Galileo keeps a watchful eye on Jupiter

Heather Couper and Nigel Henbest review the information that has emerged from last year's spectacular comet impact

All eyes are on Jupiter this month as it reaches opposition - its closest point to the Earth this year.

Through a moderate-sized telescope, you can make out a new dark band stretching around the planet's southern hemisphere. It is a scar still remaining from the great comet impact of 1994.

Last July, 21 fragments of the disrupted comet Shoemaker-Levy 9 plunged into Jupiter's atmosphere. Most astronomers made only modest predictions of what would happen. Everyone was astounded when the impacts produced brilliant bursts of light and infrared radiation, and the aftermath was a series of huge dark scars on Jupiter's pale clouds.

During the intervening year, astronomers have made good progress in understanding just what went on.

As seen from Earth, the impact took place just round the back of Jupiter: ground-based telescopes could not see the impact sites directly but only the events taking place hundreds of miles overhead. Critical evidence has now come in from the Galileo space probe on its way to a rendezvous with Jupiter this December.

Galileo was in a position to view the impacts directly, but the pictures took months to return to Earth because the space probe has a damaged communications antenna.

This is the story of a typical impact. The first signs came minutes before the comet fragment even reached Jupiter. As the electrically charged cloud of dust sped through Jupiter's powerful magnetic field, it created mighty electric currents.

These currents sent particles in the planet's radiation belts - like the Earth's Van Allen Belt - towards the planet's opposite pole, where they produced a brilliant display of aurorae. These aurorae were detected by Rosat, an international X-ray satellite observatory.

Meanwhile, the giant Keck Telescope in Hawaii, the world's largest telescope, saw a brief flash of infrared light. The comet fragment was heating up like a meteor in Jupiter's upper atmosphere.

Then things went momentarily dim for Earth-based telescopes, as the fragment entered Jupiter's cloud layers. But Galileo, with its grandstand view, picked out a brilliant flash as the fragment slammed into Jupiter's dense cloud layers.

A few seconds later, a mushroom cloud erupted from the impact site, hundreds of miles into space. Infrared telescopes on Earth again picked out the rising cloud of hot material, and observed it spreading out. The gas from each impact reached roughly the same height, but the more powerful eruptions spread further sideways.

As this gas came crashing back down, it heated Jupiter's dense cloud tops to produce the most brilliant firework display of all. And it was this spectacular final act, captured by the infrared telescopes, which was featured on those memorable clips on the television news bulletins last year.

As the planet rotated, optical astronomers saw giant black scars around each impact site, where the erupted material had fallen back.

Winds in Jupiter's atmosphere have now spread this material around in a dark band that girdles the planet.

Astronomers are still arguing about the composition of the dark matter and where it came from. Most likely, it was formed from the primitive gases in Jupiter's atmosphere such as methane and ammonia.

The heat of the eruption could weld these gases together into more complex molecules, such as cyanides and possibly even amino acids.

These reactions are very similar to laboratory experiments where researchers have fired electrical discharges through a mixture of gases. The result is invariably a tarry "gunk" of organic molecules, including amino acids.

The comet-spawned gunk on Jupiter is sticking together in larger clumps, and sinking through the clouds. Some astronomers wonder if past comet impacts may have been responsible for the yellow and brown colours of Jupiter's lower cloud layers.

The planets

Venus and Mercury are both "morning stars" this month, though you will be hard-pressed to see either. By the end of June, Mercury will be rising more than an hour before the Sun, but it will be drowned by the twilight. Venus, being so much brighter, will be a better bet.

Mars, in Leo, is becoming less conspicuous every day as its distance from the Earth increases. Its brightness drops from magnitude 0.9 to 1.2 this month, comparable with Leo's brightest star, Regulus.

Jupiter will be found at opposition on 1 June. The giant planet is visible all night close to blood-red Antares in Scorpius. Although it is low in the southern sky, it is so bright - magnitude -2.6 - that it is a striking sight. The Moon will be four moon-widths north of Jupiter on the night of 11-12 June.

By the end of the month, Saturn is rising at about 3am as Jupiter sets. Although it is relatively dim (magnitude 1.1), the planet is in the barren constellation of Aquarius and can be easily spotted.

The stars

Another rather dim constellation is Libra, a kite-shaped group of stars next to Scorpius. In fact, Greek astronomers regarded the stars of Libra as belonging to Scorpius and marking his claws.

Libra's brightest star, the gloriously named Zubenelgenubi, means "southern claw". But in Roman times, Libra was given a separate identity and its association with the scales of justice.

Neighbouring Scorpius never looks its best from the latitudes of the British Isles. To see the long, curving constellation - complete with its sting in the tail - you have to travel south to Mediterranean latitudes at least. From there, it does not take any stretch of the imagination to make out the vicious beast whose sting killed Orion, and which is now safely on the opposite side of the sky from the giant hunter.

Scorpius lies in the general direction of the centre of our galaxy, where the line of sight goes through the thickest concentration of stars, gas clouds and star clusters.

Although the constellation is so low in the sky, it is well worth sweeping with binoculars to turn up a few of its gems. And even if all you can see of Scorpius is Antares, it is worth noting that "the rival of Mars" is an impressive beast in its own right. This red supergiant star is 300 times bigger than the Sun.

Diary

(all times BST)

1 Jupiter at opposition

6 11.25am Moon at first quarter

13 5.03am full Moon

19 11.01pm Moon at last quarter

21 9.34pm summer solstice

28 1.50am new Moon