Why comets are far from the dirty snowballs we once imagined

Space · Published 7 October 2026 Swipe more like this →

In short: For decades, scientists viewed comets as mere dirty snowballs. However, the European Space Agency's Rosetta mission has fundamentally rewritten this narrative. By orbiting and landing on Comet 67P/Churyumov-Gerasimenko, the mission revealed a much darker, more complex world hidden beneath a deceptive layer of dust.

The Rosetta mission, launched on 2 March 2004, embarked on a decade-long journey to reach Comet 67P/Churyumov-Gerasimenko. During its journey, the probe approached two asteroids, 21 Lutetia in 2010 and 2867 Steins in 2008, prior to entering deep-space hibernation during June 2011. After waking up on 20 January 2014, the orbiter arrived at the comet on 6 August 2014, marking the first time a spacecraft has orbited a comet. Before its arrival, the spacecraft entered a period of deep-space hibernation. This phase lasted from June 2011 until its reactivation in early 2014. Such a long journey required precise navigation past various celestial bodies. This successful approach set the stage for its historic encounter with the comet.

The mission's findings have forced a significant rethink of cometary composition. While scientists previously expected to see significant ice on the surface, Rosetta's instruments—including OSIRIS, VIRTIS, ROSINA, and ALICE—found that ice is actually buried beneath a thick layer of dust. This surface coating makes the comet exceptionally dark, reflecting only a few per cent of the light that hits it.

In addition to orbiting, Rosetta achieved a historic milestone on 12 November 2014, when it deployed the Philae probe to perform the first-ever landing on a comet. The mission eventually concluded on 30 September 2016, when the orbiter made a controlled impact onto the comet's surface. Although the mission has ended, the vast amount of data collected is expected to sustain scientific research for decades to come.

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