Passage Three
When jack Davis, 9, looks up at the night sky, he doesn’t see just the stars. He sees his future. The fourth grader from New Jersey has a deep interest in outer space and the alien(外星的) beings he thinks might live there.
In July, Jack heard that NASA was looking for a new planetary protection officer and decided he was the right person for the job. The position, which focuses on preventing astronauts from accidentally bringing extremely small, potentially dangerous alien life-forms back to Earth and ensuring that astronauts do not take Earth-based germs with them into space, pays from $124,406 to $187,000 per year.
Jack wrote a letter to NASA, noting his qualifications for the job. “My sister says I am an alien,” Jack wrote, pointing out that since he is young, he can “learn to think like an alien.” He signed the letter “Jack Davis, Guardian of the Galaxy.”
Within days, Jack received a personal letter from NASA’s planetary science director, James L. Green. “We are always looking for bright future scientists and engineers to help us,” Green wrote.”I hope you will study hard and do well in school. We hope to see you here at NASA one of these days!”
Jack said he was “shocked” when Green’s response arrived. “I shouted, I got a letter from NASA!”
Green says he loves to answer letters from children. He calls his response a “gravity assist.” Spacecraft use gravity assistance from objects like planets and moons to help power the craft toward another celestial(天上的) body. Green hopes a reply can influence a person’s path in life in a similar way.

What is the job responsibility of the planetary protection officer?