What explosion had an altitude of 539 km?
CREATE TABLE "list_of_artificial_radiation_belts" (
  "explosion" text,
  "location" text,
  "date" text,
  "yield_approximate" text,
  "altitude_km" text,
  "nation_of_origin" text
);
SELECT "explosion" FROM "list_of_artificial_radiation_belts" WHERE "altitude_km"='539';