While all eyes are on the upcoming rendezvous of the Juno spacecraft with Jupiter on July 4, final preparations are being made for the 12-year old Rosetta mission. Rosetta, currently orbiting the comet 67P/Churyumov-Gerasimenko will complete its mission in a controlled descent to its eternal resting place on the surface of its comet on Sep 30. In doing so, it will join its companion, the stranded robot probe Philae, which landed on the comet on Nov 12, 2014.
Rosetta, along with Philae, the lander module, was built by the European Space Agency and was launched on March 2, 2004. Ten years later, on Aug 6 2014, the spacecraft reached the comet and performed a series of maneuvers to be captured in its orbit.
The lander is named after the Philae obelisk, which bears a bilingual inscription (Egyptian hieroglyphs and ancient Greek) and was used along with the Rosetta Stone to decipher Egyptian hieroglyphs. In a direct analogy to its namesake, Rosetta also carries a micro-etched nickel alloy Rosetta disc inscribed with 13,000 pages of text in 1200 languages.
Like Juno, Rosetta uses solar panels instead of the radioisotope thermoelectric generators used by previous deep space missions. The solar arrays generate 400 to 1,500 watts at depending on the distance from the sun. Power is stored in four 10-amp-hour NiCd batteries. Heaters are used to keep its systems warm while the spacecraft is distant from the Sun.
See the fascinating animation below of Rosetta's 12-year 6.5 billion km journey through the Solar System, using gravity slingshots from Earth and Mars to reach its final destination.
The animation below shows the unusual final maneuvers used to enter the orbit around the comet.
For the next few months, Rosetta mapped the comet in detail and allowed scientists to determine the landing site for the Philae lander. On Nov 12, when Rosetta was within 25 km of the comet’s nucleus, Rosetta released the 100-kg Philae probe on a 7 hour journey to the comet's uneven surface. The washing machine-sized robot, with 10 instruments for sniffing and probing, bounced several times after its harpoons failed to fire and secure it to the comet’s surface. It ended up in a ditch shadowed from the Sun's battery-replenishing rays, but still managed to run about 60 hours of experiments and send home valuable data before entering standby mode.
As 67P neared the Sun on its elliptical orbit, Philae emerged from hibernation in June 2015 and sent a two-minute message via Rosetta, eliciting great excitement on Earth. But after eight intermittent communications, the lander went permanently silent in July 2015. Rosetta continued to circle and probe the comet, studying its plume of dust and water vapor, as it turned around the sun in August 2015.
67P/Churyumov–Gerasimenko is a comet with a current orbital period of 6.45 years, its elliptical orbit extends to just beyond Jupiter’s orbit, a rotation period of approximately 12.4 hours and a maximum velocity of 135,000 km/h. It is approximately 4.3 by 4.1 km at its longest and widest dimensions. The two-lobe shape of the comet is the result of a gentle, low-velocity collision of two objects.
The comet is believed to have formed 4.6 billion years ago, from material leftover as Earth and the solar system's other planets were coalescing. As a result, understanding the composition of comets could help us better model the formation of the solar system and the source of water on Earth.
The composition of water vapor from Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found in terrestrial water. This makes it unlikely that water found on Earth came from comets such as Churyumov–Gerasimenko.
Measurements by the COSAC and Ptolemy instruments on the Philae's lander revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate and propionaldehyde. Astrobiologists have speculated that some of the physical features detected on the comet's surface by Rosetta and Philae, such as its organic-rich crust, could be explained by the presence of extraterrestrial microorganisms. Neither Rosetta nor Philae is equipped to search for direct evidence of organisms.
One of the most outstanding discoveries of the mission so far is the detection of large amounts of free molecular oxygen (O2) gas surrounding the comet. Current solar system models suggest the molecular oxygen should have disappeared by the time 67P was created, about 4.6 billion years ago in a violent and hot process that would have caused the oxygen to react with hydrogen and form water. Molecular oxygen has never before been detected in cometary comas.
In principle, Rosetta can be left in orbit around 67P and it will reappear a few years from now, but it will not likely survive the journey to the cold outer regions of the comet’s orbit, just beyond Jupiter’s orbit. Hence, ESA decided to perform a controlled descent (slow crash) of Rosetta into 67P and gather additional scientific data in the process.
A number of dedicated maneuvers in the closing days of the mission will conclude with one final trajectory change at a distance of around 20 km about 12 hours before impact, to put the spacecraft on its final descent. The final hours of descent will enable Rosetta to make many critical measurements, including very-high-resolution imaging, boosting Rosetta’s science return with precious close-up data. Unlike Philae, Rosetta is not designed to land and will not survive the crash landing.
Once Rosetta reaches 67P’s surface, it will bid its final goodbye and close its eyes for the last time, having so proudly completed its fantastic voyage.
Further reading at rosetta.esa.int and blogs.esa.int/...