Solar panels carry Rosetta's lander to comet


After 10 years travelling a cumulative distance of over 6.4 billion kilometers, the European Space Agency's (ESA) Rosetta mission was the first ever to land on a comet on Wednesday. As no launcher was capable of directly injecting Rosetta into the comet's orbit, gravity assists were needed from four planetary flybys – one of Mars (2007) and three of Earth (2005, 2007 and 2009) – a long circuitous trip that took 10 years to complete.

But on November 12, 2014, at 16:03 GMT (17:03 CET), only a second later than expected, the Rosetta lander, called Philae, touched down on the 67P/Churyumov-Gerasimenko comet's surface, making history. Philae's science observations and measurements started immediately and are initially planned to last for two and half days, providing scientists a wealth of information that will help humans understand the origin and evolution of the solar system.

Since yesterday evening, the world has been staring at the first ground-breaking images beamed back from Philae, revealing a dramatic landscape of pits and precipices, craters and boulders.

Rosetta's innovative solar panels

Seven hours before Philae's landing on the comet, at 09:03 GMT (10:03 CET), the lander was separated from the Rosetta spaceship according to ESA's plans.

For its entire 10 years and 6.4 billion-kilometer-long trip, the Rosetta spacecraft relied entirely on the energy provided by its innovative solar panels that powered all of its onboard instruments and subsystems.

ESA said "the solar cells in Rosetta's solar panels are based on a completely new technology, so-called low-intensity low-temperature cells. Thanks to them, Rosetta is the first space mission to journey beyond the main asteroid belt relying solely on solar cells for power generation."

Previous deep-space missions used instead nuclear radio isotope thermal generators (RTGs). However, for the Rosetta mission, ESA decided to design a new technology of solar cells that allow the spacecraft to operate more than 800 million kilometers from the Sun, where levels of sunlight are only 4% those on Earth. The technology will also be available for future deep-space flights, such as ESA’s upcoming Jupiter Icy Moons Explorer, ESA said.

The spacecraft dimensions are 2.8 by 2.1 by 2.0 meters. Rosetta is equipped with two 14-meter-long solar panels totaling 64 square meters. From tip to tip, the spacecraft spans 32 meters.

The reason behind ESA's solar panel technology innovation was to ensure Rosetta could survive the hazards of travelling through deep space for more than 10 years, from the benign environment of near-Earth space to the frigid regions beyond the asteroid belt. Temperature control in particular was critical, and the spacecraft was put through stringent pre-launch tests in ESA’s facilities in the Netherlands, which involved heating the outside surfaces to more than 150 degrees Celsius and then cooling them to minus 180 degrees without damaging the instruments.

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Furthermore, to limit Rosetta's consumption of power and fuel, ESA decided put the spaceship into hibernation for 31 months. During hibernation, Rosetta "was spinning once per minute and faced the Sun, so that its solar panels could receive as much sunlight as possible. Almost all of the electrical systems were switched off, with the exception of the radio receivers, command decoders and power supply," ESA said.

Philae's battery storage

Although initial observations by Philae are planned to last only two and a half days (powered by its primary, fully-charged battery), a second three-month long observation phase is also planned to follow (powered by its secondary batteries). In both cases, Philae's batteries are charged by the energy from the solar cells on the lander. However, "no one knows precisely how long the lander will survive on the comet," added ESA, noting that this depends on a series of factors such as power supply and surface activity on the comet. "For example, dust may cover the solar panels, preventing the battery from recharging. In any case, by March 2015, when the comet is closer to the Sun, it is likely that the lander will become too hot to operate," ESA said.

In fact, ESA scientists realized that Philae's two harpoons had not fired to fasten the craft down in the ultra-low gravity yesterday, making them think Philae bounced when landing onto the surface, then moved under what it seems like the shadow of a cliff. The latest information by ESA says its scientists are attempting to move Philae into sunlight so that its solar panels can be charged. Until now Philae remains working properly and keeps transmitting data to the Rosetta spacecraft, however longer operation beyond the first two and a half days might be terminated if its solar panels do not charge its secondary batteries.

Nevertheless, Philae is a very light object weighing only 100 kilograms, while Rosetta weights around 3,000 kilograms.

Rosetta's contractor is Astrium Germany and it leads the entire industrial team. Major subcontractors are Astrium UK (spacecraft platform), Astrium France (spacecraft avionics) and Alenia Spazio (assembly, integration and verification).

??Philae was financed by a European consortium under the leadership of the German Aerospace Research Institute (DLR). Other members of the consortium are ESA and institutes from Austria, Finland, France, Germany, Hungary, Ireland, Italy and the United Kingdom.??

The Rosetta mission solar panels particularly were made by Dutch Space, a Netherlands-based company, specialising on solar panels for satellites. Dutch Space is fully owned since 2006 by Airbus Defence and Space, and operating as a subsidiary within Airbus Defence and Space’ Business Division Subsystems and Equipments.

Regardless of what happens to Philae, the Rosetta spaceship, which analyses the comet from orbit will continue until December 2015.

By the end of the Rosetta mission next year, ESA scientists will have collected a wealth of data whose analysis may trigger new discoveries regarding the evolution of the solar system and the role of the complex organic molecules contained in comets in the origin of life on Earth after the Big Bang, about 13.7 billion years ago.

Solar technology will have played an important role in delivering new discoveries and reaching new limits. It remains to be seen whether solar developments on Earth's surface, through its vast spread of solar power, or the knowledge gained from the outer parts of space system, transform our world faster.

For updated news and information direct from the mission operations and science teams, follow this link or @ESA_Rosetta and @Philae2014 Twitter accounts, which post unique pictures from the space and comet.

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