Europe's long-awaited Rosetta space probe is being readied for a second time to begin its ambitious mission that will see it embark on a decade-long journey through the solar system before reaching its mysterious icy objective.

An artist's concept of the Rosetta orbiter and its tiny lander nearing the comet. Credit: ESA/AOES Medialab

Originally scheduled for January 2003, the comet-chaser's launch was ultimately postponed in the final weeks before liftoff after an upgraded version of its Ariane 5 launcher crashed in the Atlantic Ocean just minutes into flight the month before.

Officials wanted to make sure the Ariane 5 tagged for Rosetta was up to the task after an investigation determined the first stage main engine was the cause of the December 2002 failure. Despite differences in design, the Ariane 5 baseline version underwent an exhaustive review, which took longer than the time available for Rosetta to launch in its first window.

Because the $1 billion mission only had a select few weeks to launch in order to reach its target named comet Wirtanen, the postponement forced officials to choose a new comet for the craft to explore when it arrives after a journey spanning over ten years.

After several months of intensive studies, scientists on the project chose a new target for Rosetta, a much larger ball of rock and ice called comet Churyumov-Gerasimenko.

Using both the Hubble Space Telescope and ground-based observatories, the science team evaluated several key characteristics of Churyumov-Gerasimenko, including nucleus size, its evolution, and the gas and dust production rates, or "all you want to know about your target object," Rosetta project scientist Gerhard Schwehm said.

Because this new comet is somewhat larger than the previously targeted Wirtanen, engineers responsible for a small 220-pound lander carried by Rosetta had to confirm that it would be able to land as planned. "It was concluded that as long as the comet isn't of very high density, landing can be achieved with very high confidence," Schwehm told Spaceflight Now.

"Although Churyumov-Gerasimenko is roughly three times larger than the original Rosetta target, its elongated shape should make landing on its nucleus feasible, now that measures are in place to adapt the lander package to the new configuration before next year's launch," Dr. Philippe Lamy of the Laboratoire d'Astronomie Spatiale in France said last year.

After arriving in orbit around the comet in August 2014, scientists will seek to understand the properties of comets and the processes that take place inside them using Rosetta's numerous instruments aboard both the orbiter mothership and small lander, due to land three months later.

Comets are believed to be time capsules holding important clues about the birth of the solar system and its early development, so planetary scientists around the world are quite interested in studying the objects. NASA's Stardust probe successfully flew through the coma of comet Wild 2 earlier this month to collect microscopic dust particles to be returned to Earth in 2006.

"Working in unison, the lander and the orbiter will revolutionize our understanding of comets," Schwehm said. "They will lead to amazing discoveries about the most primitive building blocks of the solar system."

Once it is anchored to the nucleus, the Rosetta lander sends back high-resolution pictures and other information on the nature of the comet's ices and organics in the crust. Credit: ESA/AOES Medialab

The new comet will not be in the right position for Rosetta to reach it until late February, over 13 months after the mission's original launch date. The delay ended up costing ESA almost $100 million in additional costs due to storage and maintenance, re-planning, and a second launch campaign.

This unforeseen funding issue contributed to the need for the ESA science program to borrow about $125 million last summer, science director David Southwood said in response to written questions. "This led to a cancellation of Eddington and delay of other missions." The loan will have to paid back during 2005 or 2006, officials say.

Eddington was an extra-solar planet-hunter that had been tasked with observing stars and searching for planets even smaller than Earth around other stars throughout the galaxy.

ESA has an aggressive plan for exploring space using probes over the next decade, with six missions firmly in development for launch over the next three years. In addition, about ten missions are in progress and many more are being studied for implementation.

Further clouding the funding picture were budgetary problems with the Herschel and Planck space telescopes currently under construction, but those along with Rosetta now seem to be in the clear and the science program's $430 million annual budget is back on track, officials say.

Despite having encountered a number of potentially mission-ending obstacles, the costly Rosetta now stands just over six weeks from its scheduled liftoff on February 26.

Workers at the European spaceport in Kourou, French Guiana, began readying the spacecraft for launch in late October. Since then, a series of tests have proven key systems still operate as planned after sitting in a clean room for almost a year.

Rosetta's two solar array masts and high gain antenna were mounted on the spacecraft late last year, Rosetta project manager John Ellwood told Spaceflight Now. "All systems are getting ready for the first launch attempt on February 26."

The spacecraft will be re-fueled this month with a load of almost 3,500 pounds of fuel and oxidizer used by the probe's maneuvering system, which constitutes over half of the craft's total liftoff weight. Propellant loaded aboard the spacecraft in late 2002 was drained after the launch attempt in January 2003 was called off.

The Ariane 5G+ rocket will boost Rosetta onto its Earth escape trajectory using two burns of the storable propellant upper stage -- the first time such a launch sequence has been used on the Ariane 5. Separation from the upper stage is expected over two hours after liftoff.

The flight also marks the first launch of the Ariane 5G+, which features an enhanced upper stage and several lighter composite structures to improve performance.

Rosetta will fly by Earth three times and Mars once during its 10-year journey to Churyumov-Gerasimenko. Officials say the probe should venture near at least one asteroid on the trek.

From start to finish, the Rosetta project should span over three decades since its initial approval in 1993. Work to develop the world's first comet orbiter and lander mission has been a centerpiece of the ESA space science program since the project's inception.