Birmingham July 2012

Friday 6th July

The second lecture was entitled Space, Science and SOHO given By Anu Ojha, who is director of the National Space Centre.


He began by asking, where does space begin? The answer is we don’t actually know although the most commonly accepted boundary is about 100km above the Earth’s mean sea level.

Why is space exploration so important? For a start without it we wouldn’t have satellites giving us TV programs, weather monitoring etc.

He then continued with a look at our solar system. Our Earth has a diameter of 12800km, our Moon has a diameter of 3460km and the Earth-Moon distance is 384000km.


Anu then went on to talk about comets.

A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust and small rocky particles.

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Halley’s comet. Comet Holmes in 2007 showing a blue ion tail on the right.

Comets have a wide range of orbital periods (Halley ’s Comet is every 76 years) and most of them have elongated elliptical orbits. The coma or tail distinguishes them from asteroids. They may be remnants from the making of the Solar System. From the Earth the tail of a Comet can cover 40% of the sky.


Comet McNaught


What happens to the speed of a comet as it moves along its orbit? As it gets closer to the Sun its speed increases.

A gravity 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 has a far-reaching and deep gravity well. Anything on the surface of a planet or moon is considered to be at the bottom of the gravity well. Entering space from the surface of either means climbing out of the gravity well. The deeper the gravity well the more energy it takes to achieve escape velocity. In astrophysics, a gravity well is specifically the gravitational potential field around a massive body.


Plot of a two-dimensional slice of the gravitational potential in and around a uniformly dense, spherically symmetric body. In a uniform gravitational field, the gravitational potential at a point is proportional to the height.

In classical mechanics, the gravitational potential at a location is equal to the work per unit mass that is done by the force of gravity to move an object to a fixed reference location. By convention, the reference location is usually taken at infinity, so the gravitational potential is zero infinitely far away from any mass and negative at any finite distance: V = -GM/x where V is the potential, G is the universal gravitational constant and M is point mass (the origin of the gravity well).

The gravitational field, and thus the acceleration of a small body in the space around a massive object (within the gravity well), is the negative gradient of the gravitational potential. Thus the negative of a negative gradient yields positive acceleration towards the massive object: acceleration or gravitational field = GM/x^2. The acceleration/field strength follows the inverse square law.




Before continuing to look at comets we took a look at Lagrange points. These are semi- stable points (not gravitationally balanced) in orbit relative to the Earth and are the places where certain artificial satellites such as ACE, SOHO, Herschel and Planck are located.


L1 is suitable for making observations of the Sun-Earth system (SOHO is here). L2 is suitable for space based observations (Herschel and Planck are here). L3 is more famous in science fiction books for having another Earth but since probes have reached here it has been found to be too unstable to contain an object for very long. L4 and L5 are known as Trojan points because of asteroids found in the equivalent position around Jupiter. L4 is 60 degrees ahead and L5 is 60 degrees behind the Earth as it orbits the Sun.

L1 is 1.5 million km from the centre of the Earth. For a satellite to be placed at L1 the following formula must apply:

(GMsm/a^2) – (GMem/b^2) = mωe/r

G is the universal gravitational constant, Ms is the mass of the Sun, m is the mass of the satellite, a is the distance between L1 and the centre of the Sun, Me is the mass of the Earth, b is the distance between L1 and the centre of the Earth, ωe is the angular velocity of the Earth (and the satellite) and r is the radius of the satellite’s orbit. ωe must be the same for the Earth and satellite in order for the satellite to keep the same position in relation to the Earth. The total distance a + b = 150 million km. The mass of the satellite cancels out from the equations so the position of the satellite is mass independent.

The James Web Telescope will be placed at L2 and will be about ten times the Earth-Moon distance (i.e. 1.5 million km from the centre of the Earth).

We think that over time material has accreted at the Lagrange points. The Lagrange points are semi stable due to the effect of other planets.

We briefly looked at the methods of getting satellites in orbit. Do we use solid fuel rockets like the shuttle or liquid fuel rockets? Liquid fuel rockets are preferred as they are easy to stop but very low temperatures are needed for storing the fuel.


The thrust of a rocket = change in momentum/time and this can be modelled using the following experiment.


Whoosh Bottle Demo See CLEAPS supplementary risk assessment SRA06EthanolSafety screen and specswater cooler bottle metre stick set of scales video camera stop watch

Pour about 10cm^3 of ethanol into the bottle and roll it around so as to fill the bottle with vapour. Pour out the excess ethanol and place it well away. Place the bottle on the scales to measure the thrust.

Attach a splint to the end of a metre rule so that it is held at a slight downward angle. Light the splint and hold the end of the rule so that the burning splint is over the neck of the bottle.

It has been known for bottles to explode (perhaps because they’ve become brittle?) so carry this demo out behind a safety screen and with all students wearing safety specs.


The reaction is C2H5OH + O2 –> 2O2 + 3H2O

The bottle is about 250ml. 20% of that is oxygen. You can then calculate the number of moles of carbon dioxide and water (mass of expelled gases). Measuring the thrust and time should allow you to calculate the gas velocity.

The next part of the lecture involved looking at spectroscopy. This enables information about what the comets are made up of to be obtained.

Spectroscopy has identified water, various isotopes and organic compounds in comets.




Spectra from the Sun including fraunhofer lines.

Some of the chemicals found in comets. Iron and Nickel were found in an asteroid that landed in Arizona. Density was 7gcm^-3. 300000 tonnes in an object of radius 16m. In 1908 event was a very large, powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Russia. The explosion was believed to have been caused by the air burst of a large meteoroid or comet.


Trees knocked over by the Tunguska blast.


The Southern swamp – the epicentre of the Tunguska explosion (2008)




The effects of a comet hitting a planet was seen when a piece of comet Shoemaker-Levy collided with Jupiter in 1994. The scar lasted four months.


Earth has unique isotopes of oxygen and these isotopes have also been seen in comet Hartley. Could life on Earth have been seeded by comets?

The final part of the lecture involved Anu making a comet.

Comet Recipe

Comet Ingredients

Water in a jug (about half the amount of dry ice)

Bin liners

Dry Ice (about 2-3 x 600ml container fulls)

1 Spoonful of sand 1 Spoonful of carbon dust

Few dashes of worcester sauce (organic component)

Few dashes of whisky/red wine (optional – the methanol/ethanol component)

Bowl Disposal Bucket

Rubber Gloves Wooden Spoon

Clear screen Polysterene container for dry ice


  1. Take a bin liner and use it to line the bowl.
  2. Add the ingredients of your comet – water, sand, carbon dust, worcester sauce. These replicate the compounds that real comets are composed of. Volunteers of the audience can add some of these. Mix well with wooden spoon.

A note on the significance of the ingredients:

§ WATER – how comets have large amounts of H2O, and in the past it is believed comets could have brought water to earth!

§ The Sand, The carbon, alcohol, and the Worcester sauce for the organic component. Comets have all the right ingredients for life, but the mixture is not under the correct conditions for life to exist.

§ The dry ice – frozen CO2, the frozen gas that holds together our comet and sublimates when it interacts with the solar wind and solar radiation to form the coma and gas tail.

  1. Finally add the dry ice. Wearing gloves feel around the bin liner and mould the comet into one lump. Don’t compress it too hard as the comet may break, allow steam gas to escape.
  2. When the demonstration is completed place the comets inside the bin liner in a bucket.

Safety Precautions

  • When handling the dry ice wear gloves and goggles. Do not touch, swallow or taste the dry ice. Do not allow students too near the dry ice. Give the audience clear instructions on the hazard and the distance they should be seated form the dry ice as the comet may ‘spit’
  • Do not seal the dry ice into a container as explosive outgassing may result!
  • Transport dry ice in a plastic bag inside a box.
  • Dispose of comet outside in a well-ventilated area where students can’t access it.

The Science

Dry ice is frozen Carbon Dioxide (-78.5oC, or -109.3oF), or CO2, which is a gas under standard temperature and pressure conditions. The atmosphere contains about 0.035% of this gas. CO2 is a greenhouse gas, which means it absorbs light at infrared wavelengths. An increase in the concentration of this gas may cause an increase in the atmosphere’s average temperature. The high concentration of CO2 (>96%)in the atmosphere of the planet Venus contributes to that planet’s high average temperature.

At normal atmospheric pressure on this planet, frozen CO2 doesn’t melt into a liquid, but rather evaporates directly into its gaseous form -hence the name ‘dry ice’. This process is called sublimation and is responsible for the formation of a comet’s coma. We can see CO2 gas subliming away from our comet from where water vapour in the air condenses around it.



Anu putting the ingredients together


Anu mixing everything together including the dry ice.


The finished comet


A piece of meteorite and a piece of Mars.


The lecture should have continued with information about SOHO but we ran out of time so what follows is stuff you can investigate yourself.

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