Since antiquity, we have been fascinated by comets. These objects appear as if from nowhere and shoot across the sky with spectacular trails before disappearing again. Many scientists believe comets have an intimate connection with life.
Comets are dirty balls of ice, dust and organic matter, swinging in highly eccentric orbits. When one comes close to the sun, heat causes the matter to vaporise and this turns into the long gaseous tail characterising them. Cometary material includes many of the organic compounds and amino acids that are precursors to life.
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When a comet passes near the Earth, debris causes meteor showers like the annual Perseid Showers every August. Some theories suggest cometary material may have triggered life on Earth and meteor strikes may also have been responsible for mass extinctions.
There is a great deal of interest in investigating cometary composition and behaviour. Observing a comet close-up isn't easy. Many have orbits of millions of years and approach the sun only for brief periods.
NASA's Deep Impact smashed into comet 9P/Tempel in July 2005, the first time a comet's nucleus had been reached. Other missions like Giotto and Stardust were fly-bys that took pictures from a distance.
The European Space Agency's (ESA) Rosetta, is much more ambitious. It will stay with Comet 67P/Churyumov-Gerasimenko for 19 months and land a probe on the nucleus. Rosetta was launched as long ago as 2004. It has involved several complicated technical operations, including a hibernation of 31 months.
The craft is currently about 670 million kms from the sun, heading for a rendezvous with the comet in May 2014. It will land a module in November. It will orbit the comet through 2015, as it makes its closest approach to the sun and moves away. A total of 21 instruments will be deployed to conduct experiments and collect data.
Rosetta is named after the stone that helped archaeologists to decipher ancient Egyptian hieroglyphics (the same message was inscribed in Greek and Hieroglyphics). The spacecraft was launched in March 2004 from French Guiana on a Ariane 5 rocket.
The launch had been delayed several times. It eventually took place over a year behind schedule. The original target (comet 46 P/Wirtanen) had to be changed. The spacecraft was put through a complicated series of gravity-assist manoeuvres.
It completed a first Earth slingshot in 2005 and a Mars Slingshot in 2007. The Mars slingshot was tight. The craft was in the fourth planet's shadow and it came as close as 250 km to the surface. There was no communication. The solar panels were useless. So there was a critical period when the craft was operating purely on battery power and out of reach. The ESA called it a "billion-Euro gamble". It worked.
That was followed by a second gravity assist involving Earth, and Rosetta then approached asteroid 2867 Steins in September 2008. This was followed by yet another Earth gravity-assist in November 2009. In March 2010, the craft approached the unnamed asteroid, P2010/A2 and confirmed that it was indeed an asteroid. This object has a small cometary-style tail but this appears to be debris caused by some sort of impact. In June 2010, Rosetta did a fly-by of Asteroid 21 Lutetia.
After that, the craft was put into hibernation to conserve power. It was at around 800 million km from the sun where its solar panels received only about four per cent of the light available near the Earth's orbit. Almost everything was switched off, except for the hibernation heaters (which would wake up the craft again) and the on-board computer.
Putting a computer into sleep mode is a common way to conserve power on electronic devices. When the machine wakes, it needn't reboot, and it reverts to the state in which it went to sleep. Putting a spacecraft to sleep for 31 months and waking it up again remotely, at a distance of 670 million kms, is tricky. Although Rosetta had been put into hibernation during the Mars fly-by, that was only for a brief while and nobody could be certain it would work.
On January 20, 2014, the internal alarm went off. There was tension until NASA's Goldstone Center and ESA's Canberra Centre received a message "Hello World", which indicated Rosetta was up-and-running. The next 10 days involved various tests of equipment and everything seems to be functioning normally so far.
Rendezvousing with the comet in May 2014 will be another tough process. There are always error factors in judging cometary size and orbits. The tails are very long and orbits can change. Once Rosetta gets into orbit, it will spend several months deploying experiments and carrying out a global mapping of the comet.
In November 2014, after it has a clear idea of the surface and relevant conditions, it will launch a washing-machine-sized module, Philae. That 100-kg craft will land on the four-km-long nucleus and deploy 10 instruments at close range. It will help to provide major insights into the changes a comet undergoes as it approaches the sun. In August 2015, the comet will be at its closest distance to the Sun, about 185 million km away. Rosetta will continue to monitor what happens until December 2015 as the comet moves away.
This long odyssey should provide a clearer picture of what happens during the "active phase" of a comet's orbit, and also a more exact idea of the chemical composition. It also demonstrates a new set of technological capabilities. There will undoubtedly be surprises involved.