This post is part of a series on understanding the meaning behind the values contained in a Two Line Element set, or Keplerian Elements. The main article is here.
International Designator (Year)
This is a two digit number showing the year of the launch.
Values between 57 and 99 are for the years 1957 to 1999 while values between 00 and 56 are for 2000 through to 2056 (This range is used as the first satellite, Sputnik 1 was launched in 1957).