Fri, Dec 17, 2004 - Page 4 News List

Australia's Dish ready to for another brush with fame


For Australians, the "Dish" as they like to call the giant antennae of the Parkes radio telescope is nearly as well known as the Sydney Opera House.

Back in 1969, it sent the world 99 percent of the live television imagery from the lunar Sea of Tranquillity when Neil Armstrong and Buzz Aldrin became the first men to walk on the Moon.

Then, 13 years later in March 1986, it was the vital link when the European probe Giotto pierced the coma of obscuring gas and dust surrounding Halleys Comet to reveal its unexpectedly dark heart. And now, on Jan. 14, it will be the anchor for a massive international program to pinpoint the rapidly changing location of another pioneering European probe, Huygens, as it descends toward the mysterious surface of Saturn's moon Titan.

"These are three of the most historic of all space missions to date," said Chris Phillips, the Australian team leader.

"But this time we are not decoding and relaying the images and data from a space craft. In conjunction with 16 other telescopes in Aus-tralia, China, Japan and the US, we are actually creating a virtual radio telescope with a `dish' nearly as wide as the diameter of the earth. This will allow us to locate and track in minute detail the descent trajectory of Huygens even though it is about 80 light minutes [1.4 billion km] away from earth in the realm of Saturn," he said.

By comparison, the Sun is little more than eight light minutes from the Earth, and the moon less than twolight seconds -- or 600,000km away.

Phillips says all of the observations made by Huygens will be relayed to its mothership Cassini which has been orbiting and studying the ringed planet Saturn since July, but the signal from Huygens at that distance will be quite weak, akin to a mobile phone trying to call home from Venus.

He says the Cassini will hear Huygens loudly and clearly, yet not know exactly where it is, but that this missing information will be essential for fully understanding what-ever Huygens relays.

In fact Cassini's trajectory past Titan means it can only listen to Huygens for a maximum of three hours on Jan. 14. It will be in range for the nominal two and a half hours Huygens could take to parachute to the surface through the dense murk swirling around Titan, and up to 30 minutes more if it survives touch down. No one is sure if Huygens will sink into oily slush, or break through a brittle crust of frozen hydrocarbons, or come to rest on a hard surface. Despite the low gravity environment on Titan's surface, its atmospheric pressure at ground level could be at several times higher, or denser, than Earth's.

Cassini has sent back fuzzy images of broad blurry surface features that have so far defied explanation. It has captured radar images of about 1 percent of the surface including a glimpse of what might be a 40km-wide blob of material oozing across a grooved plain.

On Dec. 25, Huygens will be released from Cassini to coast for 21 days to its encounter with Titan.

"The batteries on Huygens can last up to six hours," Phillips said. "If it doesn't sink or shatter on impact the irony is that we will be able to hear its faint whispering for maybe three hours longer than Cassini, but never fully decipher what it was saying."

Titan is, midway in size between our Moon and Mars yet gravitationally captive to the giant ball of ringed gases that is Saturn. It has a thick chemical rich atmosphere that extends more than three times as far into space as Earth's. Astronomers have already determined that much of it comprises nitrogen, as does Earth's lower atmosphere, but laden with complex hydrocarbons including methane, rather than oxygen, and a particle rich smog that re-sembles that of Los Angeles.

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