Rosetta continues to help astronomers better understand the way comets form and how they interact with the universe around them.

The European Space Agency’s (ESA) Rosetta mission to comet 67P/Churyumov–Gerasimenko (Chury), launched in 2004, has allowed astronomers to get the closest and most detailed look at a comet ever. Two recent studies, borne of Rosetta’s heaps of data, are particularly eye-opening, and offer clues about comet formation and the complicated chemistry at work above a comet’s surface.

How to Make a Comet

Have you ever wanted to see how a comet forms? Now you can! Based on topographic and structural data collected by Rosetta and other comet-related missions, Martin Jutzi (University of Bern) and Erik Asphaug (Arizona University) have created a 3D simulation of how bi-lobed comets form.

In the video, two balls of ice collide at bicycle speed, bounce off each other, and begin to rotate.

The Chury Comet
This composite is a mosaic comprising four individual NAVCAM images taken from 19 miles (31 kilometers) from the center of comet 67P/Churyumov-Gerasimenko on Nov. 20, 2014. Image credit: ESA/Rosetta/NAVCAM

Due to mutual gravity, the smaller one begins to lose momentum and is eventually pulled back toward the larger. About 14 hours later they collide again, and this time they stick together for good. The result is one bi-lobed comet. Approximately half of all the comets astronomers have observed so far are bi-lobed.

Jutzi and Asphaug argue that these types of low-velocity collisions were likely very common in the early solar system, before the Sun and planets were formed, when there weren’t any large objects around to whip smaller bodies into high-speed frenzies.

Electron and Photon Chaos

In addition to helping astronomers understand how comets might form, Rosetta has also revealed more about the chemistry behind what happens at a comet’s surface as it approaches the Sun.

When a comet gets within a certain distance of the Sun, ultraviolet (UV) photons bombard the comet’s surface, causing it to spew gaseous plumes of water and carbon dioxide molecules. These molecules then break up into their atomic parts (hydrogen, oxygen, carbon and nitrogen), surround the comet like a nebula (known as the coma), and emit UV light. Our Earth-orbiting telescopes have been able to detect this light, and the solar photons, but are too far away to observe the interaction at a detailed, atomic level.

Rosetta has allowed for a much closer look at this process during its orbit around the Chury comet. In an analysis of the craft’s data, Paul Feldman (Johns Hopkins University) and colleagues identified an additional culprit involved in this coma chaos: electrons.

Rosetta carries a special instrument onboard, a spectrograph dubbed Alice, that detects UV light and splits it into its individual color components. Since each color corresponds to an element, researchers can use the spectrograph to identify the composition of the object emitting the UV light. In Chury’s case, they were also able to map out exactly how the water and carbon dioxide molecules were ripped apart. This is how they discovered the electrons’ role.

First, photons from the Sun slam into the comet’s water and carbon dioxide molecules. The collisions cause these molecules to shoot out high-energy electrons, which in turn smash back into the original molecules, causing them to split into zillions of atomic bits. The result is a horde of free-floating hydrogen, oxygen, carbon, and nitrogen atoms that emit the UV light our Earth-orbiting telescopes have captured.

Astronomers are thrilled to be able to observe this chemical process in such detail, but Alice’s success is exciting for another reason. A second Alice instrument, aboard New Horizons, is on its way to explore Pluto and the Kuiper Belt. The spacecraft is scheduled for a Pluto flyby on July 14th, 2015, with its own Alice spectrograph ready to collect data just as valuable to scientists.


Martin Jutzi and Erik Asphaug. “The Shape and Structure of Cometary Nuclei As a Result of Low-Velocity Accretion.” Science Express, May 28, 2015.

Paul Feldman et al. “Measurements of the Near-Nucleus Coma of Comet 67P/Churyumov-Gerasimenko with the Alice Far-Ultraviolet Spectrograph on Rosetta.” Astronomy & Astrophysics, June 1, 2015.

Relive Rosetta's long-awaited arrival at the comet it had chased for a decade before entering orbit — catch the full story in Sky & Telescope's August 2014 digital issue.


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