Despite their small masses, comets have played an extraordinary role in enhancing our understanding of cosmic physics. It was the calculation of comet Halley’s orbit and the successful prediction of its return in 1758 that firmly established the correctness of Newton’s law of universal
gravitation. It was the morphology of the dusty tails of comets that provided the earliest information of the nature of the interaction of solar electromagnetic radiation with dust, and it was the orientation and structure of the plasma tails of comets that led to the discovery of the solar wind.

More recently, the role of the changing dusty plasma environments of comets as natural space laboratories for the study of dust-plasma interactions, and their physical and dynamical consequences, has been recognized. The forthcoming Rosetta-Philae rendezvous and lander mission will provide a unique opportunity to revisit the entire range of earlier observations of dusty plasma phenomena in a single comet, as it moves around the Sun. Rosetta will rendezvous with comet 67P/Churyumov-Gerasimenko in May 2014, at about 4 AU from the Sun, deploy its Philae surface lander in November 2014, and escort the comet around the Sun until at least December 2015.

In a topical review motivated by the Rosetta mission, NLSI Scientist Mihaly Horanyi and prof. Mendis discuss the varying modes of interaction of the comet as it approaches the Sun, and the different dusty plasma phenomena that are expected in each case, drawing on the earlier observations, including their interpretations and prevailing open questions. The team’s paper appeared in the Reviews of Geophysics.

Many of the dusty plasma issues discussed for the Moon will be of interest for comets and asteroids. Comets provide excellent laboratories to study dusty plasma phenomena in space. Perhaps most remarkable is the fact that these laboratories are not static. As a comet approaches the Sun from a large distance, its particles and fields environment changes dramatically. This leads to a wide range of dust-plasma interactions with both physical and dynamical consequences for the dust, as well as the dusty plasma ensemble as a whole via possible collective effects.

Dusty plasma phenomena have been observed at a wide range of cometary activities, beginning with an inactive distant comet where the solar wind flows unimpeded on to the surface, to comets closer to perihelion, when the solar wind interacts with an extended, dense commentary atmosphere. However, all of the existing observations pertain to different comets at different heliocentric distances. Also, the methods have ranged from ground-based and Earth orbiting observations, to in-situ measurements from fast fly-by spacecraft.

Due to Rosetta’s impressively complete set of instruments to observe the nucleus, the evolving fields and particles, and dust environment of the comet, researchers will gain unprecedented insight into a range of interconnected dusty plasma phenomena, observed so far only in fragments, at different comets at different heliocentric distances.

Initially, the scientists expect to learn about surface interactions, the charging, mobilization, and transport of dust on the nucleus. These processes have relevance to all airless bodies in the solar system, including Mercury, the Moon, asteroids, and the martian moons Phobos and Deimos, for example. Rosetta will follow the emergence of the cometary atmosphere and ionosphere, the dramatically changing interaction of the comet with the solar wind plasma flow, and its effects on the size and spatial distributions of dust.

The multi-spacecraft observations of comet 76P/ Halley during its last apparition in 1986 greatly enhanced our knowledge about comets, and gave rise to the recognition of the important role dusty plasma processes can play at comets. The Rosetta/Philae rendezvous mission to comet 67P/Churyumov-Gerasimenko will likely bring closure to many open questions about comets, and the physics of dusty plasmas in cometary environments, while possibly observing a range of as of yet unseen and unpredicted phenomena.

Rosetta’s Philae lander on comet nucleus. Credit: ESA

The Rosetta Lander
The 100-kilogram Rosetta lander is provided 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, Hungary, Ireland, Italy and the UK.

The box-shaped lander is carried on the side of the orbiter until it arrives at Comet 67P/Churyumov-Gerasimenko. Once the orbiter is aligned correctly, the lander is commanded to self-eject from the main spacecraft and unfold its three legs, ready for a gentle touchdown at the end of the ballistic descent.

On landing, the legs damp out most of the kinetic energy to reduce the chance of bouncing, and they can rotate, lift or tilt to return the lander to an upright position.

Immediately after touchdown, a harpoon is fired to anchor the lander to the ground and prevent it escaping from the comet’s extremely weak gravity. The minimum mission target is one week, but surface operations may continue for many months.

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Posted by: Soderman/NLSI Staff
Source: NLSI Team

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