We can be destroyed by many kinds of cosmic catastrophes, completely unrelated to what is happening on the surface of the Earth. The star can pass through the solar system and absorb our planet, or throw us out of orbit and freeze. A supernova or a burst of gamma radiation can occur in a dangerous vicinity of us, and destroy all life on the surface of the Earth. Or, as has already happened, at least once, 65 million years ago, a huge and fast moving object such as a comet or an asteroid can collide with the Earth. If we can foresee this, we will be able to take the necessary measures. But what if there is no chance that if the comet flying to us does not work out? Our reader has heard about this possibility and wants to know:
I recently read a few articles about dark comets, and it frightened me, to put it mildly! Is Bill Napier right about the dark comets? Are they really threatening us on Earth?
Comet Lovejoy, photographed with the ISS, does not pose a danger to the Earth
Bill Napier is a scientist studying Potentially dangerous space objects. He correctly points out that although most attempts to create a list of potential hazards for the Earth are related to objects close to Earth such as asteroids leaving the main belt and crossing the Earth's orbit, this may not be a realistic description of what can be for us Danger. It will not necessarily be an asteroid located inside the orbit of Jupiter, or a comet orbiting Neptune, who are just waiting to be knocked out of the way and sent to the inner part of the solar system. Between the orbits of the four gas giants, there are many objects known as centaurs, which without warning can be knocked down from the orbit to the center of the solar system; Most of them are not cataloged. Napier claims that many such centaurs may not be visible to us even after they have rushed to our side, and until it is too late.
Asteroids ( Gray) and Kuiper belt objects (blue and orange) are considered the main threat to the Earth. But the number of centaurs (gray) exceeds 44,000.
This leads us to an important question: why can a comet become dark, or invisible? It is not just a comet flying towards us from the outer margins of the solar system, which is poorly reflecting the sun. Centaur, of course, for billions of years could evaporate all the ice from its surface, which would reduce its appearance. But the Sun emits so much light that even a medium-sized comet, or centaur, absorbing 99.9% of the Sun's light percentage, would still be easily distinguishable for us in Saturn's orbit. Moreover, comets basically consist of ice that actively reflects light and appears on the surface of the comet during its warm-up. The really dark bodies in our solar system are more like the Moon, which still reflects a lot of light – anyone who watched the night sky will tell you about it. Any object consisting of natural dark chemical elements or mixtures would still be visible in reflected light, especially in the infrared part of the spectrum.
But it is necessary to consider other possibilities. What if a light-reflecting comet flying toward us is rotated somehow in a special way? What if it is icy, but it will reflect all the light not in the direction of the Sun, like some unusual crystal? But even this will not work, although the reason for this is not so obvious. When such an object enters the part of the solar system occupied by the planets, it warms up. Heat melts the ice, and a long tail appears, directed away from the Sun, which can be seen easily and in advance through many types of professional, and through several types of amateur equipment.
Possible , Will nature cheat and make it so that this tail will not be visible from our position? To be hidden from us, the comet must be directed directly to our planet, and should be located on a straight line from the Sun to the Earth. If the tail is directed from us and hidden behind a comet, it will not be visible to us, and the comet will not be visible to us either, is it?
But this is also not true. The tail of the comets is not just directed away from the Sun, they expand as they move away from the comet's core. Even a comet directed to our forehead will have a coma visible to us. Professional astronomers, and amateurs, quickly recognize this fact.
Coma 17P / Holmes coma is visible, although it flies almost to the forehead.
The danger from an invisible comet exists, and is very different from the forms that Napier represents. Imagine that a bright, reflective comet with a tail and a coma is heading towards us. Will there be such a direction from which it can come to us invisible? Will be – in the direction from the Sun.
Telescopes, even space ones, do not dare to go even close to the Sun, since the ray of direct sunlight will roast the entire optical system. If any object – a comet, an asteroid, a centaur, even a fragment from a collision with Mercury – either approaches the Sun from behind (from our point of view) or sweeps past it due to the effect of a sling, then a suitable trajectory can send it to the Earth. That's why it's so important to have NASA STEREO satellites in working order.
To date, there has not been developed a technology to repel a fairly large asteroid or comet flying to us for a short time. But, at least, thanks to the observatories located in different parts of the solar system, we can see everything that flies in our direction. In the future, more sensitive infrared devices for a full heavenly view will give us more complete centaur lists in the solar system, and the launch of WFIRST in the 2020s will help us point out all potentially dangerous objects located at a significantly longer distance than all those that are known to us today . But the chances that a remote object will rush to our side after any impact are extremely small. The prospect of a terrible one is a comet with a very long period of circulation, which is slightly pushed toward the orbital path of the Earth.
The orbital path of the Swift-Tuttle comet passing dangerously close to the Earth's path around The Sun
Comet Swift – Tuttle, the ancestor of the Perseids, is the most dangerous object known to mankind. After 4400, she has a chance to encounter us with energy 20 times higher than the energy from the legendary event that destroyed the dinosaurs. But until that moment, we have a lot of time. In the meantime, you can be comforted by the fact that in addition to asteroids and comets flying from the side of the Sun, we can see all the large bodies heading toward us. And if we are lucky, and our civilization will exist for another thousand years, our technology will most likely develop to the point that the reflection of a comet or an asteroid will not be such a daunting task.
Comet Swift – Tuttle in 1992
Well, there is always a plan "B" – to clone Bruce Willis.