Like other amateurs, I observed the brightening of Comet Tempel 1 after it was hit by Deep Impact on July 4, 2005. Was this the most distant human-caused event that’s been easily verifiable with ordinary optics? (I used my 12-inch Dobsonian reflector.)
Yes, we think so. The Deep Impact collision occurred 0.89 astronomical unit from Earth — more than 300 times the distance to the Moon.
When the Soviet rocket Lunik II struck the Moon south of the crater Autolycus on September 13, 1959, three Hungarian observers saw the expanding impact cloud with a 7-inch refractor (S&T: November 1960, page 265).
— Alan MacRobert