Closest approach to Earth by a comet

On 12 June 1999, the tiny comet P/1999 J6 passed within 0.012 astronomical units (1,795,169 km) of Earth – only 4.7 times further away than the Moon. This is an extremely close pass in astronomical terms, but the diminutive size of the comet (it was estimated to be no more than 48 m (157 ft) in diameter) meant that it was invisible to the naked eye, and would have been extremely hard to spot even with a powerful telescope.

P/1999 J6 is thought to have returned in 2004, when it was given the designation P/2004 V9. Measurements taken during this pass (which did not approach as close as the 1999 event) suggested a diameter of only 23 m (75 ft). It is not clear whether this discrepancy is caused by the comet losing mass due to outgassing or simply the result of more accurate measurement during the second pass.

Before P/1999 J6 appeared, the record for closest approach had been held by comet D/1770 L1, better known as Lexell’s Comet, since July 1770. During this pass Lexell’s Comet came within 0.015 au (2,258,927 km, 5.9 times the distance between the Earth and the Moon). Lexell’s Comet was far larger, however, reaching a peak brightness of 2 on the apparent magnitude scale (about as bright as Polaris – easily visible to the naked eye) and appearing as large as the planet Jupiter in the night sky.