Time and again huge rocks pass by relatively close to Earth. In April 2018, for example, an asteroid named 2018GE3 with a diameter of 50 meters (164 feet) got dangerously close to us. It came from the dark depth of space and astronomers discovered it only 21 hours before the flyby.

Five years before that, a 20-meter meteorite hit the Earth close to Chelyabinsk in Russia. Catastrophy was avoided and consequences were relatively mild. Thousands of buildings were damaged by the blast's wave and more than a thousand people were injured, especially by flying glass fragments. But fortunately there were no deaths.

Astronomers are quite confident that they know where the even more lethal objects are, rocks that are several kilometres in diameter. They hope that planet Earth is not threatened by any such huge danger for at least the next 100 years. 

Read more: Japanese spacecraft Hayabusa2 lands on asteroid

Medium-sized projectiles are difficult to predict

But the "smaller" rocks with only several hundreds of meters in diameter can already cause devastating regional damage. The cases of the two asteroids mentioned above also show that we cannot possibly know them all.

For this reason, 300 astronomers, space engineers and other experts from the US, Russia, China, Germany, France and Israel are discussing the "space situation" this week.

At the 6th IAA Planetary Defense Conference  in Maryland, USA, the main focus will be on possible defense strategies. The North American Space Agency NASA is organizing the conference together with researchers from the Johns Hopkins Applied Physics Laboratory (APL).

The scientists have devised a hypothetical asteroid up to 300 meters in size, which races towards the Earth at a speed of 14 kilometers (8.6 miles) per second, i.e. around 50,000 kilometers per hour, from a distance of 57 million kilometers. The probability that it will hit us is estimated at one percent. One way to deal with this would be to evacuate the threatened regions on Earth. 

Read more: Asteroids and comets: How to tell them apart

Evacuate or deflect?

At the conference, however, the participants will also demonstrate various methods by which mankind can deflect the asteroid from its dangerous path, such as NASA's Double Asteroid Redirection Test (DART)developed by NASA's Planetary Defense Coordination Office in Washington, together with APL. In 2022, a real 150-meter-diameter asteroid, which is not posing a current danger for Earth, is to be deflected from its orbit by a collision. The researchers want to find out whether such a method promises success. 

Read more: Greenland: Massive 31-kilometer crater discovered beneath ice

DART is part of a National Near-Earth Object Preparedness Strategy and Action Plan. Asteroids whose orbit around the sun is closer than 50 million kilometers to the Earth's orbit are considered "near-Earth." More than 20,000 of them are known and 700 are added every year.

"We have to make sure people understand this is not about Hollywood," said NASA administrator Jim Bridenstine at the opening of the conference, according to AFP.

His colleague Detlef Koschny from the European Space Agency ESA agreed with him: "The positive aspect of Chelyabinsk is that it made the public aware, it made the political decision makers aware."

Not visible in the dark

The greatest uncertainty emanates from objects close to the sun, which are virtually invisible from the Earth due to the light conditions. These can only be discovered, if at all, with special telescopes that are located in Arizona, Hawaii, Chile, Spain and Sicily.

Now astronomers are also discussing the construction of a space telescope for this purpose. It could search from a different perspective. Alternatively, a telescope could be erected on the far side of the moon. It would have a much better view into the depth of the universe, with the moon shielding it from radiation and light coming from Earth.