Under local twilight conditions, a 6-mile-diameter object coming straight in should easily be visible with ordinary eyes at a distance of ten-thousand miles; that would be 20 minutes before impact. It would be small but visible. Two minutes before impact, at a thousand miles out, it would be the brightest thing in the twilight sky, almost as bright as the full moon.
If 
the asteroid were to arrive at a location that is
one hour before sunrise or after sunset, it would enter the earth’s shadow at an altitude of
140 miles and wink out of view for ~8 seconds until it hit the upper atmosphere -- from which point
onward, the hot air in front of it would get more than eight times hotter than the surface of the sun.
If it were to arrive at a location that is two hours before sunrise or after sunset, it will enter the earth’s shadow at an altitude of 610 miles and become invisible and remain out of view ~65 seconds before reaching the upper atmosphere.
The Thermal Blast Before Impact
A speed of 30,000 mph is ~40 times the speed of sound in air at sea level. This means that air molecules in the path of the asteroid cannot possibly move laterally out of the way. Instead, the air gets compressed in front of it. I do not know how hot it would get, but the calculation for temperature as a function of the kinetic energy of air molecules moving at the asteroid’s speed is more than 300,000 degrees K, or more than 500,000 F.
Given that the energy of the hot gas would radiate forward and laterally, it seems reasonable to assume that the layer of compressed air would be at least 50,000 K, which is about eight times the temperature of the sun’s surface. Energy radiates from surfaces in proportion to the fourth power of absolute temperature, therefore, and assuming similar emissivities for the sun’s surface and super-hot compressed air, the white-hot air in front of the asteroid would radiate 4,800 times as much light per unit of surface area as the sun.
Final Seconds and Milliseconds
Regardless of the local time of day, when the asteroid is 30 miles out, anyone with functioning eyes at that point would see it as 45 times larger than the sun in linear dimension, and its surface area would appear, to anyone able to peer into the brightness, 2,000 times that of the sun. People near the point of impact would feel 10-million times as much radiant energy as comes from the sun. Combustibles on the earth will begin to smolder within a tenth of a second, and then explosively ignite, including trees, houses, clothes, and skin; even rocks would fracture because of the thermal loading. Nearly four seconds would still remain before “impact.”
“Impact” is in quotation marks because up to this point it has been reasonable to think of the impending collision as being between two solid objects, similar to the surface of a rock hitting another rock or concrete or some other hard, solid surface. But the layer of 50,000 K air in front of the object would, in the eight seconds of atmospheric transit time, cause the asteroid’s rocky or metallic surface to melt and begin to boil and merge with the gaseous plasma state of the super-hot air. The situation on the ground becomes increasingly extreme in the final seconds and milliseconds.
The speed of 30,000 mph is 44 feet each millisecond. In the final millisecond, essentially all of the atmosphere that had been in the path of the asteroid would be compressed between it and what had been the earth’s solid surface. The density of the air would approach that of water. At one-tenth of a millisecond before the now hypothetical surfaces meet, the density of the air would be more than twice that of granite and approaching the density of iron. In the final microseconds before “impact,” the density of the trapped air would become equal to that of the adjacent materials and would soon far exceed the density of any ordinary substance as everything becomes a super-hot, high-pressure gas.
Then comes the “collision,” in which the asteroid merges with the matter of the earth.
Aftermath
Were such an asteroid to hit at Washington, D.C., the resultant crater would initially be about 10 miles deep. Its radius would exceed 50 miles. The material in the crater would be liquid hot for days to weeks.
The atmosphere would be pushed out of the way in the region of the impact; space would come down to the surface of the earth, even below what had been the surface. An atmospheric tsunami would radiate outward at an initial speed of thousands of miles per hour. The pressure wave would race across the continent at a slightly slower rate than the ground wave and acoustic waves in the earth. Within an hour, the ground waves and initial air wave would get to St. Louis, Dallas and Chicago; the ground waves would be sufficient to destroy all buildings. The atmospheric pressure waves would sound to people in those cities initially like a rumble, on top of the rumble of the shaking earth; the rumble would rapidly increase to a tornadic blast of wind, followed by the first appearance of airborne dust. Debris falling in from space would arrive shortly.
Washington, D.C., is close enough to the Atlantic Ocean to cause a tsunami to radiate outward. The oceanic wave would trail the atmospheric pressure waves, which would be of sufficient magnitude to disrupt the ocean’s surface. People in Britain and in the western coastal regions of Europe would hear the rumble first, then the disrupted ocean would begin washing inland. Within a few hours at most, the main oceanic tsunami would arrive, well after the great tornadic blast of air.
Acoustic waves traveling through the earth would reach the far side of the earth over an extended period, causing sustained shaking in the region of south Asia, Australia and island in the Indian Ocean. Buildings would be destroyed, but many people would survive the initial shaking. Debris would then fall from the sky.
Within maybe six hours, no part of the world would be unaffected by the shockwaves traveling through the core of the planet itself. Atmospheric shockwaves would traverse the planet many times, reverberating as the hole that would have been punched in the atmosphere moved back and forth in a tornadic oscillation that would decay over days back to equilibrium. For several days, secondary and tertiary shockwaves and earthquakes would continue over the whole planet as previously pent up stresses at tectonic boundaries were released by the flexing of granitic continental and basaltic oceanic crusts.
Some ten-thousand cubic miles of continental rock and underlying mantle material from as deep as 50 miles would have been blown into space. Rocks returning from space would be reheated in passing through the atmosphere. Slower settling, smaller pieces of dust and gravel would fall over the entire planet. The atmosphere would cloud up and block sunlight for several years, causing the earth’s surface to cool down while, at the same time, the low-albedo dust would collect solar energy and heat up, the temperature difference between air at altitude and near the surface resulting in intense winds and storms over the earth.
Within the first few hours, the physical plant of civilization would be destroyed. Cities, communications lines, roads, commerce linkages, all wrecked and burning.
And so it would stay for several years, until the atmosphere became clear enough for the surface of the earth to begin to warm again, plants to come out of stasis, and for small animals to crawl out of the ruins of the former earth and into a new geologic age.
It could end that way, suddenly, without warning. What is so for individuals is so for species and planets: It can all end without warning. It's happened before on the earth. The evidence in the rocks of the earth is compelling: heavy-duty changes have been delivered fast, new ages begun, previous ones have ended without warning.
When It Last Happened
The asteroid that is said to have killed the dinosaurs had enough energy to throw white-hot blobs of matter into space. Some people say that much of the hot material fell back all over the earth, igniting all the forests in the world; others say not. From the point of view of an observer on the moon, the impacting asteroid would have been nearly invisibly small during its approach to impact, but within a few days, maybe even a few hours, the earth would change in color from blue and green to gray and brown.
The kinetic energy of the asteroid considered here would correspond to nearly as much energy as all human beings would use in 300 years at our present rate of energy usage.
Any person or thing situated near the center of the impact zone would, within hours, a day at most, get spread every square centimeter of the earth’s surface -- same as happened to the creatures at that earlier point of impact 65-million years ago, including the ancestors of human beings at that time. It’s plausible that atoms that had once been in dinosaur bodies are now on the moon and some of the atoms would have fallen into the sun; some of those atoms might still be in orbit around the sun. Some of that debris that would have been lifted into space, the finest atomic fractions of it, would by now have been pushed into interstellar space by the solar wind and would be in other stars and on other planets that perhaps tens of light-years away.
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