Tuesday, September 18, 2007

Cosmic Lobster Pot

By Deirdre Kelleghan Published Realta March/April 2006

“There are infinite worlds both like and unlike this world of ours.

For the atoms being infinite in number, as was already proved, are borne on, far out into space. For those atoms which are of such nature that a world could be created by them, or made by them, have not been used up either on one world, or a limited number of worlds… So that there nowhere exists, an obstacle to the infinite number of worlds”

Epicurus 4 Century BC

I always visualise Cassini’s journey through the Saturnian system as a kind of orchestrated cosmic dance.Cassini moves silently at great speed in its petal shaped overlapping orbit. This precisely executed dance brings Cassini frequently through the icy ring plane north to south and then back again on the opposite side of the planet, south to north. This robot ship continues on its unparalleled odyssey of exploration. Onboard, Cassini is the custodian of twelve science instruments all primed to seek, gather, and process the offerings of this unique planetary system. Collectively they are performing one of the most important scientific probing’s of Saturn and its many moons in the history of space exploration.

One of these science instruments is the Cosmic Dust Analyser. The CDA looks a bit like a golden lobster pot, and that is not such a bad analogy. This apparatus is trawling the interplanetary ocean for particles of cosmic dust, micro particles that are the messengers of the origins of life.

Cosmic dust what is it; Cosmic dust are clouds or particles of dust and gases occurring throughout interplanetary ,and interstellar space this is a physical material that occurs in our solar system and in our universe. Interplanetary cosmic dust is only temporarily about as it is dissipated from its sources and swept out of the interplanetary sea, to interstellar space. Interstellar cosmic dust particles, the ejecta of dying stars, thrown out and mixed with interstellar gases and other gathered elements in space, recycle and create new stars and new beginnings of possible life. Interstellar dust and gas the building blocks of stars, planets and all life forms. This is I think a kind of the circular system that demonstrates one of the basic laws of the conservation of matter “In any physical or chemical change, matter is neither created nor destroyed but merely changed from one form to another”

The Cosmic Dust Analyser is a passive instrument, it does not send out anything to detect what it is designed for. It simply waits at its station onboard Cassini and its quarry comes to it.

The CDA instrument when it is struck by one or some of these cosmic particles, not only analyses its composition but amazingly enough it has to ability to determine its orbit, its trajectory i.e. where it has originated from, and therefore its source i.e. the particle or particles may be from a place outside the Saturnian system. Particles possibly from the Jovian system of from comets or meteoroids may stray into the trawling instrument. The CDA may at first seem to be a very straight forward instrument but it has a dizzying array of electronic and chemical sensors.

The Cassini orbiter has been cruising in the Saturnian environment for over one year now and has travelled billions of miles since it left Earth in October 1997. Cassini glides through space at an average speed of 12,000 miles per hour According to the Cassini website Giovanni Cassini (the gentleman that the Cassini spacecraft is named after) in the 17th century would have gazed at cosmic dust in his telescope, probably looking at the wonderful Sagittarius or some of the larger nebulous objects like M42 or even Andromeda M31. Giovanni Cassini was also the first person to explain the zodiacal light phenomenon, demonstrating that it is of cosmic origin.

Giovanni - Dominique Cassini was born in Perinaldo Italy, on June 8, 1625. He became the head of the Observatoire de Paris in 1671; it was in this wonderful city he is credited with the discovery of the Saturnian moons Iapetus in 1671, Rhea in 1672, and Tethys and Dione in 1684. Imagine being able to do such observing with what would have been a comparatively small telescope. Imagine four hundred years later a space craft is in orbit in the Saturnian system sending back some of the greatest photographic images of all time. Cassini the ingenious robot bringing us the discoveries of Cassini the genius man.

Giovanni Cassini’s greatest gift to science was probably the discovery of the gap in Saturn’s rings, which now carries his name,’ The Cassini Division’. So the Cosmic Dust Analyser onboard the Cassini orbiter, is carrying on a science investigation that has its origins through his eyes, in the 17th century, and now in the 21st century through Cassini’s science packages.

The CDA is a cylindrical golden vessel, within the chamber lie two separate impact analyzers, a large one the High Rate Detector Target (HRD) and a smaller one the Chemical Analyser Target (CAT). Particles of matter traveling through space enter the Cosmic Dust Analyzer at random. The detection of particle impacts is achieved in two ways; one is through a High Rate Detection system, used for the high impact rates hitting the CDA during the ring plane crossings. This is for cosmic dust particles with a known speed. The High Rate Detector is primarily for carrying out measurements of particles in Saturn’s ring system.

The CAT system is for measuring the electrically charged dust particles, their speed, mass, chemical composition and even impact direction. A particle may impact on the big gold target (HRD) or the smaller (CAT) plate. These impacts cause the particles to vaporise and send there ions and atoms upward to be amplified by a multiplier device and analyzed and processed into data that tells the story of the particles in detail for Cassini’s scientists here on Earth. The Cosmic Dust Analyzer is reliably designed to measure impacts from a frequency as low as 1 impact per month to up to 10 4 impacts per second.

The CDA also has an articulation mechanism that enables it to move in accordance with the space crafts direction. It is an essential function, and has been shown to be most effective when Cassini swooped within one hundred and nine miles of the icy moon Enceladus in July 2005; CDA is profiled as “think, touch or taste” as it is a direct sensing instrument. The articulation mechanism at the base of the CDA enables it to achieve a 0 –270-degree sweep of its environment; it is attached to the obiter just below the High Gain Antenna.

The CDA needs very little power that in itself is a great quality for a science instrument operating in space. The CDA is producing output information that is invaluable to the understanding of the Saturnian system and that information combined with the other instruments is vital to the purpose of the mission. The CDA was a main player in the evidence for the venting of water ice vapour and particles discovered emanating from the south pole of Enceladus, in that close fly by of July 2005.

One of the objectives of the CDA instrument is to map the size and distribution of Saturnian ring material and to search for particles outside the extensive E ring. The quest is to analyze the chemical composition of ring particles and to determine dust and meteoroid particle distribution in the rings and in interplanetary space. These tasks are just a sample of the work set for the CDA. Because it has the ability to workout the direction a particle may have arrived from, it can therefore tell whether a particle is cometary or astroidial in origin.

The smaller inner chemical analyzer target plate is made of Rhodium; this element is the rarest of all non- radioactive metals on Earth. Rhodium is a rare lustrous silvery hard metal, a member of the platinum group and its properties include a low electrical resistance and a high resistance to corrosion.

The larger HRD is plated in Gold a soft metal which is chemically unreactive. Gold is one of the few elements to occur in a natural state. Gold is unaffected by air, water, all acids and alkalis. It is a good conductor of heat and electricity. Gold is a good reflector of infra red radiation and as it’s inert it makes an excellent coating for space satellites and instruments.

An electrically charged particle flying through the entrance grids at the front of the Dust Analyzer will induce a charged signal on the grids. The charge induced is directly proportional to the charge of the particle. This allows direct determination of its electric profile .The design of the plates leaves the signal shape created by the particle to be symmetrical allowing ease of measurement of the particle direction.

The CAT measures the electric charge carried by the dust particles, and also the impact direction, impact speed, mass and chemical composition.

In the case of the CAT plate, the dust particles’ hitting it generates ejecta made up of atoms and ions. The curved shape of the target plate at the bottom of the CAT focuses the ejecta toward the multiplier which amplifies the signal and it is within this action the information about the elemental components of the micrometeoroids is extracted and stored. The chemical analyzer has a special instrument controlled heater to decontaminate the analyzer target by heating it to 90 degrees. Thereby purifying it for its next tiny subject.

The sensor plate for the HRD is different from the CAT. It deals with High Velocity dust particles which create a large fast pulse, which is in turn also proportional to the impact particles mass and speed. The HRD is an independent instrument with its own memory and processor. Data from both detector plates is sent to Earth through the Deep Space Network and is gladly received and analyzed by awaiting scientists. It should be interesting to compare the cosmic dust analysis from the Saturnian system with those samples returned by Stardust from Comet Wild 2, which consists of cometary and interstellar dust particles. A fantastic task achieved recently bringing us closer to an understanding of our solar system and the cosmos, and the correlation of cometary dust, its origins and the solar bodies that we know.

The Chemical Analyzer Target is a development of the University of Kent in the United Kingdom. The High Rate Detector is a development of the University of Chicago. The Cosmic Dust Analyzer is cruising through space over one billion miles from planet Earth.

Just like the pixels in a photograph the tiny micro particles of cosmic dust will come together and play a part in revealing an image of the soup, the mix,the ingredients of the Universe. The Cosmic Dust Analyser continues to gather vital data on the rich tapestry of interplanetary space. It will weave a fine, complex but fascinating image of the Saturnian system at micro almost nano level.

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