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Planetary News: Asteroids and Comets (2004)

Rosetta To Launch This Week on 10-Year Journey to Comet Churyumov-Gerasimenko

By A.J.S. Rayl
23 February 2004

Rosetta, the European Space Agency's comet chaser, is on the launchpad, aboard an Ariane 5 rocket, in Kourou, French Guiana, waiting to begin its 10-year, 4.4 billion-mile journey to rendezvous, orbit, and land on Comet 67P/Churyumov-Gerasimenko. Blast-off is scheduled for 11:36 pm Pacific Standard Time on Wednesday, February 25th, 7:36 Greenwich Mean Time, Thursday, February 26.

One of the most ambitious and complex robotic space projects ever undertaken, Rosetta is the first spacecraft to ever attempt to orbit a comet's nucleus, and then deliver a lander to its surface. It is slated to arrive at the comet in May 2014.

As the countdown nears, the mood for many involved with the project, not surprisingly, is one of "[a]nticipation mixed with anxiety," as ESA Science Director David Southwood puts it.

"I think that the launch is going to quite tense," he adds. "After all we have quite a wait -- because of the delayed ignition of the upper stage -- until we really know we have a mission. Seeing the rocket leave the ground is not going to be enough."

Despite the tension that comes with the territory of space exploration, Southwood says the Rosetta team is ready and "there is a quiet confidence about the place."

Rosetta's mission is to study Churyumov-Gerasimenko up close and uncover the remaining clues that will more completely explain the origin and development of our solar system. Teams of scientists hailing from countries around the world are taking part in the various experiments to be conducted with a host of instruments onboard both the orbiter and lander.

"This mission will turn science-fiction into science fact," offered 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. "Every aspect of comet Churyumov-Gerasimenko will be analyzed, resulting in the most comprehensive set of scientific measurements ever obtained of a comet."

The Rosetta spacecraft -- which is about the size of a Hummer -- will be able to make observations from as close as 2 kilometers, or 1.2 miles," notes Claudia Alexander, of the Jet Propulsion Laboratory (JPL), the project scientist for the U.S. role in the mission.

After its arrival, the spacecraft will spend six months mapping and observing Churyumov-Gerasimenko, which is only about 2.5 miles, or 4 kilometers, in diameter. Then, if all goes as planned, Rosetta will release its approximately 220-pound --100-kilogram, washing machine-sized lander to touch down right onto the core for the first-ever intimate study of a comet's nucleus.

A total of 21 experiments -- from 11 instruments on the orbiter and 10 instruments on the lander -- will be used to study the composition and analyze the nature and distribution of the cometary surface materials. A drilling device on the lander will collect samples of the subsurface material.

While the orbiter's instruments will detail the overall characterization of the comet's dynamic properties and surface morphology, the lander's suite of scientific devices 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 mission to unlock the secrets of how life began on Earth.

Rosetta is already something of a technological marvel, and a source of great pride for the European space community. Since it will take nearly an hour for radio signals from Earth to reach Rosetta once the 'chase' begins, engineers had to make the spacecraft autonomous enough to be able to 'think' for itself in some situations, as well as respond to its team's commands from Earth.

In addition, both the spacecraft and all the instruments have been built to survive not only the decade-long trek, but temperatures that fall to -235 Fahrenheit [-150Celsius] and rise higher than a blistering summer day in the Arizona desert as it orbits in nearer to the Sun.

Beyond the orbit of Jupiter

Even though Rosetta will launch with 1,650 kilograms of propellant on board --which accounts for more than half of its mass at lift-off -- the pioneering 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.

For much of its journey, the comet chaser will be placed in a hibernation mode to limit power and fuel consumption, although scientists will command the ship's instruments to conduct some science observations along the way, especially as it cruises through the asteroid belt between Mars and Jupiter.

In the farther reaches of the solar system, Rosetta will be the first spacecraft to rely entirely on the power of the Sun, 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.

On approach to Churyumov-Gerasimenko, Rosetta's camera will return images that will enable scientists to improve calculations of the comet's position, orbit, size, and shape. As it speeds into the comet's vicinity at 75,000 miles per hour, in the spring of 2014, Rosetta will move in to the nucleus as the comet moves towards the Sun, and scientists will consider those calculations, homing in on the best possible landing site on the comet's nucleus before deploying the lander in November 2014.

"This comet only has about three-hundred-thousandths the gravity of Earth," points out Alexander. 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, the lander 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.

From November 2014 to December 2015, the Rosetta orbiter will continue on, escorting 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.

On that adventure, Rosetta will 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. 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.

High hopes abound

Rosetta was to have launched in January 2003, on an eight-year trek to Comet Wirtanen onboard an Ariane-5 rocket from Kourou, but that date 'slipped' after the rocket -- one of the few in the world with the payload lift capability needed to launch the three-ton spacecraft into orbit and send it on its way to the distant comet -- failed in a pre-launch test in December 2002. It was the fourth Ariane 5 failure in 16 attempts since the model's inauguration in the mid-1990s, and it led to the postponing of the mission, and something of a budgetary crisis for the space agency.

The delay in Rosetta's launch came with a hefty price tag for ESA, estimated to be somewhere between 50 and 100 million Euros. As Science Director Southwood told The Planetary Society then, and as every space scientist knows: "Delay always means the price rises. You just shift the problem into the future."

It also meant that the mission's original target, Comet Wirtanen, could not be reached. By March 2003, however, Comet 67P/Churyumov-Gerasimenko had been singled out as the leading contender, for a launch in February 2004. And ESA's ministers in charge of space affairs in Europe agreed during their summit in Paris in May 2003, to resolve the budgetary crisis that had been threatening Rosetta, essentially green-lighting the mission.

First proposed in 1985, Rosetta has cost an estimated 893 million Euros or 600 million British pounds, which converts by today's rates to about $1 billion U.S. dollars. But, by most accounts in the space exploration community, the potential payoff of knowledge is priceless. Once the money woes were overcome and the new target chosen, only high hopes remained for the comet chaser.

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 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.

The primary contributors to the lander -- Germany, France, Italy, and Hungary -- held national competitions to name its three-legged charge. The name 'Philae' was proposed by 15-year-old Serena Olga Vismara, of Arluno, a town near Milan, Italy.

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 final clues he needed to decipher the hieroglyphs of the Rosetta Stone that unlocked the secrets of civilization in ancient Egypt.

When her suggestion was chosen, Vismara noted that one of her hobbies is surfing the net where she got the idea of naming the lander. She is being flown to Kourou to attend the launch.

Austria, Finland, Ireland and the UK also made significant contributions to the lander.

The data returned by this mission will open a window to the past, to 4600 million years ago and an epoch when only a vast swarm of asteroids and comets surrounded the Sun, before the planets swirled into being.

The quest to unlock the secrets of the solar system

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. "Comets are primordial remnants of the early solar system," elaborates Paul R. Weissman, JPL, an interdisciplinary scientist on Rosetta.

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.

No one really knows just how many comets there are, but planetary 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.

What scientists do know, and what draws them to study comets, is that these icy bodies brought "volatile" light elements to the planets, played an important role in forming oceans and atmospheres, and 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, of course, make up nucleic and amino acids, the building blocks of life as we know it.

For decades now, scientists have pondered whether life on Earth was spawned by a chance comet encounter, in a transfer process known as panspermia. Once only whispered about behind closed doors, the concept of panspermia has risen to greater acceptance during the last decade, and Rosetta just may find the answer and unravel that grand mystery.

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