Download Which Way is Up?

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,
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information from organisations within UK Space, explaining the importance of several
themes of space exploration and technology,
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AstroPi competition details and entry form,
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Competition entry project planning guidance for teachers and students.
Curriculum Links
Science:
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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
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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.
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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