Tài liệu tham khảo WikiPedia: 9691 Zwaan http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9691 http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9691+Zwaan http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9691;orb=1;c...