CREATE TABLE table_19624 ("Explosion" TEXT, "Location" TEXT, "Date" TEXT, "Yield  ( approximate ) " TEXT, "Altitude  ( km ) " TEXT, "Nation of Origin" TEXT)

What explosion had an altitude of 539 km?
SELECT "Explosion" FROM table_19624 WHERE "Altitude (km)" = '539'
SELECT "Explosion" FROM "table_19624" WHERE "Altitude (km)" = '539'