Behind the discovery of the 19 new interstellar asteroids

The discovery of the first population of interstellar asteroids was not made on the basis of observations but with amazing numerical simulations requiring millions of clones of orbits and dozens of days of calculation. Fathi Namouni, astronomer and CNRS researcher at the Lagrange Laboratory of the Côte d'Azur Observatory explains to us how, with his colleague Helena Morais, they developed these very precise simulations of the orbits of these asteroids making it possible to “go up the time "and rediscover their past positions, beyond our Solar System.

The astonishing discovery of 19 interstellar asteroids was made possible thanks to simulations carried out by Fathi Namouni, CNRS researcher at the Lagrange laboratory at the Côte d'Azur observatory (France) and his colleague, Dr. Maria Helena Morais, from theUniversidade Estadual Paulista (Brazil).

These simulations were carried out at Sigamm Intensive Computing Center (Intensive simulations in geophysics, astronomy, mechanics and mathematics) of the Observatoire de la Côte d'Azur. They consisted in reproducing the Solar system and simulate the orbit of these objects until the time when the formation of the planets ended, more or less 4.5 billion years ago.

In addition to space and ground observatories, the numerical simulations are another way of doing astronomy. This modern technique, which is based on big data and intensive computing, has become, as in other fields, a job full-fledged research in astronomy.

If numerical simulations are mainly used to Review theories, Fathi Namouni and Maria Helena Morais show us that they can be used to reproduce past scenes, billions of years old. Scenes that no space or terrestrial observatory could imagine, whatever the wavelengths.

Let Fathi Namouni Speak

ABSMARTHEALTH: How do you simulate the story of 19 asteroids?

Fathi Namouni: For each asteroid, we replicate its orbit in 1 million clones to cover with great precision the observational error of its orbit. Then we simulate the movement of each of the clones by going back in time to 4.5 billion years in the past, the time when the disc which had given birth to all the asteroids born around the Sun was compact. Compact means that it was almost flat and did not extend beyond the current orbit of Neptune (located 40 times the Earth-Sun distance).

What data do you get?

Fathi Namouni: We obtain a swarm of clones, for a given asteroid, a statistical (or probability) distribution which represents the attributes of the original orbit. The statistical dispersion of the clones is due to the phenomenon of chaos. We use the statistical distribution to know the properties orbital the original asteroid in question 4.5 billion years ago.

How much computing power was needed?

Fathi Namouni: We used the Sigamm machine hosted at the Observatoire de la Côte d'Azur. Its computation capacity is 52 Tera-Flops (52 x 10 ^ 12 Floating Operations per Second).

How long did these simulations last?

Fathi Namouni: For each asteroid, the calculation would have taken two weeks if I had had the machine for myself. In practice, as the machine is shared with other users, one month per asteroid was required, or 20 months for the entire study.

How to ensure that the results are not distorted or even that they do not contain calculation errors?

Fathi Namouni: The results are robust, because our simulation is not a training model with parameters, the outcome of which depends on the value of these parameters, such as, for example, the models where we start from a disk of planetesimals, and we try to see how to get the current configuration of the solar system. We precisely simulate the movement of each clone. The only parameter that enters this study is the size of the sample of clones for each object (1 million). If I could put 1 billion clones, the probability distribution curves will be much smoother but will not change. On the contrary, if I had put 100,000 clones per object, the curves would have been more "pixelated". The curves do not change because there are no parameters in the simulation of the movement of a clone.

Did this numerical simulation allow other discoveries?

Fathi Namouni: Yes. It highlighted facts that we did not expect. For example, one of the surprises was that 8 of the 17 Centaurs have very similar median life times (grouped around 2.4 million years with a dispersion of 0.2 million years). This kind of grouping has never been highlighted and could indicate that these bodies were captured in the same single event. We cannot currently prove that this event took place because the resolution 1 million clones per object is not sufficient for this type of diagnostic.

Has the planetary system of origin of these asteroids been located?

Fathi Namouni: No. We stop the simulation 4.5 billion years ago. To get an idea of ​​the origin of these objects, it would be necessary to simulate their process of capture in the star cluster where the Sun was born.

What's the next step?

Fathi Namouni: In the immediate future, it means increasing computing power (Peta-Flops) and increasing the number of clones to 100 million in order to smooth the distribution probabilities and discover the events where the simultaneous captures take place. Like this, we can associate the centaurs with common catch groups. This will help us understand their origin.

One of these 19 objects seems interesting to you to consider sending a probe around, or even a lander?

Fathi Namouni: We don't really know masses precise; suddenly, we can not make a choice. When these objects are better observed, we will be able to define the optimal target not only in relation to the size but also in relation to its speed relative to the Earth so that you can not only get close to it but maybe land there.

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