This single frame Rosetta navigation camera image—processed to bring out te details of the comet’s activity—was taken from a distance of 124 km from the center of comet 67P/Churyumov-Gerasimenko on February 6, 2015. (Photo courtesy European Space Agency, ESA)

Rosetta’s stone

Scientist Thomas Stephan catches up with a comet.

In August 2014, the European Space Agency’s spacecraft Rosetta arrived at comet 67P/Churyumov–Gerasimenko, settling into orbit around the dusty ball of ice. Thomas Stephan, a senior scientist in the Geophysical Sciences Department, is part of the team collecting data and investigating how the comet changes on its journey around the solar system, particularly when it gets close to the sun.

Rosetta has several instruments on board, including COSIMA (Cometary Secondary Ion Mass Analyser), a mass spectrometer that Stephan and his research group use to study the characteristics of the dust grains emitted by the comet. Rosetta also deployed robotic lander Philae, which made the first controlled touchdown on a comet nucleus in November.

“On Earth, geological processes permanently lead to formation and destruction of terrestrial rocks,” Stephan says. “But comets, which formed 4.6 billion years ago when our solar system formed, spend most of their lifetime in the outer parts of the solar system, far away from the sun, and remain largely unchanged. To learn about the formation and early history of the solar system, comets or asteroids—where most meteorites come from—are the ideal samples.”

Rosetta’s mission will draw to a close in December 2015, after escorting the comet for more than a year.