Saw this on Astronomy Picture of the Day:
Comet Hartley 2 Flyby
They say it’s 2km long (1.2 mi).
How bad would that be if it hit our beautiful blue planet?
Would it wipe out a city? A state?
Would it destroy the world as we know it?
I’m no expert by any means (but I’m sure one will be along shortly) but my research found the object that killed off the dinosaurs is estimated to be about 10 km or about 6.2 miles in diameter and killed off anywhere from 50-90% of the world’s different species, so an object the size you describe probably wouldn’t end life on Earth but I’d imagine it would be a pretty shitty day for everyone.
I would have imagined that a comet would lose an awful lot of its mass before it struck the surface of the Earth.
Just eyeballing it, that’s a volume of about 1 cubic kilometer, or 1 billion cubic meters.
That’s equivalent to a sphere about 1240 meters in diameter.
Feeding a rocky asteroid of 1.2 km diameter travelling at 20 km/sec into the Solar System Collisions Calculator gives an impact energy of 102,909 megatons,
and a crater diameter of 15 kilometers.
Meteor Crater in Arizona is about 4000 feet across and 570 feet deep and it was formed by a meteor about 50 meters across.
While there are differences in composition that may affect things I suspect a comet of this size would cause substantial damage wherever it hit.
It would easily wipe any city off the map.
You can check out Purdue University’s Earth Impact Effects Program to see how bad it would be. For comparison, here are the presumed results from the Chicxulub impact.
The problem is that mass brings its kinetic energy with it even if it ends up in the atmosphere. There’s still going to be a shockwave from it.
:smack: Yes, of course. I should’ve recognized that.
I think it would be bad day no matter where it struck. If it hit in the middle of the Pacific, there would probably be tsunami wreaking havoc all along the Ring of Fire, which has quite a few major cities.
Right. I think the current thinking with regard to any major impact is that it would be far, far better for the thing to hit land, even smack-dab on top of a city, than it would be for the thing to hit ocean.
I tried using the Earth Impact Effects Program, but it generated a 404 error when I clicked the submit button. 
I couldn’t get it to work either.
The comet would be mostly ice and not rock, which reduces the damage somewhat. I used your other figures and fed them into an impact calculator, and got this set of results.
At a distance of 100km from the impact, the thermal radiation would set your clothes on fire. 20 seconds after impact you’d be hit by a major (but not extreme) eathquake. The air blast would probably be the biggest killer, 5 minutes after impact you’d be hit by a 350mph wind, which would level practically every building. 200km out, there would still be many casualties, but probably many survivors.
The energy released would be (very roughly) 1000 times as much as the largest nuclear weapon detonation. But I don’t think this would be anywhere near an extinction level event.
Huh. Works fine for me. This is what I get when I put in some basic parameters:
Your Inputs:
Distance from Impact: 161.00 km ( = 100.00 miles )
Projectile diameter: 1.93 km ( = 1.20 miles )
Projectile Density: 1000 kg/m3
Impact Velocity: 51.00 km per second ( = 31.70 miles per second )
Impact Angle: 45 degrees
Target Density: 2500 kg/m3
Target Type: Sedimentary Rock
(N.B.: the speed, angle, density, etc. were based on the suggested “typical” values for a comet, from the website. I picked a sedimentary rock impact site, and a distance of 100 miles away from the impact site just because.)
Energy:
Energy before atmospheric entry: 4.90 x 1021 Joules = 1.17 x 106 MegaTons TNT
The average interval between impacts of this size somewhere on Earth during the last 4 billion years is 5.2 x 106years
Major Global Changes:
The Earth is not strongly disturbed by the impact and loses negligible mass.
The impact does not make a noticeable change in the tilt of Earth’s axis (< 5 hundreths of a degree).
The impact does not shift the Earth’s orbit noticeably.
Crater Dimensions:
Transient Crater Diameter: 19.8 km ( = 12.3 miles )
Transient Crater Depth: 7 km ( = 4.35 miles )
Final Crater Diameter: 29.3 km ( = 18.2 miles )
Final Crater Depth: 818 meters ( = 2680 feet )
The crater formed is a complex crater.
The volume of the target melted or vaporized is 30.3 km3 = 7.26 miles3
Roughly half the melt remains in the crater, where its average thickness is 98.4 meters ( = 323 feet ).
Thermal Radiation:
Time for maximum radiation: 669 milliseconds after impact
Visible fireball radius: 31.7 km ( = 19.7 miles )
The fireball appears 44.8 times larger than the sun
Thermal Exposure: 8.18 x 107 Joules/m2
Duration of Irradiation: 7.31 minutes
Radiant flux (relative to the sun): 186
Effects of Thermal Radiation:
Clothing ignites
Much of the body suffers third degree burns
Newspaper ignites
Plywood flames
Deciduous trees ignite
Grass ignites
Seismic Effects:
The major seismic shaking will arrive approximately 32.2 seconds after impact.
Richter Scale Magnitude: 8.7
Mercalli Scale Intensity at a distance of 161 km:
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.
Ejecta:
The ejecta will arrive approximately 3.07 minutes after the impact.
At your position there is a fine dusting of ejecta with occasional larger fragments
Average Ejecta Thickness: 32.8 cm ( = 12.9 inches )
Mean Fragment Diameter: 5.41 cm ( = 2.13 inches )
Air Blast:
The air blast will arrive approximately 8.13 minutes after impact.
Peak Overpressure: 277000 Pa = 2.77 bars = 39.4 psi
Max wind velocity: 356 m/s = 796 mph
Sound Intensity: 109 dB (May cause ear pain)
Damage Description:
Multistory wall-bearing buildings will collapse.
Wood frame buildings will almost completely collapse.
Multistory steel-framed office-type buildings will suffer extreme frame distortion, incipient collapse.
Highway truss bridges will collapse.
Glass windows will shatter.
Up to 90 percent of trees blown down; remainder stripped of branches and leaves.
1,000 miles from the impact site, there is some seismic activity and a fine dusting of ejecta, but no significant thermal effects, and relatively minor blast wave effects (e.g., the sound intensity would be as “loud as heavy traffic”, according to the site). So definitely a bad day for anyone at ground zero, but not a planet killer by any means.
Pidcture looked to me like a couple of rocks with some ice deposits on them, and a region of dust between.
A bigger impact on the impact energy is the velocity of the object when it hits atmosphere. 20 Km/sec is sort of the standard value for space objects, but you’d get 4 times the energy if it was moving at 40 km/sec. Someone could probably look up the orbital parameters and give us a range of plausible impact velocities.
You guys are giving the object way too much mass in these simulations.
Figuring on the low end of the object’s estimated size, I went with the 1200m diameter. Given its estimated mass that only works out to an average density of ~331.57 kg/m[sup]3[/sup]. Now, I know I calculated that density for a 1200m spherical object and this object is not spherical, but the impact simulator pages seem to be expecting a spherical object, so I calculated the right density to enter to get an appropriate mass of the comet for the simulation.
According to the JPL’s Small Body Database, 103P/Hartley’s velocity relative to Earth during the recent close-approach on October 20th was around 11.88km/sec. What I did not account for was any acceleration due to the earth’s gravity as the comet approaches. I’m not sure how significant this would be… anyone?
Anyway, depending on where it hit, it wouldn’t be that bad. Say it was 100km offshore, and hit in 1000m deep ocean:
I’d be interested to see what effect gravity acceleration has, though. Next time it’s scheduled to pass us, it’s predicted to have a 18.66km/s relative velocity. Then things start catching on fire 100km away, you wind up with up to a 64m tall tsunami, etc.
I’ve seen estimates that anything over 1 km in diameter impacting would have global effects. This is big enough to do that. We think there would be an impact winter. I’d guess the global effect might be on the scale of what the 1883 eruption of Krakatoa or the 1815 eruption of Tambora did. You might get some weird weather (such as snow and frost in June in places that normally don’t get it), rising food prices possibly causing famines in some areas, that sort of thing.
The local effects would, of course, be much worse.
The pictures are quite deceptive, what you’re looking at is a pile of rubble, not a solid chunk of rock. Comets are mostly ices, although with a rocky crust. NASA definition of a comet is that it must be at least 85% ice. Halleys comet, for example, is estimated to be less dense than solid ice, as isn’t solidly packed.
Yes, that would help pin it down. I can’t find the figures now, but there is a well defined range of plausable velocities, due to orbital mechanics.
Yes, there would be climatic effects, but I’m not sure how serious they would be. The mass of the comet and the energy released would be a small fraction of of the chixulub impact. IIRC, the chixulub was particularly bad because it hit rocks rich in sulphur compounds, which led to a large release of sulphur dioxide.
Very roughly, something that size would kill a small country. A much smaller object could potentially be a city killer. The Tunguska impact in 1908 was about 30 megatonnes, only a single nuclear weapon test was more powerful (the impractical Tsar Bomba). The impactor was about 50-100m across.