Fantastic Voyage: Europe Prepares First Comet Landing
On Wednesday, astronomers with the European Space Agency will attempt a maneuver that has never been tried before. A probe from the spacecraft Rosetta is to land on a comet some 500 million kilometers away. The mission could teach us about life on Earth.
Achim Zschaege's hands are calm as he steers the spaceship through the vast expanse that no human has ever seen before. "We are currently somewhere between Mars and Jupiter," he says.
A physical engineer by training, Zschaege isn't, of course, sitting on board the spacecraft himself. His work station is in an open-plan office not far from the central train station in Darmstadt. It is from this sparsely decorated space that he controls the unmanned space probe Rosetta.
On this night, Zschaege is the "Spacecraft Controller" (Spacon) on duty at the European Space Operations Center. Dressed casually in jeans and a sweater, his eight-hour shift has just gotten underway. He regularly checks the columns of numbers streaming across his computer screen, helping ensure that the probe stays on course.
When speaking about his job, Zschaege sounds as though he sees nothing particularly abnormal about his occupation. "In the final analysis, what I do here is not much different from a technician at a waterworks," he says casually. "Just with the difference that my machine is more than 500 million kilometers (310 million miles) away."
In a sense, he is right. The same bothersome mishaps and annoying computer screw-ups that we all know from our home computers happen in deep space too. Sometimes a photo isn't transmitted correctly, at other times a sensory instrument shuts down unexpectedly. But the most important rule to keep in mind in such moments, says Zschaege is: "Do as little as possible. Keep your hands away from the keyboard."
Zschaege is one of the veterans in the European Space Agency's (ESA) control room and he has been accompanying Rosetta on its trip through the solar system for more than a decade. Now, finally, the mission is nearing its climax: On Wednesday, a landing vehicle released by Rosetta is set to actually touch down on a comet.
A maneuver like this has never before been attempted, partly because of the extreme difficulties associated with such a landing. Researchers are essentially trying to land a probe on an object with a surface area roughly equal to Manhattan as it speeds through space 20 times faster than a rifle bullet. If they're unlucky, the comet's surface could be as crumbly as a cracker.
Zschaege's superior, Italian flight director Andrea Accomazzo, has been waiting for Wednesday's landing for 17 long years. "For all of us, it feels like a second moon landing," he says.
Even measured against other voyages into space, Rosetta's rendezvous with Comet 67P/Churyumov-Gerasimenko took a long time. When Rosetta was first fired into space, Gerhard Schröder was still Germany's chancellor and America's invasion of Iraq was just a year old. Since its launch on March 2, 2004, the probe has traveled 6 billion kilometers, roughly 40 times the distance between Earth and the sun. Many of those who worked on the mission in its early years have long since retired.
As part of Rosetta's marathon flight, it traveled to the outer edges of our solar system, so far away from the sun that its solar cells were unable to power the probe's systems. Ground controllers plunged the spacecraft's electronics into a kind of hibernation.
For two-and-a-half years, there wasn't a peep from Rosetta and it was only reawakened at the beginning of this year. "We had to wait almost an hour before we received the signal" indicating the maneuver had worked, Accomazzo says. "It was totally silent in the control room."
A Big, Initial Surprise
A couple of weeks later, the probe's propulsion system fired up and set the craft on its final course toward its deep-space rendezvous with Comet 67P. It began orbiting the object, which is named for its discoverers, at the beginning of August. Because of its impossible-to-pronounce name, the astronomers refer to the comet lovingly as "Churi."
Even establishing an orbit around the comet set a precedent. Never before had scientists managed to put a satellite in orbit around an object with such a weak gravitational field. Rosetta closed in on Churi to a distance of just 7,000 meters, the altitude at which many hobby pilots fly.
The high definition images that Rosetta has since transmitted back to Earth have fascinated experts and non-experts alike. The comet's irregular shape turned out to be a big, initial surprise. The expectation had been that the heavenly body, presumably like most comets, would look something like an oversized potato. Instead, however, it has two parts connected by a thin neck. "The comet," says Accomazzo, "looks like a rubber duck in a bathtub."
Because of its complex form, landing on it will be extremely risky. "It is not easy to calculate a definitive course," Accomazzo says. "The danger is very real that we will miss it altogether."
Rosetta itself will not land on the comet, remaining instead in a low orbit. The actual landing vehicle, a refrigerator-sized robot named Philae, will be jettisoned from the mother ship. The plan calls for it to slowly sink to the surface of the comet over the course of seven hours.
There is little concern that Philae might break apart on landing. On Earth, the lander weighed around 100 kilograms (220 pounds). But on Churi, with its extremely weak gravity, the research robot -- built primarily in Germany -- will weigh less than a mole. To affix itself to the ground, Philae will fire two anchor harpoons into the comet's surface. Philae should start transmitting panorama photos within minutes of its landing -- pictures that will be seen around the world.
Presuming, of course, that the landing is successful. "It will likely be even more difficult than expected," believes Stephan Ulamec, who is leading the 50-person landing team from the German Aerospace Center (DLR) in Cologne. "The landscape is very challenging. But at least the area we have chosen is lit sufficiently by the sun."
'Makes Me Uneasy'
Last week, the landing site received a name: Agilkia, after an island in the Nile River where an ancient Egyptian temple complex is located. ESA chose the name from among thousands of submissions from across Europe and the world.
The photos of the comet that Rosetta has thus far sent back show several house-sized objects near the landing site, presumably made of dirty ice. More dangerous for the lander, however, are the steep, rugged slopes jutting up all around. "If Philae hits one of them, it will flip over -- and that will be that," says Ulamec.
He too has spent about half of his career preparing for Wednesday, and the moment of touch-down will determine whether or not he has been successful. "The idea that the whole thing could end on the side of a cliff makes me uneasy, of course," he says.
But even if the landing doesn't work, the Rosetta mission has already been a huge success from the perspective of comet researchers. Even the measurements taken from its orbit around 67P have provided a wealth of new knowledge. It had largely been unknown, for example, what the surface of comets was like. Now, researchers know that it is made of a mixture of dust and ice. But is it solid beneath the surface or soft like power snow? Using thermal imaging, Rosetta has been able to look through at least the very top layer. "Now we know," says Ulamec. "Churi is covered by dust, a lot like our moon. But it could be that we'll encounter hard ice a few centimeters down."
To the researchers' surprise, Churi already has a thin tail stretching out behind it. Every day, the comet releases the equivalent of a dump-truck load of dust. Normally, tails first appear when the cosmic vagabonds approach the sun and begin thawing out. A significant amount of the comet's body evaporates, creating a cloud of gas and dust. Once it is lit by sunlight, it appears from the Earth like a torch in the night sky.
Today, of course, comets are welcomed as a stunning spectacle of nature. But in earlier times, they spread fear and horror. Nobody knew just how far away from Earth comets usually are and they were blamed for all manner of disease. Their tails, people believed, contaminated well water, spread the plague or withered the fields.
The Beginnings of Life Itself
Thanks to measurements undertaken by Rosetta, we now know much more about the tails of comets, including how they smell. The answer? Not good at all. Two mass spectrometers on board have been able to determine the make-up of the Churi's (still extremely thin) coma, as the cloud extending behind the comet is called. According to the measurements, it contains ammonia (which smells like a pig sty) and hydrogen sulfide (rotten eggs). In addition, there is a hint of highly poisonous hydrogen cyanide.
But the most decisive riddles -- questions of far-reaching importance -- can only be solved from Churi's rugged surface. The lander is equipped with instruments to take ground samples and to analyze them on site. Knowing the precise make-up of the comet could help us to learn more about the beginnings of our planet -- and of life itself.
At the same time, the history of our solar system is frozen into the comet. The heavenly bodies are created from the same material which gave rise to our sun and its planets 4.6 billion years ago. Since then, the icy clumps have changed very little.
Earlier spectroscopic analyses undertaken from great distance have indicated that comets contain organic molecules. Even amino acids, which are among the building blocks of life, have been found. Could it be that comet strikes on Earth deposited such life giving particles, allowing plants, animals and humans to develop?
Astrobiologists believe that such a scenario is possible. "Should Philae help us to decipher the complex organic molecules on the comet, then we will hopefully know the answer," says comet researcher Hermann Böhnhardt from the Max Planck Institute for Solar System Research in Göttingen. The exact design of the molecules present on the comet is decisive, he says.
"All life forms on Earth contain exclusively left-handed amino acids, whereas right-handed amino acids exist elsewhere," Böhnhardt says. "Should amino acids on comets also be left-handed, that would be a substantial indication that these chunks of ice indeed brought the building blocks of life to our young Earth."
A further theory holds that water from our oceans could also have come from comets, at least partially. Hail storms of comets could have rained down on our planet in primeval times, with the comets melting after impact, giving rise to the seas. Astronomers refer to the theoretical period of heavy cosmic strikes as the Late Heavy Bombardment.
But could it be true? Earlier, comparatively inexact measurements indicate that ice in comets tends to be made up of a mixture of hydrogen isotopes that is different than that found in water on Earth. "But it isn't for certain," says Böhnhardt. "We hope that the Rosetta mission will finally bring clarity."
It is probable that the comets that bombarded the primeval Earth contained enough frozen water to fill the planet's seas. Even a relatively small one like Churi contains an estimated 10 cubic kilometers of water, a fifth of the volume of Lake Constance. Perhaps, then, life did indeed arise out of comet slurry.
It is also possible that an impact from the cosmos contributed to the development of mankind. Sixty-five million years ago, a massive object, measuring 10 kilometers in diameter, fell from the skies over present-day Mexico. The dust kicked up by the impact blacked out the sun, leading to a plunge in temperatures around the globe. The extinction of the dinosaurs was the result.
Warm-blooded mammals were the profiteers of the disaster because of their ability to keep their body temperatures constant. With the disappearance of the dinosaurs, they were able to evolve into increasingly complex species -- and ultimately into Homo sapiens.
Comets will likely trigger such a disaster again; statistically, such a strike occurs every few million years. A sudden ice age caused by an impact could spell the end of human civilization.
Churi, though, doesn't represent a danger at all for the time being. "For the next 200 years," says Böhnhardt, "it won't cross the path of the Earth's orbit."
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