Rosetta's daring descent to within 6 km of a comet's surface
In short: On 14 February 2015, the Rosetta spacecraft executed a high-stakes maneuver, swooping just 6 km above Comet 67P/Churyumov-Gerasimenko. This transition from orbiting to flyby trajectories allowed scientists to sample the comet's atmosphere and observe its dark, mysterious surface in unprecedented detail.
After completing its initial orbiting phase, the European Space Agency's Rosetta probe transitioned to a series of flybys. This strategic shift was planned to account for increasing cometary activity and to optimize the scientific return from the spacecraft's 11 instruments. During the closest encounter on 14 February 2015, Rosetta passed over the larger lobe of the comet, specifically above the Imhotep region, at 12:41 GMT. This change in flight path was a pre-planned response to rising levels of cometary activity. The spacecraft moved from a distance of 26 km to a much closer proximity during this period.
The mission aimed to capture high-resolution measurements across various wavelengths and to 'taste or sniff' the innermost parts of the comet's atmosphere, known as the coma. By passing over the most active regions, researchers sought to understand how gas and dust accelerate from the surface and evolve as they move further away. The mission also aimed to study the comet's surface reflectivity; the nucleus is remarkably dark, reflecting only about 6% of incident sunlight. Scientists specifically focused on the outflowing gas and dust to see how these constituents change at greater distances. This approach helped clarify the link between surface activity and the surrounding coma.
This phase of the mission was critical for mapping the comet's surface, as previous orbits had only covered roughly 70% of the area due to rotation and orbital constraints. By using the Sun as a backlight during the flyby, scientists could collect shadow-free images, providing a more detailed look at the dust grains on the surface. Meanwhile, mission planners were simultaneously evaluating the feasibility of reactivating the Philae lander, a 100kg instrument designed to touch down on the surface to continue this string of mission-firsts. The lander was equipped with ten sensitive instruments to record data upon touchdown. Researchers had to carefully select a landing site that was safe and sufficiently illuminated by sunlight.
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Sources
- Summary by Intellect · source: Rosetta's closest encounter
- Also: Rosetta swoops in for a close encounter - Rosetta - Science Portal
- Also: www.nature.com — Historic Rosetta mission to end with crash into comet - Nature
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