How well can astronomers anticipate when shooting stars pass through the sky? This is sometimes possible down to the minute, but not for all swarms. However, it doesn’t make much sense to advise you to look up exactly at this time.
« Rain of shooting stars: at 7:09 p.m. and 9:57 p.m., look up! », Could we read this October 9, 2021 about the Draconids in The Dauphine. The title may surprise, as astrophysicist Eric Lagadec, from the Observatory of the Côte d’Azur, pointed out with a note of humor, on Twitter : « Some newspapers can accurately predict when to see shooting stars! It’s impressive “. There seems to be some confusion with the times shown in 2011, when this generally calm swarm of shooting stars experienced a period of rarer activity.
But, above all, we can wonder: can we really predict the observation of shooting stars with such precision? And does it make sense to indicate such precise times to observers of the sky? The answers, in order, are: yes and not really.
“Yes, we are able to say it”
« It’s my job to predict meteor showers, explains to Numerama the astronomer Jérémie Vaubaillon of the Observatory of Paris. As long as we know the parent body [ndlr : de l’essaim], generally a comet, and that we know its orbit well, the answer is: yes, we are able to say, either to the minute or to the nearest 30 minutes, that such and such a day and such an hour, there will be visible rain. And we are able to say on which part of the Earth it will be observable or not. »
As a reminder, a shooting star or a meteor (the luminous phenomenon), are caused by a meteoroid (a small solid body moving in space), when the latter comes into contact with the Earth’s atmosphere – it can happen that it causes a meteorite, if the body is massive enough. ” The meteor is literally a grain of sand, about a millimeter in size. We have no way to see them, except when they fall on us in the form of shooting stars », Explains the astronomer. However, it is possible to be able to anticipate shooting stars with some precision, because ” as long as we know which comet ejected the grains, we can simulate it on a computer, as well as their entire trajectory as a function of time, and examine whether the Earth will enter the cloud of cometary dust, or not ».
The moment of the maximum of the swarm
But what are we talking about exactly, when we indicate times like ” 19h09 ” Where ” 21h57 “? Jérémie Vaubaillon tells us that this corresponds ” at the time of maximum rain. But in general, they last a few tens of minutes. Rains that last less are rare. The order of magnitude is a few tens of minutes, or even a few hours ».
It was not always easy to determine the instant of the peak intensity of the swarms. In question, a certain gas giant of the solar system: ” Jupiter plays the spoilsport by shifting comets and associated clouds, says the expert. For decades, astronomers have broken their teeth on it. Since then, we have understood that the orbit of the comet was not the same as that of the grains, because they are smaller, they are ejected and that Jupiter does not disturb the orbits in the same way. Their position in the solar system is therefore not the same. When we understood this, we were able to determine the moment of the rains. »
“But we always have surprises”
Thus, during the passage of the Leonids in 2002, the forecasts could have been particularly close to reality. ” We were within a minute of precision, for a rain that lasted at least two hours. For the Arides this year, we were a few tens of minutes away », Explains Jérémie Vaubaillon. Anticipating timetables with such precision is possible with bodies that have been known for a long time and whose orbit is relatively stable, for example comet 55P / Tempel-Tuttle (parent body of the Leonids) or 109 / Swift-Tuttle ( parent body of the Perseids). ” But we always have surprises, recalls the expert. This year the Perseids have seen a surge of activity that no one had anticipated until now. »
In addition, these indications are of little interest when you want to do a little amateur or amateur astronomy, observing the famous shooting stars. Look up to the sky exactly at 7:09 p.m., hoping to see ” an exceptional rain of shooting stars “Falling down is not very timely advice (you might be disappointed). ” Often it is said to watch for an hour or half an hour, around the moment of the maximum », Recommends the astronomer. When you want to hope to see a swarm, it is anyway advisable to let your eyes get used to the darkness for at least several minutes.
In addition to peak times, astronomers can also predict the position of shooting stars in the sky, with varying degrees of accuracy. ” It is orbital geometry: we are able to tell in which region of the sky the shooting stars seem to come from. By extending the trajectories of shooting stars in the sky, they all intersect at a place called the radiant », Sums up Jérémie Vaubaillon. This is where you can turn when attempting to observe a swarm. Sometimes it is possible for scientists to be even more precise about their location, but not all the time: ” For the Arids this year, we were up to 0.1 ° from the forecast position. It’s not always the case. »