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Ärende: Cassini
===============
http://www.thestatesman.net/page.news.php?clid=24&theme=&usrsess=1&id=54892

The Statesman - Calcutta, India
Tuesday, September 21, 2004

   Another World Away

   With the Cassini-Huygens probe settling down to business on Saturn,
   Amalendu Bandyopadhyay looks forward to an avalanche of exciting data
   JUST under seven years and 3.3 billion kilometres later, the
   Cassini-Huygens probe is where it was meant to be. Launched from Cape
   Canaveral in the USA on 15 October 1997, final countdown began when,
   on 1 July 2004, it burned its engines for an hour and a half and went
   into orbit around Saturn. For the next four years it will explore the
   planet, its rings and moons as never before.
   The two-spacecraft Cassini-Huygens endeavour is a $3.2 billion
   cooperative project of the National Aeronautics and Space Agency, the
   European Space Agency and the Italian Space Agency. For this arguably
   most sophisticated and challenging mission that Nasas Jet Propulsion
   Laboratory has ever launched, Cassini is the largest and most
   technologically advanced interplanetary vehicle. More than 6.7 metres
   long and weighing some six tons, Cassini is very much an international
   effort since many of its components and most of the 350-kg Huygens
   probe were built by the ESA and ISA. Scientists from the Mullard Space
   Science laboratory in the UK built part of the Cassini Plasma
   Spectrometer while the Radio and Plasma Wave science instruments were
   developed with help from scientists from Sheffield University.
   Cassinis scientific teams include 122 European researchers.
   The probes journey to Saturn has been a circuitous one, including two
   flybys of Venus (April 1998 and June 1999), one of Earth (August 1999)
   and one of Jupiter (December 2000). The planets gravitational pulls
   acted as slingshots to accelerate the spacecraft towards Saturn.
   Having fired its braking engine to go into Saturns orbit, Cassini has
   already employed a dozen instruments in its scientific reconnaisance.
   Later, on 25 December, the Huygens lander will separate from its
   mother ship for its rendezvous with Titan, Saturns large,
   cloud-enshrouded moon. On 14 January 2005, it will slam into Titans
   thick atmosphere at 22,500 km an hour and parachute through the haze
   and clouds, perhaps splashing down in a sea of liquid methane. Cassini
   will orbit Saturn 76 times during its nominal four-year mission and
   will have 52 close encounters with seven of its 31 known moons. And
   given its dimensions, it can study time variations and interactions
   between diverse phenomena in a manner that would prove impossible for
   smaller spacecraft.
   How was the mission named? In early 1655, Dutch mathematician,
   physicist and astronomer Christian Huygens had his first look of
   Saturn. His telescope wasnt really much better than those of many of
   his contemporaries but from his observations he discovered Saturns
   largest satellite, Titan. In 1675, Italian astronomer Giovanni
   Domenico Cassini, who became the first director of the Paris
   Observatory, discovered a dark gap in Saturns rings, now known as
   Casinnis Division. Therefore Cassini-Huygens.
   But why? Because between 1979 and 1981, three unmanned probes flew
   past Saturn Pioneer 11 and Voyagers 1 and 2, all launched by Nasa but
   all this afforded was a real quick look. Scientists never had the
   chance to make an in-depth examination and theyre now relying on
   Cassini to be able to do just that. For many of them, Titan will be
   the star of the show. Apart from carrying out its own investigations,
   Cassini will deliver the Huygens probe that will map the clouded moon
   using radar.
   Now to the objectives. The mysteries Cassini is expected to resolve
   include: what creates the so-called zontal jets the horizontal bands
   that cross Saturns cloudy upper atmosphere? The planets winds reach
   speeds of 1,760 kmph, but what drives these winds? The probe is fitted
   with a powerful array of instruments, including a high-resolution
   camera that will document the motion of Saturns clouds.
   Scientists are also anxious to investigate the planets atmosphere in
   the vertical dimension. Cassini will not release a probe into this
   atmosphere but it will use other methods, one of which takes advantage
   of the fact that the crafts radio signals to earth will pass through
   Saturns atmosphere each time the orbiter disappears behind the planet
   and then reappears. By analysing variations in the radio signals,
   scientists will obtain data on temperatures, pressures and
   compositions within Saturns upper atmosphere.
   Many scientists believe heat from the interior is a major influence on
   Saturns atmospheric circulation. The planet radiates about 80 per cent
   more heat than it receives from the sun, an as yet unexplained fact.
   Most of its gaseous bulk is composed of a mixture of hydrogen and
   helium and the speculation is that over its 4.6 billion years of
   existence, heavier helium atoms have migrated towards the planets core
   to produce this heat. Should Cassini be able to confirm this theory,
   it will not only explain this excessive heat but will also help
   explain the evolution of gas-giant planets.
   Saturns ring system makes for a wondrous sight, far beyond the beauty
   of other planets rings as detected by space probes. But scientists
   find Saturns system most puzzling and Cassini will provide the best
   chance yet to investigate its composition. The crafts sensors, with
   their greater range of spectral response, will allow for more detailed
   analysis than was provided by Voyager. Probing the rings at
   far-infrared and microwave wavelengths, Cassinis instruments should
   even be able to detect any rocky material that may lie beneath the
   ring particles icy surfaces. One of the biggest expectations is that
   Cassini will be able to determine where the rings came from. Was the
   rings parent world one of Saturns satellites or was it an interloper
   from another region of the solar system?
   Of the planets 31 known satellites, Titan will get the most attention.
   With a diameter of 5,150 km, it is the second-largest moon in the
   solar system after Jupiters Ganymede and it is the only planetary
   satellite enveloped by a thick atmosphere. Titan invokes interest
   mostly because of its resemblance to earth in terms of atmospheric
   composition and surface pressure. Both atmospheres are dominated by
   nitrogen (77 per cent for earth, 90-97 per cent for Titan) and Titans
   atmosphere produces a surface pressure that is 50 per cent greater
   than earths at sea level. Adding to the intrigue, Titans rich organic
   chemistry makes it a planetary-scale laboratory for studying prebiotic
   processes that may have led to the origin of life on earth. The
   question of how life began on earth makes Titan a particularly
   attractive place. Moreover, its atmospheres chemistry appears to
   closely resemble that of early earth, which makes it a much more
   promising place to search for how the transition from chemistry to
   biology occurs. Huygens may throw some light on this mystery.
   In addition to 45 planned flybys of Titan, Cassini will do
   approximately six flybys of Saturns medium-sized iced satellites at
   altitudes of between 500 and 2,000 km encounters that should produce
   remarkably detailed images. For example, the craft should be able to
   map nearly all of the surface of Iapetus and Enceladus. Half of
   Iapetus surface is covered with bright ice and the other half is as
   dark as asphalt and Cassinis images should help determine whether the
   dark substance comes from the satellites interior or from an outside
   surface. Cassini will also try to determine whether active ice
   volcanoes exist on Enceladus.
   Saturns magnetic field is very powerful, its total energy some 540
   times stronger than earths, and scientists are keen to learn more
   about its structure and study its interaction with the rings,
   satellites and the stream of charged particles emanating from the sun,
   known as solar winds. Cassini will employ several instruments,
   including a magnetometer and a plasma spectrometer, to study the
   highly ionised gas within Saturns magnetosphere.
   Following its release from Cassini, the Huygens probe will spend about
   two hours boring through the atmosphere before finally making contact
   with Titan. Slowing enough, it will then deploy a parachute, descend
   and make a variety of measurements of the atmospheres physical and
   chemical properties. Having spent nearly seven years between earth and
   Titan, a fancy little laboratory within Huygens will spring to life
   that includes a gas chromatograph and a mass spectrometer in a capsule
   only nine feet across. With these instruments, the probe should be
   able to identify the chemicals it runs into. Huygens also has an
   aerosol collector and pyrolyser because when it gets low enough it
   expects to find aerosol particles in the air. It will feed these
   aerosols to the pyroliser a high tech oven that will cook them and
   forward the resulting gas to the chromatograph and spectrometer for
   final analysis. The findings will ultimately be fed back to Cassini,
   which will beam the data back to earth. Scientists will then be able
   to get a much clearer picture of just what kind of chemistry Titan is
   all about. In addition, Huygens will image the atmosphere and surface,
   measure temperatures and reveal a thing or two about Titans winds as
   probes record their effect on its descent.
   Within hours of passing deftly through the rings of Saturn on 1 July
   2004, the Cassini-Huygens mission sent back new images of Saturns most
   distinguishing feature. With eyes sharper than any that has peered at
   the planet before, the spacecraft has already discovered two new moons
   approximately three and four kilometres across located 194,000 km and
   211,000 km from Saturns centre. These are between the orbits of two
   other Saturnian moons, Mimas and Enceladus.
   Cassini has confirmed the rings are mainly boulder-sized lumps of
   water ice, though the ice is purer than expected. An analysis of the
   size of the particles lumps, using the crafts visual and mapping
   spectrometer threw up a surprise the grain size gets bigger and the
   water ice purer farther away from the planet. But ice isnt the only
   component of the rings. There is also something called dirt or dark
   material and what is interesting is its distribution in the rings
   there is proportionately more dirt in the thin, dark parts of the ring
   system such as the Cassini Division, and much less in the lighter
   parts, which are mainly ice. This suggests there is some unknown
   sorting mechanism.
   From data received, scientists speculate that Titan may preserve in
   deep-freeze many chemical compounds that preceded life on earth.
   Before and during the 1 July flyby, Cassinis visible and infrared
   mapping spectrometer pierced the smog that enshrouds Titan and
   revealed an exotic surface bearing a variety of materials in the south
   and a circular feature that could be a crater in the north. And so for
   the first time were scientists able to map the mineralogy of Titan.
   On 30 June 2004, Cassini passed closest to Phoebe, one of Saturns
   moons, before entering the planets orbit, and found dark material.
   This heavily cratered moon appeared to be mainly ice, with patches of
   water ice, water-bearing materials, carbon dioxide, possible clays and
   primative organic chemicals on the surface forming an overall dark
   crust. One of the large impact craters on Phoebe revealed dark and
   light layers near the surface of the moon and a lighter interior. But
   Cassinis 1 July flyby at the closest distance of 339,000 km allowed
   for the best view of Titan so far, and over the next four years the
   orbiter will do 45 flybys as close as approximately 950 km.
   In 2000, a flaw was discovered in the communications system that the
   Huygens probe will use to send data to Cassini which will relay it to
   earth. Apparently, the probe will be moving at varying velocities that
   will Doppler-shift its frequencies largely out of the passband of the
   orbiters receivers, whose bandwidth is narrower than expected.
   Engineers are working on the problem to save as much as possible of
   the Titan data. Should the mission succeed, we shall have spectacular
   news about Saturns interior structure, its rings, its magnetic fields
   and its intense radiation belts.
   (The author is a senior scientist with the MP Birla Institute of
   Fundamental Research, MP Birla Planetarium, Kolkata.)
   The Cassini-Huygens spacecraft at the Kennedy Space Centre; Huygens
   parachute (top) will slow the
   probes descent in Titans atmosphere to a leisurely 25 kmph. The high
   resolution view (above right)
   of Saturns moon, Phoebe, was captured by Cassini on 30 June 2000

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