messenger is currently 18 miles -lrb- 29.1 km -rrb- after nasa engineers used the last of its fuel in a last ditch effort to push the spacecraft higher into orbit after it dropped to within nine miles -lrb- 15 km .
among the images sent back by messenger as its mission draws to an end , the final weeks of nasa 's messenger spacecraft are revealing the planet in unrivaled detail .
the say a steady dusting of carbon from passing comets has slowly painted mercury black over billions of years .