| Subject: June 30 orbit: scientists prepare to be amazed |
Author:
Betty
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Date Posted: 09:45:28 06/27/04 Sun
In reply to:
Betty
's message, "Hubble turns towards Saturn as Cassini nears it's objective" on 10:10:57 05/30/04 Sun
Cameras poised as scientists `prepare to be amazed'
JOHN NOBLE WILFORD
NEW YORK TIMES
Saturn and its creamy pastel bands of thick atmosphere shimmer in pale sunlight, its majestic rings of dust and rock setting it apart from the sun's other worlds.
Dancing about in rhythmic orbits are 31 known satellites, of which the most mysterious and inviting is the planet-size Titan.
After a nearly seven-year voyage from Earth, the Cassini spacecraft is fast approaching the moment that scientists have dreamed of and planned for over the better part of their careers. The spacecraft is scheduled to swing into orbit around Saturn on Wednesday evening.
Expectations are high for the American-European mission, which is planned to last at least four years but could keep going for as much as a decade.
"The Saturn system represents an unsurpassed laboratory, where we can look for answers to many fundamental questions about the physics, chemistry and evolution of the planets and the conditions that give rise to life," NASA official Edward Weiler said in a statement.
Scientists dare not predict the discoveries waiting to be made as the spacecraft focuses its cameras and instruments repeatedly on Saturn and its signature rings and takes the measure of the icy moons during at least 76 orbits.
"Prepare to be amazed," advises Carolyn Porco, a planetary scientist at the Space Science Institute in Colorado and chief of the mission's imaging team.
Anxiety also is rising, though a compensating air of optimism seems to prevail. The spacecraft's performance over the 3.5 billion-kilometre flight has been virtually trouble free.
"There's not a single thing to point to and say, `I'm worried about that,' ''says Robert Mitchell, project manager at the Jet Propulsion Laboratory in Pasadena, Calif., where the mission is being directed.
Still, Mitchell confesses, when the time comes for the Cassini to thread the gap between two dust rings and the main engine to kick on and burn for 96 minutes, slowing for capture by Saturn's strong gravity, "I'll be in mission control with my fingers crossed."
The spacecraft has already had its first Saturn encounter, passing by Phoebe, the planet's outermost moon, on June 11.
Scientists theorize that in Phoebe they may be getting their first close examination of an object from the outer reaches of the solar system.
Everything about its appearance and motions suggests that the 220-kilometre-diameter Phoebe originated far beyond the outer planets and that it was flung toward Saturn, which captured it into its orbit.
Saturn itself, second to Jupiter in size, now looms so large with respect to Cassini that the planet's full girth no longer fits inside the frame of the craft's narrow-angle camera. The Cassini's two cameras are expected to take as many as 500,000 pictures in the next four years.
If all continues to go well, the 2,130-kilogram Cassini orbiter and the attached 315-kilogram Huygens probe — to be released in December to investigate the atmosphere and surface of Titan — will arrive at Saturn below the plane of the spreading rings.
It is to pass through the gap between the F and G rings.
Recent photography, including observations by the HUBBLE SPACE TELESCOPE (!!!), shows no sign of hazardous debris in Cassini's path, Mitchell says.
Three previous spacecraft have flown through the region without harm.
Once through the passage, the 96-minute firing of the engine is to begin braking the Cassini's velocity. Should the main engine fail, there is a backup.
As soon as Cassini settles into orbit, it is programmed to turn its cameras on the rings, searching for evidence of how particles of dust cluster there, then dissipate and gather again in ever-transient structures.
The spacecraft also will be taking some of its closest pictures of Saturn at this point, as it begins the first of its orbits among the planet's family of moons.
Imaging team leader Porco says the initial four-year orbital tour has been plotted in detail "to give us great flexibility in observing all the important targets in the Saturn system."
In the 45 years of space-age exploration of the solar system, spacecraft have visited all the planets, save Pluto. They've orbited Venus, Mars and Jupiter, and landed on Venus and Mars.
Three spacecraft have flown by Saturn, but this will be the first attempt to orbit it.
The object of greatest curiosity for the mission is Titan. Forty-five of the 76 Cassini orbits will include Titan flybys, coming as close as 965 kilometres to its surface on some encounters.
The icy moon is larger than Mercury or Pluto and somewhat smaller than Mars. Its substantial atmosphere, like Earth's, is thick with nitrogen, though it has no free oxygen.
The other atmospheric components are methane, ethane, propane and organic materials containing hydrogen and carbon. Titan has a greenhouse-warmed climate with methane clouds, rain, perhaps even lakes.
"Titan is like a time machine taking us to the past to see what Earth might have been like," says Dennis Matson, chief project scientist at the Jet Propulsion Laboratory.
"The hazy moon may hold clues to how the primitive Earth evolved into a life-bearing planet."
But scientists caution that they do not expect the Cassini mission to find life on Titan.
The large moon has not made it easy for inquiring scientists. A dense haze obscures the surface, except for glimpses of shadowy dark regions that contrast with the occasional bright patch.
The approaching cameras of the Cassini have taken some pictures that show surface traces, but no details. In October and again in December, the spacecraft is to fly close enough for its radar to penetrate the clouds and haze for the best mapping yet of a broad swath of Titan's topography.
The European-built Huygens probe will undertake the mission's most ambitious effort to explore Titan. On Dec. 24, the Cassini is to release the wok-shaped craft on its 20-day freefall journey to enter Titan's atmosphere.
It is to deploy parachutes and begin 150 minutes of intensive observations of the atmosphere.
No one is making promises as to if or how long Huygens will continue transmitting data from the surface — perhaps three minutes, possibly a half-hour.
The European Space Agency managed the development of the probe and is in charge of its operations from a control centre in Germany.
"This is going to be a mission the likes of which has never been seen before," says Porco.
"This is the mission of my career."
And so it may also be for many others of the 200 flight planners and controllers and the 250 science investigators who wait expectantly and eagerly for a spacecraft, seven years from home, to find its way through a gate in the rings of Saturn and emerge with wonders to report and pictures to post of places as never seen before.
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