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Which Way is Up? Living in Microgravity on the International Space Station An AstroPi resource for secondary school teachers. Introduction The International Space Station is in freefall. Being inside the ISS is, in many ways, like being in a lift that is falling down a shaft. But there is no need for panic – it isn’t going to crash into the Earth any time soon! It is moving fast enough so it orbits the Earth around 16 times a day. This Astro-Pi resource examines gravity and weightlessness. It considers the action of forces, and how bodies moving with sufficient speed under the force of gravity can orbit planets. It is suitable for study of qualifications at age 14-16. How to use this guide: This teacher guide, and the resources that accompany it, can be used in different ways: 1. Following the activities in sequence will cover all the curriculum links listed below. This might be done as part of a collapsed timetable day, or over a series of sessions. This would give a thorough preparation for meeting the challenges and entering the competition, regardless of prior learning. 2. Teachers can pick and choose which activities, resources and links to use and when – they can be used independently of each other. This might enhance the ways in which space and magnetism topics are currently taught. If teachers have specific challenges in mind that align with their interests and those of the children, the supporting learning activities might be selectively chosen. 3. Teachers may wish to present students, in class or as part of an extra-curricular activity, with the challenges only. Please note – the challenges are merely suggestions, and schools are completely free to use the AstroPi in any way they see fit to enter the competition. Other information accompanying this guide available at Astro-Pi.org: information from the Raspberry Pi foundation, all about the AstroPi and Raspberry Pi, information from organisations within UK Space, explaining the importance of several themes of space exploration and technology, AstroPi competition details and entry form, Competition entry project planning guidance for teachers and students. Curriculum Links Science: Gravity as a force between two objects Calculations of kinetic and gravitational potential energy Forces in pairs Representing forces as vectors Relationship between speed and orbital radius (qualitative) Maths Moving between algebraic, numerical and diagrammatic representations Expressing relationships between variables Multi-step problem solving Modelling situations mathematically. Learning Activities 1. Revising and consolidating prior learning about gravity The National STEM Centre gravity resources listed here and here can help students review earlier learning of gravity. The Gravity & Space Traffic Lights activity in this resource can be used as a quick review or informal assessment and are useful to to dispel misconceptions before moving to more rigorous consideration of gravity. 2. Gravity as a force between all masses The gravity force lab simulation from PhET shows the small force of gravity between small masses (i.e. orders of magnitude less mass than a planet) - be aware, the strain showed by the two ‘people’ holding the masses is hugely exaggerated and should be used as a discussion point to help students appreciate the miniscule forces involved with two masses of this size. 3. Projectile and Orbital Motion This fun PhET simulation allows children to launch a variety of objects at varying speed and angle to trace their path, providing a qualitative understanding of the effect of speed, mass and angle of launch. On this short scale the curvature of the Earth is ignored, so all the projectiles will eventually hit the ground – discussion might lead to how, as the distances involved increase greatly, the curvature of the Earth might affect where the object lands. This series of Wikipedia animations shows Newton’s Cannon, a thought experiment that considered what would happen if projectiles were fired from an elevated position with increasing speed. This takes into account the curvature of the Earth. It is recommended that the animations are clicked-on and viewed individually. It is important to note that the force of gravity does not act ‘downwards’ in the animations, but towards the centre of the Earth – a detail that becomes critical as the projectile distance increases and orbital motion is achieved. This PhET animation shows orbiting bodies, including the ISS, with arrows to show instantaneous velocity and the gravitational force acting towards the centre of mass of each body. Note: A common misconception is that the planet stays still while the orbiting body simply moves in a circle around it - by showing the animation demonstrating the orbit of the moon around the Earth, it can be shown that the two bodies rotate around a common centre of mass resulting in a ‘wobble’ in the Earth’s position (A search online for ‘hammer throwing videos’ will show a more down-toEarth example of this). It is this wobble that is being used by astronomers to identify planets orbiting stars outside our solar system (further details of this can be found here) The ISS, due to its relatively small mass, causes no noticeable wobble! Note also that the animations are fixed on the centre of mass of the system – in reality the system is orbiting the Sun, which is also orbiting the galaxy, meaning that actual motion paths are quite complex – some students might consider how frames of reference affect study of motion – something that Einstein considered when formulating the Special Theory of Relativity. 4. Conservation of Energy for Orbiting Bodies The steady orbit of an object, such as the ISS, can be considered either in terms of forces (i.e. motion in a circle, details of which are not required at this level) or conservation of energy. This PhET animation introduces the idea that Gravtitational Potential Energy (GPE) and Kintetic Energy (KE) can change while conserving the total amount of energy in the system. Examples where this is demonstrable include ski jumps, roller coasters, and falling bodies (neglecting air resistance). 5. Calculating Changes in Gravitational Potential Energy The equation ΔGPE=mgΔh represents the change in gravitational potential as the height (h) of a known mass (m) changes in a constant field (g). Using a simple diagram of an orbiting body (i.e. concentric circles), some discussion of the following points is possible: If the orbital height remains constant (i.e. circular motion) what does this imply for the change in GPE during an orbit? (answer: it does not change. This circle - or sphere, in the case of a real planet is known as an ‘equipotential’). Neglecting other energy changes (such as thermal), changes in the GPE would result in a corresponding change in the kinetic energy of the ISS. A loss of GPE would result in a gain in KE. What effect does this have for bodies with elliptical orbits? As KE = ½ mv2 (treating v is the scalar speed value) what implication does constant height have for the speed of the ISS? (constant speed – no GPE is transferred when orbiting at constant height, so there is no change in KE). In reality, energy is dissipated from the ISS through collisions with gas particles in the upper reaches of the atmosphere. What effect does this have on the speed and height? (KE is decreased, leading to a drop in speed. Referring back to Newton’s Cannon shows that sub-orbital speeds result in crashes!) 6. Kinetic Energy of the ISS It is possible to calculate the average KE of the ISS using known values – this can be presented as a mathematical challenge to students. Mass = 19,323 kg Height above the Earth (approx.) = 410km Mean radius of the of the Earth = 6371 km Orbital period = 92 minutes This gives a kinetic energy value of 1,151,181,028,730 Joules, or 1151 GJ (or 92 million times the kinetic energy of a 1000kg family car moving at 5 m/s). Boosting the Kinetic Energy of the ISS The ATV (automated transfer vehicle) is a European Space Agency craft that (until recently) has delivered supplies and provided boosts to the orbital altitude. The ATV can carry up to 4700kg of propellant for these rocket-powered manoeuvres, which also include debris avoidance and attitude control (turning the ISS) As a further mathematical physics challenges, a calculation of the energy input required for a 1km uplift can be calculated using ΔGPE=mgΔh n and works out at around 193,230,000 J (193 MJ). 7. Freefall Other than the occasional orbital boosts by the ATV and the tiny amount of air resistance from the upper atmosphere, the ISS is in freefall – meaning there is a negligible effect of gravity within the frame of reference of the ISS. Objects therefore have no apparent weight, and float around if allowed to. While students may have seen weightlessness effect effects in films, these are sometimes presented in confusing ways (the limited budget of most film-makers requires them to invent a ‘gravity field generator’ or other method of putting people and objects on ‘the ground’, although scenes from 2001: A Space Odyssey and, more recently, Gravity have given convincing portrayals of life in microgravity). Some of these archive videos of the ISS and Soyuz show astronauts carrying out everyday duties in microgravity and this stunning video shows astronauts literally running head-over-heels using only centripetal force to provide traction. Circular motion in microgravity is demonstrated again in this short clip. Some further effects of microgravity, including behaviour of liquid droplets, can be seen in these short video clips. ESA contributed the Microgravity Science Glovebox, a sealed environment for experiments including material science, biotechnology, fluid science and research into combustion and crystal growth. INSPIRATION? SEEKING Accelerometers measure accelerations in all directions. They are used in mobile devices for detecting shaking and rotation events which affect output on the screen. Can you create an ‘app’ for the Astro-Pi that makes use of the accelerometer? Think about the applications you have used on mobile devices – would these be useful (or fun) in space? INSPIRATION? SEEKING Accelerometers detect the size and direction of the force due to gravity when held still. Mobile devices make use of this, to ensure the screen display is always the right way up. The ISS, and the objects inside it, are in freefall, so an accelerometer will read zero in all three axes… or will it? Small but measureable accelerations may act on the ISS – can you find a way to detect and identify them? Make use of other inputs and sensors to contribute additional data. Image: ESA INSPIRATION? SEEKING The orientation of the ISS, in 3-dimensions, is known as its attitude. This must be carefully steered to control exposure to heat, to maximise solar power production, to allow communication with Earth and to manage microgravity conditions for experiments. A set of massive spinning steel wheels can be rotated for attitude adjustment, supplemented by several small rockets when required. What data can be collected by the Astro-Pi to record these events? Can the Astro-Pi gyroscope be used? How can the data be presented – in real time or after the event? Image: ESA INSPIRATION? SEEKING Traces of Earth’s atmosphere are found even at the height of the ISS, more than 400km up. Just as on Earth this causes drag that slows moving objects down, and the ISS is no streamlined sports car! Can you detect the force of air resistance? Left unchecked this would cause the orbit to decay, losing height and putting the ISS in great danger. Orbital boosts are provided by rocket engines on the ATV, Progress module, Soyuz and Service Modules. Boosts might be staged 34 times each month – can they be captured? How can the full capabilities of the Astro-Pi be used? Image: ESA