Every so often there are reports that an asteroid is passing close to Earth, with the most recent story claiming one the size of the Eiffel Tower will be zooming past our planet. Dinosaurs also famously became extinct after an asteroid collided with our planet 66 million years ago. But what is the likelihood of it happening again? 

To answer this question, CGTN Europe spoke to Doug Millard, the space curator at the Science Museum in London.

What is an asteroid?

According to Millard's description, an asteroid is "essentially a rock ... orbiting the sun a little bit further out than we are." 

"It may be fairly small, anything from a meter upwards, or it may be very big," he says, adding that the biggest asteroid is "about the size of the UK." These minor planets, which can also be referred to as planetoids, are normally found in the inner solar system.

The majority of asteroids – at least the ones we know about – are located in the asteroid belt between Mars and Jupiter and the first one to be known to humans, Ceres, was originally considered to be a new planet. The discovery of other similar objects and the equipment of the time then led to the objects appearing like stars but without an orbital disk. Astronomer William Herschel then suggested the term asteroid, which means star or star-shaped in Greek and star or planet in ancient Greek.

What is the difference between an asteroid and a comet? 

"They have different compositions," Millard explains, adding that there are "millions and millions" of both asteroids and comets. 

Asteroids form closer to the sun, where it is far too hot for ice to remain solid. Due to this, as Millard says, comets are essentially "dirty snowballs" which are "made of ice and dirt" unlike asteroids, which are composed of metals and rocky material. 

Millard also explains that comets are a lot further out, only paying occasional visits to Earth. When they are near the sun, they also sprout tails composed of vapor from the comet's surface melting. Despite such differences, one similarity between the two is that both were formed during the earliest history of the solar system, which is approximately 4.5 billion years ago.

When did an asteroid last hit Earth?

"Earth is very old and during its lifetime, it has been hit by many asteroids," says Millard, adding that the most famous asteroid collision with Earth is of course that which led to the extinction of dinosaurs and nearly all other living beings on Earth at the time. 

When the asteroid, which was bigger than Mount Everest, hit our planet nearly 66 million years ago, fire rained from the sky and the ground shook far worse than any other modern earthquake. The asteroid impact was so great that it triggered a chain of natural disasters, and as the planetoid landed in water, tsunamis developed, which traveled thousands of miles in multiple directions. The explosive blasts that resulted from this sequence of events would have destroyed everything in their path for hundreds of miles.

The impact of the collision with Earth also caused volcanic eruptions, earthquakes and worst of all, deadly dust particles to form. These shot into the air and became red hot when falling back down to the Earth's surface. Such dust falling from the sky not only led to forest fires across the globe but the sheer quantity of particles in the atmosphere meant that the Sun was blocked for years, killing plants and animals reliant on it.

Unfortunately for dinosaurs, such a sequence of events was hard to avoid, just like the collision. 

Millard eases fears by saying that if an asteroid were to be on course to hit Earth today, "we would have some warning and we would know what we had to do about it, which would probably involve sending a spacecraft just to nudge it away from hitting Earth," which is commonly referred to as "asteroid impact avoidance." 

How often do asteroids pass our planet?

According to Millard, asteroids pass by Earth relatively rarely and even if they do, it's important to remember the distance and scale of such encounters, with a "near pass" still being "quite a long way away." 

Asteroids with a 1km diameter are likely to hit Earth every 500,000 years. With its 7km diameter, the Apollo asteroid is the largest known potentially hazardous object, despite its faint magnitude. Such large collisions happen once every 20 million years.

Putting fears to rest once again, Millard shares some good news. 

"There are several organizations around the world that track asteroids and the ones we're particularly interested in are called near-Earth objects (NEOs) ... those are the ones, which perhaps might come a little bit too close to Earth. 

"By tracking them, we can have some warning and then hopefully do something about it."

Should we worry about an asteroid hitting Earth?

Millard says: "There's no point in worrying about these sorts of events. We do have the technologies now to hopefully do something about any asteroid that might be getting a little bit too close to Earth."

He emphasizes that, before our imaginations run away with us, we need to remember the time frames involved and the unlikelihood of the event before fearing a repeat of events when dinosaurs walked the planet. 

"The Earth has been here for billions of years... I think we can rest easy for the time being."

Video editor: Paula Harvey