Science: Stars And Planets: September

EVENING SKIES this month are dominated by the bright planet Jupiter, currently in a barren patch of sky underneath the Square of Pegasus. On 16 September, the planet comes closest to the Earth (although it is still more than 600 million kilometres away) and shines in the sky all night long.

Over the past three years, a spacecraft has been privileged to a rather closer view of this, the giant among the planets. The Galileo space probe has been in orbit around Jupiter since 7 December 1995, and its mission was due to end in December last year. However, the US Congress has allowed the National Aeronautics and Space Administration (Nasa) to extend the probe's studies, and Galileo will be returning data until the last day of 1999.

The extended GEM mission (for "Galileo Europa Mission") has three phases and three main objectives: to study ice, water and fire in the Jovian environment. Until February it will make a detailed study of Jupiter's ice-covered moon, Europa. Then, from May to September 1999, it will send back data on water in Jupiter's thunderclouds and its role in shaping the planet's weather.

Finally - in something of a kamikaze manoeuvre - Galileo will swing within 300km of Jupiter's innermost moon, Io. This violently volcanic moon is the most active body in the solar system and lives inside Jupiter's intense radiation belts. The strength of radiation is easily enough to kill a human being and it is debatable whether Galileo's sensitive cameras and computer systems will survive the two-month encounter.

For the moment all eyes are on Europa. The next close fly-by with this enigmatic moon takes place on 26 September and scientists hope to see details as small as 6 metres across (the size of a bus). Europa is the most favoured location for finding "life off Earth" - a prediction first made (fictionally) in 1982 by Arthur C Clarke in his novel 2010.

This moon, just a little smaller than our own, is brilliantly reflective and as smooth as a billiard ball. It appears to be covered in a global ice-sheet, under which a huge ocean may slosh. Europa is gravitationally "pummelled" by Jupiter and its fellow moons, which has the effect of warming it up. Even at Jupiter's distance from the Sun, Europa's lukewarm ocean could contain the seeds of life.

Images returned from Galileo last December reveal a world covered in huge ice-cliffs and icebergs - rather like the frozen terrain in Alaska that you fly over when taking the great circle route to the States. Galileo also found a fresh impact crater, Pwyll, which is geologically young - no more than 10 to 100 million years old. On the next encounter, scientists hope to seek out volcanic craters, too, like the ice-volcanoes active on Neptune's moon, Triton.

There are also darker, wedge-shaped gaps between the icy plates which researchers believe are similar to the volcanic mid-ocean ridges on our own planet. Some of the Galileo team have visited the Pacific Ocean floor in the submersible craft, Alvin, and concluded that Europa's ridges are probably made of slushy ice or liquid water that has welled up from inside the moon.

Nasa researchers hope to take their Earth-analogue studies of Europa still further by experimenting with a probe that will seek out life around hot undersea vents. Later this year they will test the Lo'ihi Underwater Volcanic Vent Mission Probe, in the Monterey Bay Aquarium in California, before taking it to investigate an undersea volcano 1,300 metres below the ocean in Hawaii.

WHAT'S UP: Get up before dawn if you want a good view of Mercury. In the first week of September, it rises 1.5 hours before the Sun, and passes very close to Venus on the 11th. True to its name, Mercury travels very quickly and by 25 September it will have moved into the Sun's glare once again. Venus is also a major item in the morning sky, but by the end of the month it rises only three-quarters of an hour before the Sun. Mars, too, is in the morning sky, rising at about 3am mid-month. But it is Jupiter that rules the sky this September. It is visible all night long, just below the Square of Pegasus. Binoculars reveal its four biggest moons, including Europa - you can watch them changing position as they orbit from night to night.

Finally, Saturn lies just a short distance away to Jupiter's left - use a small telescope to see its famous rings.

Diary

6th 12.21pm BST Full Moon

13th 2.58am Moon at Last Quarter

16 Opposition of Jupter

20 6.02pm New Moon

23 6.37am Autumn Equinox

28 10.11pm Moon at First Quarter