Thousands of things could have gone wrong to prevent this amazing achievement; and several things did go wrong. Let's take a moment to reflect on the magnitude of this event.
This is not the first time that the human race has had probes interact with comets. In 1986 NASA flew their "Ice" mission through the tale of the famous Halley's comet, and in 2005 they fired a massive copper block at Tempel 1 in the "Deep Impact" mission. But a landing on the less memorably named 67P/Churyumov–Gerasimenko is the culmination of ten years of space flight, let alone the preparation time of the mission before takeoff.
67P orbits the sun once every 6.45 years, it's closest approach to the sun is outside of Earth's own orbit and the farthest point it reaches is outside of Jupiter's orbit. In fact, before 1959 its orbit was significantly different - having been altered by a close encounter with Jupiter.
The Rosetta probe was launched in March 2nd 2004 atop an Ariane 5 rocket, after several delays due to weather and a piece of detached foam. Getting an object into orbit around earth requires a massive amount of energy, and whilst we're now quite adept at managing this feat; the recent incidents of the unmanned ISS resupply Antares rocket explosion and Virgin Galactics tragic crash of SpaceShipTwo serve to highlight the difficulty of this first step into space.
Over the last 10 years, Rosetta has orbited the sun 4 times, visited two asteroids, changed trajectory using the gravitation forces of Mars and Earth (3 times in the case of Earth), before rendezvousing with 67P somewhat inside Jupiter's orbit on August 6th 2014. It has also observed another comet and had to fix a problem with its star tracking system caused by being hit by the energy release by a solar flare.
67P itself travels at a speed of up to 84,000 mph - and Rosetta had to perform braking manoeuvres on approach to shed 1,700 mph of its own velocity.
Rosetta is the first man-made object ever to be placed in orbit around a comet, and is now moving at a relative velocity of 2mph - approximately walking pace - around the 2.5 mile irregularly shaped nucleus of 67P.
Early in the morning of 12th November 2014 a final go decision was made to activate and separate the Philae lander module from Rosetta, to slowly fall the approximately 19 miles to the surface to a landing site carefully selected by scientists from Rosetta's observations.
The separation of Philae from Rosetta caused a change in Rosetta's alignment, and it lost contact with mission control here on Earth - taking a few minutes to re-align itself and re-establish contact.
Philae itself has drills on its landing legs, and two harpoons designed to attach it to the surface of the comet, without which it could just drift away from the surface. A thruster atop Philae intended to prevent it from bouncing off 67P after landing has already been found to be inactive, and no-one knows whether the harpoons will actually work.
Over the course of 7 hours, the lander descends to the surface - with no human pilot in control it's landing is entirely automated. With a communication delay of 28 minutes between Rosetta and Earth, it would be impossible for a human pilot to control the landing anyway.
Philae continues down, it's low power communications being repeated to us via Rosetta, and we are treated to images of both craft taken by the other.
Eventually, we receive confirmation that Philae has landed on the soft surface of 67P, and its twitter account announces the occasion in dozens of languages - but the story does not end there.
Communications with the lander are sporadic, and it eventually becomes apparent that the harpoons failed to fire, and Philae bounced off the surface not once but twice - finally settling on its third 'touchdown', this being confirmed only on the morning of the 13th.
I now look forward to seeing some amazing footage from the surface, and hearing about details of the scientific investigations that will be carried out. Here's hoping that Philae manages to keep itself attached to 67P and that we find out more about the nature of our solar system! Well done ESA!
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