discovery of multiple moons around Saturn, Uranus, and Mars. The numbers initially designated the moons in orbital sequence, and were re-numbered after each new discovery; for instance, before the discovery of Mimas and Enceladus in 1789, Tethys was Saturn I, Dione Saturn II, etc., but after the new moons were discovered, Mimas became Saturn I, Enceladus Saturn II, Tethys Saturn III and Dione Saturn IV. In the middle of the 19th century, however, the numeration became fixed, and later discoveries failed to conform with the orbital sequence scheme. Amalthea, discovered in 1892, was labelled "Jupiter V" although it orbits more closely to Jupiter than does Io (Jupiter I). The unstated convention then became, at the close of the 19th century, that the numbers more or less reflected the order of discovery, except for prior historical exceptions (see Timeline of discovery of Solar System planets and their natural satellites); though if a large number of satellites were discovered in a short span of time, the group could be numbered in orbital sequence, or according to other principles than strictly by order of discovery. The convention has been extended to natural satellites of minor planets, such as (87) Sylvia I Romulus. The outer irregular satellites of Jupiter (VI through XII) were left officially unnamed throughout this period, although as stated above some unofficial names were used in some contexts. From 1975 to 2009, the International Astronomical Union was assigning names to all planetary satellites, and Roman numerals were usually not assigned to satellites until they are named. (An exception is Saturn's moon Helene, which received the Roman numeral XII in 1982, but was not named until 1988.) During this period, the use of Roman numeral designations diminished, and some are very rarely used; Phobos and Deimos are rarely referred to as Mars I and Mars II, and the Moon is never referred to as "Earth I". However, since 2015 some moons have again been numbered without being named, starting from Jupiter LI. The thirteen named satellites of Saturn from Aegir to Surtur were named in alphabetical order corresponding to their Roman numerals. Provisional designations When satellites are first discovered, they are given provisional designations such as "S/2010 J 2" (the 2nd new satellite of Jupiter discovered in 2010) or "S/2003 S 1" (the 1st new satellite of Saturn discovered in 2003). The initial "S/" stands for "satellite", and distinguishes from such prefixes