the apollo 12 mission landed south of copernicus on mare basalts of oceanus procellarum that were believed to have been in the path of one of the crater's rays, and scientists hoped cosmic ray exposure ages of soil samples would help constrain the crater age. the results were inconclusive, but not inconsistent with the estimated 800 million year age of crater formation. copernicus itself was a possible landing site for the canceled apollo 20 mission.