Rosetta space probe captures comet outburst

European Space Agency's Rosetta space probe has unexpectedly captured a dramatic comet outburst that may have been triggered by a landslide. 

Nine of Rosetta's instruments, including its cameras, dust collectors, and gas and plasma analysers, were monitoring the comet from about 35 kilometres in a coordinated planned sequence when the outburst happened on February 19 this year.
"Over the last year, Rosetta has shown that although activity can be prolonged, when it comes to outbursts, the timing is highly unpredictable, so catching an event like this was pure luck," said Matt Taylor, ESA's Rosetta project scientist. 

"By happy coincidence, we were pointing the majority of instruments at the comet at this time, and having these simultaneous measurements provides us with the most complete set of data on an outburst ever collected," said Taylor. 

The data were sent to the Earth only a few days after the outburst. Subsequent analysis has allowed a clear chain of events to be reconstructed. 

A strong brightening of the comet's dusty coma was seen by the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) wide-angle camera, developing in a region of the comet that was initially in shadow. 

Rosetta recorded outburst signatures that exceeded background levels in some instruments by factors of up to a hundred. 

For example, the ultraviolet brightness of the sunlight reflected by the nucleus and the emitted dust increased by a factor of six, while a significant increase in gas and plasma were also detected around the spacecraft. 

MIRO (Microwave Instrument for the Rosetta Orbiter) recorded a 30 degree Celsius rise in temperature of the surrounding gas. 

Rosetta was also blasted by dust. Almost 200 particles were detected in three hours during the outburst, compared with a typical rate of 3-10 collected on other days in the same month. 

The startrackers, which are used to navigate and help control Rosetta's attitude, measured an increase in light scattered from dust particles as a result of the outburst. 

The startrackers are mounted at 90 degrees to the side of the spacecraft that hosts the majority of science instruments, so they offered a unique insight into the 3D structure and evolution of the outburst. 

"From Rosetta's observations, we believe the outburst originated from a steep slope on the comet's large lobe, in the Atum region," said Eberhard Grun of Max-Planck-Institute for Nuclear Physics, who led the study. 

The fact that the outburst started when this area just emerged from shadow suggests that thermal stresses in the surface material may have triggered a landslide that exposed fresh water ice to direct solar illumination. 

The ice then immediately turned to gas, dragging surrounding dust with it to produce the debris cloud.
The research appears in the journal Monthly Notices of the Royal Astronomical Society.