Journey to the beginning of time

After a voyage of 2.3 billion miles, lasting six years and costing about $1.6bn, the Galileo space mission to Jupiter has ushered in a new age of astronomy.

The results from Jupiter, just released by Nasa, represent "a major contribution to our understanding of the development of the solar system," says Dr Paul Murdin, head of astronomy for the Particle Physics and Astronomy Research Council. Astronomers are no longer passive, Earthbound observers of the planets and stars - the technology of spaceflight means they can now reach out and touch parts of the nearby universe.

Galileo's observations are not of arcane interest. Jupiter and the other "gas giant" planets such as Saturn and Uranus are important because they are so far from the Sun that gases in its atmosphere have not boiled off and so their composition should reflect the primordial stuff from which the solar system was made. They also have strong gravity, which holds the gas more tightly. In contrast, the inner rocky planets such as the Earth and Mars are much more differentiated. So in theory, at least, sending a probe into Jupiter is the closest we can get to sending it into the very earliest moments in the life of the solar system.

Galileo found that Jupiter was quite different from the planet scientists had expected to find based on observations from telescopes on Earth and passing spacecraft such as the Voyager missions to Jupiter and the outer planets. The finding suggests that either Jupiter has developed more than we had expected and that it isn't a good guide to the way our planets were formed or that we must rethink the very origins of our solar system.

Hydrogen and helium are the most abundant elements in the universe, but astronomers believed the disc of dust and gas from which our solar system evolved must also have contained carbon and sulphur and other heavier elements forged in the hearts of stars and then ejected across the galaxy in supernova explosions. We are, quite literally, stardust. But Galileo's findings do not accord with a simple interpretation of this theory. It found that Jupiter's atmosphere is drier than Nasa scientists had anticipated and it contained about half of the expected helium concentrations. In addition, neon, carbon and sulphur were less abundant than predicted.

There are also fewer lightning storms on Jupiter than there are on Earth, the probe discovered. This is consistent with the lack of water vapour, says Professor Alec Boksenberg, former director of the Royal Greenwich Observatory and now professor of experimental astronomy at Cambridge University. Lightning occurs when electric charges have been transported across the atmosphere. If there are not enough water droplets to act as the carriers of charge, lightning will be that much rarer.

Jupiter is a fearful place, according to the observations sent back by the little probe that the Galileo mother ship dispatched on a suicide mission into the planet's atmosphere.

Dr Richard Young, the project scientist in charge of the mission at the Nasa-Ames research centre, says: "The probe detected extremely strong winds and very intense turbulence during its descent through Jupiter's thick atmosphere. The origin of Jupiter's winds appears to be an internal heat source that radiates energy up into the atmosphere from the planet's deep interior."

This is in stark contrast to Earth, whose weather is driven by heat from the Sun warming the atmosphere. Jupiter is so far from the Sun that it gets little solar heat, so the driving force for its weather is its gravitational energy and the heat from radioactive decay of elements deep within the planet.

The probe measured wind speeds of up to 330mph. The characteristic swirling patterns visible on Jupiter's surface appear to come, Dr Young says, from "a jet stream-like mechanism rather than swirling hurricane or tornado-like storms".

But European astronomers are less convinced that Galileo's findings mark a new era in our understanding of the universe. They point out that Nasa, increasingly hard pressed to defend its budgets, has an interest in promoting the significance of the findings. "They would have to be startling discoveries for Nasa to get its money from the US government," one scientist remarks.

They also question whether the apparent anomalies in the composition of the planet's atmosphere would really require rethinking theories of how the solar system formed. One planetary scientist points out that water is abundant on Earth but absent from the Moon. All planets have evolved since the formation of the solar system according to their local circumstances, and some gases might have been lost while others may have settled more deeply in the atmosphere of the gas giants. In other words, the probe may tell us a lot more about Jupiter's evolution but not that much about the solar system.

But there is no doubt that the probe marks a new departure in astronomical observation. Professor Boksenberg explains:"For the first time one is getting real information rather than conjecture about Jupiter."

His analysis is endorsed by Dr Murdin: "For millennia astronomers have had to put up with being passive observers. They had to sit on Earth and detect radiation by building bigger and bigger telescopes.

"What's starting to emerge now is going and getting bits of the nearby universe. We've been to the Moon and brought some bits back to Earth. Landers have been to Mars and brought stuff back into the vehicle for analysis. For the future, the European Space Agency is looking at the Rosetta mission, landing on a comet and digging around on it. I would view Galileo in that developing story. We now have real data on how the atmosphere is, not just a spectrum of the cloud tops."

From Ptolemy to the Jupiter probe: the advance of astronomy

2nd century AD Ptolemy's Almagest was the definitive treatise on astronomy until Copernicus in the 15th century. Ptolemy's theory proposed the Earth as the centre of the universe, around which all other planets, stars and the Sun revolved. He only had his eyes, Euclidean geometry and maths to work with.

1610 Invention of the telescope allowed magnification 30 times greater than human eyesight. Galileo (1564-1642) is thus able to observe Jupiter's moons and prove the heliocentric (Sun-centred), theory of the universe.

1877 As telescope technology improves, further observations are possible, and Giovanni Schiparelli (1835-1910, right), details observations of Mars. He thinks he sees linear markings organised into a network of irrigation canals, supposedly dug by intelligent beings.

Percival Lowell (1855-1916) takes up the Schiparelli theory, and builds the Lowell Observatory at Flagstaff, Arizona, to study Mars. Although his ideas meet with great opposition by the time of his death, he is still taken seriously in some quarters.

With the development of photography, pictures taken after Lowell's death fail to depict the canals. The advent of space travel also puts paid to many ideas formed by passive observation from Earth, and in the Mariner flights of the Sixties, Mariner 9 conclusively proves he was wrong.

1948 The astronomers Fred Hoyle, Hermann Bondi and Tommy Gold, reconciling observations on the evolution of the universe with theories of relativity, advance the steady state theory. This proposes that the universe has existed for ever, and that new material is constantly being created as it expands.

1965 The invention of a specially sensitive amplifier for Bell Telephone Laboratories allows Arno Penzias and Robert Wilson (right) accidentally to discover cosmic microwave background radiation: essentially an echo of the "Big Bang", which began the universe 15 billion years ago. The Big Bang theory refutes the steady state theory that the universe has always existed.

1996 The Hubble Space Telescope revolutionises our understanding of the universe. The photos sent back by the telescope show clusters of stars that form a nucleus around areas of gravitational instability, or black holes. The photos may challenge earlier astronomical views about the evolution and the formation of galaxies.

Tiffanie Darke