On the 12th of November 2014, if all goes according to plan, the European Space Agency will land a probe named Philae on the surface of a comet. This is the first such attempt to do so and will not only be a gigantic milestone in our understanding of these heavenly bodies in particular, and the origins of the solar system, but it also marks one of the most daring and brilliant engineering efforts of the modern era. Philae will be dispatched from the Rosetta space probe which went into orbit around comet 67P/Churyumov–Gerasimenko (hereafter 67P/C-G) in August this year. What it has taken even to reach this point and achieve these initial goals is truly extraordinary.
Rosetta was launched on March 2, 2004 and finally reached the comet with which it was designed to rendez-vous on August 6, 2014.
Powered only by its solar panels, Rosetta has been forced to make a number of complex manoeuvres in order to save energy and accelerate sufficiently to chase a speeding comet. Thus, after launch, Rosetta has relied on gravity assists from a number of planet flybys – first swinging around Earth in March 2005, then Mars in February 2007, Earth again in November 2007, before flying by an asteroid – 2867 Steins – in September 2008, back around the Earth in November 2009, then out past another asteroid, 21 Lutetia, in July 2010. Having finally picked up sufficient momentum and been set on the right trajectory, in mid 2011, as it swung out toward the orbit of Jupiter, Rosetta was shut down and entered a 31-month period of hibernation.
For almost three years Rosetta floated in space, waiting patiently for the comet to swing past so that it might begin its final chase. Then, earlier this year, on January 20, 2014, Rosetta was woken by her internal alarm. The probe fired its thrusters to slow its rotation, faced its solar panels towards the sun, rotated its antenna towards the Earth and finally, after an anxious wait, sent a signal to indicate that its systems were operational. It was the first communication heard from the craft during those 31 months and mission controllers (along with fans and supporters the world over) were understandably ecstatic.
Rosetta was alive and well and the mission to pursue comet 67P/C-G was back on.
Since waking, Rosetta spent nearly eight months chasing the comet and finally caught up in August, at which point it executed a series of burns to manoeuvre into orbit around the comet by September 10. On arriving at the comet, scientists were surprised to discover that it was curiously misshapen, appearing almost to be two comets stuck together and joined by a narrow bridge.
On account of its shape, it was likened to a rubber duck and presented mission controllers with significant problems in identifying a suitable landing site for the Philae lander. A decision was finally made in October and the landing site name “Agilkia” was selected, along with the date of November 12. The name of the site carries on the Egyptian theme of the mission – Agilkia being an island in the River Nile.
Come the 12th, Philae will detach itself from Rosetta and fall slowly towards the comet, a process which will take around 7 hours to complete. The landing will be hazardous, largely as it is very difficult to pinpoint exactly where Philae will touch down within the chosen site and there is a risk that it will land on a boulder or ridge and flip over. Also, on account of the incredibly low gravity of the comet, a consequence of its negligible mass, there is some concern that even if Philae lands on a flat surface, it may rebound from the comet. Thus, Philae is equipped with harpoons which will fire into the surface, as well as feet designed to screw into the ground upon landing.
The scientific understanding that has come from this mission so far is very valuable indeed and, even if Philae fails, roughly 80% of the mission’s objectives will have been met. Rosetta will continue to orbit comet 67P/C-G until August 2015, as it swings around the sun, thus giving us an opportunity to study the material make-up of the comet and its behaviour in unprecedented detail. The images that have already been released show an inspiring and magnificently barren landscape with robust and jagged features, stark in the high contrast of the unfiltered sun.
For a mission first conceived in 1993, after rejecting plans for a sample-return mission, this data has been a long time coming. Irrespective of the scientific understanding that comes from Rosetta’s remarkable journey, its very conception, the skill and precision which has gone into its execution, and the beautiful images that have come to us already constitute a wonderful and inspiring achievement. Fingers crossed, come Wednesday, we shall be looking at the first ever images taken from the surface of a comet. That is truly something worth celebrating in a jaded world in which humanity has little to be proud of right now.
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