A series of reports in the journal Science reveals details about the shape, lifespan and evolution of 67P/Churyumov-Gerasimenko – the target comet of ESA’s Rosetta mission.

The familiar shape of the comet has now had many of its vital statistics measured: the small lobe measures 2.6 × 2.3 × 1.8 km and the large lobe 4.1 × 3.3 × 1.8 km.

The total volume of the comet is 21.4 km3. Rosetta’s Radio Science Instrument has measured its mass to be 10 billion tons, yielding a density of 470 kg/m3.

By assuming an overall composition dominated by water ice and dust with a density of 1,500–2,000 kg/m3, Rosetta scientists show that the comet has a very high porosity of 70–80 percent, with the interior structure likely comprising weakly bonded ice-dust clumps with small void spaces between them.

The OSIRIS instrument has imaged some 70 percent of the surface to date: the remaining unseen area lies in the southern hemisphere that has not yet been fully illuminated since Rosetta’s arrival.

The scientists have so far identified 19 regions separated by distinct boundaries and, following the ancient Egyptian theme of the Rosetta mission, these regions are named for Egyptian deities, and are grouped according to the type of terrain dominant within.

Five basic – but diverse – categories of terrain type have been determined: dust-covered; brittle materials with pits and circular structures; large-scale depressions; smooth terrains; and exposed more consolidated surfaces.

Much of the northern hemisphere is covered in dust. As the comet is heated, ice turns directly into gas that escapes to form the atmosphere or coma. Dust is dragged along with the gas at slower speeds, and particles that are not traveling fast enough to overcome the weak gravity fall back to the surface instead.

Some sources of discrete jets of activity have also been identified. While a significant proportion of activity emanates from the smooth neck region, jets have also been spotted rising from pits.

The gases that escape from the surface have also been seen to play an important role in transporting dust across the surface, producing dune-like ripples, and boulders with ‘wind-tails’ – the boulders act as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of them.

“Because comets have very little gravity, dust and gas flow freely into space. But we were surprised to find a cloud of particles orbiting the comet that are large and heavy enough to defy the Sun’s radiation pressure,” said Dr Dennis Bodewits of the University of Maryland.

The scientists were able to make this discovery thanks to OSIRIS’ very sensitive cameras.

“Each pixel is about 30 cm. You couldn’t see a coffee cup, but you could see a large lunchbox. The resolution is about 10 times higher than Google Earth.”

According to the team, 67P/Churyumov-Gerasimenko was releasing the earthly equivalent of 1.2 liters of water into space every second at the end of August 2014.

“In observations, made by the Microwave Instrument for Rosetta Orbiter (MIRO), over a period of three months, the amount of water in vapor form that the comet was dumping into space grew about tenfold,” said Dr Sam Gulkis of NASA’s Jet Propulsion Laboratory in Pasadena.

“To be up close and personal with a comet for an extended period of time has provided us with an unprecedented opportunity to see how comets transform from cold, icy bodies to active objects spewing out gas and dust as they get closer to the Sun.”

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M. Hässig et al. 2015. Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa0276

Hans Nilsson et al. 2015. Birth of a comet magnetosphere: A spring of water ions. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa0571

F. Capaccioni et al. 2015. The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa0628

Samuel Gulkis et al. 2015. Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa0709

K. Altwegg et al. 2015. 67P/Churyumov-Gerasimenko, a Jupiter family comet with a high D/H ratio. Science, vol. 347, no. 6220; doi: 10.1126/science.1261952

Nicolas Thomas et al. 2015. The morphological diversity of comet 67P/Churyumov-Gerasimenko. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa0440

Holger Sierks et al. 2015. On the nucleus structure and activity of comet 67P/Churyumov-Gerasimenko. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa1044

Alessandra Rotundi et al. 2015. Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun. Science, vol. 347, no. 6220; doi: 10.1126/science.aaa3905