Unfortunately, discussions on “the city” in Ireland are too often fragmented and Welcome to the real estate state in which decisions on planning, residential …

What can humans do to defend Earth against a black hole? Say, the black hole has a trajectory towards Earth, what would be our weapon against it?

Let’s say that tomorrow astronomers announce that, after having observed the perturbed orbits of certain Kuiper belt objects, and perhaps a fortuitous gravitational microlensing event or two, they have confidently determined that a stellar size black hole, a small one really, is on its way towards the inner solar system, and its trajectory will likely intercept the Earth.

At this point, let’s say, the black hole is still a considerable distance away, say, 300 astronomical units (300 times the Sun-Earth distance), but its velocity is significant, say, about 140 km/s (not at all unreasonable for an object originating from outside the solar system) relative to the Sun.

Which means that we have about 10 years before the black hole arrives.

So what can we do?

Well, first of all… this is a stellar size black hole. I.e., “small”. But small is in the eye of the beholder. The smallest black hole that can form by a known physical process weighs about three times as much as the Sun, or roughly a million (!) times as much as the Earth.

Right there, that should tell you that the question is like an ant asking another ant what they can do if a battle tank is about to roll over their ant hill.

But let’s continue with this scenario. Our black hole, quite invisible of course, approaches the inner solar system. At first, the signs are seen only by astronomers. The black hole itself won’t be seen, but its effect on planetary orbits will become increasingly noticeable. Let’s say the black hole approaches in the plane of the ecliptic, in the direction of Saturn. In 2028, this will eventually take the black hole on a trajectory between Uranus and Neptune. But we’re are not there yet; at first, the perturbations are subtle.

The world goes bonkers in the meantime. As news of the imminent approach of the black hole spread, there will be widespread panic. There will be plenty of cranks and crackpots proposing all sorts of magic schemes to save humanity. There will be political upheavals. Perhaps even wars. There will be outlandish, mad schemes to build spacecraft (for which we don’t have the technology) to escape. There will be resignation, acceptance of an unavoidable fate.

There may be more sensible plans to try to save at least some of the cultural heritage of humanity. Perhaps save a few human beings, maybe a future Adam and Eve. But these plans, too, are doomed; we will not learn in a few short years how to create a self-sustaining presence in space, and no artifact created by humanity will survive for geological time scales.

And the black hole approaches relentlessly. When it is less than 50 astronomical units from the Sun, with a year and a half left until arrival, its gravitational influence on Neptune is already comparable to that of the Sun, which means that Neptune’s orbit is changing quite substantially. Uranus is next, its orbit will be disrupted, too. Depending on the actual trajectory, both planets may permanently detach from the solar system, becoming rogue planets in interstellar space.

OK, to make it truly spectacular, let’s say our black hole actually brushes by Saturn. The result will be dramatic: as the black hole approaches, Saturn will be tidally disrupted, ripped apart. It will be quite a light show, visible to the naked eye. At this point, there will be less than five months left until the end.

As the remnants of Saturn wink out of existence, there will be no more light shows. The black hole itself will remain quite invisible as it approaches the Earth. It will first announce its imminent arrival by altering tides. These effects will become noticeable when the black hole is less than a month away. Tides will be disrupted and will increase in magnitude each day. As a further omen, we lose the Moon; even if it survives the encounter with the black hole, it ceases being a satellite of the Earth, and more likely, becomes a rogue body itself.

Finally, we get to the last day. Tides are now gigantic. Coastal cities will have been washed away. There will be seismic and volcanic activity worldwide, on unprecedented scales. Millions will have died already, with no refuge for the survivors. Our infrastructure will break down; there will be no more electricity, no more running water as transmission towers topple, water lines rupture, dams collapse.

Finally, in the last 90 minutes the Earth gets within its so-called Roche limit. Tidal forces now exceed the Earth’s own gravity, and the planet is literally ripped apart. No human will survive this on the surface (indeed by this time, there is no surface, only a planet-sized drop of molten rock that is being stretched by tides) but humans in spacecraft may witness the final moment as the material that was once the Earth is ripped apart completely and forms a rapidly rotating disk of material around the (still invisible) black hole, spiraling in.

And the black hole continues on its path of destruction. The trajectory I picked, in 2028, actually takes it quite close to the Sun. Which means that the Sun is likely to suffer a similar fate; once it is inside its Roche limit with respect to the black hole, it, too, is ripped apart. Some of it may be sucked in by the black hole, the rest may form a gas cloud that, perhaps one day, may yet coalesce into another star.

The black hole, slightly heavier, now continues on its path, exiting the solar system. Planets that it did not approach may remain largely untouched, in orbit around the remnants of the Sun. For all we know, hundreds of millions of years from now, a diminished but newly stabilized Sun may yet bring life to Venus. Or maybe not, and the solar system remains barren forever.

As to human civilization, it will be all gone. Earth is gone. Everything we ever built, ever created, is gone. One possible exception: refugees in primitive spacecraft who escaped the immediate destruction. They will float in space for weeks, maybe months, but ultimately, their supplies will be exhausted, their life support systems will shut down, and these spacecraft become tombs, possibly remaining in orbit around the remnants of the Sun.

What can we do facing such wholesale destruction? Absolutely nothing. In fact, that colony of ants has a better chance surviving being run over by a battle tank.

Again, a reminder: the mass of the smallest astrophysical black hole exceeds the mass of the entire Earth by a factor of a million.