Table:
text: Sample return mission
List of missions
Robotic missions
Launch_date Operator_ Name_ Sample_origin Samples_returned Recovery_date Mission_result_
June 14, 1969  Luna E-8-5 No.402 The Moon None - Failure , Launch failure
July 13, 1969  Luna 15 The Moon None - Failure , Crash-landed on the Moon
23 September 1969  Cosmos 300 The Moon None - Failure , Failed to leave Earth orbit
22 October 1969  Cosmos 305 The Moon None - Failure , Failed to leave Earth orbit
6 February 1970  Luna E-8-5 No.405 The Moon None - Failure , Launch failure
September 12, 1970  Luna 16 The Moon 101 grams (3.6 oz) , of lunar rock September 24, 1970 Success

text: Sample return mission
List of missions
Robotic missions
Launch_date Operator_ Name_ Sample_origin Samples_returned Recovery_date Mission_result_
September 2, 1971  Luna 18 The Moon None - Failure , Crash-landed on the Moon
February 14, 1972  Luna 20 The Moon 55 grams (1.9 oz) , of lunar rock February 25, 1972 Success
November 2, 1974  Luna 23 The Moon None - Failure , Drilling device damaged on Moon landing , Mission abandoned
October 16, 1975  Luna E-8-5M No.412 The Moon None - Failure , Launch failure
August 9, 1976  Luna 24 The Moon 170 grams (6.0 oz) , of lunar rock August 22, 1976 Success
1979 (planned) , (canceled 1977)  Mars 79 (Mars 5M) Mars - - Canceled in 1977 , Planned to return 500 grams (18 oz)

text: Sample return mission
List of missions
Robotic missions
Launch_date Operator_ Name_ Sample_origin Samples_returned Recovery_date Mission_result_
February 7, 1999  Stardust 81P/Wild , (Comet coma) Over 1 million , comet particles January 15, 2006 Success
August 8, 2001  Genesis Solar wind Solar wind particles September 8, 2004 Partial success , Return capsule damaged after failed parachute deployment , Some usable samples recovered
May 9, 2003  Hayabusa 25143 Itokawa , (Asteroid surface) 1,500 grains , of asteroid particles June 13, 2010 Partial success , Planned to return several hundred milligrams
November 8, 2011  Fobos-Grunt Phobos , (Mars satellite soil) None - Failure , Failed to leave Earth orbit , Crashed into the southern Pacific Ocean , Returned none of the 200 grams (7.1 oz) planned

Question: What was the average number of missions successful in a sample return mission?