Rosetta Mission Teacher’s Course

Rosetta is a robotic space probe built and launched by the European Space Agency whose mission is to investigate comet 67P/Churyumov-Gerasimenko. It was launched on the 2nd March 2004 and on the 12th of November 2014 the Philae lander was detached from Rosetta and successfully landed on the comet. Unfortunately the initial impact was softer than expected and Philae bounced onto rough terrain in the shadow of a cliff or crater wall. This prevented the solar panels from harnessing the Suns’ light to recharge its battery and its activities have been curtailed, at least in this phase of the comet’s orbit.


On the 31st October 2014 I was lucky enough to attend a course arranged by the National Space Academy on the science behind the mission.

The course was based in the Robert Hooke building at the Open University Milton Keynes. This was quite apt as Robert Hooke was very interested in comets. He published a paper on them in 1678 showing that they have a solid nucleus and generate their own light. He actually tried, unsuccessfully, to make one using sulphuric acid.

Robert Hooke FRS (28 July 1635 – 3 March 1703) was an English natural philosopher, architect and polymath.

The day was introduced by Ross Burgon, who is an outreach co-ordinator, who showed us a simulation program that allows you to see the effect of an object colliding with the Earth – an impact calculator.


The first speaker of the day was Professor Ian Wright, who is the principal investigator of the Ptolemy instrument.


Ptolemy is one of nine instruments on Philae and it is specialised for the analysis of so-called light elements, comprising carbon, nitrogen and oxygen. It can also be used to analyse volatiles such as water, carbon monoxide and noble gases, as well as light organic compounds. As a result of the non-optimal landing conditions experienced by Philae, Ptolemy was only operated in its “sniffing” mode, attempting to analyse material released from the comet during the lander’s descent and rebounding.


The above image shows the Philae lander and its instruments. Copyright ESA/ATG medialab.

The objectives of the Rosetta mission are:

To study the origin of comets, the relationship between cometary and interstellar material, and its implications with regard to the origin of the Solar System.

The measurements to be made to achieve this are:

Global characterisation of the nucleus, determination of dynamic properties, surface morphology and composition;

Determination of the chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus;

Determination of the physical properties and interrelation of volatiles and refractories in a cometary nucleus;

Study cometary activity and the processes in the surface layer of the nucleus and the inner coma (dust/gas interaction);

Global characterisation of asteroids, including determination of dynamic properties, surface morphology and composition

The Open University is just one of the UK contributors to the Rosetta project. The map below shows the other contributors.


Why study a comet – the small questions

How big is it? Shape? Rotation? Orbital Characteristics? Colour? What is it made of? Magnetic properties? Porosity? Temperature? What is it like inside? Electrical properties? How does it change as it heads towards the Sun? Dust? Ice? What is its strength?

Why study a comet – the big questions

How is it related to the other bodies of the Solar System?

What can we learn about our home planet by observing it up close?

What can we learn about us as sentient, conscious beings?

Why study a comet – the big answers

Because it is a surviving remnant from the events that happened about 4.5 billion years ago, when the Sun and planets formed.

It is an opportunity to gaze at our ancestral, astrological beginnings and to search for clues as to how such materials begat life.

Comets are a good indication of the early Solar System before the presence of life.

Studying comets could reveal conditions present at the birth of our solar system (4600 million years ago)

What is a comet?

Comets are usually small bodies that orbit the Sun in our Solar System.

They consist of rock, dust and ice.

The solid, core structure of a comet is known as the nucleus. Usually it is smaller than 50km; the tail (coma, etc.) can be very large up to 150 million km (larger than the sun).

The nucleus is usually made up of rock, dust, water ice and gases such as carbon dioxide, carbon monoxide, methane, and ammonia. Methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids may also be found.

Comets are usually very dark objects as only few of them reflect light.

Comets consist of material formed at the formation of the earth (other planets) frozen in time.


The main part of the Rosetta mission was to investigate one comet in particular.

67P/Churyumov–Gerasimenko (67P/C-G) is a comet originally from the Kuiper belt, a region of the Solar System beyond the planets, with a current orbital period of 6.45 years. It is approximately 4.3 by 4.1 km at its longest and widest dimensions.


Comet Churyumov–Gerasimenko as seen by Rosetta

It is the first comet to have had a spacecraft land on its nucleus.

The Rosetta Mission

The origin of the name of the mission comes from the Rosetta stone through which researchers were able to unlock Egyptian hieroglyphs and Demotic by comparing to the same story told in ancient Greek? Enabled the discovery of 3000 years of ancient history

The Rosetta Mission is a project funded by the European Space Agency (ESA)


The Rosetta Mission in a similar way seeks to reveal secrets of the origin of earth.

The mission was approved in Nov 1993 and the hardware design was completes in 2000

It was launched in March 2004 on Ariane 5G in Kourou, French Guyana

It met up with comet Churyumov Gerasimenko (67P) in November 2014

It is the first project ever to attempt in-situ analysis of comet!!

3000kg launched (100kg lander, 165kg scientific instruments)

It flew past several other comets before landing

Fully automatic landing after determining a suitable landing site

Total cost 970-980 million

Rosetta Orbiter

Mass at launch was 3000kg

Dimensions are 2.8 x 2.1 x 2.0 metres

11 experimental probes

ALICE ultraviolet imaging spectrometer

CONSERT radio wave transmission/reflection by nucleus

COSIMA ToF MS (up to 2000Th) of dust particles (organic??)

GIADA number, mass, momentum and velocity distribution

MIDAS dust environment population, size, volume and shape

MIRO mircowave to determine abundances of gases

OSIRIS optical, spectroscopic and infrared imaging system

ROSINA ion and neutral spectrometer RPC physical properties of nucleus, structure of inner coma etc.

RSI radio signal shift to measure mass, density and gravity to determine orbit

VIRTIS visible and infrared spectrometer to map temperatures, identifies gases, examine physical conditions on the nucleus and determine landing site

The Lander Philae

Mass is 100kg

Dimensions are 0.8 x 0.8 x 1.0 metres

Extreme surrounding conditions: warm environment -40 – 50C; cold environment -140 – -50C

9 experimental components

APXS alpha proton X-ray spectrometer

ÇIVA/ROLIS Rosetta Lander imaging system

CONSERT probing nucleus using radio waves

COSAC complementary sampling and composition (chromatography/ ToF MS)

MODULUS Ptolemy gas chromatograph ion trap MS

MUPUS density and mechanical/thermal property testing

ROMAP local magnetic field and comet/solar wind interaction

SD2 sample drilling below 20cm for other components

SESAME electrical, seismic and acoustical testing


Professor Wright’s project is MODULUS Ptolemy

Methods Of Determining and Understanding Light elements from Unequivocal Stable isotope compositions (MODULUS)

Developed at the Open University in the UK

Originally designed as 2 versions Ptolemy and


Only Ptolemy flew, so Bernice was incorporated into Ptolemy


Ptolemy is a mass spectrometer

A mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules. It makes use of the basic magnetic force on a moving charged particle.

After the particles are ionised they are accelerated in an electric field by an applied voltage. After a single velocity is selected the particles move into the mass spectrometer where a magnetic field causes the particles to be deflected in a circular path. The radius of the path and the position of the detector is a function of the mass of the particles.



r is the radius of curvature, m is the mass of the particle, v is the velocity of the particle, q is the charge of the particle, B is the magnetic field strength


Stable Isotope Ratio Measurements for Light Elements

Measure 2H, 13C, 15N, 17O and 18O relative to specified reference

Degree of isotope fractionation provides information about temperature range at time of formation

Material from SD2 will be pyrolysed to be converted into compounds such as H2, O2, CO, CO2 and N2

Isotopic separation by the GC/MS

Isotope ratios determined by


Ion Trap

An ion trap is a combination of electric or magnetic fields used to capture charged particles in a vacuum. Ion traps have a number of scientific uses such as mass spectrometry, basic physics research, and controlling quantum states. The two most common types of ion trap are the Penning trap and the Paul trap (quadrupole ion trap).

Ion trap as the instrument of choice

There are two main purposes for the mass spectrometer in the Ptolemy:

Detector for gas chromatography and identification and quantifying sample gases from the gas chromatogram

Isotope ratio mass spectography, which is unusual for ion traps!)

Magnetic sector (standard for those experiments) was not compatible with requirements for Rosetta mission

Because of small size, simplicity of construction, only 1 parameter control (amplitude of radio frequency drive potential), possibility to adjust time for build-up ions (precision improvement) the ion trap mass spectrometer was chosen

Ability to work at fairly high pressures (1 x E-3 mbar)

At the time of design (1995-1998), no ion-trap mass spectrometer could detect isotopes at natural abundance levels

Reaction of residual water vapour into MH+ (M = CO2,CO,N2)

No meaningful measurements this way and no way to measure hydrogen at the time.

To overcome the problem of residual water generating the MH species, a supplementary H- gas flow was introduced to allow reactions with the stored sample ions

Thermodynamics prefer the reactions involving CO2, CO and N2.

Calibration procedures have yielded values between 3-5 0 for nominally d(0). Accuracy needed are a few or tens of percents.

Due to the lack of reactivity with hydrogen isotopic oxygen ions were measured using the O2

The ion trap is insensitive for low m/z values (H), therefore to measure H (and D) the sample ions were allowed to react with stored Ar2

Hardware Setup Ion Trap

Size reduction of ion trap allows the saving of energy

Resonance frequency of trap sensitive to temperature and therefore self-tuned to achieve better resolution (upper m/z limit)

Usually at least 12 to 150 Th (e.g. water from xenon)

Focusing on low m/z range reduces complexity

Ionization through e- from etched Si wafers

Ion detection through spiral amplification channel

Supply of grade 6 helium (99.9999 pure) to actuate pneumatic valves, as carrier gas and the ion trap buffer gas is admixed with grade 6 Ar (dilution ratio 100ppm) to perform isotope analysis of H and provide calibration signal at m/z 40

Additional H- supply for conversion of sample gases into protonated ions

Chemical getter pump to reduce residual carbon dioxide

Gas manifold system delivers the gases to be analysed into the trap

These tubes are chemical reactors, which mean ceramic tubes containing solid state chemical reagents. Heating those releases carbon dioxide of known isotopic composition for calibration

The gas chromatograph is the interface between manifold and ion trap. Several columns are used to split up species


Three versions of Rosetta ensure functionality

Analysis sequences have all been preprogrammed into the instrument

Modifications were possible during the flight phase

Experimental work is possible until they are out of power

Technological spin offs of those ion traps for small-scale real world applications (under development)

J.F.J. Todd, S.J. Barber et al., J. Mass Spectrom. 2007 42 1-10












The Rosetta probe acquired these images of Comet 67P/Churyumov-Gerasimenko on the 11th July 2014. The pictures show that the comet appeared to have a double nucleus or core, making it what scientists call a “contact binary.


Below from left to right the images show Philae’s descent towards 67P before bouncing off in an easterly direction.


Rosetta – a 10 year ESA mission to land on a comet!




The future of the Rosetta mission

It is hoped that with more direct sunlight Philae will “wake up”, allows the lander team to operate the instruments and do more science.

Rosetta, itself, will continue to observe what is actually happening on the comet up close.

You can keep abreast of what is happening with the Rosetta mission in real time by visiting the web page

There is also a twitter account

and facebook


The image above shows models that the Ptolemy team used to plan the landing selection. The little blue model is the official ESA model and the duck model was bought from a well-known supermarket for 67p.





The above image is a model of Philae


The next session of the day was run by Sophie Allan of the National Space Academy. Sophie is a part time specialist Physics teacher in Leicester City Schools and develops and teaches Physics programmes for the National Space Academy, she is also actively involved in the development of new curriculum focused STEM masterclasses and CPD activities with a particular specialism in GCSE Astronomy and Applied Science. One of her aims is to fill the gap and produce more good engineers for space engineering and space science.

The national space academy and esa have produced resources that link the Rosetta mission to the science taught in schools.

Where does space begin?


There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 km above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping.

Sending objects into space and fighting against the pull of gravity costs a lot of money and is very hard. A large velocity is needed for lift and to produce an orbital velocity.

It costs roughly £17,000 per kg to send anything into space which is why missions need to be small.

Life on Earth is very fragile. We are kept safe by a very thin atmosphere. The blue haze in the image below is the atmosphere and you can see how thin it is.



What the maths really tells us

Earth diameter = 12 800 km

Moon diameter = 3 460km

Earth-Moon distance = 384 000km



People are very bad at imagining the distances from the Earth to other astronomical bodies such as the Moon. One way of illustrating the actual distance between the Earth and the Moon is to get a globe and wrap a piece of string around it ten times.


The above image shows Sophie holding the “Earth”.

Get a volunteer to unwind the string and your audience will get a much better understanding of the actual distance between the Earth and the Moon.

If you could drive to the Moon at a speed of 70mph then it would take you 142 days. It would take you 151 years to get to the Sun. Travelling at the speed of light you would get to the Sun in about 8 minutes.

People are also bad at visualising time.

The scale of “deep time”

The Cosmic Calendar is a method to visualize the vast history of the universe in which its 13.8 billion year lifetime is condensed down into a single year. In this visualization, the Big Bang took place at the beginning of January 1 at midnight, and the current moment is mapped onto the end of December 31 at midnight. At this scale, there are 438 years per second, 1.58 million years per hour, and 37.8 million years per day. This concept was popularized by Carl Sagan in his book The Dragons of Eden and on his television series Cosmos.

The Big Bang occurs on the 1st January

The Milky Way forms of the 15th March

Sun formed on the 31st of August

The oldest rocks known on the Earth appeared on the 16th September

Mammals appeared on the Earth on the 26th December

Primitive humans appeared on the 31st December at 22:24

Agriculture started on the 31st of December at 23:59

Jesus was born on the 31st of December at 23:59:55

Muhammad was born on the 31st December at 23:59:56

Modern World as we know it appeared on the 31st December at 23:59:58

Scale activities – on all scales!


1 atom has a diameter of 0.1nm. Representing the diameter of an atom as a 1 cm diameter marble the UK would be 100 million atoms across.

The scale of the Universe 2


The image above left shows a distance of 10 million light years from the Milky Way and the image above right shows actual size oak leaves.

It’s all empty space – even us

Density of a nucleon is 3.2 x E18 Kgm^-3

The mass of an average person is 50kg

There are about 7 billion people on the Earth

The volume of the world population is mass/density = 50 x 7E9/3.2E-19 = 1E-7m^-3

If you had a match box of dimensions 0.01 x 10.05 x 0.07 = 3.5E-5m^-3 you could fit the world population in it 320 times.

So how do we get into space?

You need a rocket. Two demonstrations show how a rocket takes off.

The Whoosh bottle


You need a bottle “rocket”, ethanol fuels and a source of oxygen, i.e. the air


Add some ethanol to the bottle


Swill the ethanol around the inside of the bottle

Pour out excess ethanol and cover the top of the bottle to prevent the ethanol vapour escaping


Quickly remove the top and use a lighted splint to ignite the vapour. The hot gases move upwards and by Newton’s third law there is a thrust downwards on the bottle. In a rocket the apparatus is the other way round. The gases move downwards producing an upwards thrust of the rocket.

Use a video camera to film and time the pulse. Use a set of scales to measure the size of the thrust pulse value

Use the volume of the bottle to calculate the oxygen content by volume (Remember that oxygen is about 21% oxygen)

From the complete combustion equation (balanced!) work out the mass of products you can work out the exhaust gas velocity!

Making a mini-version using 1.5 litre bottles for movement


You can tape the bottle to an electronic scale and measure the thrust. Videoing the experiment will give you the time.

The compressed air rocket


The rocket launcher is made up of various bits of plumbing materials


The above image shows the materials involved in the making of a rocket (which is really a projectile). The images below show the making and using of the rocket.


Space Launch System

The Space Launch System (SLS) is a United States Space Shuttle-derived heavy expendable launch vehicle being designed by NASA. It follows the cancellation of the Constellation program, and is to replace the retired Space Shuttle. The NASA Authorization Act of 2010 envisions the transformation of the Constellation program’s Ares I and Ares V vehicle designs into a single launch vehicle usable for both crew and cargo. It will be much larger than Saturn 5 (the rocket involved in the Apollo program).

The SLS launch vehicle is to be upgraded over time with more powerful versions. Its initial Block I version is to lift a payload of 70 metric tons to low Earth orbit (LEO), which will be increased with the debut of Block IB and the Exploration Upper Stage. Block II will replace the initial Shuttle-derived boosters with advanced boosters and is planned to have a LEO capability of more than 130 metric tons to meet the congressional requirement; this would make the SLS the most capable heavy lift vehicle ever built.

These upgrades will allow the SLS to lift astronauts and hardware to various beyond-LEO destinations: on a circumlunar trajectory as part of Exploration Mission 1 with Block I, to a near-Earth asteroid in Exploration Mission 2 with Block IB, and to Mars with Block II. The SLS will launch the Orion Crew and Service Module and may support trips to the International Space Station if necessary. SLS will use the ground operations and launch facilities at NASA’s Kennedy Space Centre, Florida.


Artist’s rendering of the SLS Block 1 crewed variant launching


Ballistics (from Greek “to throw”) is the science of mechanics that deals with the launching, flight, behaviour, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.

A ballistic body is a body with momentum which is free to move, subject to forces, such as the pressure of gases in a gun or a propulsive nozzle, by rifling in a barrel, by gravity, or by air drag.

A ballistic missile is a missile only guided during the relatively brief initial powered phase of flight, whose trajectory is subsequently governed by the laws of classical mechanics, in contrast (for example) to a cruise missile which is aerodynamically guided in powered flight.

A projectile is any object projected into space (empty or not) by the exertion of a force. Although any object in motion through space (for example a thrown baseball) is a projectile, the term most commonly refers to a ranged weapon. Mathematical equations of motion are used to analyse projectile trajectory.


For stability the centre of pressure (cp) needs to be under the centre of gravity (cg).

For a rocket you want the launch velocity to big enough to allow it to escape the pull of gravity.

Vertical motion under the influence of gravity can be described by the basic motion equations. Given the constant acceleration of gravity g, the position and speed at any time can be calculated from the motion equations:


Where Vy is the vertical velocity at time t, Voy is the launch vertical velocity, Y is the vertical height at time t, Yoy is the launch distance (which will be zero) and g is gravity (9.8ms-2). Y needs to be large enough to get into space (of course g decreases as you get further away from the Earth).

As the paper rocket is in fact a projectile you need to consider the horizontal motion as well. Putting all the relevant equations together gives the range as vo2sins (2q)/g where vo is the launch velocity.


Maximum range occurs when q = 45o and R = vo2/g

In order to escape into space forces much greater than the pull of gravity is needed. You need multiples of g force. There is a limit to the value of g force that a human can withstand although pilots and astronauts can be trained to withstand higher values. Colonel John Slapp demonstrated that a human can withstand at least 46.2G but he did experience temporary blindness after one test run.

Tour of the laboratories

Our guide was Dr Andre Morse.


The above right picture shows the space environmental simulation chamber (looks like a large pressure cooker) that is used for testing the Ptolemy Qualification Model. It is cooled using liquid nitrogen.

The group have copies of the various parts of the lander to help them work on the original. The copies are cryogenically cooled.


The above image shows Dr Dan Andrews who explained some of the things that went on in the lab.


The above centre image shows Dr Morse explaining the function of part of the equipment – an ion trap mass spectrometer



Dr Starkey is deputy Head and Principal Operator NanoSIMS laboratory. She is responsible for daily maintenance, planning of user time, and experimental design and set-up, analysis and interpretation, for internal and external scientific users of the instrument on a range of terrestrial and extra-terrestrial projects.

She told us about her work which includes analysing space dust to understand the history of the earliest times in the Solar System. Space dust (this consists of many things such as tiny pieces of comets, asteroids, planets and even spacecraft debris) literally rains down to Earth but is not easy to collect.

The largest amount of space dust can be found in the stratosphere and the NASA Cosmic Dust Laboratory uses a high altitude aircraft to collect it.


The above images show the Cameca NanoSIMS 50L. It is the latest generation of Secondary Ion Mass Spectrometer (Ion Microprobe) instrumentation, allowing high sensitivity compositional analyses of up to 7 species (elements, molecules or isotopes) simultaneously at a nominal spatial resolution of down 50 nm, and with high mass resolution.

The instrument can be used to analyse:

•Primitive meteorites,

• Martian and lunar samples,

• Cometary material brought back by Stardust,

• Interplanetary dust particles,

• Terrestrial rocks,

• Experimental analogues and simulations

• Other materials – biological and materials


The image above left is the SEM and the image above right shows a preparation area.

When Dr Starkey gets her dust the first thing she does is to uses a scanning electron microscope (SEM) to look at it. This allows her to see the particle at a high resolution so that she can understand the appearance and texture of the tiny components they contain. The SEM also provides chemical information.

The final laboratory technique she uses has the disadvantage of being totally destructive. The particle being examined by the NanoSims is totally destroyed during analysis. It has a very small (nano-sized) beam that can provide analyses with a really high spatial resolution (i.e. allows her to resolve very small features) and high sensitivity (i.e. she can produce numbers from analysing very little material).



Particles have to be pressed onto gold foil to produce a flat surface for the final phase of analysis, the NanoSIMS, and this very expensive and sensitive instrument can’t tolerate rough surfaces.

The primary goal of the Stardust mission was to collect samples of a comet and return them to Earth for laboratory analysis. The images above are some of the results of the analysis

More practical ideas

Examination of the light emitted from the tails of comets shows that one component of the tail consists of ionised gas (plasma) originating from the comet’s atmosphere (or ‘coma’). The tail plasma streams away from the comet in the direction radially away from the Sun, and appears to flow along magnetic field lines which are shaped like sharply-bent hair-pins draped over the comet’s nucleus. In fact comets actually have two tails; the ion tail is formed by the process of sublimation and ionisation of gas while the dust tail is the release of the silicate dust from the surface of the nucleus.



A plasma is created whenever atoms of a gas are heated up to very high temperatures. As a result the atoms have so much energy that when they collide, the electrons are thrown off. In fact plasma is a group of electrons and ions. The plasma ball is an electrical apparatus invented by Nikola Tesla in 1894. In the 1980s it gained popularity. It is essentially a glass globe with a central electrode. The globe is filled with a mixture of inert gases. It works just like a teals coil and is useful in conducting electrical experiments. In fact, it can be viewed as a miniature Tesla coil. Inside the ball is a coil of wire that has a very high frequency passing through them. Translated, this means the electrons in the wires are oscillating very quickly. The result is that the atoms around the coil lose their electrons and plasma is formed. Because the globe has had some of its air removed (sucked out) it is very easy to makes electric sparks and readily sees them. In short, plasma is a partially ionized gas and therefore the ability of the negative charges to move about makes it very responsive to electromagnetic fields. Plasma, having these unique properties is considered to be the fourth state of matter.

The plasma ball gives you some idea of what is going on in a comet’s tail but you can also use it to do some cool experiments.

Borrow or buy a long fluorescent tube and bring it near the plasma ball. You will notice that once a part of the mercury gas in the tube gets glowing it can stay glowing even as you extend it. There is essentially no limit to how far you can pull the tube. It also works on the household small tubes. The fluorescent tube holds ionized mercury (plasma) and that plasma is a conductor (because of the free charges) and for this reason the tube’s light can be drawn with no apparent increase in resistance (no decrease in brightness).

Also, note that the starting point of the tube must be close to the plasma ball where the Electric Field is largest (the voltage is changing the most rapidly). This can be demonstrated by moving the tube closer then further radially to the globe. At certain distances the tube will not glow. There is a minimum Electric Field required to ionize the mercury gas and if the field is not strong enough the tube will not light.

The fluorescent light is produced: the low pressure, ionized mercury gas releases mostly UV and violet light when it regains its electrons. This light falls on the fluorescent paint that coats the inside of the tube. The paint then glows white. The UV light is blocked by glass, so harmful UV light does not escape the glass tubes. Thus, the process does not work in reverse: if you shine UV light on the tube from the outside the paint won’t fluoresce.


The very high voltages of the plasma ball can easily polarise a coin (or piece of aluminium foil) placed on top of the plasma ball. By bringing your finger only a few millimetres above the penny, you will be able to produce a spark from the top of the coin. This spark will not cause pain, or electric shock, but will be hot and if you hold your finger their long enough it might begin to hurt. The tip of the finger will now show a few harmless burn marks that will rub off in a day. This sparking technique explains how lightning forms due to the Electric Field ionizing the air. You can also have fun burning small pieces of paper with the spark. If you are too shy to touch the spark with your hand, you can touch a metal key (or any conductor) to the coin and the spark will still form while providing additional insulation. You should avoid touching the spark with your fingernail. Fingernails conduct electricity better than the skin and underneath it is a tissue that is dense lined with pain nerves.


Gravity – the basics of Solar Systems orbits


Comets have an elliptical squashed orbit. Probably the most famous comet is Halley’s comet.

Halley’s Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–76 years. Halley is the only short-period comet that is clearly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime. Halley last appeared in the inner Solar System in 1986 and will next appear in mid-2061.


But what is gravity?

Isaac Newton said that the gravitational force between objects is always attractive and he produced a formula that gave a value of the gravitational force between two objects.

Fg is the gravitational force, m1 is the mass of object 1, m2 is the mass of object 2, r is the distance between m1 and m2 and G is the universal gravitational constant.


His work has served us well and enabled men to go to the Moon but Albert Einstein realised that the work could do with improving.

With his theory of relativity, Albert Einstein explained how gravity is more than just a force: it is a curvature in the space-time continuum. That sounds like something straight out of science fiction, but simply put, the mass of an object causes the space around it to essentially bend and curve. This is often portrayed as a heavy ball sitting on a rubber sheet, and other smaller balls fall in towards the heavier object because the rubber sheet is warped from the heavy ball’s weight.


You can demonstrate this with the above arrangement, pulling down the centre of the cloth instead of using a ball. By pulling the cloth down by different amounts you can mimic objects of different masses.


A 2-dimensional image of how gravity works. Via NASA’s Space Place

In reality, we can’t see curvature of space directly, but we can detect it in the motions of objects. Any object ‘caught’ in another celestial body’s gravity is affected because the space it is moving through is curved toward that object.


Sir Isaac Newton PRS MP (25 December 1642 – 20 March 1726/7) was an English physicist and mathematician (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution.

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist. Einstein’s work is also known for its influence on the philosophy of science. He developed the general theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).

Potential energy wells


The above image shows a plot of a two-dimensional slice of the gravitational potential in and around a uniformly dense, spherically symmetric body.

A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space – the more massive the body, the deeper and more extensive the gravity well associated with it. The Sun is very massive, relative to other bodies in the Solar System, so the corresponding gravity well that surrounds it appears “deep” and far-reaching. The gravity wells of asteroids and small moons, conversely, are often depicted as very shallow. Anything on the surface of a planet or moon is considered to be at the bottom of that celestial body’s gravity well, and so escaping the effects of gravity from such a planet or moon (to enter outer space) is sometimes called “climbing out of the gravity well”. The deeper a gravity well is the more energy any space-bound “climber” must use to escape it.

In astrophysics, a gravity well is specifically the gravitational potential field around a massive body. Other types of potential wells include electrical and magnetic potential wells. Physical models of gravity wells are sometimes used to illustrate orbital mechanics. Gravity wells are frequently confused with embedding diagrams used in general relativity theory, but the two concepts are distinctly separate, and not directly related.


“The gravitational potential, V, (Jkg^-1), at a point is the work done, W, per unit mass, m, to move a small object from a place far away from gravity fields”. It can also be defined as the energy required per unit mass to remove that mass from the gravity field (i.e. place it far away). This is a useful variable to define since it gives us an idea of how much energy is involved in moving objects within a gravity field. In the above equation M is the mass of the object that the small mass is a distance r from and the negative sign indicates that gravity is an attractive force, the system binds mass together and you need energy to escape the system.

So what happens to the speed of a comet as it goes along its orbit?



The equations involved

Gravitational force F = (GMm)/r^2

Gravitational potential V = – GM/r

Gravitational field E = GM /r^2 = -gradV (gradient of a potential V – r graph)

Potential energy PE = – GMm/r


Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism. Later the concept was expanded greatly to comprise any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is often represented by a spectrum, a plot of the response of interest as a function of wavelength or frequency.

Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms and molecules. Spectroscopy is also used in astronomy and remote sensing on earth. Most research telescopes have spectrographs. The measured spectra are used to determine the chemical composition and physical properties of astronomical objects (such as their temperature and velocity). Therefore it can be used to identify the constituents of comets and of course other members of the Solar System. It can also be used to identify the constituents of exoplanets.


You can demonstrate spectroscopy in a lab using gas discharge lamps. Look at the light through a diffraction grating and you will see a spectrum. The spectrum identifies the element in the gas.



An emission spectrum appears as a series of lines also called a line spectrum. This line spectrum is called an atomic spectrum when it originates from an atom in elemental form. Each element has a different atomic spectrum. The production of line spectra by the atoms of an element indicates that an atom can radiate only a certain amount of energy. This leads to the conclusion that bound electrons cannot have just any amount of energy but only a certain amount of energy. This energy is radiated when an electron jumps down from a higher energy level to a lower energy level.

image  ΔE = hf

Spectroscopy was used to identify the elements in our Sun.

Joseph Fraunhofer (6 March 1787 – 7 June 1826), ennobled in 1824 as Ritter von Fraunhofer, was a German optician.

He discovered of the dark absorption lines known as Fraunhofer lines in the Sun’s spectrum, and for making excellent optical glass and achromatic telescope objectives.


A material’s absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material. Radiation is more likely to be absorbed at frequencies that match the energy difference between two quantum mechanical states of the molecules. The absorption that occurs due to a transition between two states is referred to as an absorption line and a spectrum is typically composed of many lines.

The image below is a high resolution spectrum of the Sun showing thousands of elemental absorption lines – fraunhofer lines


The unknown element…….587.49nm (Na?)……no!



Evidence so far indicates that comets typically contain, carbon monoxide, carbon dioxide, water, nitrogen and, sulphur compounds as well as a whole zoo of carbon compounds. So all of the above are contenders.

Comets all contain similar levels of cyanogens – all except Machholz 1 that is. It has really low levels of CN – less than 1.5% of the normal level

This could be because;-

It was formed somewhere really cold, may be in the chilly outer regions of the Kuiper belt far beyond Neptune, where the low temperatures mean that most carbon gets trapped in other molecules

A second possibility is suggested by the comet’s peculiar orbit. Machholz 1 approaches very close to the Sun on its orbit, closer even than Mercury, so it is possible that repeated baking by the Sun’s heat has removed most of its cyanogen.

But a more exciting idea is that Machholz 1 is an alien. “An extrasolar origin makes it easy to explain the odd composition.


Comet 96P/Machholz or 96P/Machholz 1 is a short-period comet discovered on May 12, 1986, by amateur astronomer Donald Machholz on Loma Prieta peak, in central California using 130 millimetres binoculars.


The two main sources of comets in our Solar System is the Kuiper belt (30 to 50 AU from the Sun) and the Oort cloud (up to 10 000 AU from the Sun) Credit: ESA

Collision possibilities?

Small objects frequently collide with Earth but do little or no damage. However there have been some large impacts.


The Barringer crater was created about 50,000 years ago during the Pleistocene epoch, when the local climate on the Colorado Plateau (Arizona) was much cooler and damper. The crater was over 10,000 years old when the first humans saw it, at the earliest, 40,000 years ago.

The object that excavated the crater was a nickel-iron meteorite about 50 meters across. The speed of the impact has been a subject of some debate. Modelling initially suggested that the meteorite struck at up to 20 kilometres per second but more recent research suggests the impact was substantially slower, at 12.8 kilometres per second. It is believed that about half of the impactor’s bulk was vaporised during its descent. Impact energy has been estimated at about 10 megatons. The meteorite was mostly vaporized upon impact, leaving little in the crater.


The above left illustration gives you some idea of the size of the meteorite. The top right picture shows part of the crater.


One of the best-known recorded impacts in modern times was the Tunguska event, which occurred in Siberia, Russia, in 1908. The 2013 Chelyabinsk meteor event is the only known such event to result in a large number of injuries, and the Chelyabinsk meteor is the largest recorded object to have encountered the Earth since the Tunguska event.

Although no human is known to have been killed directly by an impact, over 1000 people were injured by the Chelyabinsk meteor airburst event over Russia in 2013 (see photo below). In 2005 it was estimated that the chance of a single person born today dying due to an impact is around 1 in 200,000. The four-metre-sized asteroids 2008 TC3 and 2014 AA are the only known objects to be detected before impacting the Earth.


10 000 tonnes – 15-20m diameter

V = 10 miles/s

Airburst at 27 000m

400kT explosive equivalence


The most notable non-terrestrial event is the Comet Shoemaker–Levy 9 impact, which provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. Most of the observed extrasolar impacts are the slow collision of galaxies; however, in 2014, one of the first massive terrestrial impacts observed was detected around the star NGC 2547 ID8 by NASA’s Spitzer space telescope and confirmed by ground observations.



Brown spots mark impact sites on Jupiter’s southern hemisphere.


Fireball on left is that of “Tsar Bomba” (King of Bombs), the largest nuclear weapon test in history, 57 megatonnes, detonated by the Soviet Union over Novaya Zemlya in the high Arctic. Airburst was at 4000 m altitude, fireball diameter was about 6-7km.This was equivalent to about 3000 Hiroshima weapons. Image on right is the G impact fireball from Shoemaker-Levy.

Impact events have been a plot and background element in science fiction.


Deep Impact was a NASA space probe launched from Cape Canaveral Air Force Station at 18:47 UTC on January 12, 2005. It was designed to study the interior composition of the comet Tempel 1 (9P/Tempel), by releasing an impactor into the comet. At 05:52 UTC on July 4, 2005, the impactor successfully collided with the comet’s nucleus. The impact excavated debris from the interior of the nucleus, forming an impact crater. Photographs taken by the spacecraft showed the comet to be more dusty and less icy than had been expected. The impact generated an unexpectedly large and bright dust cloud, obscuring the view of the impact crater.

Analysis of data from the Swift X-ray telescope showed that the comet continued outgassing from the impact for 13 days, with a peak five days after impact. A total of 5 million kilograms of water and between 10 and 25 million kilograms of dust were lost from the impact.


The above picture shows a meteorite that is on display in the Robert Hooke building at the Open University.

How big was the impactor?


Explosive yields are measured in KILOTONS (thousand-tonne equivalents of TNT). The picture is Operation Grapple – a Christmas Island British Hydrogen bomb test with a yield of a few megatons – similar in energy to the energy release from the Barringer impact.

Nuclear weapons tests/events give us a good model

Measured in KILOTONS (kT)

Hiroshima was 20kT

Meteor Crater event was 2.5 Megatons (2500kT) …..about 125 Hiroshima bombs

1kT = 4.2 x 1012 joules

Its entry velocity was 12.8 km/s

We know the entry velocity and the energy needed to blast out the crater

Use KINETIC ENERGY equation to work out MASS of impactor (½mv2)

Use DENSITY equation to work out VOLUME of impactor (m/V)

Assuming it’s spherical, use SPHERE equation to work out RADIUS of impactor! (4pr2/3)

If you don’t know the sphere equation, work it out as a cube shape!

KE = 2500kT

Entry velocity = 12.8 km/s

1kT = 4.2 x 1012 J

Density of iron meteorite = 7g/cm3 = 7000kg/m3

Remember, 1000kg = 1 ton

It’s estimated from computer modelling that the original mass of 300 000 tonnes ablated away in the Earth’s atmosphere with an impact mass of about 128 000 tonnes. The newest generation of US supercarriers have a mass circa 100 000 tonnes

Original mass was 300 000 tonnes

Radius about 16m

1908 – Tunguska, Siberia


Documented impact structures discovered thus far. Some are subsurface. “Astrobleme” = “star wound”



Collisions with Earth have caused mass extinctions on Earth. Is this good or bad?

Comet impacts may have brought Earth’s original water content?

Comet impacts may have seeded Earth with original organic materials necessary for life?

Making a comet

Health and safety

Dry ice sublimes at -78°C and will cause serious skin burns on contact, but momentary contact is unlikely to be a problem.

Do not confine in a sealed container as it will explode.

10 kg of dry ice will produce 5 m3 of gas, raising the level of CO2 from 0.035% (natural) to safe-limit (USA) of 0.5% in a room 3 m high by 19 m on a side.

Make sure there is adequate ventilation, although if the dry ice is transported in a substantial expanded polystyrene box, little will sublime.

CO2 is heavier than air therefore pools at ground level.

In theory trapped gas could fracture the comet or cause it to split, but this has never been recorded.

Do this in a well-ventilated area. Wear safety spectacles and gardening gloves.

The essential ingredients are dry ice, sand and water. The other items represent the organic molecules thought to be present in a comet.


Line a mixing bowl with a bin liner


Pour in half a litre of water and several handfuls of sand (silicates).


Stir and add Worcestershire sauce or other organics, ethanol (or any excess alcohol lying around), carbon (charcoal) and smelling salts or other ammonia compounds.


Add about 2kg of crushed dry ice and, wearing insulation gloves; squash all the materials in the bin bag until you can feel a solid mass forming.


Then you have a comet.

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