By Jove, I think we'll make it!

After a six-year odyssey the Galileo spacecraft is days away from Jupiter. Peter Bond assesses one of the last of the megabuck space missions

A robot ambassador from Earth is about to pay a rare visit to Jupiter's realm. On Thursday, 7 December, America's Galileo spacecraft will brake into orbit around the giant planet at the end of its 4 billion kilometre odyssey to the outer solar system. The outcome of this operation will remain unknown for a nail-biting 52 minutes as the tell-tale telemetry wings its way at the speed of light across more than 800 million kilometres of space.

If all goes well, Galileo - named after the 17th-century Italian scientist who discovered Jupiter's four largest moons - will spend the next two years surveying the Jovian system. Its first big task will begin just before it enters orbit - recording data from a small probe released from the mother craft last July. Travelling at 170,000 km/h, the fastest speed achieved by a man-made object, the probe is scheduled to plunge into Jupiter's orange and white clouds little more than two hours before the main Galileo craft enters its orbit. Scientists hope that up to 75 minutes of unique data about the temperature, pressure and composition of Jupiter's atmosphere will be transmitted to the orbiter before the radio link is broken.

If the 48-minute burn of Galileo's main engine successfully slows the spacecraft, it will swing into the first of 11 elliptical orbits around the gas giant. Over the next 23 months, it will bounce around Jupiter like a tiny billiard ball, using the gravity of each planet-sized Galilean moon to change its course and redirect it to the next target.

Despite the highly successful fly-bys of the two Voyager spacecraft in 1979, many questions about the largest planet remain unanswered. It has thin rings whose fine structure Galileo should reveal. Other instruments will probe the powerful magnetic field with its intense radiation belts and energetic charged particles. One of the main targets will be Jupiter's swirling clouds and storm systems, such as the Giant Red Spot, which has been raging incessantly for at least 300 years.

Among the retinue of at least 16 satellites, the four moons will take pride of place. Three of these are huge ice balls whose surfaces have been shaped by cosmic impacts and strange ice flows. Most intriguing of all is the pizza-like Io, its orange surface pockmarked with volcanoes spewing sulphur into the rarefied atmosphere.

According to the chief of mission planning, Jan Ludwinski, the best pictures of Io will have a 3-4km resolution. "But we will be down to tens of metres for Europa, Ganymede and Callisto," he says. "Galileo will be flying just a few hundred kilometres from these moons."

Scientists at the Jet Propulsion Laboratory (JPL) in California have endured years of delay and frustration while waiting for the fruits of these experiments. Originally scheduled for launch from the space shuttle in 1982, the start of the mission was put back seven years by development problems with the space shuttle and then by the disaster aboard the Challenger, which exploded shortly after take-off from Cape Canaveral, killing seven crew members. During the post-Challenger safety review, Galileo's liquid- fuelled Centaur upper stage was deemed too volatile for carriage inside a crewed shuttle.

Eventually, it was decided to replace the Centaur with a solid-fuel rocket system, which was safer but less powerful. Further periodic boosts to Galileo's velocity were required through "gravity assists" during a fly- by of Venus and two close encounters with Earth. The original modest transit time of just over two years was suddenly increased to six years. Galileo would be 13 years old before its primary mission at Jupiter could even begin.

In April 1991, the spacecraft's umbrella-shaped main antenna failed to unfurl properly, severing Galileo's main communications link. Mission planners were obliged to fall back on the low-gain antenna. Since the received signal is 10,000 times weaker from the antenna, data must be sent back at a much slower rate to ensure the contents are clearly interpreted.

Yet another, possibly fatal threat to the mission surfaced in October this year when the on-board tape recorder malfunctioned. Fears that the tape may have broken appear to be unfounded, but project manager Bill O'Neil has decided to limit its use in the initial stages of the Jupiter approach. Plans to take close-up pictures of the volcanic moon Io have been scrapped in order to ensure that the unique data from the atmospheric probe are safely stored.

Despite these setbacks, Mr Ludwinski says the mission will still achieve 70 per cent of original targets. "We'll be like a tourist in a city with a single roll of film," he says. "We can still document our trip and record the most important sites."

Great efforts are being undertaken to ensure the highest possible data harvest. New flight software being uploaded to the spacecraft's computer includes data compression methods that will allow it to transmit the most important scientific information while editing out less valuable data such as the dark background of space. In addition, ground engineers are modifying the receiving stations in Australia, California and Madrid.

Together, these changes will enable them to collect the strongest possible signal and achieve the highest possible capture rate for the incoming data. Although still a far cry from the intended 134,400 bits per second (bps) with the high-gain antenna, these alterations will allow the information from Jupiter to arrive at 1600bps, 100 times higher than the low-gain antenna's original performance level.

Despite all the hard work and ingenuity of JPL staff in rejigging the $1.4bn mission, the days of megabuck space probes, with hundreds of scientists, engineers and technicians in support, are numbered. Nasa boss Dan Goldin is among those who regards Galileo as a space-age dinosaur. After fears for the Galileo tape recorder, Mr Goldin commented: "The faster, better, cheaper approach to planetary programmes is not putting all your eggs in one basket."

Nasa is already racing ahead to turn Mr Goldin's vision into reality. The 1997 Cassini mission to orbit Saturn and deploy a probe into the atmosphere of its moon, Titan, will be the last of the old-style missions to be flown. They will be replaced by a new generation of task-oriented smallsats. Starting next year, the first Discovery craft, each budgeted at less than $150m, will be sent on their way to explore Mars and the asteroid Eros. Before the end of the century, the agency's New Millennium Programme intends to send three high-tech spacecraft weighing just a few hundred kilograms into the solar system.

Meanwhile, despite all the problems, Galileo has chalked up some notable successes, including the first close-up pictures of two asteroids and the discovery of a small asteroid moonlet.

The writer is the space science adviser for the Royal Astronomical Society.