2.We read about an astronaut. The astronaut travelled into space in 1961.

→ We read about an astronaut who travelled into space in 1961.

3.This is the man. He works for NASA.

→ This is the man who works for NASA.

4.The team plays on the left. The team has never won the championship.

→ The team who plays on the left has never won the championship / that plays on the left has never won the championship.

5.This article describes a ground-breaking space mission to land on a comet. The mission is called Rosetta.

→ The ground-breaking space mission which this article describes is called Rosetta / that this article describes is called Rosetta / X this article describes is called Rosetta.

6.The Rosetta mission has a task. The task is comparable to a fly trying to land on a speeding bullet.

→ The task which the Rosetta mission has is comparable to a fly trying to land on a speeding bullet / that the Rosetta mission has is comparable to a fly trying to land on a speeding bullet / X the Rosetta mission has is comparable to a fly trying to land on a speeding bullet.