Harrow and Hillingdon Geological Society
Micrometeoroids
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Dr. Anton Kearsley spoke to us about his role in analysing dust in space. Near to the Earth this dust contains space debris both natural particles and man-made space debris. When one considers that the speed of a rifle bullet is less than 1km/sec, dust travels at 10 – 70km/sec and leaves billions and trillions of tiny impact craters. These pores vary from less than nm to mm in size. They can do considerable impact damage to satellites and space stations.
Meteoroid dust may be from comets, asteroids or from outside the Solar System. Smaller meteoroids burn up to become shooting stars, whereas meteorites actually land on Earth. Cosmic dust contains the most primitive material in the Solar System and much dates to before the Earth was formed 4.5 billion years ago.
The material which lands on Earth can be collected from anywhere, but is much cleaner from areas such as Antarctica and Greenland where large volumes of snow are melted. If the sample contains Fe and MnO2 it is probably artificial (for example exhaust from an internal combustion engine). Aircraft such as U2 spy planes are used to collect the dust from the atmosphere.
The instruments used to analyse rocks from space are very precise. Thin sections of the Allende meteorite were cut and an elemental map was constructed by bouncing beams of electrons off the sample with X-ray emission showing the position of eg. Mg, Al, Si, Ca, Fe etc. The energy spectrum showed elemental composition of O, K, S, C etc.
The Columbia space shuttle, in March 2002, went to the Hubble telescope, collected and rolled up two of the solar collection panels and brought back them back to Earth. These were sent to Holland and examined for damage and the material which was deposited. Scanning electron microscopy, which is non-destructive and demonstrates a high special resolution of µm scale, was used.
The elements in micrometeoroid dust can be determined and the holes defined as natural or manmade. The distribution is about 38% Space debris, 37% natural and 25% is unresolved.
Magnesium and Iron rich residues dominate and Ferrous sulphide is the next most common. Metals are relatively rare. Pre solar silicon carbide grains might come from giant red stars or could be from the shuttle but are most likely to be inter-stellar. Most space debris is rich in aluminium. Tiny Al2O3 does not do a lot of damage but larger metal and carbon fibre grains may do. Large impact craters containing aluminium alloy on top of other aluminium alloy has been shown and emphasises the need to put adequate protection on the rocket.
Research on a Japanese space craft has resulted in a collection technique using insulation polyamide material (Kapton). This is essentially multi layered polymer layers of plastic.
Genesis Solar wind collectors use an array of highly pure silicon wafer, which may also have collected cometary and interstellar dust.
Other analysis techniques include: Raman spectroscopy – Laser is fired at the material and the minerals can be mapped within the particle; high energy protons are fired at the grain which emits x-rays and these rays determine the presence of Fe, Si, Mg etc. Particles of known materials and known size were fired at metal foil to produce standards with which to compare the specimens under test. Analytical SEM is a very versatile way to analyse dust in space.
Anton also told us about the Stardust mission that was due to return to Earth on the following Sunday. This mission used aerogel blocks to collect particles from the corona and tail of a comet. It also collected interstellar dust on the reverse side of the aerogel blocks.
This was a particularly interesting and utterly absorbing lecture. It was well illustrated and pitched at a level which everyone could understand. The speaker brought some examples of the materials he had described including pieces from the Hubble telescope which we were encouraged to handle. It was obviously a very popular talk as demonstrated by the large number of questions it generated.