Irish scientists have contributed to an ambitious mission that will attempt to land a spacecraft on the surface of a comet, writes John Moore
What do a slab of basalt, a dirty snowball, life on Earth, and an international team of scientists involving more than 50 industrial contractors from across Europe and the US have in common? The answer is the launch of the European Space Agency's €1 billion mission to land a spacecraft on a comet.
Named "Rosetta" after the Rosetta Stone, the inscribed piece of basalt discovered 200 years ago which helped Egyptologists to decipher hieroglyphics, the ESA hopes the project could unravel many mysteries associated with comets.
Rosetta actually includes two crafts, an orbiter that will travel to the distant comet, 46P/Wirtanen, and which will then deposit the second craft, a lander, on its icy surface. Rosetta is built and ready to fly but the big question is whether it will get off the ground during the current launch window, which ends on January 30th.
It should be on its way already but Rosetta's planned launch in early January was put back. Last December, ESA controllers deliberately exploded an Ariane 5 rocket 170 seconds after lift-off when an equipment failure sent it off trajectory. The launcher was similar to the one that will carry the Rosetta mission and a review is still underway to reduce any risk to the costly spacecraft.
Rosetta will rendezvous with comet Wirtanen, in November 2011, when it will be about four times further away from the sun than we are. By that time it will have orbited the sun nearly four times and undergone speed boosting fly-bys of Mars in 2005 and Earth in 2005 and 2007.
It will also visit two asteroids - Siwa and Otawara - for closer inspection. If it survives the long and hazardous trip, Rosetta will have broken the record for the longest solar cell-powered spacecraft flight yet.
By the summer of 2012, and some 800 million kilometres from the sun, Rosetta will enter orbit around Wirtanen, coming within a few kilometres of its coal-black surface. Comets are often referred to as "dirty snowballs" because of their assumed composition - loosely packed ice and tar-like hydrocarbon compounds. Some scientists suggest that comets hitting Earth might have delivered both water and the chemical substances that allowed life to form.
Comets heat up as they approach the sun, releasing gases and dust particles that form a characteristic cometary "tail". The orbiter's scientific payload of 11 experiments will begin to analyse gases coming from the comet's surface and measure its production of water and carbon monoxide/dioxide. It will also map the temperature and composition of the comet's nucleus.
Having identified a suitable location, a lander will then be released for a gentle touch down on the surface. It will approach at walking speed - less than one metre per second and then use one of two harpoons to anchor itself in position.
Immediately, all of the lander's 10 instruments will spring into action, taking at least one contingency measurement in case something goes wrong. It will send back high resolution pictures and other information about the comet's icy surface and organic crust.
It also will take samples down to 30 centimetres below the surface and will feed these into small ovens for more detailed analysis of what this "dirty snowball" is made of. Other instruments will measure properties such as near-surface strength, density, texture, porosity, ice phases, and thermal properties.
Ireland has produced a key element of the Rosetta spacecraft. An Electrical Support System Processor Unit, designed and built by Space Technology Ireland Ltd at St Patrick's College, Maynooth, Co Kildare, will handle communications between the orbiter and lander. This is the conduit that will carry data from Wirtenan's surface back to Earth. Though the minimum mission target is 65 hours to gather this vital information, operations may continue for months afterwards.
Over a period of 12 months, as Wirtanen's orbit comes closer to the sun, the orbiter will observe the extraordinary metamorphosis of its icy nucleus as it begins to warm and vaporise. It will also monitor changes to the coma - the plasma of matter and anti-matter that develops about the nucleus when the comet encounters solar winds and interstellar dust as it closes in on the sun. One important question that many of the scientists want answered is will the material be smoothly released from the surface, or does it crack open at certain areas to release the material underneath?
"The exciting part of Rosetta is that we want to see the core's structure of the nucleus because that will give us an insight into the accretion [cometary formation] processes which took place," says Dr Gerhard Schwehm, Rosetta's project scientist.
"We can look at this from the orbiter but also get a microscopic look from the lander to actually see the structure of the dust grains and ices on the nucleus, because it helps to understand the processes much better."
Rosetta's journey will end in July 2013, after 10.5 years, when the comet is closest to the sun.