There are many different cosmic catastrophes that can condemn us, not caring at all about what is happening on Earth. A star can penetrate the solar system and swallow our planet, or knock us out of orbit into the permafrost. A gamma-ray flash or supernova can happen too close to us (or towards us) and destroy all life on the planet, sterilize it. You don't have to go far - just 65 million years ago, a large fast object like a comet or an asteroid fell to Earth, incinerating almost all life. It would be nice to at least prepare for fate, find out what it will be. But what if there is no chance? If an invisible comet flies to us? There is such a possibility.
Dark comets? Seriously? Can they threaten the Earth?
Obviously, a fast moving unexpected object crashing into the Earth is a threat to life.
Bill Napier is a scientist who studies potentially dangerous objects from space. He rightly points out that while much of the effort to catalog the potential hazards to Earth is focused on near-Earth objects like asteroids leaving the main belt and crossing Earth's orbit, none of this reflects a real threat. You don't have to wait for an asteroid near Jupiter or a comet near Neptune to feel the need to change its orbit and head into the inner solar system. Between the orbits of the four gas giants, there are many objects (centaurs) that can head inward without any warning, and most of them are not included in the catalogs. Napier claims that many of these centaurs are invisible to us, so when they do decide to come to us, it may be too late.
An important question arises: how to see an invisible, dark comet? It is not just a comet that rushes towards the Sun and reflects light in such a way that it can be seen even with the naked eye. Over billions of years, a centaur can lose all volatile ice, and with it its reflectivity. However, the sun emits so much light that even a medium-sized comet (like a centaur) that absorbs 99.9% of the sunlight will still be visible from a distance of Saturn. Moreover, comets, as a rule, consist of ices, which reflect light well and come to the surface together with the heating of the object. The darkest objects in our solar system, like our moon, will reflect a lot of light, especially in the infrared spectrum.
But there are other possibilities to consider. What if a highly reflective comet flying towards us is in a strange position, for example, icy, but so that the reflected light will leave the Earth? This option is less obvious, but it won't work either. When an object enters the planet-inhabited part of the solar system, it heats up. The heat acts on the ice and results in a long tail that points away from the Sun and which will be immediately recognized even by non-professional sky watchers.
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Perhaps nature has conspired with the comet and will make it a tail invisible from our point of view? To hide the tail, the comet must be pointed directly at us and fly so that the Sun, Earth and the comet are in a straight line. If the tail is pointing away from us and hiding behind a comet, we will not be able to see it? Alas, this is also far from the truth. Comet tails not only point away from the sun, they also radiate away from the comet. Even around the comet in the above scenario, there will be a visible coma around it. And again, astronomers will quickly notice.
An invisible comet could pose a serious threat, but it will differ in shape from that proposed by Napier. Imagine, if you can, that a bright, reflective comet with a tail and a coma is striving towards us at full steam. Could there be a direction in which we cannot see it perfectly? Maybe: away from the Sun.
Telescopes don't dare look too close to the Sun, even in space, as even a glimpse of direct sunlight can ruin and fry the optical system. If an object - a comet, asteroid, centaur, or a fragment of Mercury - goes around the Sun and accelerates away from it, the correct trajectory can send it towards the Earth. Therefore, it is very important to keep NASA STEREO satellites afloat.
At the moment, the technology to deflect an incoming asteroid or comet in a short period of time has not been developed, but with many observatories in different places in the solar system, we could see everything that is directed towards us. In the future, a thorough infrared survey of the entire sky would allow us to make a complete census of the centaurs of our solar system, and the launch of WFIRST in the 2020s will help us map potentially dangerous objects at great distances. But the chances that an object will fly at us after something disrupts its orbit is extremely small. It is much more likely that a long-period comet will overtake the Earth in its orbit.
Comet Swift-Tuttle, which gave birth to the Perseids, is the single most dangerous object known to mankind. When dropped, it will release 20 times more energy than the legendary dinosaur killer 65 million years ago. But we have a lot of time to prevent this from happening. In general, no dark comets threaten us if we are careful enough. In any case, we must have at least another thousand years to build sufficiently developed defense mechanisms.
ILYA KHEL