Arp 81: 100 Million Years Later . ... NASA Explanation: From planet Earth, we view this strongly interacting pair of galaxies , cataloged as Arp 81, as they were only about 100 million years after their mutual closest approach. ... Also known as NGC 6622 (left) and NGC 6621, the galaxies are roughly equal in size but are destined to merge into one large galaxy in the distant future , making repeated approaches until they finally coalesce . ... NASA Web Site Statements, Warnings, and Disclaimers . ...