The world must be prepared for an asteroid strike, which is just a matter of time, warns a leading astrophysicist from Queen's University Belfast in Northern Ireland.

It is a case of when an asteroid collision will happen, rather than if it will happen, said Alan Fitzsimmons.

Fitzsimmons was highlighting the threat for Asteroid Day, a global event on June 30.

On that day in 1908, a small asteroid exploded over Tunguska in Siberia and devastated 800 square miles. 

A similar unexpected strike in today's world could easily destroy a major city and a larger asteroid could be more dangerous, Fitzsimmons warned. 

"It is important to know that scientists and engineers have made great strides in detecting Near-Earth Asteroids and understanding the threat posed by them. Over 1,800 potentially hazardous objects have been discovered so far, but there are many more waiting to be found," Fitzsimmons said in a statement released by Queen's University Belfast.

"Astronomers find Near-Earth Asteroids every day and most are harmless. But it is still possible the next Tunguska would take us by surprise, and although we are much better at finding larger asteroids, that does us no good if we are not prepared to do something about them," he said. 

Fitzsimmons will be joined by other astronauts and experts for discussions and presentations to be streamed live from Luxembourg on June 30.