flight campaign entirely dedicated to life science experiments. Between 4 and 7 June, eight experiments were run in three different levels of partial gravity, another first for a parabolic flight campaign. 6)
During our more common zero-gravity parabolic flights, research teams are subjected to 20-second bursts of weightlessness during which they run experiments ranging from life sciences, to technology demonstrations, to material physics. Results offer an indication of how various mechanisms work without gravity and are compared to results on the ground. But what happens at varying degrees of weightlessness?
To help fill in the graph, scientists were offered a unique opportunity to run experiments at one-quarter, one-half, and three-quarters gravity. The aim is to better understand biological dependence on gravity. Ultimately, if humans are to embark on long-term spaceflight and live on the Moon and Mars, we need to determine the levels of gravity in which humans can live and work.
Another team subjected baby plant roots to doses of partial gravity and monitored root growth using lasers to investigate how the roots manage to stay "grounded" in the absence of gravity. We know plants adapt to weightlessness rather quickly, but researchers still need a clearer picture of what's happening on a cellular level. Extra-terrestrial farming is vital to human survival off-planet, and adapting agriculture to altered gravity is an important step to making this possible. For a full list of experiments, see here.
Another team subjected baby plant roots to doses of partial gravity and monitored root growth using lasers to investigate how the roots manage to stay "grounded" in the absence of gravity. We know plants adapt to weightlessness rather quickly, but researchers still need a clearer picture of what's happening on a cellular level. Extra-terrestrial farming is vital to human survival off-planet, and adapting agriculture to altered gravity is an important step to making this possible.
Parabolic flights are one of a few ways to recreate microgravity conditions on Earth, but how is this achieved? The A310 Zero-G aircraft, operated by Novespace in Bordeaux, France, repeatedly performs a special maneuver. After pulling up sharply to 50 degrees, the pilots reduce the thrust and pitch of the airplane to cancel air-drag and lift. This places the plane on a parabolic flight path, exactly as if it has been thrown upwards and released. It then essentially falls over the top of the parabola, creating 20 seconds of 0g. When it reaches 50 degrees nose-down, the plane then pulls out of the descent to normal flight.
To achieve partial gravity, the angle at which the plane pulls up and pulls out is shallower, and the pilots carefully cancel out only part of the lift. This creates about 25 seconds of one-quarter gravity, or 35 seconds of half-gravity, or 50 seconds of three-quarters gravity. The maneuver is performed every three minutes for a total of 31 times per flight.
In addition to this unique collaboration between ESA, DLR, and CNES, the partial gravity parabolic flight campaign also featured a special guest experiment by NASA and pilot-turned-ESA-astronaut Thomas Pesquet.
"It was a real privilege to work on this unique campaign, not only because of the constructive collaboration with my colleagues from DLR and CNES, but also to provide such an interesting suite of experiments with rare and much-needed data," said Neil Melville, Coordinator of Parabolic flight and Drop Tower campaigns. He is pictured on the left, alongside Katrin Stang of DLR and Sébastien Rouquette of CNES (Figure 5).
Figure 5: From left: Photo of Neil Melville of ESA, Katrin Stang of DLR and Sébastien Rouquette of CNES (image credit: Novespace)
Brain experiments in low gravity
The brain is a sensory sponge. It absorbs information from the environment to help you orientate yourself at every turn and perform at your best. After adding small doses of weightlessness, scientists are collecting surprising data about the structure and behavior of the most complex organ in your body. 7)
Three different studies during the latest ESA parabolic flight campaign showed how shots of weightlessness wake up the brain, but disorient it, too. Participants volunteered to throw their brains off balance by experiencing microgravity without leaving Earth, onboard the Zero-G plane.
The aircraft, operated by Novespace in Bordeaux, France, offers up to 90 periods of weightlessness, 20 seconds at a time.
Removing the sense of weight for short periods of time has a direct – and varied – impact on cognition and spatial memory. While volunteers got better at multitasking and solving demanding equations, they found it more difficult to navigate in new environments.
Removing the sense of weight for short periods of time has a direct – and varied – impact on cognition and spatial memory. While volunteers got better at multitasking and solving demanding equations, they found it more difficult to navigate in new environments.
Recordings of brain activity and tests to monitor attention and arithmetic skills confirmed that there is a significant increase in performance during short-term exposure to microgravity.
Figure 6: Three different studies during the latest ESA parabolic flight campaign showed how shots of weightlessness wake up the brain, but disorient it, too. Participants volunteered to throw their brains off balance by experiencing microgravity without leaving Earth, onboard the Zero-G plane (image credit: ESA, A. Conigli) 8)
Figure 7: Two candidates test their skills on Computers during a parabolic flight (image credit: Novespace) 9)
Growing plants for food in space and on other planets will be necessary for exploration of our Universe
June 11, 2018: Plants are quicker to react and more sensitive than you might think – they can detect light changes in a fraction of a second and can bend towards light sources within minutes – and they respond equally fast to gravity. For the first time last week, European scientists filmed roots growing in real-time on a plane (ZeroG-A310) that recreates different gravity levels. 10)
Guided by gravity on Earth, roots find their way down into the soil, but how do plants keep their ‘feet' on the ground when in space?
"Altered gravity has a big impact on plant growth. We do not fully understand how their cells cope," says lead scientist Franck Ditengou from the University of Freiburg in Germany. His research team involves five universities across Europe.
Seven-day-old roots were put through repeated shots of microgravity – from a quarter to three quarter g – during the first partial-gravity international life sciences parabolic flight campaign held in Bordeaux, France, in June 2018.
Figure 8: This microscope image shows the individual cells that make up the root of an Arabidopsis thaliana plant. Highlighted in green is one of the hormone auxin carriers, a protein that has a crucial role in coordinating many growth and behavioral processes in a plant's life cycle. This hormone is important for gravity perception and regulates the asymmetric growth between the upper and low side of the root (image credit: University of Freiburg) 11)
Figure 9: Charting the very first reactions of young plants down to the millisecond in partial gravity was a new feat for the team. The baby roots were germinated in a Petri dish and transferred to the plane in a microfluidic chip where they continue to receive nutrients. Scientists used a high-resolution microscope to monitor up to eight plants each flight. The device was equipped with lasers and a spinning disc that scanned each root in great detail. Fluorescent markers helped to track the cellular changes happening at the root tip in real time (image credit: Novespace) 12)
Charting the very first reactions of young plants down to the millisecond in partial gravity was a new feat for the team. Baby roots were germinated in a Petri dish and transferred to the gravity-aircraft in a microfluidic chip where they continued to receive nutrients.
Scientists then used a high-resolution microscope to monitor up to eight plants each flight. The device was equipped with lasers and a spinning disc that scanned each root in great detail.
Fluorescent markers tracked the cellular changes happening at the root tips in real time. "This experiment is unique because it allows us to see how different g forces affect the gravity perception machinery," explains Ditengou.
Arabidopsis thaliana is a well-known plant for biologists. Its rapid growth and concentric cells make it easy to work with. "They offer good views through the microscope and they are easy to genetically modify," adds Ditengou. The team used transgenic plants that were modified to have problems in perceiving gravity. These mutants are key to find out exactly how and where plants lose their ability feel gravity.
Previous experiments on sounding rockets, long-arm centrifuges and zero-g parabolic campaigns showed huge changes in gene expression and some deformation in the roots structure. Results are soon to be published.
The long journey of the roots is not over. Understanding plant growth is the first step to adapting crops for more productive agriculture.
Figure 10: This microscope image taken at 400 times magnification shows the individual cells that make up the root of an Arabidopsis thaliana plant. The cells responsible for sensing gravity (statoliths) react to changes moving inside the root. 13)
CIMON, the birth of stars and the secret of plasma crystals
March 9, 2018: The 31st DLR (German Aerospace Center) parabolic flight campaign ended successfully on 8 March 2018. Twelve experiments in the fields of human physiology, biology, physics, technology testing and materials science took place on board the A310 ZERO-G, dealing with investigations into the birth of stars, plasma physics, the behavior of molten materials in zero gravity and blood circulation in the human body, amongst other things. The campaign took place in Bordeaux, where the company Novespace, which conducts parabolic flight campaigns on behalf of the DLR Space Administration, is based. "The experiments remain exciting, even after 18 years of research on parabolic flights, as many participants are breaking new scientific or technological ground in their questions," said Katrin Stang, Program Manager for DLR's parabolic flights. 14)
Figure 11: Exploration of complex plasmas (image credit: DLR)
• CIMON – an astronaut assistance system for the ISS
CIMON (Crew Interactive MObile companioN) will be used on the ISS (International Space Station). CIMON is mobile, equipped with artificial intelligence and is designed to support and take some of the load off astronauts in their everyday tasks. The aim of the parabolic flight test was to test the basic properties of CIMON in microgravity. In particular, its spatial orientation, navigation and maneuvering were tested, in order to be optimally prepared for deployment on the ISS – in permanent microgravity. Christian Karrasch, CIMON Project Manager at DLR , was on board and is satisfied with the results of the parabolic flight. "CIMON has demonstrated that it can maneuver safely in microgravity, and passed all the tests with flying colors – we are really looking forward to its first deployment on the ISS."
• The birth of stars in a parabolic flight experiment
How are stars born? The INKA (unstable protoplanetary bodies in a low-pressure wind tunnel) experiment performed by scientists from the Faculty of Physics at the University of Duisburg-Essen addressed this question. Sand dunes migrate when the wind removes particles from one side, which are then deposited by gravity on the downwind side. Indeed, what would happen without gravity? The dunes would simply break up in a cloud of sand grains.
Similar situations are conceivable during the formation of planets, in which loosely linked particles the size of sand grains form a body a kilometer or so in size with very little gravity of its own – a planetesimal. To discover the conditions under which such bodies are stable, scientists observed a sample made of particles one millimeter in diameter in a low-pressure wind tunnel, in which the pressure and wind speed could be varied. The wind tunnel was simultaneously on a centrifuge, in order to simulate different levels of planetesimal gravity (planetesimal size).
• Plasma crystals – a phenomenon that only occurs in microgravity
Complex plasmas are electrically conductive gases – similar to those used in fluorescent tubes – into which 'dust particles', also referred to as microparticles, are introduced. The microparticles, with a diameter of up to ten microns, receive a strong negative charge in a plasma chamber due to electron attachment and are made to float using electric fields. As a result of this charge, the electrostatic interaction between the microparticles is very strong, so that new, scientifically interesting phenomena can occur, such as the formation of a plasma crystal – a regular arrangement of microparticles in the plasma.
In their parabolic flight experiment, scientists from Justus Liebig University in Gießen wanted to investigate the electrorheological properties of the liquid phase in complex plasmas. In addition to this basic research, complex plasmas are also ideal for use as modelling systems for other areas, such as crystallography, the physics and technology of liquids and gases, as well as nanotechnology. Since 2014, this experimental system has had a 'duplicate' in the PK-4 facility on the ISS, in order to facilitate longer-term research under microgravity.
• The design of new materials through unique research opportunities
Like the PK-4, the DLR TEMPUS facility for researching melting in microgravity has a twin on the ISS. In the TEMPUS experiments, liquids can be examined in unique ways; metals and alloys can be positioned and melted using electromagnetic fields. In this way, the molten material is not contaminated through contact with another material, such as a crucible. In microgravity there are no disruptive convective flows in the molten material, as would occur under normal gravitation.
The measurements performed by scientists from DLR and various universities are focusing on new findings concerning the thermophysical properties of materials, such as density, viscosity, electrical conductivity and thermal expansion. These results form the basis of model calculations for technical processes in the design of new materials. In the 31st DLR parabolic flight campaign, a new thermal imaging camera was used. "The thermal imaging camera has allowed us for the first time to observe TEMPUS samples at temperatures below 600 degrees Celsius," explained Julianna Schmitz from the DLR Institute of Materials Physics in Space in Cologne. "This expands our research capabilities and allows us to study metals that melt at low temperatures. We are very pleased with the results."
• Human microcirculation in microgravity
Microcirculation is the blood flow in the smallest vessels in the human body. It has great significance for the human organism as an important blood reservoir, and affects blood pressure, promotes heat exchange and transports oxygen and vital nutrients to cells. Researchers at the University Hospital of Düsseldorf have examined changes in microcirculation in microgravity on a parabolic flight using a special manual microscope the size of a smartphone that takes measurements under the tongue. The findings from the parabolic flight could help in the development of new diagnostic alternatives, in order to identify people at an increased risk of circulatory problems and thus prevent them in good time. The flight safety of astronauts and jet pilots, for example, could also be significantly improved in this way.
Pointing the way to augmented reality in space
May 17, 2017: The directions are simple, the conditions less so: press the corresponding physical button indicated on the headset display while experiencing weightlessness. Participants in an experiment running on ESA's 66th parabolic flight campaign are helping researchers to develop augmented reality as a useful tool for astronauts on the International Space Station. 15)
Detailed instructions displayed on a laptop often require astronauts to interrupt their workflow and concentration to refer back to checklists. By replacing static displays like laptops with augmented reality headsets, a team at the University of Rostock aim to increase astronauts' efficiency and accuracy when working science experiments on the Station.
To begin creating a useful device, developers need to understand how users interact with their environment. In the case of space, the lack of a perceivable up or down makes for a unique frame of reference that affects hand–eye coordination and similar skills. Researchers have boarded an aircraft to work with augmented reality systems in the few seconds of weightlessness these specialized flights provide.
Figure 12: The participant pictured is wearing an augmented reality headset displaying 12 targets arranged in a circle as well as motion-capture sensors to track body movement. During weightlessness, she is shown a target that must be touched on a fixed, vertical board in front of her. Developers track her performance to understand field of view and visual-motor skills under weightless conditions. This feedback is then used to tailor the augmented reality software to the subject's performance in this unique environment (image credit: ESA, A. Conigli)
Essentially, the augmented reality headsets aim to replace the laptop displays that the astronauts currently consult for instructions during science operations on the Station. Developers expect to make technical changes based on the performance measures and to run the experiment in future parabolic flight campaigns, with the ultimate goal of getting the hardware ready for testing on future astronaut missions on the Station.
The 66th parabolic flight campaign is being run by Novespace in Bordeaux with sponsorship from ESA, DLR (German Aerospace Center) and France's CNES space agency.
Student experiments on parabolic flights performed successfully
November 11, 2016: An extraordinary experience just concluded for four teams of university students. In the frame of the Fly Your Thesis! 2016 program, they were selected to conduct experiments during the 65th ESA parabolic flight campaign. This gave the opportunity to the participating students not only to execute their experiments in weightlessness conditions, but also to get the direct experience to float in weightlessness, which otherwise – apart from parabolic flight campaigns – is a privilege practically reserved only to astronauts. 16)
The four teams investigated different aspects of space science. The"TEPiM" team from the Universidad Politécnica de Madrid (UPM) in Spain studied the melting process of Phase Change Materials in weightlessness conditions. The "CFVib" team, also from UPM, investigated the behavior of fluids subjected to high frequency low amplitude vibrations. The Italian team "PoliTethers" from Politecnico di Milano, tested the control dynamics and algorithms for tether-based systems, in view of possible future applications to tow space debris to be deorbited, and the team from the Universität Duisburg-Essen in Germany, "Anemoi4" examined the wind speeds needed to lift dust in a Martian-like atmosphere.
The teams were selected in December 2015 and spent about 8 months developing their experiment. During this period, they were closely supervised by ESA Education, Novespace, and by an assigned mentor from ELGRA (European Low Gravity Research Association).
Parabolic flights on board the Novespace Airbus 310 Zero-G:
To offer weightlessness on board the Airbus Zero-G, the pilots have to fly the airplane in a special maneuver following the form of a parabola. This maneuver starts in a steady flight at 6km altitude. The pilots then increase the thrust and gradually pull up the aircraft to a pitch angle of about 50º. This maneuver lasts about 20 seconds and causes the local acceleration to increase to 1.8 g (1.8 times the gravity level at the Earth's surface). Once the maximum pitch angle is reached (altitude around 7.5 km) the engine power is reduced to only compensate for air drag and the aircraft falls freely following a parabolic path with the highest altitude point at around 8.5 km. During this free fall state, which lasts about 22 s, experiments and experimenters on board the aircraft experience weightlessness just like astronauts on board the ISS (International Space Station). The maneuver is concluded with another 1.8 g pull out phase eventually bringing the aircraft back to a steady horizontal flight. This is repeated 31 times per flight and the campaign consists of three flights.
To fly this special manoeuvre 3 pilots are controlling the aircraft at the same time. One pilot is responsible for the yaw movement, one for the pitch and one for the thrust.
The plane with only 40 seats and an overall length of 46.4 m has nearly half of its total length (20 m) dedicated to accommodate the experiments to be executed in weightlessness conditions. The floor and all walls are specially padded to prevent that experimenters get hurt during the transitions from weightlessness to 1.8 g.
Figure 13: The entire team of Fly Your Thesis! 2016 poses in front of Novespace's A310 also known as ZERO-G (image credit: ESA)
Figure 14: Novespace's A310 ZERO-G cabin can get very busy with experiments and experimenters. Experiments are mostly automated and only require operator intervention in between parabolas allowing for the weightless experience to be fully enjoyed (image credit: ESA)
The Fly Your Thesis! program:
After selection in January 2016, the student teams worked hard and had to pass specific reviews and milestones. Designing experiments for weightlessness and to be operated on board the Zero-G aircraft is very different than just building to operate them in a lab. On one hand, it is very challenging to meet all safety requirements, since the experiments are flown on board an aircraft, and are operated in weightlessness conditions. On the other hand, the experiments must be operated in a very limited amount of time, and their design must allow for this ease of operations. All student experiments of FYT! 2016 were very successful throughout the campaign and the scientific data which was collected will keep the students busy for quite some time as they carefully evaluate of the results. The results will then be disseminated in papers and at international conferences.
1) PCM (Phase Change Materials):
The TEPiM team developed an experiment, which studies the Marangoni effect during the melting of PCM (Phase Change Materials) in weightlessness. This effect is very hard to study on Earth because convection would govern the phenomena occurring during phase transition. However, in weightlessness, convection is absent, and the team wanted to observe the influence of the Marangoni effect on the melting behavior of the PCM. Phase Change Materials can be used for thermal control of spacecraft but also have many application on Earth. Andrés Cobos, team leader of TEPiM said, "The sensation of the first ZERO- g parabola will be remembered for the rest of our lives and more importantly, the experience of the whole project has been really enriching".
Figure 15: The TEPiM team from UPM during the 2016 FYT! program parabolic flight campaign in the Novespace A310 Zero-G aircraft (image credit: ESA)
TEPiM (Thermocapillary Effects in Phase Change Materials in Microgravity) project:
PCMs take advantage of their high latent heat of the solid/liquid phase transition to store and release a large amount of heat energy. This way, PCMs absorb energy from their environment during the melting process and release this energy during the solidification. In these processes the temperature remains constant. 17)
Thanks to this feature, PCMs are useful for passive thermal control systems and extreme temperature dampers on ground and in space. Unfortunately the melting process in microgravity conditions is slower than that in the presence of gravity. This is due to the fact that in space the process is only based on conductive heat transmission. However, on ground a thermal gradient always generates a convective motion. This convective heat transmission is more effective than pure conductive heat transmission.
With the aim of quickening the melting process, the TEPiM team proposes to include an air layer in the PCM cells to generate a Marangoni flow in the liquidated PCM (based on thermocapillary forces) and thus a convective heat transmission.
The team has been working in a numerical model that takes into account the energy equation, convective transport and tracking of the solid/liquid interface to predict the PCM behaviour under microgravity conditions with a free surface. With this model, the team will be able to predict the behaviour of the PCM during its melting process in presence of an air layer under a microgravity environment.
The experiment that is going to be carried out in the Fly Your Thesis! program will allow to test and validate the numerical model. This will permit the creation of improved designs of passive thermal control systems based on PCMs with better performance in thermal control under microgravity.
The experimental set up is a prismatic cell filled with octadecane paraffin in solid state and air. During the micro-g phases of the parabolic flight, it will be heated at one of its sides to produce the liquation of the paraffin under microgravity conditions. The melting process will be recorded in order to monitor the movement of the solid/liquid interface. This will be compared to the one predicted by the model.
Figure 16: Predicted behaviour of n-octadecane in microgravity with the TEPiM experiment set-up. White curve indicates the solid/liquid interface (image credit: Santiago Madruga´s research group (ETSIAE-UPM Madrid-Spain)
2) Control of Fluids with Vibrations:
Fluids in weightlessness behave quite differently than under normal gravity conditions; for instance sloshing of fuel in spacecraft tanks is a great challenge yet to be resolved. The CFVib team looked at how they could control fluids in weightlessness using high frequency vibrations. A first look at their data shows some promising but also surprising results. "From the first parabola to the end one the microgravity and having our experiment in ZERO g were two of the most wonderful experience of our life. We are looking forward to see the science results!! " said Jose Javier Fernandez, team leader of CFVib.
Figure 17: The CFVib team, which is affiliated with the Escuela Técnica Superior de Ingeniería Aeronáutica y del Espacio at UPM, has been selected to design, build and operate a fluid science experiment that will investigate the complex behavior of liquids under high frequency vibration in a weightless environment, and evaluate the potential of small amplitude excitations for controlling and managing liquids in space (image credit: ESA)
CFVib (Control of Fluids in Microgravity with Vibrations) project: 18)
The CFVib team has designed and proposed an experiment to fly in the autumn 2016 Novespace parabolic flight campaign promoted by ESA's Fly your Thesis! program.
There are special challenges associated with managing fluids in space, something that is required, for example, in life-support and propulsion systems. In particular, it is important to understand the effects of the small vibrations, often called g-jitter, that come to the fore in the absence of a strong gravitational field. These may be due to on-board machinery, crew movements, orbit and docking manoeuvres, etc., and are problematic for scientific experiments that assume a zero gravity condition. Furthermore, without a strong gravitational field to pull fluids down, surface forces dominate, leading to more complex static configurations even in the absence of g-jitter. Depending on the fluid, container, and their surface properties, multiple solutions may coexist, especially in non-wetting systems where disconnected (partial) volumes of fluid can be established. The relatively large energy barrier between these states means that hysteresis will be much more prominent than it is in ordinary gravity.
A fluid mass with at least one free boundary will generally respond to vibrations via surface motion (waves) oscillating on the same timescale as the vibrational forcing. In addition to this more familiar response, vibrations induce a slow reorientation of the (average) fluid position toward a new (quasi-static) equilibrium determined by a balance between surface and vibrational energies. These new surface configurations, which are known as vibroequilibria, may differ substantially from the unforced equilibrium shape.
Figure 18: Photo of vibrating ceramics in weightlessness to move liquids (image credit: ESA)
The CFVib project will make use of piezoelectric devices to vibrate, at high frequency, representative fluid-gas and fluid-fluid configurations confined in relatively small transparent acrylic containers. The induced oscillatory velocity field will drive the reorientation of the fluid interface. The dependence of this vibroequilibria phenomena on control parameters (frequency, amplitude, phase) will be investigated, as well as subsequent instabilities and coupling to other dynamical modes, like sloshing. It is hoped that the results of the project will allow engineers to take advantage of the vibroequilibria phenomenon, in certain systems, to help control and manipulate confined fluids, setting up desired initial conditions for microgravity fluid experiments, for example, or managing fluids in life-support and propulsion systems.
In order to fulfill the experiment objectives the CFVib team is investigating two different container shapes: cylindrical containers that are 3 cm in diameter and 6 cm tall, and cuboid containers with edges 3 cm long. Some experimental containers hold water, some oil, and some a mixture of the two. The piezoelectric devices are attached to the ends of the cylindrical containers and to three of the six sides of the cuboid containers. Forcing frequencies range from tens of kHz to MHz, and a set of cameras record the resulting movement and surface deformation.
A first look at their data shows some promising but also surprising results.
3) Towing a Satellite:
Just like ships and cars are towed if they broke down or are out of fuel, it might be possible that one-day satellites are towed for deorbiting purposes. The Politethers team therefore investigated how a tether strained between two satellites would behave in weightlessness, and what control strategies and algorithms for the tether would be most suitable to allow a "space tug" to execute "orbital towing" maneuvers to deorbit other satellites. The experiment showed very promising results at first sight, and now the arduous work of 3D data image analysis starts for the team. "The few hours spent in microgravity have largely paid off the efforts and hard work from the past months. It was in an amazing and unforgettable experience for the whole team from both personal and professional points of view. The experiment was a success: a huge amount of data was collected. Preliminary analysis show that the control system is effective and robust but they also highlighted a different tether behavior with respect to ground tests. More analysis and publications will follow" said Riccardo Benvenuto, team leader of Politethers team.
Figure 19: The Politethers team from Politecnico di Milano during the 2016 Fly Your Thesis! program parabolic flight campaign in the Novespace A310 Zero-G aircraft (image credit: ESA)
The dynamics of fixed-length tethered-systems (SatLeash - Experiment): 19)
The Politethers team is composed of three Ph.D. candidates and two M.Sc. student from the Politecnico di Milano, Department of Aerospace Science and Technologies (PoliMi-DAER), in Italy. They will investigate the dynamics and control of tethered-tugs, for space transportation and active debris removal.
Space tethers are long cables, made of high strength fibers strands, used to connect two or more end-bodies in orbit. Many applications have been proposed for space tethers, and among them the team is focusing on Active Debris Removal and space transportation using the tethered-tug concept, i.e. two objects, one passive and one active, connected by a flexible link, the motion of the system being excited by the active spacecraft thrusters.
Because of their overall flexibility and when placed in a zero-g environment, tethered-systems undergo a complicated set of three-dimensional librations and vibrations. Therefore, it is necessary to study their three-dimensional behavior in microgravity and to this end, parabolic flights are the most suited facilities for both time-span and available test area.
Figure 20: Depiction of the experiment set up (image credit: Politethers team)
Tethered system will play a crucial part in future missions. Hence, validated models, simulation tools and stabilizing control laws, describing tethered-tugs orbital and attitude dynamics, are considered of primary importance to design future missions.
Whiplashes or bounce-back effects are an example of these highly complex dynamics. Therefore, PoliMi-DAER has developed simulation models to describe the tethered-satellite-systems dynamics and design their control. The experiment goals are the validation of these models and the implementation of control laws to stabilize the system, avoiding whiplashes or bounce-back effects. The team is proposing to fulfill these objectives by testing a reduced-scale tethered floating system, released and retrieved with different conditions. Its three-dimensional trajectory will be reconstructed using stereo-cameras and acceleration sensors. Different tether stiffness will be tested as well as different control strategies.
Figure 21: Use of tethers in space: deorbiting is a possible application for space-tethers (image credit: Politethers team)
4) Sand storms on Mars:
In the novel (and later movie) ‘The Martian', a dust storm endangers on a human base on Mars. We know that there are dust storms on Mars, sometimes even covering a large proportion of the planet. However, our current models regarding dust lifting predict that the wind speeds and atmospheric pressure on Mars are not sufficient to lift dust off the ground. The Anemoi4 team hoped to be able to solve this contradiction with the results of their experiment, which worked flawlessly on board. The team is very pleased with their participation and their results. "Developing this experiment taught us so much and brought us so much further" said a team member at the end of the campaign, Maximilian Kruß from the Anemoi4 team explained, "Developing this experiment taught us so much. Besides the educational value, feeling weightless was a unique experience. We definitely wouldn't mind to fly again ....."
WINDMILL (Wind Induced Dust Movement in Low-Gravity Location): 20)
The Anemoi4 team is composed of two PhD and two Master students from the University of Duisburg-Essen in Germany. They want to study wind induced dust lifting in different gravitational environments, such as the Martian surface, to get a better understanding of adhesive dust properties.
Dust storms on other planets are often used as a dramatic element in sci-fi movies, most recently in the Martian. In fact dust storms on Mars are not a product of the imagination of some Hollywood directors but can regularly be observed on the Martian surface. In the early 80's experiments in wind channels on earth were made to investigate this phenomenon and lead to the conclusion that dust cannot be lifted up by typical wind speeds on Mars. There have been many discussions about the mechanisms responsible for dust lifting and new ideas are introduced until recently. Nevertheless no experiments have been conducted under altered gravitational levels. Results from experiments under 1g were solely extrapolated to Martian levels. Until today reduced g experimental data is still missing.
The Anemoi4 team designed an experiment to measure the wind velocities which are needed for dust to be lifted up in various gravitational environments. As dust sample they use so-called JSC which is usually used as Martian soil analog. The dust bed is placed inside of a small wind channel filled with CO2 at a pressure of 6mbar to recreate the Martian atmosphere. This wind channel will be placed inside a centrifuge to simulate different gravitational potentials. This setup offers the possibility to investigate dust lifting in the range of 0 -1 g during the state of microgravity in a parabolic flight. The dust bed is observed optically by a camera with up to 100 frames/s to determine the threshold wind velocity at which dust starts to lift up.
Figure 22: CAD - model of the vacuum chamber used to simulate Martian atmosphere. The chamber is rotated to create different g-levels (image credit: Anemoi4 Team)
The results of the student team from Duisburg will give an insight how the sticking properties in a dust bed behave in different gravitational environments. In particular they will be able to measure experimentally, whether solely air flow can be responsible for dust saltation on Mars. The results will provide new important findings and could be relevant for rover missions or even manned space missions to Mars in the future.
Figure 23: WINDMILL experiment schematic (image credit: Anemoi4 Team)
The team with their WINDMILL experiment, worked on identifying the necessary conditions required for effective dust grain saltation often seen on Mars which leads to the famous seasonal dust storms observed on our red neighbor.
WIND MILL (Wind Induced Dust Movement In Low-Gravity Location)
Sept. 2016: Mars is a dusty place and you might not think it is surprising that we regularly see dust storms on its surface. But the phenomenon has puzzled scientists since the 1980s when experiments showed that typical wind speeds recorded on Mars are not strong enough to lift the dust. Many theories have been suggested to explain the dust storms but few experiments have investigated them. 21)
WIND MILL is a project heading to participate in the parabolic flight campaign Fly Your Thesis! 2016 which is organized by ESA. The WIND MILL experiment was designed by four students from the University of Duisburg-Essen in Germany as part of their thesis project. It will fly on ESA's parabolic flight campaign that offers repeated 20 seconds of weightlessness.
Figure 24: Photo of the WIND MILL assembly (image credit: ESA)
Inside the canister is a small wind channel filled with carbon dioxide at low pressure to represent the atmosphere found on Mars. The canister spins like a centrifuge and recreates different levels of gravity – the faster it spins the heavier the contents will be. This experiment cannot be done on the ground because the team wants to recreate Mars gravity – around two thirds of gravity on Earth.
In this WIND MILL experiment, the student Anemoi4 team (4 physics students of the University of Duisburg-Essen) investigates adhering forces of granular matter under the influence of wind and physical conditions as they prevail on Mars. The goal is to find a dependency between the critical drag force of the wind on the sand grains, which is needed to lift them up, and the gravitational force. 22)
So far, similar experiments were only performed on Earth so that predictions for other gravitational environments are hard to make. The results could be important for future rover or even manned space missions to Mars as it is important to have a conception of the possible atmospheric environment.
Figure 25: Illustration of a sand grain simulation experiment on Earth (image credit: University of Duisburg-Essen)
On Earth, the wind threshold velocity for lifting up 100 µm grains at 10 mbar is 15 m/s. A linear scaling to lower g-levels cannot explain dust storms on mars or rill structures on comets.
In the flight campaign experiment, different gravitational environments will be created within a vacuum chamber while rotating it at different velocities. Depending on this attractive force, cohesion properties of dust exposed to a wind flow can be then examined. Besides, this is the first time in history for a g-adjusted wind channel to be installed in a plane for parabolic flights.
Figure 26: Schematic view of the WIND MILL experiment assembly in the aircraft (image credit: University of Duisburg-Essen)
The next parabolic flight campaign series with the ZeroG-A310 aircraft, organized by ESA, is scheduled for the autumn of 2016 at the Bordeaux Airport, conducted by Novespace.
Mixing fluids in parabolic flight
Nov. 23, 2015: Researchers from the Free University of Brussels (ULB) recently discovered a natural phenomenon when mixing liquids of different viscosity in microgravity on ESA's parabolic flight campaigns. They now want to do more in-depth experiments but must keep their liquids separate during bumpy rocket launches and then mix them as needed. 23)
The device (Figure 27)results from a collaboration between the scientists and students of two schools to involve youngsters in science, technology, engineering and maths. The Brussels Engineering School ECAM worked on the design while mathematics students from the Saint-Michel college programmed the microcontroller under supervision from their teachers and the researchers from the Free University of Brussels.
Parabolic flights offer researchers hands-on access to microgravity in refitted aircraft as they fly up and down at 45º. At the top of the curve, the passengers and experiments experience around 20 seconds of weightlessness. Before and after the weightless period, increased gravity up to 2 g is part of the ride.
Figure 27: This picture shows a prototype container. The articulated base keeps the contents as stable as possible during launch while a motor vibrates the container at specific frequencies for the experiment (image credit: ESA/MRC/ULB)
Debris Detention Demonstration on Zero-G-A310 Flight
On June 9, 2015, ESA carried out their 62nd parabolic flight campaign. On this occasion, GMV Aerospace and Defence S. A. (Spain) flew their PATENDER (Net Parametric Characterization Parabolic Test) experiment with the goal of demonstrating the launch of nets and capture of satellites in zero-gravity conditions similar to outer space. PATENDER is an ESA-funded activity within the Clean Space program, which aims to encourage the development of space-debris reduction projects, using custom-built technology to capture decommissioned satellites still orbiting the Earth. 24) 25)
Within space-debris capture techniques, GMV has been studying nets as one of the most promising non-rigid methods. The project is based on the development of a software simulator that recreates the net deployment dynamic and contact with the target satellite. This simulator has been vetted by means of a real parabolic-flight experiment, filmed with high speed cameras that enable a 3D reconstruction of the trajectories of the net itself and each of its nodes/knots.
This GMV-led multidisciplinary activity is being carried out within a consortium formed by the Polytechnic University of Milan, in charge of mathematical net models and 3-D reconstruction, and the Asturian Foundation PRODINTEC, responsible for manufacturing the pneumatic-electric net-launching system. GMV's remit is to develop the software simulator and coordinate all project phases, from project acceptance and commissioning to validation of the final results.
The system, developed over one year, was taken in early June to Novespace, at the Bordeaux-Mérignac airport (France), to conduct low-gravity tests by means of a parabolic flight aboard an Airbus A-310. Net launch trials were carried out using a mock-up of ESA's earth-observation satellite, ENVISAT.
The A-310 parabolic flight operations of Novespace involved the execution of 31 parabolas, achieving about 22 zero-gravity seconds per parabola. The first trial parabola showed that all experiment components were firmly anchored, so a start was then made on launching the first and second set of nets, gradually upping the launch pressure until hitting the satellite mock-up. Throughout the whole flight the nets were successively launched in each of the parabolas performed, obtaining over 15 deployments and a 100% capture rate.
After completion of the experiment ESA declared its satisfaction with the results, in the sure knowledge that they will help to mature non-rigid, net-based space-debris capture technology. The next step will be the orbit trial of a complete net totally representative of a space net aboard an atmospheric rocket (to obtain a longer zero-gravity time).
Figure 28: GMV's PATENDER being tested aboard an Airbus A-310 (image credit: GMV)
First scientific campaign using the new Airbus A310 ZERO-G
May 2015: When the Airbus A310 ZERO-G landed at Bordeaux-Mérignac Airport at 12:35 CEST on 7 May 2015, after three days of flying, the first campaign, using the new parabolic flight aircraft was successfully concluded. This first joint parabolic flight campaign by DLR (German Aerospace Center), ESA (European Space Agency) and the French Space Agency CNES (Centre National d'Etudes Spatiales), marked the inauguration of the new A310 ZERO-G parabolic flight aircraft for experiments under altered gravity conditions. This makes the converted 'Chancellor Airbus' the new bridge for experiments heading for space. The eight German research projects in this campaign have the potential to become experiments on the ISS (International Space Station). Five to six scientific research campaigns will be conducted on the new parabolic flight aircraft each year. 26)
The flight campaign was conducted by Novespace, a subsidiary of CNES, of Bordeaux-Mérignac, France. 27)
Figure 29: Photo of the cardiovascular system experiment during a parabolic 'microgravity' flight phase (image credit: DLR)
Legend to Figure 29: When transitioning from 'normal' gravity to microgravity, the distribution of blood in the human body suddenly changes. This experiment, being conducted by the DLR Institute of Aerospace Medicine in Cologne and the Hannover Medical School (Medizinische Hochschule Hannover, MHH), is designed to clarify the consequences of this change for the heart and the aorta.
Figure 30: Photo showing the preparations for a human physiology experiment for a parabolic flight (image credit: DLR)
Legend to Figure 30: Stefan Schneider of the German Sport University in Cologne (Deutsche Sport Hochschule) applies contact gel to the scalp of a subject so that the electrodes in the EEG (Electroencephalography) cap can record the brainwaves. The experiment tested here is scheduled to be carried out on the ISS from 2016 onwards.
Legend to Figure 31: ESA, France's space agency CNES and DLR (German Aerospace Center) inaugurated the Airbus A310 ZERO-G refitted for altered gravity by running 12 scientific experiments in the first campaign of early May 2015.
Legend to Figure 32: Conducting hands-on experiments in weightlessness and hypergravity is enticing for researchers in fields as varied as biology, physics, medicine and applied sciences. To turn the A310 into a parabolic science aircraft, most seats were removed to provide as much space as possible inside, while padded walls provide a soft landing for the researchers – the changes in ‘gravity' can be hard to handle. Extra monitoring stations have been installed for a technician to monitor the aircraft system's as it is pushed to its limits – this is no transatlantic cruise.
1) "Zero-G A310," ESA, April 28, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/04/Zero-G_A310
2) "From the Chancellor Airbus to a new parabolic flight aircraft," DLR, April 24, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13451/year-all/#/gallery/19306
3) "Airbus A310 Zero-G," ESA, Dec. 17, 2014, URL: http://www.esa.int/Our_Activities/Human_Spaceflight/Research/Airbus_A310_ZERO-G
4) Dan Thisdell, "Zero-G flying means high stress for an old A310," Flightglobal, March 23, 2015, URL: http://www.flightglobal.com/news/articles/zero-g-flying-means-high-stress-for-an-old-a310-410416/
5) "Two student teams conduct science in parabolic flights," 8 November 2018, URL: http://m.esa.int/Education/Fly_Your_Thesis/Two_student_teams_conduct_science_in_parabolic_flights
6) "Gravity for the loss," ESA, 12 June 2018, URL: http://m.esa.int/spaceinimages/Images/2018/06/Gravity_for_the_loss
7) "The surprising lightness of the brain," ESA, 4 May 2018, URL: http://m.esa.int/Our_Activities/Human_Spaceflight/Research/The_surprising_lightness_of_the_brain
8) "Brain study during parabolic flight," ESA, 4 May 2018, URL: http://m.esa.int/spaceinimages/Images/2018/05/Brain_study_during_parabolic_flight
9) "Brain science during parabolic flight," ESA, 4 May 2018, URL: http://m.esa.int/spaceinimages/Images/2018/05/Brain_science_during_parabolic_flight
10) "Plants on a gravity rollercoaster," ESA, 11 June 2018, URL: http://m.esa.int/Our_Activities/Human_Spaceflight/Research/Plants_on_a_gravity_rollercoaster
11) "Green for gravity," ESA, 11 June 2018, URL: http://m.esa.int/spaceinimages/Images/2018/06/Green_for_gravity
12) "Seeds on a chip," ESA, 11 June 2018, URL: http://m.esa.int/spaceinimages/Images/2018/06/Seeds_on_a_chip
13) "The roots of gravity," ESA, 11 June 2018, URL: http://m.esa.int/spaceinimages/Images/2018/06/The_roots_of_gravity
14) "The 31st DLR parabolic flight campaign in Bordeaux has ended successfully after two weeks," DLR, 9 March 2018, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-26426/year-all/#/gallery/29993
15) "Parabolic point break," ESA, Human spaceflight and robotic exploration image of the week, May 17, 2017, URL: http://m.esa.int/spaceinimages/Images/2017/05/Parabolic_point_break
16) "FYT (Fly Your Thesis)," ESA, Nov. 11,2016, URL: http://m.esa.int/Education/Fly_Your_Thesis/Student_experiments
21) "Windmill of dust," ESA, Sept. 20, 2016, URL: http://m.esa.int/spaceinimages/Images/2016/09/Windmill_of_dust
23) "Mix as needed," ESA, Nov. 23, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/11/Mix_as_needed
25) L. Cercós, R. Stefanescu, A. Medina, R. Benvenuto, M. Lavagna, I. González, N. Rodríguez, K. Wormnes, "Validation of a Net Active Debris Removal simulator within parabolic flight experiment," 2014, URL: http://robotics.estec.esa.int/i-SAIRAS/isairas2014/Data/Session%202b/ISAIRAS_FinalPaper_0079.pdf
26) "Parabolic flight – stress test for future experiments in space," DLR, May 7, 2015, URL: http://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10081/151_read-13586/year-all/#/gallery/19455
27) "First parabolic flight for the new aircraft Zero-G," Novespace, May 7, 2015, URL: http://www.unlockpwd.com/first-parabolic-flight-for-the-new-aircraft-zero-g-novespace/
28) "European space agencies inaugurate altered-gravity aircraft," ESA, May 8, 2015, URL: http://www.esa.int/Our_Activities/Human_Spaceflight/Research
29) "Weightless in space," ESA, May 8, 2015, URL: http://www.esa.int/spaceinimages/Images/2015/05/Weightless_space
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).