Simplify the following text for a Grade 8 native English-speaking person.
Original: They orbit each other with a period of 15.4 hours , which roughly indicates that the binary is very close â '' the separation being of the order of 10 â `` 20 km , if typical asteroid albedo and density values are assumed .
Simplified: It takes 15.4 hours to orbit each other , which says that the binary is very close â '' the separation being of the order of 10-20 km , if normal asteroid albedo and density values are assumed .