What is the largest mass for Tycho Crater?
CREATE TABLE "u_s_unmanned_soft_landings_1966_1968" (
  "u_s_mission" text,
  "mass_kg" real,
  "booster" text,
  "launched" text,
  "mission_goal" text,
  "landing_zone" text,
  "lat_lon" text
);
SELECT MAX("mass_kg") FROM "u_s_unmanned_soft_landings_1966_1968" WHERE "landing_zone"='tycho crater';