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Planetary News: Asteroids and Comets (2004)

Rosetta Launches on 4.4 Billion-Mile Trek to Comet 67P/Churyumov-Gerasimenko

For Rosetta -- the third time was the charm. The European Space Agency's comet chaser -- aboard an Ariane 5 rocket -- lifted-off the launchpad at the spaceport in Kourou, French Guiana, at 7:17 Greenwich Mean Time (GMT) Tuesday, March 2. [8:17 Central European Time (CET), 11:17 p.m. on Monday, March 1, Pacific Standard Time.]

Some 19 years after it was first proposed, 10 years after it was approved, and after surviving one mission-threatening postponement, and two minor launch delays, Rosetta has finally begun its 4.4 billion-mile journey to rendezvous, orbit, and land on a comet. And, it went like clockwork.

The launcher successfully placed its upper stage and payload into an eccentric coast orbit (200 x 4000 km). About two hours later, at 9:14 GMT [10:14 CET, 1:14 a.m. PST], the upper stage ignited its own engine to reach an escape velocity in order to leave the Earth's gravity field and enter heliocentric orbit. The Rosetta probe was released 18 minutes later.

It will take Rosetta 10 years to make the trip to Comet 67P/Churyumov-Gerasimenko - which is, essentially, a big, dirty iceball about 2.5 miles, or 4 kilometers in diameter orbiting way out there in the solar system, past the orbit of Jupiter.

On arrival -- sometime in the summer of 2014 -- the spacecraft will drop into the comet's orbit, spend six months studying it, then deposit its approximately 220-pound [100-kilogram] washing machine-sized lander, hopefully, right onto its nucleus. It is one of the most ambitious and complex robotic space projects ever undertaken.

At the end of that long journey, the potential reward is intellectually tantalizing -- the revelations of the final mysteries as to how the solar system formed - and perhaps even the final clues as to how life on Earth began.

"After the recent success of Mars Express, Europe is now heading to deep space with another fantastic mission. We will have to be patient, as the rendezvous with the comet will not take place until 10 years from now, but I think it's worth the wait" said ESA's Director General Jean-Jacques Dordain witnessing the launch from Kourou.

ESA's Operations Center (ESOC) in Darmstadt, Germany, has established contact with the probe as it flies away from Earth at a relative speed of about 3.4 kilometers per second. ESOC will be in charge of Rosetta operations and orbit determination throughout the mission.

During the next eight months, the spacecraft's onboard systems will be checked and its science payload will be commissioned. Then, it will be placed into hibernation mode for most of the 10 years of its journey through the Solar System.

To achieve mission success, Rosetta had to be something of a technological marvel. Since it will take nearly an hour for radio signals from Earth to reach the spacecraft once the 'chase' begins, the spacecraft had to be autonomous enough to 'think' for itself in some situations, as well as respond to its team's commands from back home.

Then, there was the issue of survival. Rosetta -- and all the scientific instruments on board had to be designed and built to survive not only the decade-long trip to the comet, but temperatures that fall to -235 Fahrenheit [-150 Celsius] and rise higher than a blistering summer day in the Arizona desert as it orbits nearer to the Sun.

Named for the historic Rosetta Stone, the key to decoding the hieroglyphics of ancient Egypt, which is on display in the British Museum, ESA is projecting its Rosetta and its lander, which just two weeks ago was named Philae, as the space equivalents -- the keys to decoding the secrets of the solar system. [Philae is the name of the island in Nile River on which an obelisk that featured a bilingual inscription, including the cartouches / Egyptian hieroglyphs of Cleopatra and Ptolemy, was found. This discovery is what gave the French historian Jean-Francois Champollion the last clues he needed to finally decipher the hieroglyphs of the Rosetta Stone that unlocked the secrets of civilization in ancient Egypt.]

Romancing the stone

Even though Rosetta -- which is about the size of the super-sized Hummer -- will launch with 1,650 kilograms of propellant on board -which accounts for more than half of its mass at lift-off -- the spacecraft will make three flybys of Earth and one of Mars to get the gravity assists and momentum necessary to 'slingshot' it out to the far reaches of the solar system, beyond the orbit of Jupiter and its distant target.

The spacecraft will be ordered out of hibernation and reactivated for these planetary and asteroid flybys. Observation of asteroids is one of the mission's secondary objectives.

In the farther reaches of the solar system, Rosetta will be the first spacecraft to rely entirely on the power of the Sun to stay up and running, and it will carry the largest solar panels ever flown in space to pull that accomplishment off. Each of Rosetta's two giant solar panels is about 46 feet, or 14 meters, in length. Together, the panels boast a combined light-collecting area of 210 square feet, or 64 square meters - every meter of which is needed since the spacecraft will be venturing into an area of space where the light levels are merely 4% of those near Earth.

By mid-2011, when it is about 497 million miles [800 million km] from the Sun, Rosetta will ignite its main engine for a major deep-space maneuver that will place it on an interception trajectory with the comet. That will take nearly three years.

In January 2014, Rosetta will be reactivated for good, as it enters a six-month approach phase, closing in slowly on the nucleus of comet Churyumov-Gerasimenko. The comet will then still be far from the Sun and its nucleus should be dormant.

As Rosetta speeds into Churyumov-Gerasimenko's vicinity at 75,000 miles per hour, the camera will return images that will enable scientists to improve calculations of the comet's position, orbit, size, and shape. The spacecraft will enter an orbit about 15.5 miles [25 km] above the nucleus in August 2014, and spend the next few months conducting detailed maps of its surface, and scientists will use those to home in on the best possible landing site on the comet's nucleus before deploying the lander in November 2014.

Philae will be dropped from an altitude of about .6 mile [1000 meters] and, due to the tiny gravity of the nucleus, it will touch down at walking speed. "This comet only has about three-hundred-thousandths the gravity of Earth," points out Claudia Alexander, of the Jet Propulsion Laboratory (JPL), the project scientist for the U.S. role in the mission. So, the lander -- which will be commanded to self-eject from the orbiter and unfold its three legs -- will make a graceful, controlled descent to the surface in a maneuver that's been likened more to a docking in space than a landing.

In fact, the scientists and engineers anticipate the landing will be so soft that Rosetta has been previously described as the mission that will "kiss" a comet. The lander's legs can rotate, lift or tilt to return it to an upright position, should it teeter or land wrong side down.

Immediately after touchdown, Philae will fire a harpoon into the surface as an anchor to keep it from floating away, since the comet's extremely weak gravity alone won't be enough to hold it down. The lander will operate from the surface for at least a week, sending high-resolution pictures and other data about the upper crust of the nucleus back to Earth via the orbiter.

Rosetta will continue its observations of the comet's nucleus for more than a year, at least until December 2015, and will have a ringside seat to monitor the "awakening" of the comet's activity as it comes closer to the Sun and reaches its perihelion, in October 2015. Then, from November 2014 to December 2015, the orbiter will escort the comet - with the lander riding along atop it like a cowboy on a horse - into the sunset, so to speak.

If the plan works and all systems are performing nominally, Rosetta -- beyond being the first spacecraft to orbit the nucleus of a comet and to put a lander on its surface -- will also be the first spacecraft to ride alongside a comet as it makes its journey around the Sun. As Comet 67P/Churyumov-Gerasimenko, nears the Sun, its icy surface will sublimate into gas, stripping off as much as a meter of its outermost layer, and Rosetta will bear witness and measure this eons-old process at close range for the first time.

On this adventure, the comet chaser would log yet another milestone as it chronicles how a clump of ice and dust from the freezing outskirts of the solar system transforms as it moves into the sunlight of the inner solar system into the ethereal vision of a shooting star with a glowing tail that humankind has acknowledged and recorded since time immemorial.

Peering into the stone

Rosetta's mission is to study the comet up close and uncover the last clues to the mystery of the origin and development of our solar system -- and even, perhaps, how life began on Earth. Like most ventures in space exploration these days, this effort is an international one. Scientists from countries around the world are taking part in the various experiments to be conducted with a total of 21 experiments - from 11 instruments on the orbiter and 10 instruments on the lander.

In addition to the Philae lander, Rosetta is carrying a 364-pound [165 kg] science payload consisting of 11 instruments developed in partnership by ESA member countries, the United States, Greece, Hungary and Taiwan.

"Every aspect of comet Churyumov-Gerasimenko will be analyzed, resulting in the most comprehensive set of scientific measurements ever obtained of a comet," summed up Ian Halliday, chief executive of Particle Physics and Astronomy Research Council (PPARC), the United Kingdom's strategic science investment agency, during a press briefing in London last week. "This mission will turn science-fiction into science fact."

The orbiter's instruments will detail the overall characterization of the comet's dynamic properties and surface morphology, and "will be able to make observations from as close as 2 kilometers -- or 1.2 miles," points out Alexander.

Four of the orbiter instruments are dedicated to observation of the nucleus: the ALICE ultraviolet spectrometer, the OSIRIS high-resolution camera, the VIRTIS imaging spectrometer and the MIRO microwave radiometer/spectrometer. Three more instruments will study the composition of the nucleus and its emanations; the COSIMA and ROSINA spectrometers and the MIDAS microscope. The GIADA collector will analyze dusts in the vicinity of the nucleus while the RPC group of sensors will characterize the internal structure of the comet's coma and its interaction with the solar wind. The last two instruments, CONSERT and RSI, will use radio waves, one to probe the internal structure of the nucleus and the other to determine the distribution of masses inside the nucleus and the structure of the coma.

Developed under the leadership of Germany's DLR aerospace research agency, the lander boasts a suite of nine scientific instruments, which includes a drilling device. These instruments -- provided by ESA member countries in partnership with the United States, Hungary, Poland and Russia -- will determine the physical properties of the comet's surface and subsurface and their chemical, mineralogical and isotopic composition - and perhaps provide the final hints enabling the Rosetta mission to unlock the secrets of how life began on Earth.

Among Philae's scientific arsenal, the ÇIVA/ROLIS set of cameras will provide panoramic and stereoscopic high-resolution views; the APXS, COSAC and Ptolemy instruments will analyze soil compounds; the SESAME seismometer will probe the surface to a depth of 6 feet [2 meters]; the MUPUS instrument will study its characteristics with sensors on the anchoring harpoon. The ROMAP magnetometer and a second model of the CONSERT experiment will study the magnetic field and its interactions with the solar wind.

"Rosetta carries more instruments than any previous scientific spacecraft -- that makes it challenging and one of the most exciting missions ever," Alexander notes. "We are anticipating major discoveries, just like Galileo and Cassini."

The data returned by Rosetta will open a window to the past, to 4600 million years ago and a time when only a vast swarm of asteroids and comets surrounded the Sun, a time before the planets in our solar system swirled into being.

As the oldest and most primitive bodies in the solar system, comets are, basically, icy preserves of the material present during the formation of the solar system, relics of a time long, long ago. Since they have remained unchanged by comparison to other bodies within our solar system, comets provide the earliest record of materials in a pristine form, and, hence, they are the keys to unlocking the secrets of the our universe. "Comets are primordial remnants of the early solar system," explains Paul R. Weissman, an interdisciplinary scientist working on Rosetta.

No one really knows just how many comets there are, but scientists have estimated that there may be billions of them out beyond the orbits of Pluto or Neptune when it orbits farther out than the small planet.

Scientists have uncovered evidence that comets brought "volatile" light elements to the planets, played an important role in forming oceans and atmospheres, and that they carry complex organic molecules -- including hydrogen, carbon, oxygen, and nitrogen -- that may have been involved in the origin of life on Earth. These elements make up nucleic and amino acids, the building blocks of life as we know it.