66 million years ago, a space bomb ten kilometers, or six miles, wide raced through the atmosphere at 20 kilometers per second and landed on the Gulf of Mexico at an angle of 90 degrees. The result was a crater 180 kilometers, or 110 miles, wide.

In an alternate Earth, a ball of pure iron is due to slam into the Great Lakes region and create a crater 300 miles wide. What would be the needed size, speed and angle to create such a hole on Earth?

  • $\begingroup$ does it have to be so recent as 66million y.a.? are you looking to kill off an entire dinosaur era or are you just looking for a geological feature for your modern day setting which could possibly be a remnant from a lot earlier in time? $\endgroup$ – EveryBitHelps Feb 13 '17 at 21:50
  • $\begingroup$ Nope. Being general. $\endgroup$ – JohnWDailey Feb 13 '17 at 22:35
  • $\begingroup$ A while back, I asked a related sort of question on astronomy SE, about how big a crater can be while still retaining a globe like shape. As long as its far enough in the past to allow life to form and evolve after impact the answer seems to be 'fairly big'. May be of some use to you in determinig if your crater size is feasible or not. astronomy.stackexchange.com/questions/18257/… $\endgroup$ – EveryBitHelps Feb 13 '17 at 23:23

Looking at this XKCD What If to see that 11 meters per second is a very reasonable number, then plugging the parameters for asteroid Psyche into this handy impact crater calculator, you get a crater about 250 miles across. So your impactor would have to be either slightly bigger or slightly more dense than Psyche or it must travel slightly faster than 11 kilometers/second.

Edit: I got to thinking about Newton's shortcut for determining impact depth and realized an asteroid the size of Psyche (~132km) would shatter the Earth's crust and actually get in to the mantle. I'm not sure this is desirable. Given that the thickness of the Earth's crust in the Great Lakes region is about 45 km, we'll limit the size of the impactor to 20 km. We'll also give it a density of 4 g/cc (4000 kg/m3) which is slightly denser than diamond but a little short of titanium. This means it must hit the Earth at 170 km/s (or about 380,000 mph). This is faster than the escape velocity of our Solar System, so your impactor would have to be an extrasolar object, but the Sun orbits the galactic core at around 200 km/s so 170 km/s is not unreasonable.

  • 1
    $\begingroup$ Upvoting this for referencing What If. $\endgroup$ – CaM Feb 13 '17 at 21:54
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    $\begingroup$ Erm, that’s 11km/s in your first sentence, not 11m/s, right? $\endgroup$ – Dubukay Mar 6 '18 at 5:13


Enter your variables and get back a crater size.

Here is an example output using your Inputs:

  • Distance from Impact: 300.00 km ( = 186.00 miles )
  • Projectile diameter: 40.00 km ( = 24.80 miles )
  • Projectile Density: 8000 kg/m3
  • Impact Velocity: 20.00 km per second ( = 12.40 miles per second )
  • Impact Angle: 90 degrees
  • Target Density: 2750 kg/m3
  • Target Type: Crystalline Rock
  • Energy before atmospheric entry: 5.36 x 1025 Joules = 1.28 x 10^10 MegaTons TNT

The average interval between impacts of this size is longer than the Earth's age. Such impacts could only occur during the accumulation of the Earth, between 4.5 and 4 billion years ago.

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.

What does this mean? Crater Dimensions:

  • Transient Crater Diameter: 304 km ( = 189 miles )
  • Transient Crater Depth: 108 km ( = 66.9 miles )
  • Final Crater Diameter: 644 km ( = 400 miles )
  • Final Crater Depth: 2.07 km ( = 1.29 miles )

The crater formed is a complex crater.

  • The volume of the target melted or vaporized is 477000 km3 = 114000 miles3.
  • Roughly half the melt remains in the crater, where its average thickness is 6.55 km ( = 4.07 miles ).
  • $\begingroup$ Can you split these into different paragraphs or lists so I can read it better? $\endgroup$ – JohnWDailey Feb 13 '17 at 21:05
  • $\begingroup$ @Will, I edited your paragraphs into lists to seperate out the facts. if I messed anything up, please feel free to change back. $\endgroup$ – EveryBitHelps Feb 13 '17 at 21:40
  • $\begingroup$ Energy and global changes are not the focus here. 400 miles seem too much for a crater to be the core of the creation of an alternate Great Lakes. And I said "iron", not "crystalline rock". $\endgroup$ – JohnWDailey Feb 13 '17 at 22:37
  • $\begingroup$ This is the best calculator I'm aware of. Playing with numbers I get exactly your 300 mile crater with a 25km asteroid of dense rock hitting at 12.5 km/sec. Note that this is far beyond the dinosaur killer, it would almost certainly be an extinction event for all higher life. $\endgroup$ – Loren Pechtel Feb 14 '17 at 2:09
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    $\begingroup$ @JohnWDailey Yes, you said iron asteroid. The answer states the target is crystalline rock. The target is the Earth's crust. The Earth's crust is largely composed of olivine. Olivine is a crystalline rock. This is all correct: An iron asteroid impacting the Earth. $\endgroup$ – Tim Feb 14 '17 at 18:45

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