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by Sheyna E. Gifford, MD for Astrobiology Magazine
Moffett Field CA (SPX) Jun 03, 2014
The European Space Agency's Rosetta spacecraft has been traveling for a decade to meet comet 67P/Churyumov-Gerasimenko (C-G). Rosetta is expected to finally catch up with C-G in August. Then in November, Rosetta will eject a lander called Philae onto the comet's surface: a one-way trip to a totally unknown landscape.
You might be wondering: How does one build a lander for an unknown land?
To start with: prepare for a hard landing and a soft landing at the same time. To that end, Philae's 100 kilogram (220 lb.) payload includes a spring-tipped landing tripod and two harpoons.
That's right, harpoons. If the comet has a hard surface, Philae's harpoons will hopefully keep it from bouncing off the comet and floating away. In case harpoons alone don't suffice, Philae also has a jet full of cold gas that can fire to keep it pinned to C-G's surface.
How much dust lies on the surface of a comet is anyone's guess. It could be a few inches, a few feet or few meters. In case comet 67P/is covered in a thick coat of soft dust, Philae's three human-sized tripod legs have drills on their tips to keep the lander from sliding around or sinking too far down.
"We're landing in complete unknown. That's where the tricky engineering is. There's a lot of dust on the surface brought up by the gas," said U.S. project manager Art Chmielewski during a teleconference in March 2014. "It will be pretty exciting to see how deep these [tripod] legs go into the dust."
After the lander's release from the orbiter, the rest is up to automation, chance, good luck and incredible engineering, and example of which is its solar panels. Philae has a solar panel on each of its six sides. If it hits the dust and sinks or rolls into a gully, one of the solar panels will hopefully still be able to take in enough energy to continue making observations.
For now, we don't know how long the landing itself will take or where Philae will land. For many of the questions that you would ask before kicking a satellite the size of a dishwasher onto an active comet, the answer is likely, "We don't know yet, but we will soon."
Sometimes the best way, or the only way, to learn about what's going on the Universe is to reach out and touch it.
Scientifically speaking, comets are complex remnants from the very earliest times in our Solar System. They accreted, or grew, out of the same spinning gas cloud that gave rise to the Sun. Comets are believed to be at least as old as the gas giants Jupiter and Saturn, yet were recently discovered to be rich in organic molecules.
Credible theories maintain that comets may have introduced the basic components of life to Earth. If that's true, it's likely that they have introduced life's building blocks to other bodies in our solar system, such as Mars and the moons Enceladus and Europa. From an astronomer's perspective, we have many reasons to be chasing after comets.
In terms of the strictly human experience, comets captivate not just our curiosity, but also our attention, in a very unique way.
"Seeing a comet in the sky is a powerfully visceral experience," said Claudia Alexander, the Rosetta project scientist at NASA's Jet Propulsion Laboratory. "It appears as something that shouldn't be there, and if you didn't know better it's easy to see them as scary."
With comets, part of the fear factor is that they come streaking through the darkness seemingly out of nowhere. Most of the time comets slip along unnoticed. We only see them clearly for what amounts to an astronomical split-second as they come charging towards the Sun or go streaking away from it.
Because of their stealth and seeming unpredictability, comets have traditionally invoked terror, suspicion and awe in human kind. In many ways they resemble tigers, not just in their quiet approach and dramatic charge, but also in appearance. The similarity, of course, is their tails. Tails are the feature most people think about first when they think about comets.
Tails add vastly to comets' mystique and render them unique as astronomical phenomena. Tails mark the approach to the solar apex of a comet's long orbit, when surface temperature, dust halo thickness and geologic activity all change. Most of these attributes contribute to the production of the tail, which can be observed from Earth with the naked eye over the course of weeks and, in the case of Halley's comet, from one generation to the next.
Throughout the ages, whenever these bright-tailed visitors have come by Earth we wished we could keep them longer, look at them closer, and understand them better. After millennia of watching in wonder from the ground-and more than 15 years after the Rosetta mission was approved-we are swiftly approaching our long-awaited rendezvous.
Whether or not the lander Philae makes it down in one piece, a year from now we will have a whole new world of knowledge about these tigers of space.
Rosetta: Humanity's first attempt to land on a comet
"They chose that name because they wanted the mission to provide a 'bridge' to the past of our solar system, the same way that the original Rosetta Stone provided a bridge to an ancient culture that had previously not been understood," said Alexander.
Previous comet-centered missions have revealed just enough to allow us to unseat prevailing theories, but not enough to firm up new ones.
In 1986, Images of Halley's comet nucleus showed us a dark, jelly-bean-shaped object very distinct from the traditional "dirty snowball" postulated by astronomer Fred Whipple in the 1950s. In 2005, NASA threw a coffee-table-sized spacecraft at comet Temple. The resulting Deep Impact explosion was shallower than anticipated, but nonetheless implied that comets may be more porous the previously thought and not armored with an icy exterior or shell.
Then, there's the "dirty" part of the "dirty snowball" comet theory. Comet surfaces are covered in some inky-colored material, which may be hydrocarbons (the result of interactions between nitrogen, methane and energetic particles) or may be something else entirely. Whatever the dark material is composed of, after examining the aftermath of Deep Impact we discovered that comets may not only be coated in dark material: they may be entirely composed of it.
The Deep Impact and the EPOXI missions also showed us that comets possess distinct geology - ridges, cracks and even cliffs that sprout jets. NASA's Stardust spacecraft chased down comet Wild 2 and discovered glycine, one of the basic building blocks of life, streaming out of its nucleus.
The list of comet missions goes on. Each revealed an abundance of surprises. None have been as ambitious as Rosetta.
Rosetta is essentially a flying comet laboratory. It carries 16 experiments on 11 instruments jointly managed by the US and Europe. These will measure the volume and contents of the gas in the comet's coma, bounce radio signals off the comet's nucleus, measure thermal and electrical properties of the surface, count water, carbon monoxide, ammonia, and methanol molecules, and take high-resolution infrared images. Experimenters will try to figure out what causes the famous jets on comet surfaces by comparing maps of surface temperature to where the jets appear, in the hope of discovering whether or not hot spots create jets.
One Rosetta instrument called COSIMA (Cometary Secondary Ion Mass Analyser) will literally put comet C-G under the microscope by examining the dust from Philae's landing. Another instrument called CONSERT (Comet Nucleus Sounding Experiment by Radio wave Transmission) lives partly on the orbiter and partly on the lander. By sending and transmitting radio signals back and forth, CONSERT will effectively ultrasound comet C-G's nucleus.
Philae's landing could easily end up less than ideal: with two legs in a rut or with the lander lying on its side. But as long as CONSERT arrives in working order, Rosetta should be able to discover whether comet Churyumov-Gerasimenko is loosely packed, made of discrete layers or dense all the way through. Even if Philae crash-lands, or its sensors become coated in sticky, electrically-charged dust, CONSERT will take initial readings of C-G's surface layer during the descent to the surface.
If Philae lands upright and all systems are a go, we may get answers to many long-standing mysteries, including, to use Whipple's analogy, what's making the snowball dirty. Philae's SD2 instrument is a drill and sample collector with three instruments all its own. Between two gas analyzers, seven micro-cameras, an infrared spectrometer and a light microscope, we may come to learn far more about cometary "dark stuff."
Above all, Rosetta's cameras will watch the C-G come alive in real time at it approaches the Sun.
"The whole idea of the mission is to be close enough to see its changing activity from very nearby, and also to periodically take in particles that are coming off the surface," said Alexander. "So 'escorting' the comet means that we'll be that close all the time, as the comet comes to life underneath us."
This transition is important for many reasons, one of which involves the search for life itself. Several experiments are designed to replace long-held speculation with facts about life's components on comets and in the Solar System at large. For astrobiologists, this is where things get really interesting.
Life in Deep Space
"That's part of the purpose of the Rosetta mission - to give us concrete evidence that we can use to replace speculation," said Alexander. "Some sophisticated molecule fragments found in DNA are seen in asteroid chemistry. We really are at the beginning of understanding how some of this chemistry is related to life's origins (if at all)."
In a manner reminiscent of a forensics lab, Rosetta will look for the chemicals associated with the comet and with life as we know it. The experimenters are asking basic scientific questions about comets: Do comets, like asteroids, harbor basic amino-acid precursors? Are molecules on comets left or right handed?
All life on Earth uses left-handed amino acids to make proteins. In 2011, left-handed amino acids were discovered on asteroids, but so far not on comets, perhaps because on comets the outer layers are destroyed as they heat up. How a comet's chemical components change as it becomes active it another major focus of Rosetta research.
In the meanwhile, comet C-G has also started to stir, growing a dusty veil that makes it visible against the starfield.
Rosetta caught first site of C-G on March 20th. Now the navigation camera is actively taking the observations and allowing course corrections that will bring the two objects closer together. A few short months from now, in August 2014, Rosetta will be close enough to the comet to chose a target landing site for Philae. Then in November, Philae is expected to be dropped from 1 kilometer above C-G's surface, with its drills, solar panels and harpoons at the ready.
The Rosetta orbiter itself bears a striking resemblance to a passenger airplane with the wings cut off. Coincidentally, the plan is to to release the lander onto comet C-G from a cruising altitude similar to an airplane on Earth.
"The lander drops. It just drops, but not like the some landers [Mars Curiosity Rover]," said Chmielewski. "Imagine a sheet of paper. Lift it up over your head and let go of it. That's a better idea of the kind of speed and connotation of what this landing's going to look like."
With Philae perched upon its back waiting to fly, and with all of its experiments, sensors, and other in-case gadgetry, Rosetta is speeding towards a long-awaited date with ancient history. In the coming year, this spacecraft will unveil the shape, structure and contents of a comet. Through its infrared eye we will watch C-G respond to solar radiation as it comes closer and closer to the Sun. In the process, we will discover the many differences between a 4 billion-year-old piece of the Solar System quietly moving through space and a 4 billion-year-old piece of the Solar System evaporating into a visually stunning stream across the sky. Which of life's chemical constituents comets can carry into the Inner Solar System will be explored, and we may, with a little bit of luck, land upright on the surface, dig in our heels and ride the tiger across the sky.
"Comets... are the travelers," said Alexander. "They come from a distant place in space, and because of we think they represent pristine, unchanged remnants of the distant past. They come to us as ambassadors, if you like, from a different time. At present, we don't have the capability to travel to those distant solar system locales or to go backwards in time. So comets present a unique 'archeological dig' opportunity, so to speak - and they travel to us."
Research partially funded by Cardiff Centre for Astrobiology, ESA member states and NASA. Airbus Defense and Space built the Rosetta spacecraft. NASA's Jet Propulsion Laboratory, Pasadena, California, manages the U.S. contribution of the Rosetta mission for NASA's Science Mission Directorate in Washington.
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