Key to the Heavens
Richard Macey. Photo: Edwina Pickles.
SMH July 18 2002

Walking on the moon was fun while it lasted. Now there's a $1.4-billion plan to hitch a ride on a passing comet, writes Richard Macey.

The date is 10 years away but Warwick Holmes, a Sydney-born electrical engineer, doesn't need a crystal ball to know how anxious he will be come July 2012.

That month, a television-sized robot from Earth will, if all goes well, attempt history's first landing on the icy surface of a comet.

Although the $1.4-billion mission is being funded and managed by the 15 nations of the European Space Agency, Holmes and his small company of mostly Australian engineers have been assigned a vital role. Their job is to ensure two vital systems aboard the mechanical explorer - one that controls the direction it points during its 10-year voyage and another that allows it to automatically detect and fix faults without Earth's help - work as planned.

The probe has been named Rosetta after the stone, found in 1799 in the Nile delta, that allowed the deciphering of Egyptian hieroglyphs. Scientists believe comets, rubble left over from the birth of the solar system, hold the secrets that will help decipher the physics and chemistry behind the birth of the planets.

To be launched in January from Kourou, a space port nestled between the Amazon and the Atlantic in South America's French Guiana, Rosetta's target is a 1.2km-wide ball of ice and dust called Wirtanen, discovered in 1948 by an American astronomer, Carl Wirtanen. While the better known Halley's comet takes a leisurely 76 years to loop the solar system, Comet Wirtanen whips around in just 5 years.

"It's dim ... not as dramatic as other comets," says Holmes, in Sydney yesterday to brief the Australian Space Development Conference on Rosetta. Wirtanen, he explains, was chosen as Rosetta's target because its relatively small orbit, taking it no further from the sun than the planet Jupiter, makes it one of the few comets within reach of even the world's most-powerful rockets.

"Comets are the oldest things in the solar system. They are the original clay from which the planets were made. Their surfaces are basically water-ice, carbon dioxide, carbon monoxide and methane." Only recently regarded as cosmic "snowballs", it now appears that comets are covered in a black crust of "tar" produced by intergalactic dust and carbon raining down for billions of years. "Halley's comet turned out to be the blackest object ever seen in space - blacker than coal dust, rather than a nice snowball," Holmes says.

As they approach the sun, ice inside starts to vaporise, producing jets of gas and dust that envelop the comets in a cloud, or coma, of water vapour, hydrogen and cyanide, thousands of kilometres wide. The pressure of sunlight on the coma causes the gas and dust to stream out for millions of kilometres, producing spectacular tails that terrorised ancient civilisations.

In 1986 four probes briefly glimpsed Halley's comet as they zoomed by at speeds of up to 79 kilometres per second.

To land gently on Wirtanen, Rosetta will have to gradually sneak up from behind, like the horse-riding outlaws who robbed trains in so many westerns. But even Europe's biggest rocket, the mighty Ariane 5, does not have the power to hurl the three-tonne Rosetta directly on to Wirtanen's 37.4 kilometre per second orbital path.

Instead, it will take the long route, heading first to Mars. The red planet's gravity will then fling Rosetta back to a close fly-by of Earth. Two years after passing the home planet Rosetta's path will intersect Earth's orbit once more. This time Earth's gravity will finally slingshot Rosetta towards its destination. Along the way the probe will take close-up looks at two asteroids.

Although the circuitous cosmic road reduces the rocket power needed to toss the probe into deep space, there are tolls to be paid. All previous spacecraft that have ventured into the outer solar system - NASA's Pioneer, Voyager, Galileo and Cassini missions - have relied on nuclear power. However Rosetta's gravitational slingshot path will bring it to within 1400km of Earth, a feat requiring extraordinary navigational skills to avoid a collision.

"If anything goes wrong it could burn up in the atmosphere," Holmes says, explaining why the European Space Agency opted to power Rosetta using solar panels rather than risk showering Earth with plutonium.

But out where the probe will meet the comet, the sun shines 30 times weaker than on Earth. So that Rosetta can soak up the energy it needs it will be fitted with 70 square metres of solar panels, creating a craft stretching 32 metres from tip to tip. The revolutionary solar collectors will be covered in tiny glass pyramids to catch as much sunlight as possible.

Another downside will be journey time. "We have to keep the thing alive for 10 years," Holmes says.

By the time Rosetta catches the comet it will be a billion kilometres from Earth. If anything goes wrong, news of the glitch - travelling at the speed of light - will take 50 minutes to reach mission control in Germany. Even if flight controllers have an immediate solution, it will be another 50 minutes before new orders reach the probe.

"A 100-minute delay could be fatal," said Holmes. As a result, Rosetta will be fitted with an electronic brain giving it the "intelligence" to detect and respond to problems as they arise. If its eyes, the navigation camera, fail, on-board computers will have to decide which back-up system to activate to restore order.

To ensure Rosetta's brain can cope with the job, Holmes and his Holland-based team have been subjecting the craft to pre-launch tests. "We put it through a whole series of nightmare scenarios."

In 2011, out near the orbit of Jupiter, Rosetta will finally find itself trailing Wirtanen. That November it should be just 100,000km behind, creeping up at a mere 90kmh.

Rosetta will continue to close in until May or June 2012, when it will glide past the comet at 7kmh, at a distance of 35km. But no attempt will be made to go into orbit.

"We don't want to catch the comet just yet as we won't know enough about it," Holmes says. "First we want to measure its shape and gravity."

About 200km past Wirtanen, Rosetta will fire its rocket motors and allow the comet to catch up. Again the aim will be to glide past, slightly closer, carefully studying the comet as it goes by. The dance around Wirtanen will be repeated up to 10 times until Rosetta is just one kilometre above its surface.

"A puff of gas from the engines will be all that is needed to put us into orbit around it. The first thing will be to start looking for a landing site," he says.

As Rosetta drifts over the comet it will have to be wary of jets of gas erupting from the surface at speeds of up to one kilometre per second. Although the density of the jets will be extremely thin, they could batter Rosetta's solar panels, blowing it like a sailboat in a gale.

"Nobody has ever had to deal with this. The engines will probably have to be constantly firing, constantly fighting against the dust and the gas."

In July 2012 the orbiting Rosetta will jettison its 100 kilogram, three-legged lander. It will drop straight down. In the comet's gravity, 100,000 times weaker than Earth's, the lander will weigh just one gram. About 60 minutes after separation the lander should settle on the surface.

"It has to be able to land on soft snow or rock-hard ice," Holmes says, adding they do not want the lander to bounce back into space.

To ensure the lander does not bounce, a gas thruster and an electronic shock absorber will fire to hold the craft down. Screws under each landing leg will be driven into the comet, allowing the legs to grip the surface. Then two 10-centimetre harpoons, attached to wires, will be fired into the ground. "The wires will then be retracted, tying us down like an anchor. There is a good chance we will break through the crust."

It is hoped the legs will be long enough to keep the craft above the surface. The lander's battery should provide enough power for about three weeks of surface experiments. An array of 10 cameras will snap panoramic and close-up pictures. Microphones will listen for the echo of pips emitted by speakers fitted to each leg. By studying how the sounds bounce off the surface scientists hope to learn what lies inside the comet.

A drill will dig 25 centimetres under the surface to retrieve samples to be analysed by a suite of on-board instruments and then photographed through a microscope. In another experiment, radio waves beamed from the lander, through the comet and then on to Rosetta, will help scientists produce a virtual CAT scan of Wirtanen's interior.

The lander will fall silent when the battery dies. However scientists hope that a year or so later, as Wirtanen nears the sun, light falling on tiny solar panels will revive the craft.

Holmes, who recalls watching as an eight-year-old as Neil Armstrong walked on the moon, joined British Aerospace in England after completing science and electrical engineering degrees at Sydney University 16 years ago.

He volunteered to work for nothing to gain experience in the space industry.

Twelve years ago he set up his own company, SatAsInt Consultants - an acronym for satellite assembly, integration and test. His company has been assigned to a number of European Space Agency projects, including an orbiting X-ray telescope, for which he was the lead launch engineer. "It's the most interesting job in the world," he said, adding he was certain Rosetta would capture the public's imagination. "Comets are very romantic."

But he is very aware of the time and money invested in Rosetta.

"I am already biting my fingernails ... by [2012] there will be nothing left. If Rosetta doesn't work there will be an awful lot of unhappy people. But it won't fail for want of trying."