Hunting asteroids

An Astronomer for Observer Support for Near-Earth Object (NEO) Observation, Marco Micheli has been studying NEOs for over 20 years. He works at the ESA Space Situational Awareness (SSA)-NEO Coordination Centre in Frascati, Italy, where he observes those asteroids and comets that are considered most dangerous for our planet. We spoke with him about NEOs and the risk of collision with Earth.

What was your first encounter with astronomy? How did you become interested in NEO observation?

My interest in astronomy started when I was about six or seven years old. I convinced my mother to bring me to a small outreach observatory in my home town, where the local amateur astronomers were showing the night sky through telescopes a few nights every week.

A few years later the same amateur astronomers association, the Unione Astrofili Bresciani, got a new and bigger telescope, and started organizing summer camps that were more research-oriented. When I was 15 I attended the first of these courses and I found out about NEOs, and how they were observed and tracked with telescopes. I immediately became so fascinated by them that I decided this would be my future dream job.

What are the current trends in your field of research?

During the past few years the discovery rates of new NEOs has grown significantly, with new and more powerful telescopes finding many smaller objects, including asteroids as small as 10 meters or less. Those objects are obviously not as threatening as the km-sized ones that can cause global consequences if they impact our planet, but they can still cause significant local damage if they impact a populated area. An example of such an impact happened in 2013 over the Russian city of Chelyabinsk, where about 2000 people were injured by the shock wave caused by the explosion of a 20-meter asteroid in the atmosphere.

Those smaller objects can now be discovered with a few days of advance warning, which would be sufficient to organize civil protection activities on the ground once their impact point is known. Obtaining the observations of these incoming small asteroids, and predicting their impact location accurately and rapidly, is probably the main recent trend of my field.

"Smaller objects can now be discovered with a few days of advance warning, which would be sufficient to organize civil protection activities on the ground once their impact point is known."

What NEOs are you currently researching?

My activities at the ESA Space Situational Awareness (SSA)-NEO Coordination Centre focus mostly on the specific subset of NEOs that are considered most dangerous for our planet. Those are the ones for which we cannot yet exclude the possibility of an impact during the next century or so. This includes small objects, but also larger and better characterized ones, for which we can predict possible impacts in the near future.

How do you track NEOs at the ESA SSA-NEO Coordination Centre? 

We mainly use telescopes distributed all over the world to obtain images of the most dangerous asteroids, and use these images to accurately measure the position of the asteroid in space. Those positions form the basis from which the trajectory of the asteroid can be calculated and extrapolated to the future, in order to predict close approaches and possible collisions with our planet.


What would be the risks of a NEO hitting the Earth? Why is it important to monitor NEOs?

The actual threat of an NEO hitting the Earth depends mostly on its size. On the smaller end, objects a few meters in diameter often just result in a bright meteor in the sky, and sometimes the fall of fragments on the ground. Larger objects from 10 to 20 meters in diameter can already cause significant damage, especially if they explode in the low atmosphere and create a shock wave that hits the ground, as it happened in Chelyabinsk. An object of a few hundred meters in diameter can already cause a major disaster on a regional scale, and anything above a few kilometers will have global consequences, which can be as dramatic as what happened 66 million years ago when the dinosaurs became extinct after a 10 kilometer asteroid hit the Earth.

Fortunately, astronomers have discovered the vast majority of these large km-sized asteroids and we are confident none of them are in a collision course with our planet at least for the next century. However, a large fraction of the smaller objects of a few hundred meters in diameter are still unknown, and new ones are being discovered every day. Those are the most important ones to spot as quickly as possible, because today’s technology would be sufficient to deflect or destroy them if they turned out to be on a collision course.

Once you find a threat, how do you evaluate the probability of a collision?

The first ingredient of a calculation of impact probability is a set of accurate measurements of the position of the asteroid, taken from images collected by telescopes. All these positions are then used to compute a possible orbit for the object, which is then extrapolated to the future to see if it comes close to the Earth.

However, since the set of measurements we have may not be perfect, and may not cover a very long time span, it is impossible to determine the exact orbit of the asteroid with large precision. In most cases, a set of possible orbits, all similar and all compatible with the measurements, can be derived. All of these orbits are extrapolated to the future, and then we can check how many of them actually result in a collision with the Earth. The fraction of colliding trajectories and the total number computed gives us the probability of a collision happening at a given time.

Every time more observations are included in the computation, some of the orbits may now show to be incompatible, and can be excluded. If those are all the ones that resulted in a collision, we can then say the collision is excluded. If not, the process is repeated to compute a new probability of impact.

This shows why it’s extremely important to keep getting more observations of all asteroids that have a small chance of colliding with the Earth. It is the only way we can clarify if the collision will really happen.

What are the odds of a NEO impact happening in the near future? 

Among the objects we know, the one with the highest impact probability is about 10%. However, this is a tiny object of about 10 meters in diameter that will come close to Earth in 2095. Even if it were to collide, it would likely disintegrate in the atmosphere and not cause any damage.

Among the unknown objects, statistically we can expect that an object of about 50 meters in diameter may hit Earth every few centuries. Larger objects are rarer, but can cause such a large devastation that it’s important to discover them as far in advance as possible.

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