In district 9 how did the mothership arrive on earth?

In the film "District 9," the mothership arrives on Earth in 1982, hovering over Johannesburg, South Africa. It appears to be stranded and immobile, ultimately leading to the discovery of its inhabitants, the extraterrestrial species known as the "Prawns." The mothership's arrival and subsequent malfunction catalyze the events that unfold in the story, highlighting themes of xenophobia and social segregation.