What Would Actually Happen If a Large Asteroid Hit Earth Tomorrow: A Science Breakdown

A 10-kilometre asteroid, roughly the size of the one that ended the Cretaceous period 66 million years ago, travelling at around 20 kilometres per second would release energy equivalent to several billion nuclear warheads on contact with Earth. The impact itself lasts less than a second. The crater it leaves behind can be 150 to 200 kilometres wide.
The shockwave moves outward faster than sound. Within minutes, everything within a radius of several hundred kilometres is incinerated. The ejecta, superheated rock and debris launched into the upper atmosphere, re-enters at hypersonic speed, heating the air so intensely that forests ignite from the thermal pulse alone, with no flame ever touching the ground. NASA models based on the Chicxulub impactor (the asteroid that struck what is now Mexico's Yucatan Peninsula) suggest wildfires would spread across entire continents within hours of a comparable collision.The debris thrown into the stratosphere is where the longer catastrophe begins. Soot, sulfur dioxide, and pulverised rock block sunlight on a planetary scale. This is not a temporary dimming. Studies of the Chicxulub event, published in journals including the Proceedings of the National Academy of Sciences, estimate global temperatures dropped by 10 to 15 degrees Celsius within weeks. Photosynthesis collapses. The food chain follows.
The atmosphere also absorbs an enormous nitrogen oxide load from the impact, which reacts with water vapour to produce acid rain across surviving land surfaces. Soil chemistry changes. Freshwater sources acidify. The ocean surface, absorbing sulfuric acid from above, becomes hostile to the calcium-carbonate shells that marine organisms depend on, the base of the ocean food web begins to dissolve.
If the asteroid strikes ocean rather than land, a tsunami with wave heights potentially reaching 300 to 500 metres radiates outward. Coastal cities, Mumbai, Chennai, Kolkata among them, would have minutes to hours of warning depending on distance from the impact zone. No evacuation at that scale is logistically possible.The people who survive the initial impact and its immediate thermal and blast effects face a different problem: an impact winter that can last one to three years. Agriculture fails globally. The USGS and various climate modelling teams have estimated that a Chicxulub-scale event would reduce surface sunlight to a fraction of normal levels for months, killing crops across every latitude.

Stored food, grain reserves, canned supplies, becomes the only food source. India's national grain reserve, maintained by the Food Corporation of India, typically holds around 50 to 70 million tonnes. That sounds substantial until you calculate 1.4 billion people consuming it with no resupply. At basic caloric minimums, it runs out in weeks to a few months, not years.
Underground water sources survive better than surface ones. Geothermal energy remains available. These are not trivial facts, they define where and how a small fraction of the population could sustain itself through the impact winter. Bunkers, mines, and deep infrastructure become the geography of survival.The Chicxulub impact killed an estimated 75 percent of all species on Earth, including every non-avian dinosaur. It did not kill everything. Crocodilians survived. So did many mammals, birds, and a wide range of insects. The survivors shared certain traits: small body size, dietary flexibility, and the ability to live on decaying organic matter when living plants were gone.

Humans are large-bodied, highly specialised in their food systems, and almost entirely dependent on agricultural supply chains built for a stable climate. By the metrics the fossil record gives us, we are not well-positioned for a Chicxulub-scale event. The species that made it through 66 million years ago were not the dominant ones. Dominance, in that extinction, was a liability.
A smaller asteroid, in the 1 to 2 kilometre range, produces regional rather than global extinction. The 1908 Tunguska event over Siberia involved an object estimated at 50 to 80 metres and flattened roughly 2,000 square kilometres of forest. Scale up to a kilometre-wide object and the regional devastation becomes continental, with global atmospheric effects that stop short of full photosynthesis collapse.NASA's Planetary Defense Coordination Office tracks near-Earth objects continuously. The Double Asteroid Redirection Test (DART) mission, which successfully altered the orbit of the asteroid Dimorphos in September 2022, demonstrated that kinetic impactor technology can work, provided the threat is identified years or decades in advance. ISRO has not yet announced a dedicated planetary defence mission, though Chandrayaan and Aditya-L1 have built the deep-space tracking infrastructure that any future Indian contribution to this effort would depend on.

The honest constraint is lead time. DART worked because Dimorphos was a known target with years of preparation. An asteroid discovered six months before Earth collision, a scenario that remains possible for smaller, darker objects, leaves no time for deflection. The physics of orbital mechanics require years of nudge, not a last-minute push.
The difference between a civilisation-ending event and a survivable regional catastrophe is almost entirely a function of asteroid size and lead time. A 10-kilometre object is extinction-class regardless of where it lands. A 140-metre object is catastrophic for a region and survivable for the planet. Most of the detection effort currently focuses on objects above 140 metres, and as of the most recent NASA census, roughly 40 percent of that population remains untracked.
The impact winter, the acid rain, the collapse of photosynthesis, and the failure of food systems do not happen because the asteroid is uniquely destructive. They happen because every system humans depend on, agriculture, trade, freshwater, energy, was built for a planet where the Sun shows up every morning. Remove that assumption and the infrastructure is revealed for what it always was: a set of arrangements that work only inside a narrow band of atmospheric stability the asteroid erases in a second.