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Chasing a Comet

The European Space Agency’s spacecraft, Rosetta was launched into space from Kourou, in French Guiana on 2 March 2004.

Its mission: To meet up with Comet 67P/Churyumov-Gerasimenko, travel along with it to study it from close quarters, and to drop a robotic laboratory on its surface.
Cost of the project: 1.2 billion Euros (Rs 9183 crores).

Why spend so much to study a comet?
Comets are believed to be left-over material from the time the Sun and planets were formed, so studying their composition would reveal what materials went into the making of the planets of the solar system and what material was available in the space environment at the time.
Some scientists are of the view that hydrocarbons (the building blocks of life on Earth) and water were brought to our planet by comets crashing on its surface after it was formed. Studying comets, therefore, becomes essential in our quest to find how life began on Earth.
Comets travel at great speeds. Comet 67P travels at speeds of around 135,000 kmph. For Rosetta to catch up with it at launch speed would have been like a fly trying to catch up with a bullet that had been fired. Its progress had to be speeded up and this was done by putting it on a circuitous route that brought it close to Earth three times and to Mars once.
Whenever it drew close to these planets, their gravitational pull increased its speed tremendously and though its journey to the comet took longer because of these detours it gained the required velocity.
By June 2011, Rosetta was travelling at the right speed and was on track for its rendezvous with the comet but now it was almost 700 million kilometres away from the Sun, too, too far for a spacecraft powered solely by solar cells. At this point nearly all its electrical systems were shut down to save on power and fuel and Rosetta was put into deep sleep hibernation. Only its computer and heaters were kept active. The heaters were necessary to ensure that the instruments on board were not affected by the severe cold as the spacecraft moved further and further away from the sun.
Rosetta was in hibernation and cut off from Earth from June 2011 to January 2014 when its orbit brought it back to within 673 million kilometres from the Sun and there was enough solar energy to power the spacecraft fully again. An internal alarm woke up the sleeping systems and Rosetta sent a signal to let mission operators know that it had survived the most distant part of its journey. There was an outburst of joy and relief at mission control!
They were hearing from Rosetta after 31 months and during that time they had no way of knowing whether Rosetta was in good shape or whether it had gone to sleep for good.
One operator stood up and yelled: “Go, Rosetta, GO!!!”
Rosetta caught up with Comet 67P/Churyumov-Gerasimenko in August 2014 at a point in the solar system which is 405 million kilometres from Earth and 540 million kilometres from the Sun. It had travelled a total distance of 6.4 billion kilometres for its rendezvous! The comet and the spacecraft are now travelling together.
Soon after meeting up with the comet, Rosetta mapped the comet’s surface and located a suitable landing spot for the probe it was carrying. Then on 12 November, the spacecraft which had come to within 23 kilometres of the comet’s surface, executed the manouevre that the whole world was waiting for: it released a robotic laboratory
which had been named Philae onto the comet’s surface. It was a historic moment for though space probes had been crashed on comets earlier, a guided soft landing had never before been attempted. The event generated almost the same amount of interest as the first landing of human beings on the Moon in 1969.
Comet 67P is small, only about 4 kilometres in diameter, and its gravity is several hundred thousand times weaker than that of Earth. This was dramatically illustrated by Philea’s fall. Though it weighs 100 kilogram (on Earth), the robot probe took all of 7 hours to fall through a distance of 22.4 kilometres to the comet’s surface. The plan was that the moment it touched the ground, Philae would shoot harpoons from its three legs to anchor itself on the surface. Unfortunately the system failed and as a result, the moment it touched the ground, it bounced a kilometre high and took two hours to fall back to the ground when it again bounced, though to a lesser extent. The upshot was that though it initially landed at the spot chosen for it, it eventually ended up in a less than ideal place for a device dependent on solar energy for its working— in the shadow of a cliff!
Philae started gathering and relaying images and data to mission control in Darmstadt, Germany from the moment it left Rosetta and finally touched down to the time its primary charge ran out two and a half days later.
For the first time the world got to see high resolution pictures of the surface of a comet taken from the comet itself.
One of the first things scientists learnt from the landing was that underneath the dust that covers the surface, the crust of the comet is extremely hard. We can expect to learn many new things about comets in the months to come as scientists throughout Europe carefully and meticulously analyse the data sent back by Philae. There is expectation too that as the comet draws even nearer to the Sun by March or April, some light may fall on Philae’s solar panels and it may get recharged enabling it to send back more data and carry out some experiments that it had been scheduled to do before its batteries ran out. Ultimately, of course, it will be destroyed by the Sun’s heat.
Meanwhile, Rosetta continues to ride alongside Comet 67P. It has a ringside view of the changes taking place on the comet’s surface, notably in its coma and its streaming tails, as it streaks into the increasing intensity of the Sun’s radiation. In August 2015, the comet will make its closest approach to the Sun, and Rosetta will be right there, recording it all!
It will accompany the comet till December 2015 when the celestial visitor will head back to the outer solar system and Rosetta’s mission will officially come to an end.

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