"There is a case to be made that dark, dormant comets are a significant but largely unseen hazard," he said
Scientists estimate that there should be around 3,000 comets in the solar system, but only 25 have so far been identified.
"Dark" comets happen when the water on their surface has evaporated, causing them to reflect less light.
Astronomers have previously spotted comets heading towards earth just days before they passed.
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