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updated 2/4/2004 4:14:51 PM ET 2004-02-04T21:14:51

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.

Set for a Feb. 26 launch, with two potential windows spaced 20 minutes apart, Rosetta should finally begin its trek to Comet 67P/Churyumov-Gerasimenko, a ball of ice, dirt and dust that sweeps through the Solar System almost every six years or so. The mission is twofold, consisting of an orbiter expected to circle the comet upon arrival and a small lander to touchdown on the icy traveler shortly thereafter.

"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, mission 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.

A mission built for two
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.

Colliding softly with a comet
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.

More than one of first
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.

Long-distance solar power
The key to Rosetta's success ultimately depends on its power supply, two massive solar panels that stretch out 45 feet (14 meter) from each side of the orbiter. Project engineers specially designed the panels to operate at low temperatures and cull the most power from the low-intensity sunlight it will receive during its mission.

"I think we're setting a record for the farthest use of solar panels from the sun," Schwehm said. "[NASA's] Stardust used them from outside 2 AU, but we will reach out even farther."

Rosetta's solar panels are responsible for keeping the spacecraft primed for its cometary rendezvous for its 10-year transit through the Solar System, generating about 395 watts of power at the low end and 850 watts at the high. At its farthest point form the Sun, the orbiter/lander duo will be about 5.25 AU away - to the orbit of Jupiter - before it falls back Sunward towards its date with Comet Churyumov-Gerasimenko.

Through much of the pre-rendezvous trip, Rosetta will be in a hibernation mode, generating only enough power keep its sensitive instruments from freezing in the cold of space. Mission controllers plan to awaken the spacecraft briefly in 2008 during its pass through the asteroid belt to study nearby space rocks, though definite targets won't be announced until later.

A rocky road into space
Despite its upcoming launch, Rosetta has certainly had its share of mission headaches.

The spacecraft's original flight plan called for a 2003 launch with a Comet Wirtanen in Rosetta's crosshairs. But ESA scrubbed that launch after an Ariane-5 rocket failed in December 2002.

The launch failure cast a pall over Rosetta scientists after their mission was put on indefinite delay, cutting their chances of a successful Wirtanen rendezvous. After some quick number crunching, mission planners settled on Churyumov-Gerasimenko, one of a number of original targets they had to choose from, as a viable alternative target.

"Last year we were pretty glum knowing that we were facing launch delay," Alexander told SPACE.com. "But now we're really there and I'm really looking forward to the extended mission. The years in between are going to go by fast."

The comet target switcheroo, however, was not without cost. To get to Churyumov-Gerasimenko, Rosetta has to swing by Earth not once but three times, starting in 2005 with subsequent passes every two years after. A Mars flyby in 2007 is also required to fling the spacecraft through the asteroid belt.

"The first step is to get into orbit and have an excellently working spacecraft," Schwehm said. "But even if it takes 10 years, [the science] is worth waiting for. After such a long launch delay, our scientists deserve it."

Rosetta is ESA's second dedicated comet rendezvous. The first, Giotto, visited two of the icy space snowballs starting with a historic pass by the famed Comet Halley in 1986. Six years later Giotto made its second cometary rendezvous during a flyby of Comet Grigg-Skjellup.

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