Rosetta probe set to meet comet after 10-year chase
After a journey that has lasted a decade, Europe's Rosetta spacecraft is now on its final approach to a comet.
The tiny probe is set to rendezvous in a few hours with one of the strangest objects in the solar system.
The latest in a series of manoeuvres will bring Rosetta to within 100km of Comet 67P/Churyumov-Gerasimenko.
One of the scientists leading this European Space Agency (Esa) venture described it as "the sexiest, most fantastic mission ever".
Confirmation of Rosetta's rendezvous with 67P - the start of its extraordinary trek alongside the comet - should come by 09:35 GMT (10:35 BST).
Throughout human history, comets lighting up the night sky have triggered fascination and fear but their speed and distance have made them difficult to investigate.
One theory is that they delivered water, carbon and other essential building-blocks for life to the early Earth.
Previous missions have had to be fly-bys - brief encounters crossing a comet's path to gather data or collect samples of dust.
By contrast, Rosetta is designed to fly around comet 67P in a form of orbit for more than a year, its 20 instruments providing unprecedented information about the comet's structure and composition.
If all goes according to plan by November, mission managers will pick a spot for what will be an audacious attempt to send a lander, known as Philae, to touch down.
For the moment though, all eyes will be on Wednesday's landmark manoeuvre which should bring Rosetta into a controlled flight in a triangular pattern around the comet.
With 67P hurtling along at 55,000 km per hour (34,000 mph), the spacecraft's speed has been adjusted so that in relative terms it will be flying beside the comet at a slow walking pace of 1m/sec (2.2mph, 3.6kph).
This has never been tried before - and because radio signals take more than 22 minutes to travel between Earth and the spacecraft, the main moves have to be pre-programmed.
Project scientist, Dr Matt Taylor, said: "We have to make baby steps as we approach it, because we don't know exactly how the comet is behaving and how the spacecraft will behave around it.
"We have a rough idea but we have to take a gradual approach to really get a handle on how to fly around a comet.
"For me this is the sexiest, most fantastic mission there's ever been. It's ticking a number of boxes in terms of fascination, exploration, technology and science - predominantly science."
One surprise is the comet's bizarre shape, with what look like two parts joined together, some of the earliest images giving an impression of a rubber duck.
This irregular structure may complicate attempts to calculate the comet's gravitational pull, vital to planning how to steer the spacecraft as it accompanies the comet through space, and how to attempt a landing.
Early measurements have shown that the comet's surface is about -70 degrees C, indicating a covering of dust rather than ice, but much about the object remains to be discovered.
The mission was named after the Rosetta stone which helped unlock the hieroglyphic language of the ancient Egyptians.
And since comets are the remnants of the earliest days of the solar system, it is hoped that the spacecraft will help reveal how the system evolved and whether there is a link to the emergence of life on the early Earth.
Planned back in the 1990s, hundreds of scientists and engineers have been on tenterhooks for this moment.
They watched the launch back in March 2004 and then waited as Rosetta was sent on a long trek through space to allow it to pick up speed, orbiting the Sun five times and using the gravity of Earth and Mars to accelerate to catch up with 67P.
During one phase as the spacecraft was on the most distant and coldest stretch of its journey, it was put into hibernation for more than two years to save power - and its reawakening last January was a cause for relief and celebration.
Prof Monica Grady of the Open University, which has designed and built an instrument for the lander, is among those excited about now getting results.
She said: "This is the culmination of a scientific project that for me has been going on for more than two thirds of my scientific career.
"It is going to be the crowning moment of something that I have been working on for so long."
Like many of the scientists involved, Prof Grady is keen to see if the comet's water, locked in the form of ice, bears any relation to water found on Earth, and to understand more about its chemical composition.
"The biggest question that we are trying to get an answer to is: where did life on Earth come from?
"How did life get going? Was it the building blocks of life that were brought to us from comets or did it get going on Earth? Did the water on Earth come from comets? Are we reliant on these bodies to have brought water to us?"
The plan is to measure the ratio of hydrogen and deuterium in the comet's ice to provide a comparison with water on Earth.
Researchers also want to see if the comet contains amino acids, molecules that are essential ingredients for life.
A Nasa mission called Stardust, which gathered samples of dust from a comet, found evidence of the amino acid glycine.
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