Why are dangerous asteroids heading towards Earth so hard to detect?


How far can a potentially dangerous asteroid be before it is detected? Credit: Shutterstock / Alexyz3d

The Earth is often in the line of sight of fragments of asteroids and comets, most of which burn tens of kilometers above our heads. But sometimes, something bigger happens.

This is what happened off the east coast of Russia on December 18th. A giant explosion occurred over the Bering Sea when an asteroid several tens of meters exploded with an explosive energy ten times greater than that of the bomb dropped on Hiroshima.

So why did not we see this asteroid coming? And why do we only hear about his explosive arrival now?

Nobody saw it

If the December explosion had occurred near a city – as in Chelyabinsk in February 2013 – we would have heard about it at that time.

But as it happened in a remote part of the world, it was not noticed for more than three months, until details were unveiled at the 50th Lunar and Planetary Conference. on global science this week, based on NASA's fireball data collection.

So where does this asteroid come from?

At risk of space debris

The solar system is littered with materials left by the formation of planets. Most of it is locked in stable reservoirs – the asteroid belt, the Edgeworth-Kuiper belt and the Oort cloud – far from the Earth.

These tanks continually spill objects into the interplanetary space, injecting fresh debris into orbits traversing those of the planets. The internal solar system is awash with debris, ranging from tiny dust spots to comets and asteroids several kilometers in diameter.

Many people have observed and captured the explosion of the Chelyabinsk meteor.

The vast majority of debris colliding with the Earth is totally harmless, but our planet still bears traces of collisions with much larger bodies.

The largest and most devastating impacts (like the one that killed the dinosaurs 65 million years ago) are the rarest. But smaller and more frequent collisions also pose a significant risk.

In 1908, in Tunguska, Siberia, a vast explosion erased more than 2,000 square kilometers of forest. Due to the remote location, no deaths have been recorded. If the impact had occurred two hours later, the city of St. Petersburg could have been destroyed.

In 2013, it was a 10,000-tonne asteroid that exploded above the Russian city of Chelyabinsk. More than 1,500 people were injured and about 7,000 buildings were damaged, but no one was killed.

We are still trying to determine how often such events occur. Since our information on the frequency of the most important impacts is quite limited, estimates can vary considerably.

Usually, people say that Tunguska size impacts occur every few hundred years, but this only relies on a sample of an event. The truth is we do not really know.

<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2019/1-whydangerous.jpg" title = "The explosion of a ball fire of 173kt off the east coast of Russia in December was the biggest fireball recorded since 2013 over the Russian city of Chelyabinsk. NASA / JPL-Caltech / Center for Near-Earth Objects Studies">
Why are dangerous asteroids heading towards Earth so hard to detect?

The explosion of a 173 kt fireball off the east coast of Russia in December is the largest recorded since 2013 over the Russian city of Chelyabinsk. Credit: NASA / JPL-Caltech / Center for Near-Earth Objects Studies

What can we do there?

Over the past two decades, a concerted effort has been made to search for potentially dangerous objects that may pose a threat before they reach the Earth. The result is the identification of thousands of asteroids close to the Earth on a few meters in diameter.

Once found, the orbits of these objects can be determined and their paths planned for the future, to determine if an impact is possible or even probable. The longer we can observe a given object, the better the prediction.

But as we saw with Chelyabinsk in 2013 and again in December, we are not there yet. While the catalog of potentially dangerous objects continues to lengthen, many are still not detected, waiting to surprise us.

If we discover that a collision is waiting in the next few days, we will be able to determine where and when the collision will occur. This happened for the first time in 2008, when astronomers discovered the tiny 2008 asteroid TC3, 19 hours before it reached the Earth's atmosphere in northern Sudan. .

For the anticipated impacts with a longer delay, it will be possible to determine if the object is really dangerous or will simply produce a spectacular but harmless fireball (like 2008 TC3).

For all items that are really a threat, the race will be launched to divert them – to turn a shot into a miss.

<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2019/2-whydangerous.jpg" title = "The meteoric trail taken about 200 km from Chelyabinsk a minute after the explosion of 2013. Credit: Flickr / Alex Alishevskikh, CC BY-SA">
Why are dangerous asteroids heading towards Earth so hard to detect?

The meteorite trail was traveled about 200 km from Chelyabinsk a minute after the 2013 explosion. Credit: Flickr / Alex Alishevskikh, CC BY-SA

In search of the heavens

Before we can quantify the threat an object represents, we must first know that this object is there. But finding asteroids is difficult.

The polls roam the sky looking for light spikes of stars moving on the stars in the background. A larger asteroid will reflect more sunlight and therefore appear brighter in the sky, at a given distance from the Earth.

As a result, the smaller the object, the closer it must be to the Earth before we can locate it.

The objects of the size of the Chelyabinsk and Bering Sea events (about 20 and 10 meters in diameter, respectively) are tiny. They can only be spotted when you are very close to our planet. The vast majority of the time, they are simply undetectable.

As a result, having impacts like those coming out of nowhere is the norm rather than the exception!

The impact of Chelyabinsk is an excellent example. Moving in his orbit around the Sun, he approached us in the sky of the day, totally hidden by the light of the Sun.

For larger objects, which impact much less frequently but cause much more damage, it is fair to expect that we receive a warning.

Why not move the asteroid?

Although we must continue to search for threatening objects, there is another way to protect ourselves.

Missions such as Hayabusa, Hayabusa 2 and OSIRIS-REx have demonstrated the ability to move asteroids close to the Earth, land on their surface and move objects.

From there, a brief jump is enough to deflect them – to turn a potential collision into a near miss.

Interestingly, asteroid deflection ideas fit in perfectly with the possibility of asteroid mining.

The technology needed to extract materials from an asteroid and return them to Earth could also be used to alter the orbit of this asteroid, thus removing it from a collision risk with our planet .

We are not there yet, but for the first time in our history we have the potential to truly control our own destiny.

This concept of artist shows the OSIRIS-REx probe in contact with the asteroid Bennu. Credit: Goddard Space Flight Center of NASA

Explore further:
The asteroid will shave the Earth on October 12: ESA (Update)

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