Comet 67P/Churyumov-Gerasimenko is nearly devoid of any cave structures, according to a new study of data gathered by the Rosetta spacecraft, which entered orbit around the frozen body in 2015. This conclusion is based on examination of how the gravitational field of the frigid body affected the spacecraft.

Astronomers have long debated whether or not caves form within comets. Just eight comets have been explored by spacecraft, and so many mysteries still remain concerning the frozen bodies. Researchers have long known that comets have low densities, but it was unknown until now if these measurements were due to a light, solid composition or a cavernous structure.

Rosetta was launched on March 2, 2004, later becoming the first spacecraft to ever orbit two extraterrestrial bodies.

The subtle effect of the gravitational field of Comet 67/P on the Rosetta spacecraft is measured by examining the Doppler shift of the signal being sent from the observatory back to Earth. This shift in frequency is similar to the change of sound heard by a bystander on a platform as a train speeds past.

"In practical terms, this means that we had to remove the influence of the Sun, all the planets – from giant Jupiter to the dwarf planets – as well as large asteroids in the inner asteroid belt, on Rosetta's motion, to leave just the influence of the comet," Martin Pätzold, principal investigator on the project, says.

Comet 67/P and other similar bodies are largely composed of dust and water ice, providing their common description as "dirty snowballs." This new analysis shows Comet 67/P is comprised of four times as much dust as ice when mass is measured, and twice by volume. These frozen bodies formed during the formation of the solar system more than 4.6 billion years ago.

Due to uncertainty about the nature of comets, mission planners kept the vehicle just 10 kilometers or 6 miles away from the frigid structure. Before the vehicle arrived, most astronomers believed it would be impossible to determine whether or not caverns existed on the comet from such a distance. The lopsided potato-like shape made gravitational differences more pronounced, providing easier analysis.

Rosetta will be sent to a crash landing on the comet in September 2016, ending the mission.

Investigation of the data on Rosetta's comet revealing a lack of caverns was detailed in the journal Nature.

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