What are UFOs?

The story

For decades, UFOs have intrigued the world, but many scientists avoided studying them. In the United States, after highly publicised sightings of unidentified objects by US Navy pilots, UFOs are moving out of the shadows and into the light as NASA pledges to study them scientifically. So what does science have to say? Though some are identified as balloons, drones, weather phenomena or optical illusions, others remain mysterious. Could they be the result of secret new technology? And what would it take for alien engineers to traverse vast distances to send probes or visit Earth from other solar systems? Explore the evidence as astrophysicists and engineers use new technologies to investigate the strangest objects in our skies.