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CSIRO MEDIA RELEASE 95/136

CSIRO SUPPORTS NASA'S GALILEO MISSION

Launched in 1989, Galileo reaches Jupiter on 8 December, Australian time. On that day it carries out its first major experiment, sending a probe plunging into the planet's atmosphere.

CSIRO's Australia Telescope will be one of two sets of telescopes that will receive signals directly from the probe; the other is the Very Large Array in the USA.

The probe's signals will also be picked up by the Galileo spacecraft, 213, 000 km above Jupiter, and re-broadcast to Earth, where they will be collected by NASA's tracking stations around the world.

This is one of the most important experiments of the Galileo mission. It is the first time a man-made probe has entered the atmosphere of one of the 'gas giants' of the Solar System. The probe's findings will build on what we learned last year about Jupiter's atmosphere when it was churned up by Comet Shoemaker-Levy.

The probe, a 340 kg cone-shaped missile, was dropped from the main spacecraft in July. It will hit the planet's atmosphere at 170,000 kilometres per hour, the fastest impact speed of any man-made object ever. In four minutes it will undergo gut-wrenching deceleration, slowing down to a mere 430 kilometres per hour. In the crucial first two minutes after it enters the atmosphere a heat shield will protect it from burnout, and then a parachute will open to slow its fall.

As it sails down towards Jupiter's solid core, the probe will be measuring the temperature, pressure and chemistry of the upper 600 km of the planet's atmosphere, locating the major cloud decks and looking out for lightning.

The probe is expected to last for only 75 minutes. At the point of destruction of the probe, the temperature will have risen to about 185 degrees Celsius and the pressure will be around 30 times atmospheric pressure on Earth.

During its descent the probe will be buffeted by Jupiter's high winds. At the latitude where it enters, Jupiter's clouds are swept eastwards at more than 400 kilometres per hour. Measuring how the wind speed varies with altitude will help us to understand the dynamics of Jupiter's atmosphere. This can be done by measuring the Doppler shift of the probe's radio signal as the probe is pushed along by the winds, and this is what the Australia Telescope will be doing.

If the experiment is successful it should improve the accuracy of our estimates of the wind speeds by a factor of five to ten.

Trial observations will be run on Thursday 7 December.

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