diff --git a/2019/day02/problem b/2019/day02/problem
index 60eef46..081725b 100644
--- a/2019/day02/problem
+++ b/2019/day02/problem
@@ -137,6 +137,7 @@ References
    . https://adventofcode.com/2019/leaderboard
    . https://adventofcode.com/2019/stats
    . https://adventofcode.com/2019/sponsors
+   . https://www.hq.nasa.gov/alsj/a11/a11.landing.html#1023832
    . https://en.wikipedia.org/wiki/Gravity_assist
    . https://en.wikipedia.org/wiki/Halt_and_Catch_Fire
    . https://en.wikipedia.org/wiki/Magic_smoke