After a year of delays and a completely reworked mission, the European Space Agency (ESA) is gearing up for Rosetta, an international effort to send two spacecraft on a cosmic date with one of the snowballs of the universe.

"All systems are green…and we are all go now," said Rosetta project scientist Gerhard Schwehm, of ESA. "I think we have a wonderful spacecraft here."

The upcoming launch is a redemption of sorts for Rosetta planners, who saw their original plans evaporate last year after the Ariane-5 rocket booster for a separate mission failed. ESA officials delayed Rosetta's launch indefinitely, a setback that kept Rosetta from visiting its initial target, the Comet Wirtanen.

However, ission planners were able to find a suitable replacement, Churyumov-Gerasimenko, a 2.5-mile (four-kilometer) wide collection of ice and dirt that speeds through the Solar System at about 83,800 miles (135,000 kilometers) an hour.

Today, Schwehm said, Rosetta's orbiter and lander is one of ESA's most complex scientific payloads ever designed for Solar System research. But the two spacecraft still have a 10-year journey ahead of them before reaching their final destination. If everything goes as planned, the mission should meet up with Comet Churyumov-Gerasimenko in 2014.

Rosetta takes its name from the Rosetta Stone, which gave archaeologists the tools to decipher Egyptian hieroglyphics. Project scientists hope their orbiter-lander mission is the astronomical equivalent of its namesake, giving astronomers the tools they need to decipher the nature of comets.

"We know very little, ultimately, about comets," said Claudia Alexander, project manager and project scientist for the NASA contingent of the mission, adding that once Rosetta arrives at its final target, the mission will enter a phase akin to the Galileo probe's arrival at Jupiter. "We are going to be learning very fast then."

Altogether, Rosetta is taking 21 scientific instruments to Comet Churyumov-Gerasimenko, which breaks down to 11 on the orbiter and 10 on the lander. Among the instrument suit are cameras to take high-resolution images of the comet's surface and shape, various spectrophotometers to study its chemical composition, as well as instruments to measure the gases and dust that spew from Churyumov-Gerasimenko as it approaches the Sun.

The mission's flight profile calls for the orbiter-lander combo to meet up with Comet Churyumov-Gerasimenko about 4.5 astronomical units (AU) from the Sun. One AU, or astronomical unit, is the distance from the Earth to Sun, about 93 million miles (150 million kilometers).

The comet should still be relatively inactive at the start of the mission, Schwehm said, allowing project scientists a ringside seat to Churyumov-Gerasimenko when it begins to expel gas and form its tail. Rosetta planned mission runs about two years, following the comet until it reaches its closest point to the Sun.

A few months after its 2014 arrival at Comet Churyumov-Gerasimenko, Rosetta's lander is designed to self-eject and start its own studies.

Since the lander doesn't use a rocket engine to control the descent, just a small reaction control system, its survival depends on the orbiter matching the relative speed of the comet, allowing it to trace a slow, ballistic trajectory to Churyumov-Gerasimenko's surface. Three landing struts are designed to absorb any impact shock, and can lift or tilt the lander to keep it in an upright position.

"We have harpoons that will anchor the lander to the comet," Schwehm said. "The comet [has] a low gravity, so if you come at it with a propulsion system, you'll jump right off again."

Schwehm said that the lander is expected to fall toward the planet slowly, and should hit the comet surface at a speed of about three feet (one meter) per second.

"It's something like a pedestrian walking into a wall, but we really don't know for sure," Schwehm said. "This [unknown] is something we have to cope with, because nobody knows what the comet is like up close."

Schwehm added that unlike Rosetta's orbiter, the lander was built specifically for its first target, Comet Wirtanen, which is smaller than Churyumov-Gerasimenko and has less gravity. While mission planners believe the lander can handle the additional stresses, they can only find out once Rosetta reaches its target.

Rosetta engineers included a drilling system among the lander's instruments, some of which mirror those on the orbiter, to study sample materials from beneath the comet surface. Data from the lander can only reach Earth after being relayed through the orbiter.

In addition to being the first mission to land on a comet, Rosetta is expected to debut a few new instruments in space, one of which is from the mission's American contingent, Schwehm said.

Researchers at NASA's Jet Propulsion Laboratory (JPL) in California have built the first microwave instrument to be used for planetary science. The instrument, aptly named the Microwave Instrument for the Rosetta Orbiter (MIRO) will be used to analyze Comet Churyumov-Gerasimenko's rate of outgassing, as well as the temperature just below the surface of the comet's nucleus.

"There have been some of these instruments used on Earth for terrestrial science and atmospheric studies, but they are enormous and costly," Alexander said, adding that those Earth instruments can each weigh up to 440 pounds (200 kilograms), more than Rosetta's entire scientific payload. JPL scientists were able to cut MIRO's weight down to about 44 pounds (20 kilograms), she added.

NASA also contributed in part to ROSINA, one of the most sophisticated mass spectrometers ever to fly in space. Led by investigators in Switzerland, Rosetta will use the ROSINA instrument to determine the composition of Comet Churyumov-Gerasimenko's atmosphere, ionosphere and the speeds of individual gas particles.

ROSINA should also be able to carbon date comet fragments, which could tell scientists how long the icy traveler has existed.

"We're going to be able to tell the age of the comet," Alexander said with enthusiasm.

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