For the first time in history, a spacecraft has landed on a comet. The momentous event represents the culmination of 35 years of research on comets by Prof. Akiva Bar-Nun of Tel Aviv University's Department of Geosciences and other scientists working for the European Space Agency.
At 08:35 GMT on Wednesday, November 12, the European Space Agency's Rosetta satellite released its lander Philae towards Comet 67P/Churyumov-Gerasimenko, a large mass of ice and dust some 316 million miles from Earth. The descent took approximately seven hours, with a signal confirming touchdown received at Earth at around 16:00 GMT. The comet, shaped like a rubber duck with a narrow neck, is 2.5 miles long and 1.2 miles wide, roughly the size of central London. Its terrain is severe, studded with cliffs, steep slopes, and fissures. The Rosetta spacecraft, launched in 2004, is now flying with the comet, hovering just six miles over the nucleus to take measurements -- and extraordinary pictures.
"The seeds for this mission took root during the 1986 visit of Halley's Comet," said Prof. Bar-Nun, a member of the Rosina group at the University of Bern. "The ESA's Giotto spacecraft passed by Halley, but remained more than 600 miles away from it. So a group of us, scientists from the US and Europe, got together to design a spacecraft that would not pass by a comet -- but would instead fly with the comet and bring samples of its ice back to earth."
Taking its time
Comets hold vital clues about the original materials that went into building the solar system 4.5 billion years ago. The prize awaiting the successful landing is the opportunity to directly sample the organic material that may have prepared Earth for life 3.8 billion years ago.
"When we proposed to ESA to bring a sample back, they said, 'You have no idea what the mechanical strength of the ice is. How are you going to drill into it?'" Prof. Bar-Nun continued. "So we shifted the emphasis to what is now known as Rosetta -- a spacecraft that could match the orbit and speed of the comet, staying with the comet for a year and a half and launch its probe at the appropriate time."
Life on ice
"Our TAU lab has been studying cometary ices for 35 years, and we are the only ones who are able to produce and study large ice samples, about eight inches wide and four inches high," said Prof. Bar-Nun. "According to data from a previous NASA mission called Deep Impact (from 2004), which recorded the imprint of a piece of metal in the comet's snowy surface, we know that the comet is covered by soft ice like newly-formed snow. This made it tricky for the Philae lander, which needs harpoons to latch onto the ice and screws to anchor the spacecraft legs to the surface.
"Comets stayed cold for 4.5 billion years, the age of the Solar System, and now one is coming right at us, heated by the sun, spewing gasses, dust, and ice particles," said Prof. Bar-Nun. "Mixed into this dust is a plethora of organic material that may have been brought to our planet by a comet and where, dissolved in the ocean, it prepared the scenario for the emergence of life on Earth."
The Rosetta mission is scheduled to last until December 2015, four months after the comet has made its closest approach to the sun and started to head back out to the more distant reaches of the solar system. The Philae lander could survive for up to three months, but its lifetime depends on whether it will be able to effectively recharge its batteries - and whether it can hang on tight as it swings through the solar system.
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