Quote of the Day
People seldom improve when they have no other model but themselves to copy.
— Oliver Goldsmith, writer
I have seen a number of articles in the popular scientific press about asteroid 1997 XF11 and the close approach it made to Earth back in June. The June approach was not that close – ~27 million kilometers. The closest approach is expected in 2028 and will be 980,000 km or 2.4 times the average Earth-Moon distance.
It seems like stories of asteroids approaching close to the Earth appear every so many months. This makes sense because astronomers have done an excellent identifying a large percentage (~93%) of the large Earth orbit crossing asteroids – back in the old days, we had no idea what was out there.
Some of the news stories focus on the destruction that an asteroid impact from an object like this would cause. For example:
- If it struck it could destroy a whole continent or potentially wipe out life on the planet. (UK article)
- If it were to hit us, would kill between a quarter and a half of the world's population. (Another UK article)
- The asteroid is about a quarter mile (400 meters) wide, large enough to cause considerable local or regional damage were it to hit the planet. (Space.com article)
In this post, I will use a web-based impact calculator to look at the potential impact of an asteroid like this on the Earth.
Background on Asteroid Impact Modeling
This paper was used to provide the various models used in the web calculator. It is quite readable and worth some time.
1997 XF11 Size
It is difficult to determine the size of 1997 XF11 because it is so small that we cannot image it. Thus, we can only estimate its size based on its brightness and assumptions about the reflectance of its surface. The following quote describes the difficulty in measuring the asteroid's size.
Better colour information obtained by imaging the asteroid through different filters may enable the size of the rock to be determined more accurately. The problem is that we cannot actually `see' details on the asteroid at this distance, and so the only way to estimate its size is by its brightness. To do this, we assume that it reflects about as much light as similar objects in the solar system. If 1997 XF11 is actually a much whiter, brighter colour, then it will be smaller for the same brightness: conversely, if it's made of especially dark rock the suggested one mile diameter could be a serious underestimate.
For those interested in the details of estimating an asteroids size from its brightness, see this post.
1997 XF11 Orbit
Figure 2 shows the orbital data from NASA.
The asteroid's speed is a critical parameter in determining its impact significance. Figure 3 shows how I use the orbital speed formula to determine the asteroid's speed.
Figure 4 shows the input dialog for the web calculator. I am assuming nominal values:
- 2 km diameter (middle of the stated range)
- nominal density of 3000 kg/m3
- 45° impact angle (the most common impact angle – Shoemaker)
- an ocean impact at point with a depth equal to the Pacific average
- an orbital velocity of 34 km/s (Figure 3)
- Effect at 1000 km distance (a continental distance)
Figure 5 shows the output dialog for the calculator. You must click on each category to see all the results.
I concatenate all the individual results into a single graphic in Figure 6.
The asteroid impact would be devastating.
- Seismic effects comparable to an 8.1 magnitude quake
- An air blast that breaks windows at 1000 km distance
- A 100 meter high tsunami
The calculator results are interesting. I can believe that the impact from an object like this would be devastating. According to the calculator, an impact like this occurs on average every 7 million years. Since the Earth is 4.5 billion years old, we must have many craters from asteroids like this – I assume most of the craters are underwater and not visible or on land and worn down by geologically processes. I did see that the Chesapeake Bay was formed from the impact of an asteroid about this size.