What is your idea of an asteroid? Many people think of them as potato-shaped, inert and perhaps rather dull, pock-marked objects—far away in deep space. But over the last ten years, two Japanese space missions – Hayabusa and now Hayabusa 2 – have dispatched that view to the history books. Asteroids are interesting bodies that may be able to explain how life on Earth came about.

The Japanese Space Agency, JAXA, is about to bring back samples to Earth from the 1km-wide asteroid Ryugu – with touchdown expected on December 6 at a military test site in South Australia. The first Hayabusa craft returned samples from asteroid Itokawa in 2010, which like Ryugu orbit the Sun near Earth. I'm one of the scientists who analyzed the grains, and am now looking forward to investigating Ryugu.

If intelligent aliens visit the Earth, it would be one of the most profound events in human history.

Surveys show that nearly half of Americans believe that aliens have visited the Earth, either in the ancient past or recently. That percentage has been increasing. Belief in alien visitation is greater than belief that Bigfoot is a real creature, but less than belief that places can be haunted by spirits.

Scientists dismiss these beliefs as not representing real physical phenomena. They don't deny the existence of intelligent aliens. But they set a high bar for proof that we've been visited by creatures from another star system. As Carl Sagan said, "Extraordinary claims require extraordinary evidence."

I'm a professor of astronomy who has written extensively on the search for life in the universe. I also teach a free online class on astrobiology. Full disclosure: I have not personally seen a UFO.

Unidentified flying objects

UFO means unidentified flying object. Nothing more, nothing less.

There's a long history of UFO sightings. Air Force studies of UFOs have been going on since the 1940s.