The data collection mission was supposed to last until December 2015 but it has been extended to September 2016. By then, the comet will have moved too far from the sun for the solar-powered Rosetta to continue functioning.
It has been an epic journey. The mission began in March 2004, with the launch of Rosetta. Rosetta's path involved slingshot manoeuvres to utilise the gravity of the Earth thrice, and Mars once, to generate speed.
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In September 2008, Rosetta investigated an asteroid, 2867 Steins, and another, 21 Lutetia, in July 2010. As it moved out further from the sun, keeping the solar-powered vehicle functional became more difficult.
On June 8, 2011, Rosetta was put into hibernation with an "alarm" set for it to wake itself up. That worked perfectly. Rosetta came out of sleep mode and contacted Earth on January 10, 2014. Then it embarked on complicated manoeuvres to match orbits with the comet between May and August 2014. Rosetta finally arrived at the comet on August 6, 2014. It fine-tuned its orbit and released the Philae lander from a distance of about 3 km in November 2014.
The lander did not anchor itself to the surface as intended - three mechanisms designed to anchor Philae failed. The lander bounced and came to rest at another spot. That in itself was lucky, given that the comet has very little gravity. However, Philae was damaged, with its communication equipment being affected. It also came to rest at a spot that did not receive much sunlight. So the solar-powered lander has been under-powered and its communications with Rosetta (which relays to the Earth) have been sporadic.
The mission involves accompanying the comet for as long as possible. The comet was at its closest to the sun (perihelion) in August 2015 and has since moved away. Although Rosetta's enormous solar panels allow it to operate even when it receives only four per cent of the solar energy received at Earth's distance from the sun, the space probe will eventually move beyond that point too.
The mission was not just about getting there and matching speeds. The lander contains nine scientific experiments and the Rosetta orbiter-vehicle has 11 experiments. This is the first time a spacecraft has been so close to a comet and an enormous array of data has been gathered. The name of the mission is taken from the Rosetta Stone, which let the ancient Egyptian language be deciphered. It is hoped the mission will decipher some of the secrets of how life began.
Comets feature in several plausible hypotheses of the origin of life. Some scientists suggest that debris falling off comets contain amino acids, the complex molecules that are key building blocks of life. Comets have highly eccentric orbits. This means massive swings in temperature as they approach the sun and then recede. They develop spectacular "tails" and then refreeze. Rosetta will collect data on those processes.
Philae gathered data during its seven-hour descent and the two hours that it "bounced". It continued experiments and sent data for about 64 hours. However, after that, there has been only intermittent contact. The lander's solar batteries may have charged as the comet neared the sun. But increased activity on the comet's surface forced Rosetta to move away, beyond the range of communication. Rosetta will move closer again now. It would be able to communicate with Philae - assuming Philae is still capable of communication - once it is within 200 km of the comet.
Experiments have so far revealed 16 organic compounds. Four of these -methyl isocyanate, acetone, propionaldehyde and acetamide - have never before been detected in comets. Apart from these, water vapour, carbon monoxide, carbon dioxide and formaldehyde have been found. These could all be precursors to life.
Philae will become non-functional in January or February as the lander will not be able to generate enough energy. But Rosetta will continue to gather data. It would be possible to put Rosetta into hibernation again but the spacecraft is also running out of propellant fuel. Hence the final attempt to land it on 67P to conclude a journey of 13 years in some style.