Type 1: Could there be a giant, unknown asteroid bearing down on Earth? I frequently see what-if questions on Quora long these lines, typically involving a Texas-size (or Europe-size, or Moon-size) asteroid that is less than a year from hitting when we discover it. This is similar to the scenario in some of the more over-the-top
Armageddon-style flicks.
Nothing to worry about here. Nothing at all. A Texas-size asteroid that will hit Earth in one year? Impossible. A Europe-size asteroid?
Completely impossible.
There is no object that big in the asteroid belt. The largest asteroid, Ceres, is about the size of Spain and France put together. That's a bit bigger than Texas, and maybe large enough to call it the “size of Europe,” but Ceres is in a stable orbit, far away from us. It is completely ruled out as a threat.
If you want to go for an all-out disaster scenario, you're better off looking to the Kuiper Belt beyond Neptune. The largest objects in the Kuiper Belt — Pluto and Eris — truly are Europe-size but, like Ceres, they are in stable locations and are not heading our way even on a geologic timescale. The big objects in the Kuiper Belt are so few in number and so widely distributed that the change of any one of them being perturbed into an Earth-intersecting orbit is essentially zero.
There are many smaller, more easily disturbed objects in the Kuiper Belt; this region is the source of some of the comets that we see in the inner solar system. Those bodies are much smaller, however, typically no more than a few kilometers wide, and even so they would be easy to see well in advance. The ones that migrate inward are also rare enough that major comet impacts on Earth happen tens of millions of years apart, on average.
Nature is kind to us in an important way: The bigger the potential impactor, the easier it is to spot and the harder it is to perturb. As a result, we would have a lot of advance warning if there was even a minuscule chance that a large object was on a collision course.
If we're talking about a continent-size Kuiper Belt object, astronomers would have already spotted it unless it was at least 50 to 100 AU (Sun-Earth distances) from the Sun. From that distance, an object would take about
200 years to reach the inner solar system. We’d have a couple centuries of advance warning, not one year. And such an object would travel through the inner solar system with little consequence unless it passed very close to Earth—an extremely unlikely event on top of another extremely unlikely event.
If you want to game out every possible disaster scenario, you can consider one more: an interstellar object entering our solar system at high velocity. The two known interstellar comets — ‘Oumuamua and Borisov — entered our solar system at a speed of about 5 AU per year. If we spotted an interstellar Pluto headed our way, and we spotted it at a distance of 50 AU, we might then have only a decade or so of warning. For 'Oumuamua and Borisov, we had far less warning, because they are both tiny. Again: The bigger the object, the more advance notice you get.
An interstellar comet impact is the disaster scenario du jour; it is the premise of the upcoming movie
Greenland, which deals with the risk in an unexpectedly plausible way. But note that we are talking about much smaller objects than the one in the original question; 'Oumuamua was only about 300 meters long! Even these little things are rare enough that astronomers had to search for years before finding one. We have no idea if continent-size interstellar Plutos even exist. At any rate, they've left no sign of their existence. If anything in the solar system had been hit by such an object in the past 4 billion years, for instance, we'd see the evidence.
Bottom line: The last time a continent-size object struck Earth was 4.5 billion years ago, when it led to the formation of the Moon. The likelihood that it would happen in your lifetime is extremely close to zero. The likelihood that it would happen with just one year of advance warning is exactly zero. It simply cannot happen.