Rosetta and 67P/Churyumov-Gerasimenko
On 6th August 2014 the European spacecraft Rosetta rendezvoused with the comet 67P/Churyumov-Gerasimenko. The probe is now orbiting the comet due to several movements from its thrusters as they orbit the Sun. This European project, of which Britain is involved, is the first time that this has been achieved. Rosetta will not only accompany the comet around the Sun, it will also send another probe, called the Philae Lander, to land on the comet. This is aimed to take place on 11th November 2014.This is a very exciting project. The aims of this mission are to:
- Better understand what the comet is made from. Some of the elements are particularly interesting because they are used for making life.
- Better understand how the coma forms as it approaches the Sun and how the coma forms into that distinctive tail (actually two tails are usually formed as I will later explain).
- Explain the origins of the comet.
You may think this is all a bit pointless though and a waste of money. However, understanding about comets is not just about satisfying scientific curiosity, it is much more than that.
Threat of impact from large objects in space
There is no reason to think that this comet is in any way on collision cause with Earth; but the threat of asteroids or comets hitting the Earth is very real.
On 15th February 2013 an asteroid struck Earth in Chelyabinsk, Russia. The asteroid was believed to be about 17 metres in diameter and exploded above ground. Eyewitnesses described of intense heat and brightness before the blast wave of the explosion. Hundreds of people were injured, two seriously. The explosion was equivalent to nearly 500,000 tonnes of TNT. The atom bomb that destroyed Hiroshima was ‘just’ 15,000 tonnes of TNT! Fortunately, the explosion happened over 12 miles above the surface and the Earth’s atmosphere absorbed much of the blast wave. To watch a YouTube video of this event click on the link below.
This is not the first time that a powerful strike from space has hit the Earth. In 1908 the Earth was hit by what is believed to be from a fragment of a comet. This fragment also hit Russia in Siberia. It was estimated to be between 20 to 60 metres in diameter. When a large object hits the atmosphere it comes in very fast and creates very high temperatures of 1000’s °C. This also caused this fragment to explode above the ground. The explosion was estimated to be the equivalent of 5000,000 tonnes of TNT! This is at least 300 times more powerful than the atom bomb that destroyed Hiroshima. An area of 850 square miles was destroyed but luckily it was not a densely populated area but in the Siberian wilderness.
The comet, 67P/Churyumov-Gerasimenko, is about 4 miles across in one direction and its mass has been calculated (from the pull Rosetta felt due to its gravity) to be 10 billion tonnes! If this comet hit the Earth, travelling at 120,000 mph, it would make a crater, estimated to be about 25 miles across. This would destroy an area of about 490 square miles due to the crater alone. This sounds less destructive than the smaller fragment that hit Siberia, but I’m just referring to the crater, not the destructive area. The destruction in Siberia was just from the blast wave.The 490 square miles destruction from just the crater is immense. To put the area of the crater into context; if it landed in the centre of Isle of Wight, it would not only destroy the island completely, the crater would even reach the main land. The blast wave from this would go far beyond that. The total destruction would be far worse. That land that was dug out to make the crater has to go somewhere. Large molten rocks would rain in the surrounding areas 100’s of miles away, depending on the angle the object hit the Earth. The High temperatures generated from the impact and the object travelling through the atmosphere would start forest fires. The large amounts of dust from the impact crater would go up into the atmosphere and block the Sun’s ray’s. This would cause global freezing. It would also severely reduce the ability for plants to photosynthesise. This would disrupt the ecosystem and food would be in short supply. The impact would set off earthquakes, where the magnitude on the Richter scale would reach double figures. Tsunamis would result and if the comet hits the oceans; mega tsunamis with waves 4 or 5 miles high would occur! In short it would be a cataclysmic event. In fact any object greater than 0.6 miles across would threaten mass extinctions. Incidentally, what killed off the dinosaurs was due to an object, possibly a comet, about 6 miles wide. The extinct type threating asteroids and comets happens about every 50 to 100 million years. The problem is the last one happen about 65 million years ago!
The more we know about these objects the more we are able to deal with them. In understanding precisely what the comet is made from would help to work out methods that would prevent them colliding with Earth.
The Scientific interest
The scientific interest is also worth knowing. Comets are thought to be formed at the same time as the planets. In understanding what the comet is made from and the structure of it will help us to better understand how the solar system was formed. The comet is essentially a time capsule from the time when the solar system was created. This information will be gathered by physically analysing the comet directly, using the lander. Also X-rays will be directed at the comet to see inside it.
It is theorised that comets helped to provide the Earth with water. Not only that it is also theorised that comets provided the Earth with organic elements; elements that are needed for life. Comets colliding with Earth may have been a crucial factor of our existence as well as all life. In analysing comets may provide us with more evidence on these theories.
What is a comet?
A comet is essentially a rocky icy object. The ice consists of water, carbon dioxide, carbon monoxide, methane and ammonia, as well as other substances. In fact comets have many substances and elements, including, as I have mentioned, organic elements.
Comets have an elliptical orbit as it goes around the Sun. As the comet approaches the Sun the ice vaporise to form an atmosphere of gases around the comet called the coma. The ice turns into a gas from its solid state, missing out the liquid state. This process is called sublimation. Dust is also pushed out into the coma.
From this coma two types of tails can form. These are:
1. Dust tail
2. Gas tail
The dust tail is formed when the dust gets pushed back due to the radiation pressure of the sunlight itself. Sunlight is a collection of electromagnet waves but these waves can also behave like particles, called photons. By the way, this phenomenon where particles sometimes behave like waves and waves sometime behave like particles is called the wave–particle duality. Anyway, using that concept explains how radiation pressure can push the dust in the coma back to form the dust tail. Those photons bombard the dust pushing them back with every strike. Radiation pressure is very weak though so the dust tail is often diffused (wispy in appearance) and curved, such as the one shown in the picture.
The second tail that is often formed is called a gas tail. This is formed by two basic sections.
Firstly, the ultraviolet region of the sunlight removes one or more electrons from the atoms that make up the gases. This makes those atoms into charge particles, called ions.
Secondly, solar winds from the Sun push these gases back to form the gas tail. The Sun is basically a nuclear fusion reactor and explosions from the Sun called solar flares throws out many tonnes of matter into space. This matter radiates out in all directions producing the solar winds. Solar winds are also charged particles. The electrostatic forces between the solar wind and the gas tail have a bigger effect on the gas tail than the radiation pressure has on the dust tail. This means the gas tail is straighter and narrower.
The tails and coma are only formed when the comet is close enough to the Sun, otherwise it returns to the icy rock like object.
The motion of the Comet orbiting the Sun and how the tail interacts with the Sun is illustrated in the video that I made below.