Identify the topic sentence in this passage.
On July 20, 1969, astronaut Neil Armstrong became the first human to touch the moon’s surface and walk on the moon. His spacecraft was the US-made rocket Apollo 11. The event was televised, and an estimated 530 million people waited to see what he would do. Once the ship landed, Armstrong climbed down the ladder to the moon’s surface. He had to steady himself to adjust to the difference in gravity on the moon. He spoke these famous words: “That’s one small step for a man, one giant leap for mankind.”