The magic hour is twilight, a few minutes window to look, as from the crack of a door that is about to close, a piece of sky where it is otherwise impossible to distinguish anything. In that segment of the celestial vault, still illuminated by the light of the Sun, astronomers have discovered 3 new asteroids. One of them, called 2022 AP7, is as tall as a mountain, a kilometer and a half, of what some call a “planet killer”. Until now they had escaped the objectives of telescopes all over the world, because they came from that part of the sky where it is more difficult to observe.
In particular, 2022 AP7 is large enough and its orbit is close enough to that of the Earth to be classified as “potentially dangerous”: it is the largest discovered in the past 8 years, but there would be no risk of collision with our planet for at least the next 100 years. The NoirLab of the American National Science Foundation gave the news by presenting the study published in The Astronomical Journal.
We have it only seen now, thanks to the observations of the Cerro Tololo telescope, in Chile, as its orbit passes within that of the Earth. Everything in this half of the sky is lost in the solar glow. Thus, finding out if there are any objects colliding with our planet becomes a challenge against time that is played in those 10 minutes after sunset and before dawn: “The asteroids of the inner Solar System orbit very close to the Sun – there has explained Marco Micheli, astronomer at Neocc (the acronym stands for Near-Earth object Coordination center) of ESA in Frascati e co-author of a paper on the subject – Obviously when the Sun is high in the sky it is not possible to use optical telescopes, so it is necessary to wait for it to set, or just before it rises, in order to observe these areas in sufficient darkness”.
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Blinded by the sun
At Neocc, Micheli is in charge of observing objects with telescopes and producing the data necessary to calculate their orbits. The largest population of asteroids is found in the belt between Mars and Jupiter and they are more easily observed because they are seen at night. It is very intuitive: when we turn our gaze from the Sun we are blinded, as when we take a photo against the light we lose details and brightness of the subjects (just think of the new moon phase). Asteroids are no exception. Furthermore, everything we look at from the opposite side of our star reflects the very sunlight, and is thus much more visible, like the full Moon in the sky. To take advantage of the magic hour, you must therefore aim very low on the horizon. This leads to various problems: “We try to observe in the course of what is called a nautical sunset, when the Sun is between 12 and 18 degrees below the horizon – added Micheli – The sky is already dark enough, but not completely dark. As you can imagine, it will therefore be more difficult to identify weak light sources, which contrast much worse than they would in the night sky ”.
The second problem is linked to the fact that the more you look towards the horizon, the more atmosphere there will be between the observer and the Space: “The air in the Earth’s atmosphere is the main source of disturbance, both because it absorbs part of the light and because it scatters the light due to the turbulence of the air – the astronomer pointed out – Generally, this situation lasts only a few tens of minutes per night, and it is therefore necessary to use wide-field telescopes , which in a few images are able to shoot large areas of the sky quickly. Furthermore, bring the telescope as low as the horizon possible often means literally taking it to its constructive limits, it is necessary to collaborate with the observers’ staff, in order not to risk ruining the instrumentation ”.
The search for “planet killers”
According to NASA, the objects at least as large as 2022 AP7 they are known for over 90%. Above all, those still to be discovered in the innermost part of the Solar System would be missing from the census. It is from there that, quite theoretically, a “planet killer” directed towards the Earth could emerge, with a certain degree of surprise: “It should be remembered that the number of asteroids above the kilometer is small, therefore also the probability of impact with one of them is almost nothing – is Micheli’s reflection – However, it is possible that a good fraction of the kilometer-sized asteroids still unknown is hiding right next to the Sun, due to orbital mechanisms similar to those of 2022 AP7 or other similar dynamics that make certain asteroids observable from Earth only every few decades, and it is therefore probable that over the next few years new ones will be made discoveries of this type “.
mind runs to a worldwide success film like Don’t look up. Even if it’s a different story: “In the case of Don’t look up it was a comet that came directly from the outer solar system, in a much longer orbit. These objects can only be discovered during theirs phase of arrival towards the Earth, and therefore it is not possible to have forewarnings of decades as for normal asteroids. However, they are much rarer, and therefore represent a very small fraction of the total risk of impact with the Earth ”.
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The asteroid playing hide and seek
Micheli explained to us the reasons why a colossus of this size was able to evade all observations: “The orbit of 2022 AP7 has only one particularity worthy of note, the fact that it takes almost exactly 5 years to make a full circle around the sun. This proximity to a whole number of years, i.e. of orbital periods of the Earth, generates a situation whereby the asteroid is always far away from us when it is observable in the direction of the night sky, while it is a little closer (and therefore is brighter, ed) only when its position in the sky it is prospectively close to the Sun. This explains why, despite being quite large, it has only now been discovered. Of the 3 asteroids it is certainly the least interesting: it neither represents a danger nor has scientific or research relevance. The media attention that has been dedicated to him is absolutely unjustified ”.
The asteroids that orbit further out, more comfortable to observe, can be tracked for longer periods of time, which allows you to refine the calculations and predictions of where they will be and when. And therefore also to know if there is a risk that in the future they will collide with the Earth. For this reason, the contribution of ESA’s Planetary Defense office was important, through the Neocc: “Our center has a network of many telescopes around the planet, ready to respond within a very short time to observational needs of this type – underlined Micheli – We managed to re-observe these asteroids for a few days just after the discovery, and with these data it was possible to determine its orbit well enough to be able to find them at the next (and always short, ed) observation window a few months later. In this way we can guarantee that, every few months, it is possible to obtain new observations of these asteroids, even if only for a few days, being able to determine their orbit with sufficient accuracy to be able to easily predict future possible collisions with the Earth “.
The first ended a few weeks ago planetary defense mission of the Dart probe of NASA, but for years the whole scientific world has been collaborating to avoid ending up like dinosaurs: the Flyeye telescope (installed in Sicily) and others under test will soon be added to the network of telescopes around the world. Micheli specified that “ESA is also studying an infrared satellite that it will monitor from Space one sky frame near the sundiscovering asteroids, generally much smaller, coming from that direction “.
The aim is to avoid collisions which, it must be reiterated, are currently excluded, at least for 2022 AP7: “The acronym Potentially Hazardous Asteroid unfortunately it is often misleading. This term indicates large asteroids (over 140 meters in diameter approximately, ed) whose orbit passes within approximately 7.5 million kilometers of the Earth’s orbit. The fact that the orbits are relatively close does not imply that the two objects never come close, especially in the time-scale of human life. For example, in the case of 2022 AP7, the minimum distance between its orbit and that of the Earth is 7 million kilometers, but the object will never be closer than 200 million kilometers for the next century at least ”.